Motor Protection

R.J. Phansalkar

R&D , C&S Electric

MOTOR PROTECTION

This presentation is to refresh the electric motor related knowledge and understand the different aspects for which motor protection Relays are used.

• General things about Electric Motors including its working and starting.

• Reasons of Motor failures• Discussions on different types of failures including

Overload, Single phasing etc.

Objective & Contents

Objective

Contents

• What is Pulsating magnetic field & Rotating magnetic field?

• Why starting current is very High?

• How motor behaves after it is started?

• What is the effect of phase sequence?

• Typical connections of Star / Delta Starter

Some Common Questions

• An ELECTRIC MOTOR uses electrical energy to produce  mechanical energy, very typically through the interaction of magnetic fields and current carrying conductors.

• The reverse process, producing electrical energy from mechanical energy, is accomplished by an  Alternator or dynamo.

Electric Motor

AC &DCAC motors – single /three Phase Induction Motors• Squirrel cage Motor• Slip ring motor• Sychronous motor

• DC motors Series wound Shunt wound

Compound wound

• Stepper Motor• Servo Motor

By Size Extremely small Small (FHP) up to 3 HP Medium 3 to 10 HP Large 10 to 100 HP Very large >100HP

By constructionBrushless or Brushed

Categorization of Motors

Types of Motors

• Motors which our Relays protect are mainly induction motors.

• About 70% to 80% of the Industrial Load is because of the induction motors.

• Motors are rated in HP and 1 HP is 746 watts.

The windings of the three phases are distributed in space in such a manner that vector sum of the three magnetic fields (each set by an individual phase voltage) will create a rotating magnetic field.

Rotating Magnetic Field

• Current will reduce

as the speed picks up.

• Torque Vs speed

What happens when a motor is started?

Cy star contactor, CΔ is Delta contactor

and CN is Main contactor

Star Delta Starter

For star Delta connections, 6 terminals of the motor must be on the terminal strip

Connection of Motor in Star Delta Circuit

1. Overloads 30%2. Contamination of winding 19%

1. Single phasing 14%2. Bearing Failure 13%3. Old age Rotor Failure 10%1. Miscellaneous 9%

44% FAILURES ARE RELATED TO HEAT

0

5

10

15

20

25

30

1 2 3 4 5 6

MOTORFAILURES

Reason of Motor Failures

Excessive current may be: - Due to either mechanical overload on the motor OR Due to electrical system, unbalance supply voltage single phasing defective starter defective motor itself.

In either case it is essential that the supply should be disconnected before any damage is done to the motor.

An overload device thus usually operates by releasing the latching-in device by disconnecting the supply to no-volt coil or in contactor starters by operating the operating coil circuit.

Over load

• Insulation breakdown is a common reason for motor failure.

• Windings in the motor are insulated with organic materials including epoxy and paper. Insulation degradation occurs when winding temperature exceeds its rating.

• The National Electrical Manufacturers Association (NEMA) states that the time-to-failure of organic insulation is halved for each 10°C rise above the motor insulation class rating

Over load & Over Temperature

• Allowing Motor to reach and operate at a Temperature100 C above its maximum temperature rating will reduce motor’s expected life by 50%.

• This reduction in expected life of the motor repeats itself for every 100 C.

• There is no industry standard that defines life of the motor but it is generally considered 20 years.

Half Life Rule

• The Locked Rotor Torque or Starting Torque is the torque the electrical motor develop when its starts at rest or zero speed.

• A high Starting Torque is more important for application or machines hard to start- as positive displacement pumps, cranes, Escalators, lifts etc. A lower Starting Torque can be accepted in applications as centrifugal fans or pumps where the start load is low or close to zero.

Locked Rotor or Starting Torque

• The National Electric Code (NEC) defines “Motor Overload Protection” as that which is intended to protect motors, motor-control apparatus, and motor branch-circuit conductors

against

excessive heating due to motor overloads and failure of the motor to start.

• Motor Overload Protection is also commonly referred to as “Running Protection”.

Motor Over load Protection

• Class ‘A’ Insulation 1050 C

• Class ‘B’ Insulation 1300 C

• Class ‘F’ Insulation 1550 C

• Class ‘H’ Insulation 1800 C

• Class ‘N’ Insulation 2000 C

Insulation Classes

Thermal Overload Relay

Overload protection for single and three-phase AC motors in the small (above 1 horse power) and

medium horse power

range is typically provided by one of two methods:

Thermal Overload Relays,

or

Solid-state Overload Relay

Overload protection for large three-phase motors is sometimes provided by Thermal Overload Relays which are connected to Current Transformers (CT’s).

However, most new installations utilized micro-controller based motor protective relays which can be programmed to provide both overload and short-circuit protection.

These protective relays often also accept inputs from Resistance Temperature Devices (RTD’s) imbedded in the motor windings(usually two per phase) and the relays are capable of displaying the winding and motor bearing temperatures, and provide both alarm and trip capability.

Over load Protection

• Electro-magnetic overload relays are old type which is not in use recent days that's why I am giving details about Thermal Overload relay. These are mostly used in Air Compressor now a days.

Thermal Overload relay:-

• This may be bi-metal strips or solder pot elements and in either case as the action is due to their heating up,a time element is always present. The action of bi-metal strips over load release depends on the movement resulting from the different rates of expansion of the two metals forming the combined strip when heated.

Over load Relays

• Winding contamination is identified by comparing the pattern of impedance and inductance. Impedance will decrease, and fall towards inductance, as the insulation system begins to degrade due to contamination.

• • In most cases, the insulation life may be extended by

removing the motor from service, cleaning, dipping and baking (re-insulating) for a limited time after detection.

• As the fault progresses, it will graduate to an insulation to ground fault or winding short. The progress is, normally, insulation degradation, changes to the failure.

Winding Contamination

• The ambient environment effects the progression of winding contamination degradation.

• Statistically, it is found that the mean time to failure after fault detection progressed by a simple natural log multiplier If ‘m’ is the Winding Contamination Multiplier

e-m * Hb where Hb = Base Hours

The multiplier ‘m‘ is based upon the ambient conditions .

How winding contamination

Affects life of the motor?

• Clean and dry environment ambient temperature <250 m=0• Clean factory environment Conditioned air with ambient

temperature < 250C m=0.5• Medium factory environment Variable ambient

temperatures and humidity. m=0.75• Harsh factory environment Variable ambient

temperatures and humidity. Enclosed motors in exterior environments. m=1.0

• Very Harsh environment. High humidity, high ambient and/or Acidic/basic environment. Includes motors mounted in cooling towers. m not applicable

Environmental Conditions

Motor bearing failures are more because of over greasing rather than under greasing !

Bearing Failures

Under lubrication can cause a bearing to wear out prematurely. On the other hand, applying too much lubricant can often lead to catastrophic results to the bearing (grease churning and overheating) or long-term damage to motor coils and windings.

Effects of Improper lubricants

• Standard Motors are designed to withstand the temperature rise produced within the Motor when delivering rated horse power and added to the industry standard 400 ambient temperature rating , will not exceed the safe winding insulation temperature limit.

• Electric motors feature a service factor, which indicates

how much over the nameplate rating any given electric motor can be driven without overheating.

• The service factor for the electric motor is defined as a multiplier which, when applied to the rated Horse power indicates a permissible horse power loading which may be carried out at permissible conditions including rated voltage and frequency.

Service Factor

• Undercurrent protection is required by some codes as a safety measure.

• A water pump that cavitates can be dangerous. The water typically provides pump cooling. Without the cooling water, case temperature can reach an extremely high value.

• If valves are opened under these conditions and cold water is allowed to reach red-hot metal parts, the resulting steam pressures can destroy the pump and pose a serious personnel hazard.

Why under current protection is required?

• NEMA designed motors are rated for two starts from cold and one start from hot per hour. Motor jogging refers to excessive starts and can cause overheating. The motor may not get up to full speed and the forced air cooling is not effective.

• Since the thermal model accurately tracks the motor’s used thermal capacity at all times, including during starts and between starts, the starts-per-hour feature may not be required.

Motor Jogging

• The NEMA standard for Electric motors and Generators recommends that maximum voltage unbalance be limited to 1%

• It is recommended that if there is unbalance then the motor load should be reduced to avoid unacceptable temperatures

voltage unbalance 1% 98% loadvoltage unbalance 2% 95% loadvoltage unbalance 3% 88 %loadvoltage unbalance 4% 82% loadvoltage unbalance 5% 75% load

Voltage Unbalance

• Over voltages cause insulation stress and premature breakdown.

• Under voltages, such as those caused by brownouts, can lead to increased motor heating. Torque developed by an electric motor changes as the square of the applied voltage. A 10% reduction in voltage results in a 19% reduction in torque. If the motor load is not reduced, the motor will be overloaded.

Effect of Under & Over Voltage

• Unequal single phase loads

• Open delta connections

• Transformer connection open

• Tap settings of the transformers not proper

• Transformer impedance is not equal

• Power factor correction capacitors are not the same

Some Causes of unbalanced voltage

conditions

• The term single phasing means one of the phase is open. This condition subjects the motor to the worst case of voltage unbalance.

• The phase current will increase by √3 times

• Nothing can prevent or eliminate single phasing

Single Phasing

• Damaged Motor starter(most likely to occur on automatically started equipments like compressors, air conditioners.

• Damaged switch or circuit breaker on the main• Open Fuse• Open cable caused by overheated lug• Open connection in wiring• Open winding in the motor• Any open circuit in any phase between secondary of

the transformer and Motor

Typical Causes of Single Phasing

• Winding-to-winding and winding-to-ground failures inside the motor are difficult to detect using the phase and ground-fault CTs due to low magnitudes of current.

• Differential protection in high-end motor protection relays use multiple CTs to compare the current entering and leaving the winding. If there is a difference in currents then leakage is occurring. This sensitive protection is used on very large motors.

High Resistance winding Faults

Motor protection System

• Typical motor protection scheme

Features…Protection and control Related

• Thermal model biased with RTD and negative sequence current feedback

• Phase and ground TOC and IOC• Start supervision and inhibit• Mechanical Jam • Current Phase Reversal• Acceleration Time• Undercurrent / Underpower• Starts per Hour

Features…Monitoring and metering related

• 

• Current Metering• RTD Temperature• Event Recorder: 256 events with 1ms time

stamping• Oscillography with 32 samples per cycle and

digital states • IRIG-B clock synchronization• Motor health diagnostics• Security audit trail

Features…Communication Related

• 4 X20 character LCD display

• Control panel with 12 LED indicator

• Front USB and rear RS485 serial communications

• Multiple Communication Protocols:

• IEC 61850, MODBUS TCP/IP, MODBUS RTU, DNP 3.0, IEC60870-5-104, IEC60870-5-103 

Features… Security related

• Access control

• Intrusion detection

• Auditing and reporting

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THANKS