Note: The source of the technical material in this volume is the Professional Engineering Development Program (PEDP) of Engineering Services. Warning: The material contained in this document was developed for Saudi Aramco and is intended for the exclusive use of Saudi Aramco’s employees. Any material contained in this document which is not already in the public domain may not be copied, reproduced, sold, given, or disclosed to third parties, or otherwise used in whole, or in part, without the written permission of the Vice President, Engineering Services, Saudi Aramco. Chapter : Electrical For additional information on this subject, contact File Reference: EEX21608 W.A. Roussel on 874-1320 Engi neerin g Ency clo pe dia Saudi A ramco DeskTop Standards Selecting Low Voltage Motor Starters
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Motor Voltage.....................................................................................50Continuous Current.............................................................................50
Special Criteria ...................................................................................51
Motor Contactor Auxiliary Devices ............................................................53
kVA Code/Locked-Rotor Amperes ....................................................88Voltage and Horsepower ....................................................................88
Figure 25. Example of AC Coil Voltage Ratings for NEMA Size 3and 4 Low Voltage Contactors ...........................................................55
A major component of a motor starter is the enclosure. To properly select low voltage motorstarters, it is necessary to understand the physical arrangements of motor starter enclosures.
This Information Sheet explains the physical arrangements of enclosures by describing
components that are common to all enclosures and by describing various types of enclosures.
Common Enclosur e Components
Motor starter enclosures have several components that are common to all types of enclosures.
These components include a disconnecting means, lock and tag features, and enclosure
interlocks. Descriptions of these common components are given in the following paragraphs.
Disconnecting Means
A common component that is included on all types of enclosures is a means of externally
operating the disconnect device that is mounted inside of the enclosure. This component is
typically a flange mounted handle located on the outside of the enclosure as shown in Figure
1. The handle is mechanically fastened to an operating mechanism that is located inside of
the enclosure and that attaches to the disconnecting device (disconnect switch or breaker).
The handle provides for external operation of the disconnecting device, and it gives positive
visual indication of its status (open or closed).
Lock an d Tag Featur es
A common component of enclosures that is very important for safety is the provision to
padlock the operating handle. This provision allows one or more padlocks to be inserted
through a hole in the operating handle to lock it in the “Off” position. The purpose of this
feature is to allow the motor starter to be locked in the de-energized position and tagged with
a “Warning” tag to provide for safe inspection and maintenance of the motor. The location of
this locking provision is identified for the enclosure shown in Figure 1. In addition to the
capability of padlocking the operating handle, enclosures also allow padlocking of the cover
Motor starters are typically mounted in NEMA-type enclosures. However, for someapplications, motor starters are mounted on flat, open panels. In accordance with NEC Article
430-132, motor starters operated at 50 volts or more between terminals must be guarded
against accidental contact by mounting in an enclosure or by locating in a controlled room,
controlled balcony, or at an elevation of 8 feet or more.
In older manufacturing facilities, open panel mounting was normally accomplished by
mounting the motor starters on pole-supported slate or micarta panels. These panels, which
are sometimes called “electric switchboards”, were also used to mount other electrical
controls needed for the facility. The switchboards, which are usually supplied by open-type
uninsulated bus, were typically located in a dedicated room where access was allowed only to
qualified electricians and to authorized managers.
For modern applications, open panel mounting of motor starters is typically accomplished by
fastening the starters to a flat, painted, steel panel. The panel is then mounted in a large steel
cabinet or in a separate control room. When the starters are mounted in this manner, their
continuous current rating is increased in accordance with the NEMA ICS2 contactor ratings
shown in Figure 2. With reference to this figure, it is noted that the ratings for open panel
mounting are 110% of the ratings for enclosed mounting.
The single wall-mount enclosure is the most commonly used type of enclosure. A typicalsingle enclosure, similar to the one illustrated in Figure 3, offers the advantage of placing
individual starters at their most convenient location while still providing all of the common
component features described above (i.e. disconnecting means, lock and tag features, and
enclosure interlocks).
Single enclosures are also designated by a NEMA-type number that indicates the
environmental conditions for which they are suitable. NEMA enclosure types and
classifications are described in the following Information Sheet (NEMA Enclosure
Classification System).
Single wall-mount enclosures are available from manufacturers in a number of sizes. Therequired size for an enclosure is recommended by the manufacturer and is determined by the
type and size of combination controller to be housed. When needed, extra space can be
requested by the user to accomodate field-mounted control components.
A group wall-mount enclosure is essentially several single enclosures designed and
manufactured as one unit but with individual internal compartments. The group-type
enclosure is designed to save time, space, and expense when installing multiple control
devices.
Figure 4 shows an example of a group enclosure with four compartments for mounting four
combination controllers. Group enclosures are typically partitioned into either four or six
compartments. Each compartment is designed to hold a combination starter, incoming or
feeder circuit breakers, fusible switches, or other auxiliary devices. The barriers between
compartments can be removed to provide oversize spaces allowing for installation of a lesser
number of larger size controllers.
In addition to the barrier compartments, the group enclosure normally contains internal wiring
troughs. Typically, one trough is located at the top and is fitted with power terminal straps forextension to adjoining compartments. Another wiring trough is located along the bottom for
interconnecting wiring and outgoing cables.
The compartments have hinged doors that are interlocked to prevent opening them when the
breaker switch is in the “On” position. In addition, the disconnect operating mechanism can
be padlocked in either the “On” or “Off” positions.
The metal enclosure of the motor control center is built with a single steel-channel frame that
has compartment-like spaces for insertion of individual combination starters. The individual
compartments of the enclosure share common bus systems and wireways. With regard to the
bus systems, a main horizontal bus is installed across the top of the unit to provide three-
phase power distribution from the incoming line or primary disconnect device to each vertical
structure. A vertical bus is mounted in each vertical unit to provide distribution of the main
bus power to each of the individual vertical compartments. Completing the arrangement of
bus systems is a neutral bus mounted on stand-off insulators across the bottom of each
vertical unit and a ground bus mounted across the top of each unit. With regard to the
wireways, the enclosure has both vertical and horizontal wireways to provide for convenient
servicing and controller change-outs. All wireways are provided with hinged panel covers for
easy access and as a barrier to fire.
For this type of enclosure, a steel compartment shell, referred to as a drawout case, is
provided for each compartment. Figure 6 shows the construction of a typical drawout case.The drawout case, comprised of three sides and a base, serves as a housing for mounting of
each starter. Four mounting points on the drawout case allow it to engage guide rails, located
near the top of the compartment space, for easy insertion and withdrawal. A quarter turn latch
located at the top of the case securely holds it in the compartment after insertion.
In addition to the handle mechanism, a control panel is mounted on the front of the drawout
case. The control panel allows mounting of pushbuttons, indicator lights and related control
devices. The arrangement of mounting both the handle mechanism and the control panel on
the front of the drawout case helps to make inspections and maintenance easier.
A final feature of this type of enclosure is the compartment door. Each compartment of the
motor control center has a separate hinged door that allows the handle mechanism and control
panel to protrude through the door when it is closed. The doors are typically secured in the
closed position using two quarter turn indicating type fasteners. As described above, an
interlock prevents the door from being opened when the handle is in the “On” position.
Saudi Aramco Applications
With regard to Saudi Aramco application of enclosures, SAES-P-114 requires that a motor
controller be either a combination motor starter or a circuit breaker. When the controller is a
combination motor starter, the enclosure for the controller is provided by the manufacturer asan integral part of the starter. The provided enclosure is designed and assembled in
accordance with NEMA Standards 250 and ICS-6 to meet specific application environmental
conditions (NEMA enclosure types and classifications are described in the following
Information Sheet). The enclosure provided by the manufacturer also includes the common
enclosure components described above (a disconnecting means, lock and tag features, and
enclosure interlocks).
When the controller is a circuit breaker, the enclosure is provided by one of two means.
Either the circuit breaker is designed and constructed with a self-encasing enclosure, or the
breaker is designed for mounting inside of a metal-enclosed switchgear compartment.
With regard to enclosures applied for low voltage motor control centers (MCC), 16-SAMSS-
503.4.2 requires that MCC’s be rigid, free-standing, metal-enclosed structures, consisting of
vertical sections assembled into a group having a common bus and forming an enclosure to
which additional sections may readily be added. The enclosures must be suitable for back-to-
wall or back-to-back mounting. Back-to-back constructions having a common horizontal bus
are not acceptable. The MCC cubicle design must be NEMA Class I, Type B, with all
ventilation openings suitably filtered or screened with a specified corrosion-resistant material
arranged to prevent entrance of rodents and other foreign matter.
and 13. Type numbers applied to enclosures for hazardous location use include Types 7, 8, 9,and 10. Enclosures covered by this classification system are nonventilated, except that Types
1, 2, and 3R enclosures may be either nonventilated or ventilated.
Figures 9, 10, and 11 give a brief overview of the types of enclosures included in the NEMA
classification system and the environmental conditions that they protect against. Figure 9
shows an overview comparison of enclosures used for indoor nonhazardous locations, Figure
10 shows a comparison of enclosures used for outdoor nonhazardous locations, and Figure 11
compares enclosures applied to indoor hazardous locations.
Detailed descriptions for selected enclosure types are provided in the sections that follow
Overload relays are protective devices that guard low voltage AC motors against a variety of abnormal conditions that can overheat motor windings. The overload relays are designed to
accomplish this protection by reflecting the heating characteristics of the motors that they
protect. The two main components of an overload relay are the relay itself and the heater
element.
When selecting an overload relay and its heater elements for application, several factors must
be considered. These factors include the motor full-load current and service factor and the
relay style, class, type, temperature compensation, and pole arrangement. This Information
Sheet describes these overload relay selection factors. Note: Work Aid 1 has been developed
to help the Participant select an overload relay.
Motor Data
Full-Load Amperes
An important factor used in the selection of the overload relay is the motor nameplate full-
load amperes. The amperes marked on the motor nameplate represents the amount of
amperes that the motor will draw continuously when delivering its nameplate-rated
horsepower at nameplate-rated voltage and frequency. When an overload relay is applied to a
motor circuit, it senses the motor line currents either directly or indirectly. For the case where
the overload relay senses the current directly, the motor amperes flow directly through the
relay and its heater elements. For the case where the overload relay senses the currentindirectly, the motor amperes flow through the primary winding of a current transformer (CT)
and allow the relay to sense the current via the secondary winding of the CT.
Because overload relays sense the line currents of a motor, they are sized according to the
amount of amperes that they are capable of handling. Each size of relay is rated with a range
of amperes that it can safely and continuously carry. Figure 13 shows an example of the
ampere rating range for a few sizes of one particular manufacturer’s overload relay. When
selecting an overload relay, the selected size must have a current range that covers the full-
load nameplate amperes of the motor to which it is applied.
As schematically shown in Figure 15, a bi-metallic overload relay has two basic components:the relay itself, which contains the bi-metallic actuated contact, and the heater elements. The
relay is available as either a single-pole relay or a three-pole (block) relay. The heater
elements are constructed of resistance wire or similar material, and they are mounted inside of
the relay body. Following is a description of each of these basic overload relay components.
Block -type relays are three-pole bimetallic, thermally actuated relays.
The physical construction of the block-type relay includes three sets of motor current-carrying
connection terminals mounted on an insulated housing and used for connection to a three-
phase motor circuit. Contained within the insulated housing (body) of the relay are provisions
for inserting and connecting interchangeable heater elements. The relay provides a circuit
that allows motor current to flow into the relay connection terminals, through the heater
elements, and back out to the motor circuit.
Also contained within the insulated housing (body) of the relay is a bimetallic strip that is
used to detect the heat generated by the interchangeable thermal elements. The bimetallic
strip is mechanically connected to and operates a single-pole, single-throw, snap action
switch. The snap-action switch is used to open the control circuit of the starter.
The block-type relay is rated in accordance with the range of full-load current that it is
capable of carrying, the NEMA size of contactor it connects to, and the interchangeable heaterelements designed for use with it.
Heater elements are constructed of resistance wire or similar material. They are designed to
be inserted into and connected to the overload relay. Each block-type relay is constructed
with three individual compartments to accept three individual heating elements. The heaters
are connected to the relay in an arrangement that allows the motor current or CT secondary
current to flow directly through them.
Individual heating elements are marked with their heater type numbers. Each manufacturer
has its own form of designating the heater ranges and ratings. The precise current that a
heater element is rated at depends on many factors, such as the number of heaters included inthe overload relay and the type of enclosure used for the starter. However, in all cases,
heaters are rated based on a range of motor amperes at which they will generate sufficient
heat to cause the overload relay to operate. Typically, the heater(s) selected will provide for
the overload relay to operate at 115% to 125% of heater rating at an ambient of 40oC.
Solder-pot overload relays are thermally responsive relays that contain two basic component:a ratchet mechanism that operates a NC contact and a heater element as schematically shown
in Figure 16. Following is a description of these basic components.
Ratchet Mechanism - With reference to Figure 16, it is noted that the ratchet mechanism is
comprised of several parts. One part is a small cylinder that contains an alloy (e.g. solder)
that will melt due to heat produced by excessive current flow. Within this cylinder is a
portion of a shaft that is prevented from turning by the holding action of the alloy. The other
end of the shaft is connected to a toothed ratchet wheel that interlocks with a pawl and holds a
spring loaded actuator in the loaded position. At the end of the actuator travel path is an NC
contact that is operated when the actuator is released and allowed to reach the end of its travel
path.
Heater - The heater element for this relay is designed in the form of a resistance wire coil that
mounts around the cylinder containing the alloy. Similar to the heater elements used for the
bi-metallic type relay, the heater elements for the solder-pot relay are designed to produce a
precise amount of heat in direct proportion to the motor current that flows through them. The
heater elements are rated in accordance with a range of motor current that will cause the
overload relay to operate when excessive motor current flows for a specified period of time.The characteristics of the heater cause the overload relay to operate with an inverse time-
current characteristic.
Oper ating Pr inciples
With reference to Figure 16, the operation of the solder-pot relay can be described by first
noting, when the overload relay is connected for operation, that its heater terminals are
connected to the motor circuit to allow motor current to flow through the heater. Prior to an
excessive flow of current, the alloy in the cylinder is in a solid state allowing the ratchet to
hold the actuator in place. When an excessive amount of current flows through the heater for
a specific amount of time, the heat generated by the heater element acts directly on the alloy
film, melting it at a precise temperature. Once the alloy is converted to a liquid state, the shaft
within the cylinder is released allowing it to turn and rotate the ratchet wheel. Rotation of the
wheel releases the pawl, which in turn releases the spring-loaded actuator. The released
actuator then travels to the NC contact, and operates it to open the coil circuit of the starter.
Solid-state overload relays monitor motor line current and use semiconductor circuits to
determine the heating effects that the level of current will have on the motor and conductors.
Components
The basic components that make up a solid-state relay are the main body (or block) and a
selection of current sensing and special function plug-in modules. Following is a description
of these components.
Block - The main body (or block) of the solid-state overload relay is physically constructed to
hold three sets of motor current-carrying connection terminals mounted on an insulated
housing. When placed in operation, the terminals are connected to the motor circuit to allow
motor current to flow through the relay.
Contained within the relay body are built-in current transformers that are used to monitor the
motor line currents and to translate them into logic level signals. Also contained within the
body of the relay is a semiconductor circuit that represents a thermal model of the motor. The
thermal model is typically calibrated to have an exponential function with NEMA overload
relay Class 10 characteristics.
The main body of the relay provides for mounting of selected plug-in modules to build in the
amount and type of protection desired. The selection of plug-in modules include current
sensing modules and special function modules.
The main body of the relay also houses an electromechanical relay contact that is used foropening the coil circuit of the starter. This contact is normally provided as a single-pole
single-throw (SPST) NC contact that is closed when the relay is energized and that opens
when the relay trips or when control power is removed.
In addition to the above features, the solid-state overload relay is ambient-compensated, has
both manual and automatic reset capabilities, and indicates overload trip operations through
use of light emitting diodes (LEDs).
Modules - In place of the type of heater elements used by thermally actuated overload relays,
the solid-state relay uses a plug-in module, shown in Figure 17, that is identified as a current
sensing module. This module, sometime referred to as a “heater” module, receives the logiclevel signals that represent the motor line current, and it determines the relative heating effect.
Figure 17. Cur r ent Sensing (Heater) Plug-In Module for
Solid-State Over load Relay
Although the current sensing plug-in module receives logic level signals but does not receive
actual motor amperes, it is still rated in units of motor line amperes. Nominal ratings for the
current sensing plug-in module range from 0.54 amperes to 150 amperes. When a currentsensing module for the solid-state relay is selected, the selection is made in accordance with
the percent of full-load current desired to trip the overload relay. Similar to thermal type
relays, the solid-state overload relay normally provides for trip operation at 115% to 125% of
motor full-load amperes at 40oC.
In addition to the current sensing plug-in module that is required for operation of the solid-
state overload relays, several special plug-in modules are available for optional selection to
provide additional types of protection for the motor. These modules are physically plugged-
in, adjacent to, or in tandem with, the current sensing module. The special function modules
available for selection include: a phase unbalance module that trips the solid-state relay when
line currents are unbalanced, an overtorque protection module that trips the relay whenovertorque conditions exist for specific periods of time, a long acceleration module that
permits extra acceleration time beyond NEMA Class 10 characteristic, and an underload
protection module that senses loss of motor load and, then, trips the relay.
Operation of the solid-state overload relay with a properly sized plug-in current sensing
module follows the inverse time-current curve shown in Figure 18. Based on this curve, the
relay will trip after 7 seconds at 600% full-load amperes for “cold” starts, after 4 seconds at
600% full-load current for “hot” starts, and ultimately at 115% of full-load current for long
periods of time.
A principle advantage of the solid-state relay over the thermally actuated type is that the solid-
state relay operates with a one percent accuracy. The thermal type relay is not as accurate
because small variations in tolerances in the mechanical elements of a thermal relay result in
large variations in performance. On the other hand, solid-state overload relays are more
expensive than thermal types, which make them less popular for smaller, less critical motors
and loads.
Operation of the solid-state relay is accomplished with the CTs monitoring all three phases of the motor current. The current signals from the CTs are transposed, via solid-state circuits, to
a logic level signal and then transmitted to the current sensing plug-in module. The plug-in
module, which also contains solid-state circuitry, receives the logic signals and, using the
thermal model circuit built into the relay, it determines the corresponding heating effects on
the motor. When the current sensing module determines that the flow of current is excessive
for a specified period of time (in accordance with Figure 18), it sends a trip signal to the NC
electromechanical relay contact in the main relay, operating the contact and thus opening the
The Type A relay is available as either ambient-compensated or non-compensated. Ambient-
compensated relays have the advantage of providing the same trip characteristics in ambient
temperature from -40oC to +77oC. Compensated and non-compensated relays are generally
identified by the color of their reset rod.
For the Type A overload relays, interchangeable thermal heater elements for single-pole and
block-type relays are available to cover motor full-load currents from 0.29 to 133 amperes in
approximately 10% steps.
Type B
Using a block-type, bi-metallic design that provides Class 20 operation in either single or
three-phase applications, the Type B overload relay is similar to the Type A overload relay in
that it is also designed to protect industrial motors against overload conditions.
Additional similarities of the Type B with the Type A relay include: available ambient-compensated and non-compensated models, inverse time delay trip operation, standard SPST
In the selection of overload relays, it is important to note and consider temperatureenvironmental conditions. Following are conditions that should be considered.
Motor-Ambient - In accordance with NEMA MG-1, the ambient temperature rating of the
motor is the maximum temperature of the medium and gases surrounding the motor that the
motor is designed to operate in and to meet the ratings of its nameplate. Increased ambient
temperature will cause an increase in motor operating temperature, which in turn presents a
risk to the motor.
In the selection of overload relays, NEC Article 430 addresses the consideration of motor
temperature rise and thus motor ambient temperature by requiring that overload relay trip
ratings be limited based on rated motor temperature rise. In accordance with NEC Article430, overload trip settings are to be limited to a maximum of 115% of motor full-load current
for motors with a temperature rise greater than 40oC.
Starter-Ambient - The ambient operating temperature of the starter should also be considered.
Starters operating in a constant ambient temperature that is within the rating of the overload
relay will allow the relay to operate properly. This operation will provide for consistent and
acceptable protection of the motor. For this condition, it is not a requirement to use a
temperature compensated overload relay.
Severe Environments - For some cases, a starter and its overload relay may be located in one
area where the ambient temperature varies, while the motor is located in a different areawhere the ambient is constant. The varying ambient temperature at the starter can result in
improper operation of the overload relay. This operation will cause the protection of the
motor to be affected. For this condition and similar conditions, where ambient operating
temperature for the starter and the overload relay vary, it is important to use an overload relay
In accordance with NEMA ICS-2, an overload relay identified as ambient temperature-
compensated indicates that the ultimate current that causes the relay to trip remains essentially
unchanged over a designated range of ambient temperatures.
The important feature of an ambient-compensated overload relay is that motor overload
protection is provided with substantially the same trip characteristics in ambient temperatures
that vary. Overload relays typically provide ambient temperature compensation for
temperatures ranging from -40oC to +75oC.
In thermally actuated overload relays, temperature compensation is typically accomplished
through use of a compensating bi-metal that is responsive only to heat generated by motor
current that is passing through the heater element. The bi-metal maintains a constant “travel
to trip” distance that is independent of ambient conditions. In this way, the operation of the
relay remains essentially unchanged by any change in ambient temperature. Thecompensating feature is fully automatic, and no adjustments are required for its use when it is
supplied with the overload relay.
An ambient-compensated overload relay should be used whenever the control is located in a
varying ambient temperature area and whenever the motor that it protects is in a constant
ambient temperature.
Non-Ambient
Non-ambient compensated overload relays are relays that do not have built-in features to
automatically compensate for varying ambient temperatures. Whenever the overload relay islocated in an area with a constant temperature, or whenever it is located in the same area as
the motor, compensation may not be necessary.
Note: 16-SAMSS-503.5 requires overload relays to be ambient temperature-compensated.
Thermally activated overload relays are available as single-pole or three-pole arangements.Single-pole overload relays can be used for application on single-phase circuits, or three
individual single-pole units can be combined for use on a three-phase application.
The single-pole unit works as an independent overload relay with its own heater element and
its own NC contact to open the starter coil circuit. Selection of a single-pole unit is
accomplished in the same manner as selection of a three-pole block unit, with the selected
relay rating and heater rating being based on full-load current. When three single-pole units
are applied to a three-phase application, the individual NC contacts of the three units are
connected in series to allow any one of the three to open the starter coil circuit.
The major advantage of selecting three single-pole units for a three-phase application is that
the arrangement provides improved protection against a single-phasing condition, where onephase of the three-phase circuit becomes open. The disadvantages of using three single-pole
units for a three-phase application in place of a single three-pole block are increased cost and
increased space requirements.
Note: 16-SAMSS-503.5 requires thermally actuated overload relays to be three-pole block
type.
Three-Pole
The use of a single three-pole overload relay for three-phase applications is the arrangement
that is commonly used. This arrangement provides for the three current carrying poles of therelay to be mounted in the same insulated housing. The relay contains only one NC contact
for use in opening the starter coil on a relay trip.
With the three-pole arrangement, overload relays can be designed to work with one, two, or
three heater elements. Most modern thermally activated overload relays are designed to use
three separate heater elements. The body of the relay is designed to allow mounting and
connection of each heater in its own compartment, with the heat generated by all three heaters
acting on the bi-metallic strip that operates the relay NC contact.
The advantages of the three-pole arrangement are that it is compact, economical, and
efficient. The disadvantage is that it is not able to provide reliable protection against a single-
Single-phasing is a conditions that occurs when one phase of a three-phase circuit supplying amotor becomes open and allows the motor to operate as a single-phase motor. For this
condition, the current in the phase that opens goes to zero while the current in the other two
phases increases. Operating in this unbalanced condition results in overheating of the motor
and can lead to damage or failure of the insulation if not detected quickly enough.
The potential problem when this condition occurs and a single three-pole block-type thermal
overload relay is connected in the circuit is that the relay may not be able to detect the
condition and operate. The operation of the relay depends on the combined heat generated by
all three heater elements. With the relay operating with one phase open, the heater in the
open phase will not generate any heat, and even though the current in the other two phases
has increased, the increased heat of the two elements may not be sufficient to result in a totalamount of heat that will activate the bimetallic strip and operate the relay.
Alternatively, if the single-phasing (or open-phase) condition occurred in a circuit using three
single-pole relays, each of the relays in the two conducting phases would immediately detect
the increase in current and, in accordance with its time-current curve, cause its relay to
operate.
The three-pole solid-state relay does not have the same problem as the thermal relay in
detecting an unbalanced current condition. For the solid-state relay, a special function plug-in
module is available to detect unbalanced current conditions. Modules are available to trip on
detecting either a maximum of 10% current unbalance or 20% current unbalance.
A major component in all motor starters is the contactor. The contactor is essentially an on-
off switch that is operated by electromechanical means and that controls the flow of current to
the motor. When selecting a contactor for application in a motor starter, several factors must
be considered. These factors include the type of contactor to be selected (air-magnetic or
vacuum), the size of contactor required for the application, the need of contactor auxiliary
devices for operation of the control circuit, and the proper contactor coil voltage rating. This
Information Sheet describes these contactor selection factors. Note: Work Aid 2 has been
developed to help the Participant select a contactor.
Motor Contactor Types
Air-Magnetic
The air-magnetic contactor is the most common type of contactor selected for motor starterapplications. Figure 20 shows a typical NEMA air-magnetic contactor with an overload relay
connected to its load terminals. This type of contactor is generally selected because it is
economical and easy to maintain and because it has a versatile design that provides for
accommodating a great many variations in the method of control.
The electrical portion of the contactor consists of an electromagnet, a coil, and a moving
armature or crossbar. Moving and stationary contacts, arranged in sets or poles, carry the
motor current. Air-magnetic contactors are often provided with three poles or sets of
contacts. However, other configurations, such as two, four, or five poles are available.
When power is applied to the contactor coil, magnetic flux is created in the electromagnet.The magnet then attracts the armature, pulling the moving contacts into the stationary contacts
and allowing power to flow through the contacts to the motor.
The air-magnetic contactor must be able to close, carry, and open normal motor current. As a
result, the contactor is rated in accordance with the size of load that it must control. NEMA
standards provide two methods of rating the air-magnetic contactor. One is a rating based on
horsepower and the other is a rating based on motor full-load and locked-rotor current.
Low-voltage air-magnetic type contactors are designated by NEMA (and available from
manufacturers) in sizes 00 to 9 with horsepower ratings from 1.5 hp to 1600 hp. Note: 16-
SAMMSS-503.4.4 requires that air-magnetic contactors be selected based on horsepower rating.
When selecting a contactor, an important selection factor to consider is the size and rating of the contactor required for the application. In accordance with NEMA ICS-2, contactors
(controllers) are rated by means of two methods. One rating is based on horsepower, and the
other rating is based on motor full-load and locked-rotor current. The method of rating
contactors based on horsepower is the one more rating that is commonly used and the one
rating that is required by 16-SAMSS-503.4.4.
Because both the full-load and locked-rotor currents are a function of the horsepower rating at
a specified voltage, motor contactors (controllers) are rated for the maximum horsepower that
they can safely handle at these voltages. The motor contactors (controllers) are classified by a
size number, and they are rated in horsepower. Figure 22 shows the maximum horsepower
ratings for three-phase, single-speed full-voltage magnetic contactors for nonplugging andnonjogging duty as designated by NEMA. As the NEMA size classification increases, so
does the physical size of the contactors (controllers), because larger contacts are needed to
carry and break the higher motor currents, and heavier mechanisms are required to open and
close the contacts.
The NEMA size horsepower ratings shown in Figure 22 are based on the mechanical and
electrical requirements for starting a NEMA design B or C motor that has normal acceleration
time and normal start/stop duty. If greater than normal duty is required such as motor
jogging, long acceleration time, or dynamic braking, a controller of larger than normal size is
used. Tables showing the recommended sizes and horsepower ratings for greater than normal
Another factor that must be considered when selecting a contactor is the voltage rating
required for the contactor. Low voltage contactors are designed for service on circuits rated
to 600 VAC. However, for a given NEMA size contactor, the horsepower rating for thecontactor is dependent on the voltage level at which the contactor is applied.
With reference to the table of horsepower ratings shown in Figure 22, it is seen that for a
given NEMA size contactor, its horsepower rating is reduced when applied at the lower
voltage levels. For example, a NEMA size 1 contactor is rated to control an AC induction
motor with a maximum nameplate rating of 10 horsepower at a nameplate voltage rating of
460V or 575V. However, the same NEMA size 1 contactor, when it is operated at a voltage
of 200 VAC or 230 VAC, is rated to control only a 7.5 horsepower motor.
When selecting a contactor, it is necessary to use both the motor nameplate voltage and the
motor nameplate horsepower for the selection process.
Continuous Curr ent
When a contactor is being selected, another factor to consider is the continuous current rating
of the contactor. In accordance with NEMA ICS-2-321, each NEMA size contactor is
designated with a continuous current rating. This rating represents the maximum rms current,
in amperes, which the contactor (controller) is permitted to carry on a continuous basis
without exceeding the temperature rises permitted for the contactor.
For example, with reference to Figure 22, it is seen that the maximum rated continuous
current for a NEMA size 1 contactor is 27 amperes. When selecting a contactor, this value
should be compared with the continuous full-load current rating for the motor.
One exception for the continuous current rating of the contactor, is the “service-limit current
rating”. The service-limit current represents the maximum rms current, in amperes, which the
contactor is permitted to carry for protracted periods in normal service. At service-limit
current ratings, temperature rises are permitted to exceed those ratings that are obtained by
testing the contactor at its continuous current rating.
For example, with reference to Figure 22, it is seen that the service-limit current rating for a
NEMA size 1 contactor is 32 amperes. This service-limit current rating implies that the
contactor may be used at this current level for reasonable periods during normal service (i.e.,
the high-current intervals of load cycles, long acceleration times, short periods of dynamic
braking, etc.), however, it is expected that the temperature rise of the contactor will exceed itscontinuous current temperature rise.
Dynamic Braking - Another special factor that may exist for a motor application is the
requirement of the motor to be used for dynamic braking. When used for this purpose, the
motor may be required to carry higher than normal nameplate current for a period of time.
When a contactor is being selected and if it is known that a motor will be used for this type of
service, an inspection should be made to determine the level and duration of current in regard
to NEMA requirements for a Class A contactor. When the NEMA limits are exceeded by the
application requirements, a larger size contactor should be selected.
Star ting Duties - Under normal starting conditions, an AC induction motor is expected to draw
approximately 6 times normal current for the starting period. However, when a motor is
accelerated from standstill to full speed with its shaft mechanical load fully applied, the
current drawn by the motor can be larger. When it is determined, during the process of
selecting a contactor, that a motor must be started with its full mechanical load being applied,
then an inspection should be made to determine the duration of the load and the maximum
current for the starting period. For this condition, as for the conditions of dynamic brakingand long acceleration time, a larger contactor size should be selected in accordance with the
NEMA limits for a Class A contactor.
Contact L ife
Another special factor that may exist for a contactor application is the requirement for greater
than normal interrupting duty. Under this condition, contact wear will exceed normal wear
rates and contact service life will be shortened. The service life of the interrupting contacts on
a contactor is directly related to the amplitude of arcing current interrupted and the number of
times interruption is required. When greater than normal interrupting service is expected for
the contactor, a larger NEMA size contactor should be selected. The general rule is to select
During the selection of a contactor, auxiliary devices are available for selection and inclusion
on the contactor. Consideration should be given to the contactor application and the control
circuit arrangement to determine if these items are needed and if they should be selected.
Two auxiliary items that may be considered are auxiliary contacts for the contactor and
interlocks. Following is a description of these items.
Contacts
Depending on the complexity of the control circuit to be used for the contactor being selected,
additional auxiliary contacts may be required in addition to the standard ones provided with
the contactor. For this case, manufacturers typically offer one or more types and sizes of
auxiliary contacts that can be added to the contactor. Some of these auxiliary contacts can be
assembled to the contactor in the field while others may require factory assembly. Figure 23
shows one manufacturer’s offering of one type of auxiliary contact that can be added to size00 through size 1 contactors at the factory or in the field. The auxiliary contact shown in
Figure 23 can be provided as a NO or NC contact, and it can be selected with either an 18
When a low voltage motor contactor is being selected, another important factor to consider is
the voltage rating of the coil for the contactor. This rating is the voltage that must be applied
to the contactor coil in order to operate the contactor. The coil voltage rating is selected to be
equal to the voltage rating of the motor starter control circuit.
Because there are many different voltage rating for control circuits, low voltage contactors are
available with a wide selection of AC and DC coil voltage ratings.
With reference to contactors with AC voltage coils, manufacturers typically offer coil voltage
ratings from 24 volts AC to 600 volts AC in a number of steps. As an example, Figure 25
shows the standard AC coil voltage ratings offered by a typical manufacturer for NEMA size
3 and 4 contactors. Other voltage ratings are usually available as a special order. In
accordance with NEMA Standard ICS 2-110, these alternating current-operated contactors
must be able to withstand 110 percent of their rated voltage continuously without injury to theoperating coil, and they must close successfully at a minimum of 85 percent of their rated
voltage.
NEMA Contactor Size Cont. Rating Amperes AC Coil Volts
3 90 120
3 90 208
3 90 240
3 90 480
3 90 600
4 135 120
4 135 208
4 135 240
4 135 480
4 135 600
Figur e 25. Example of AC Coil Voltage Ratings for NEMA Size 3
Inverse-Time (Therm al-Magnetic) MCCBs have a thermal-magnetic tripping action. The current
path within the breaker is through a bimetallic strip. A bi-metal consists of two strips of metal
that is bonded together. Each strip has a different thermal rate-of-heat expansion. As the
current passes through the bi-metal, the bi-metal strip heats up and bends. Greater current
passing through the bi-metal will generate more heat, resulting in faster bending of the strip.
The bi-metal continues to bend until it moves far enough to mechanically unlatch the breaker
mechanism, allowing the breaker to open. This thermal action is called an inverse-time
characteristic (as the current increases, the time to trip is less).
For high fault currents, the thermal action is too slow to protect the downstream devices,
therefore a magnetic trip action is used. The magnetic trip action functions by use of an
electromagnet in series with the load current. When a short circuit occurs, the fault current
passing through the circuit causes the electromagnet in the breaker to attract the armature,
initiating an unlatching action. This magnetic trip response is instantaneous. By definition
instantaneous means “no intentional time delay.” The magnetic action is usually adjustablewithin a range (5-10x) for large frame MCCBs, where x is the breakers’ ampere trip (AT)
rating.
Magnetic Only MCCBs are identical to the inverse-time MCCB except that the thermal trip
action is eliminated. Magnetic trip MCCBs are often used for motor fault protection because
the NEC also requires a separate device to provide overload protection for the motor.
Motor Circuit Protectors (MCPs) are identical to magnetic-only MCCBs except for the ratings
label. Magnetic-only MCCBs have an interrupting rating that is the same as the rating for
thermal-magnetic breakers (e.g., 14 kA, 25 kA, 65 kA). MCPs do not have a stand-alone
interrupting rating; they are rated as an assembly, which is called a combination motor starter,
consisting of overloads, a contactor, and a fault/disconnect device. The other minor
difference is that the MCP adjustments must be listed in amperes, whereas the magnetic-only
MCCB adjustments are typically listed in multiples of the trip (continuous current) rating.
Molded case circuit breakers are rated as follows:
• Frame sizes (AF) of 100 A, 225 A, 400 A, ..., 6000 A.
• Trip ratings (AT) of 15 A, 20 A, 25 A, ..., 6000 A; NEC Article 240-6 lists all
37 standard AT ratings.
• Amperes interrupting capability (AIC) ratings of 10 kA, 14 kA, 18 kA, ..., 100
kA; no standard exists for AIC typical ratings.
• Voltage ratings of 120 V, 240 V, 277 V, 480 V, and 600 V.
Inverse-Time (Thermal-Magnetic) - The interrupting rating or short circuit rating at a 400C
ambient temperature is commonly expressed in root mean square (rms) symmetrical amperes.The interrupting capability of the breaker may vary with the applied voltage. For example, a
breaker applied at 480 volts could have an interrupting rating of 25,000 amps at 480 volts, but
the same breaker applied at 240 volts may have an increased interrupting rating of 65,000
amps.
All MCCBs operate instantaneously at currents well below their interrupting rating. Non-
adjustable MCCBs will usually operate instantaneously at current values approximately five
times (5x) their trip rating. Low voltage breaker contacts separate and interrupt the fault
current during the first cycle of short circuit current. Because of this fast operation, the
momentary and interrupting duties are considered to be the same. Therefore, all fault
contribution from generators, motors, and the dc components of the fault waveform must beconsidered. Some MCCB manufacturers only list the symmetrical interrupting rating. If an
asymmetrical rating is not given, assume the following (Figure 33):
The response curves of all protective devices are plotted on common graphs so that they maybe compared at all current and time points. The standard method used to plot device T/C
characteristics is to plot the devices on log-log graph paper (Figure 39).
Standard log-log graphs show 4.5 cycles on the horizontal scale representing current. The
current axis ranges from 0.5 to 10,000 amperes. The vertical axis, representing time, ranges
from 0.01 to 1000 seconds and/or .6 to 60,000 cycles. Because current limiting fuses and
molded case circuit breakers may operate in less than 0.5 cycles (.00835 seconds),
manufacturers of these devices may reproduce T/C characteristic curves with 6 cycle vertical
scales and times ranging from .001 to 10000 seconds (.06 to 600,000 cycles). The horizontal
current scale is also often “shifted” for a particular plot by multiplying the current scale by a
factor of 10, 100, or 1000 (x10, x100, x1000).
Non-Time Delay
Non-time delay fuses are typically single-element fuses that are particularly suited for short
circuit protection of components in circuits without inrush currents, such as lighting loads. If
used on circuits with inrush currents (motors and transformers), they must be often oversized,
which sacrifices certain levels of current limitation. Figure 40 describes typical T/C
characteristic curves of both a 30 and 400 ampere non-time delay fuse.
Time Delay
Time delay fuses are typically dual-element fuses providing both overload and short circuit
protection. They are typically applied on circuits with motor loads (temporary inrush
current). They do not offer excellent short circuit protection as non-time delay fuses.
However, they provide excellent overload protection because they can be closely sized to full-
load motor currents. Figure 41 describes typical T/C characteristics of both a 20 and 400
Saudi Aramco (SAES-P-114) permits the following three types of molded case circuitbreakers (MCCB) to be used for motor phase fault protection.
• inverse-time (thermal-magnetic)
• magnetic only
• motor circuit protectors (MCPs)
Inverse-Time (Therm al-Magnetic) MCCBs are permitted by SAES-P-114 for low voltage motors
rated 1.0 hp or less. The NEC also permits their use as long as their continuous current rating
does not exceed 250 percent of the motor’s full-load amperes (IFLA) as listed in NEC Table430-150.
Although most codes and standards permit use of inverse-time MCCBs, these MCCBs are
typically not used because of nuisance tripping caused by high motor starting inrush currents
(typically 4-6 IFLA). Figure 42 shows the T/C characteristics of the MCCB protecting a 1.0 hp
motor. Note: Although Figure 42 shows, and the NEC permits, the MCCB providing both
overload and short circuit protection, it is not recommended practice.
Magnetic-Only MCCBs are also permitted by SAES-P-114 for low voltage motors rated 1.0 to
100 hp. The NEC also permits their use as long as their rating (setting) does not exceed 700
percent of the motor’s full-load amperes (IFLA) as listed in NEC Table 430-150 and if theMCCB is part of a listed combination controller. Figure 43 shows the T/C characteristics of a
magnetic only MCCB protecting a 100 hp motor.
Motor Circuit Protectors (MCPs), like magnetic-only MCCBs, are permitted by SAES-P-114 for
low voltage motors rated 1.0 to 100 hp. NEC Article 430-52 also permits their use as long as
they are part of a listed combination controller and are set at not more than 1300 percent of
the motor’s full load amperes (IFLA) as listed in NEC Table 430-150. Figure 44 shows the T/C
characteristics of an MCP protecting a 100 hp motor.
Motor nameplate data was previously discussed in Module EEX 216.02. This Module will
briefly review the following nameplate data to be used in selecting a low voltage motor
disconnect/fault protective device:
• full-load amperes
• kVA code/locked rotor amperes
• voltage and horsepower
Note: Work Aid 3 has been developed to help the Participant select a motor disconnect/fault
protective device.
Full-Load Amperes
The protective device’s continuous current rating should not exceed the motor’s full-load
amperes (IFLA) as listed in NEC Table 430-150. 16-SAMSS-503 specifies that the continuous
current ratings of MCCBs (or MCPs) shall not be less than 125% IFLA unless the MCCB is
100% rated. If a LVPCB is being used, 16-SAMSS-503 specifies a continuous current rating
no less than 115% IFLA.
kVA Code/Locked-Rotor Amperes
The code letters marked on motor nameplates show motor input kVA under locked-rotor
(starting) conditions. The code letters for determining motor branch-circuit short-circuit andground fault protection are explained in NEC Article 430-52 and Table 430-152.
Voltage and Hor sepower
The protective device’s voltage rating is based on the system’s nominal voltage rating and not
on the motor’s nameplate voltage rating. The motor’s nameplate horsepower rating is used to
determine the kVA input under locked-rotor conditions (see previous paragraph), and to
determine the motor’s full-load and locked-rotor amperes in accordance with NEC Tables
3. Determine the minimum size contactor to select.
Note 1: In accordance with NEC Article 430 (Work Aid 2A, Handout 1), the general
requirement is that the controller must have a horsepower rating that is not lower than the
horsepower rating of the motor at the application voltage.
Note 2: When special application criteria such as long-acceleration time, dynamic
breaking, or above average starting duty is specified, a larger NEMA size contactor must
be selected in accordance with the rating tables provided in NEMA ICS 2-321. In
general, these special application criteria require the selection of a contactor that is one
NEMA size larger.
4. Determine whether a reversing or non-reversing contactor is to be selected.
Note: This selection parameter is determined from the operating conditions for the
starter.
5. Determine the coil voltage rating for the contactor to be selected. The coil voltage ratingmust be equal to the control circuit voltage rating. Per 16-SAMSS-503, select a 120 V
coil rating.
6. Using the contactor selection parameters determined in steps 1 through 5:
• motor horsepower (hp) -
• motor voltage (VM) -
• number of phases, 1 or 3 -
• type of contactor -
• reversing or non-reversing -
• contactor coil voltage rating -
Select a low voltage contactor from the Westinghouse Catalog 25-000, pages 356 - 359
(Work Aid 2C, Handout 3).
7. Note: 16-SAMSS-503 (Work Aid 2B, Handout 2) requires that only combination
controllers be used for motors rated 600 V and below and 1 to 100 horsepower. As a
result, contactors are supplied by the manufacturer as an integral component of the
combination controller. These selection procedures provide for verifying the selection of
3. Collect the maximum symmetrical short circuit current available (SCA) from the system
one-line diagram:
Short circuit current available (SCA) -
4. Calculate a required breaker interrupting rating 105 percent greater than the maximum
SCA: Notes: 1) Saudi Aramco design practices require that all electrical equipment
interrupting and withstand ratings be equal to 105 percent of SCA. 2) Magnetic-only and
MCP-interrupting ratings are part of the listed combination controller ratings.
Breaker interrupting rating in amperes - Iint = 1.05 x SCA
5. If using MCP fault/disconnect protection, select the next standard size MCP (including
magnetic trip ranges) from Westinghouse Catalog 25-000 (Work Aid 3C, Handout 3),
pages 127 or 128, that equals or exceeds the voltage (V), NEC full-load amperes (FLAN)
breaker interrupting rating (Iint), and locked rotor amperes (LRAN) from steps 1, 2, and 4above. Note: All Westinghouse MCP interrupting ratings are 65 kA. If higher ratings
are required, which is considered unlikely for refinery operations, an MCP must be
selected with a current limiter attachment that increases the rating to
100 kA.
6. If using a magnetic-only breaker, follow the same procedures as in step 5. Note: This
Module limits selection to MCPs for motors less than or equal to 75 kW (100 hp).
7. If using a LVPCB controller, select the next standard size from Westinghouse SA-11647
(Work Aid 3E, Handout 8), page 7, that equals or exceeds V, 1.15 FLAN, and Iint from 1,
2, and 4 above. Note: SAES-P-114 (Work Aid 3D, Handout 4) requires LVPCBs used asmotor starters to have continuous current ratings 115 percent greater than FLA N .
9. Alternative to selecting the individual fault/disconnect protective device, select from the
vendor’s list an enclosed combination motor starter (O/L relay, contactors,
fault/disconnect device, and enclosure). Therefore, if using this option, select acombination motor starter from Westinghouse Catalog 25-000 (Work Aid 3C, Handout 3),
pages 406, 407, 415 and 416. The combination starter ratings must equal or exceed V,
FLAN, and Iint from steps 1, 2 and 4 above.
10. Verify that the MCP or LVPCB selected complies with SAES-P-114 (Work Aid 3D,
Handout 4), and 16-SAMSS-503 (Work Aid 3B, Handout 2) criteria.