Guide to Motor & Starting

Guide to motors and starting

2

Introduction

Electric motors are deservedly the mostpopular prime movers for industry andcommerce. Compared with other sources ofmechanical power, they're inexpensive,compact, reliable and versatile. As a result ofthis popularity, millions of motors are installedeach year and, except for the very tiniest, everyone needs a starter. The manufacture of thesestarters is the basis on which the control gearindustry was built, and motor starters are stillat the core of almost every control gearsupplier's business.

Starters commonly used today, however, differ from

their predecessors. Some types, such as the faceplate

starter, have disappeared altogether. Other types,

such as primary resistance starters, are fast declining

in popularity. In addition, asynchronous induction

motors are now almost universal, virtually eliminating

the need for the specialised starters used by other

types of motor.

This supplement deals with a wide range of starting

techniques for asynchronous motors, an area where

Schneider's engineers have unrivalled expertise. The

benefits and limitations of various starting methods are

explained and, unlike some ostensibly similar

publications, the information presented is right up to

date - current, useful and practical data is presented in

a clear concise form.

Your comments on the contents of this supplement are

welcome, as are your suggestions for topics which you

would like to see covered in future issues.

Introduction

3

Scope

Motors – a few basics

The scope of this publication

Principle of operation

The rotating magnetic field

This supplement has been written to provide

engineers, designers and users of motor starters with

a brief overview of current techniques to assist in their

understanding, and to help them in the design of

equipment. It by no means covers all aspects of

motor starting but, nevertheless, it deals with the vast

majority of applications likely to be encountered in

industry and commerce. For those requiring further

information, a short list of sources is included at the

end of the supplement.

Three-phase asynchronous motors are, by far, the

most widely used type. The operation of this type of

motor relies upon the creation of an induced current in

a conductor which is, itself, under the influence of a

magnetic field. It is this principle of operation which

gives rise to the commonly used term "induction

motor."

A typical motor has three stator or field windings

which are arranged at an angle of 120° relative to each

other. These windings are fed from the three phases

of the mains supply which are, themselves, offset by

120° . This arrangement produces a rotating magnetic

field which, as it turns, tends to pull the motor's rotor

round with it.

The magnetic field rotates once during each complete cycle of thesupply current. Motor speed is, therefore, directly related to thesupply frequency (f in cycles per second or Hz), and the number ofpole pairs (p) which the motor uses. The motor's so-calledsynchronous speed is given by:

Ns (in revolutions per minute) = 60f / p

The majority of motors in use are four-pole machines (2 pairs), whichhave synchronous speeds of 1500 rpm at 50Hz and 1800 rpm at 60Hz.

Motors – a few basics

4

Motorconstruction

Slip

In practice, an induction motor can never run at its

synchronous speed, since it can only generate torque if

there is an induced current in the rotor conductors. This

can only be the case if there is relative movement

between the rotor and the rotating magnetic field. The

rotor must, therefore, rotate slightly more slowly than the

field which rotates at synchronous speed. This is why

the motor is described as asynchronous.

The difference between the synchronous speed (Ns) and

the actual nominal rotor speed (Nn) is called the slip. Slip is

always expressed as a percentage of the synchronous

speed:

slip = 100(Ns - Nn)/ Ns

Slip

Motor construction

A three-phase asynchronous motor comprises two maincomponents, the stator and the rotor.

As its name suggests, the stator is the stationary part of the motor, and

consists of a strong casing (usually manufactured from cast-iron or alloy)

into which is fixed a ring of laminated silicon steel sections. The

laminations are slotted so as to accommodate the stator windings which

create the rotating magnetic field. Each of the main windings, of which

there are three in a three-phase motor, comprises a number of coils. The

magnetic coupling of the windings is arranged to give the required

number of pole pairs (and thus synchronous speed) of the motor.

The rotor is the rotating part of the motor which drives the machine to

which it is coupled. It is similar to the stator, but is made up of a greater

number of laminated sections. Together, these form a cylinder which is

keyed to the motor shaft. There are two principle types of rotor - squirrel

cage and wound.

5

Controlling speed

Controllingspeed

Motorconstruction

Squirrel cage rotors are, by far, the most common. They have straight

conductors set into slots around the periphery of the rotor. These

conductors are connected together by rings at each end of the rotor, so

that their arrangement somewhat resembles a circular squirrel cage, from

which the assembly gets its name. A popular variation is the double-

cage, which has two concentric cages and offers a higher starting torque

than single-cage versions. No external electrical connections can be

made to any type of squirrel cage rotor.

Wound rotors have windings similar to those used in the stator. One end

of each winding is connected to a common (star) point, and the other

ends are connected to slip rings. External connections to the rotor

windings are made via these slip rings, allowing additional resistance to

be added to the rotor circuit during starting. This enables the motor's

starting current and torque to be controlled.

Varying the supply voltage alone is

a comparatively ineffective way of

controlling the speed of an

induction motor. Voltage increases

raise speed somewhat, but this

effect is limited by magnetic

saturation in the windings.

Conversely, voltage reductions

decrease speed but, again, the

range of control is very limited, and

torque is adversely affected.

Today, the most popular method of

speed control is the use of a

variable frequency (inverter) drive.

These vary both the voltage and

frequency of the supply to the

motor, giving a wide range of

control over speed, without loss of

torque. With inverter drives,

standard 50Hz motors can be

operated successfully over at least

the range of supply frequencies

from 5 to 50Hz with only a slight

loss of operating torque. If

operated continuously at

frequencies of 25Hz or less, the

cooling provided by the motor's

built-in fan is likely to be

insufficient, and additional forced

cooling should be considered.

Startingcurrent

6

Starting current

If a stationary squirrel cage motor

is connected directly to the supply,

it will typically draw a starting

current of 5 to 8 times its normal

full-load current (FLC). For smaller

motors, this is often acceptable,

but for large machines, or where

supply capacity is limited, some

means of reducing the starting

current becomes necessary. This

is usually done by reducing the

voltage applied to the motor during

starting. Most of the starters

described in the remainder of this

publication have been developed

specifically to limit motor starting

current.

With conventional contactor-based

starters, however, there is a

problem - reduced starting current

means reduced starting torque

which may, in some applications,

be unacceptable. This limitation is

examined in more detail in later

sections which describe particular

starter types. It is worth noting,

however, that inverters, which

control both supply frequency and

voltage, allow starting currents of

1.5 x FLC or less, while still

providing high starting torque.

When using soft starters, starting

currents are generally between 2

and 5 x FLC.

Summary

Three-phase asynchronous induction motors are the mostcommonly used type in industry. Their speed is largely determinedby supply frequency, with voltage variations having comparativelylittle effect. Connected directly to the supply, these motors havetypical starting currents of 5 to 8 x FLC. Often, starting currentsneed to be reduced, and various forms of starter have beendeveloped to make this possible.

Starters

7

Direct-on-line (DOL) starters

With this type of starter, the stator

windings of the motor are

connected directly to the three-

phase mains supply. The motor

starts and accelerates in a way

determined by its own

characteristics. Typically, the peak

starting current is between 5 and 8

times normal full-load current, and

the peak starting torque is between

0.5 and 1.5 times the motor?s

nominal operating torque.

Overloads designed to

BS EN 60947-4-1 are based on a

starting current of 7.2 times normal

full-load current.

21

43

65

21

43

65

2 4 6

1/L

1

3/L

2

5/L

3

U V W

-F1

-KM1

-Q1

M3

Motorcurrent

Speed

1

2

3

4

5

6

7

0 0.25 0.50 0.75 1

Instantaneous motor current

Instantaneous motor current

Motor Torque

Motor Torque

Load Torque

Load Torque

Torque

2.5

2

1.5

1

0.5

0 0.25 0.50 0.75 1

Speed

Direct-on-linestarter diagram

Direct-on-line current/speed characteristics Direct-on-line torque/speed curve

Starters

Starters

8

Although DOL starters offer a number of advantages,

including simplicity, low cost and high starting torque,

their use is limited to applications where:

• low-power motors are being used,

and the supply capacity is high, so

that the starting current surge

does not adversely affect other

equipment using the same supply

• the equipment driven by the motor

is fitted with a gearbox or some

other device which will soften the

mechanical shock produced by

the high starting torque

• a high starting torque is needed -

for example, the equipment starts

against its full mechanical load.

When the limitations of DOL

starting are not acceptable, it is

necessary to use alternative

starting techniques which reduce

the peak starting current and,

therefore, the peak starting torque.

The normal approach is to arrange

for the motor to be started at

reduced voltages, and a number of

methods have been developed for

doing this.

DOL starters are not suitable when:

• the peak starting current would

result in a serious voltage drop on

the supply system

• the equipment being driven cannot

tolerate the effects of very high

peak torque loadings

• the safety or comfort of those

using the equipment may be

compromised by sudden starting

as, for example, with escalators

and lifts

This type of starter may only be used where access is

possible to both ends of all three stator windings. In

addition, the windings must be rated to withstand the

full supply voltage when delta-connected. With star-

delta starting, the

peak starting

current is typically

between 1.5 and

2.6 times the

normal full-load

current, and the

peak starting

torque is between

0.2 and 0.5 times

the motor's

nominal operating

torque.

9

Star-deltastarters

Star-delta starters

Star-delta starting torque/speedcharacteristics

On starting, the supply is first

applied to the motor with its stator

windings star-connected. As the

motor accelerates, its speed

stabilises when its developed

torque become equal to its load

torque. This usually happens at

about 75% - 80% of nominal

speed. The star contactor is then

de-energised, and the delta

contactor energised to delta-

connect the stator windings. Each

winding is now

fed with the full

supply voltage,

and the motor

adopts its normal

operating

characteristics.

The run-up time with the windings

star-connected is controlled by a

timer which, typically, can be

adjusted from 0 to 30 seconds.

This timer is adjusted during

commissioning to ensure that the

star-delta changeover occurs, as

closely as possible, at the point of

torque equilibrium. The transition

time from star to delta is also

important, and a special timer is

normally used to ensure that there

is a period of between 30ms and

50ms between the opening of the

star contactor and the closing of

the delta contactor. This allows

time for any switching arcs to be

extinguished.

Star-delta starter

Star-delta startingcurrent/speedcharacteristics

current

Speed

1

2

3

4

5

6

7

0 0.25 0.50 0.75 1

Current in star connection

Current in star connection

Current in delta connection(direct)

Current in delta connection(direct)

Torque

Speed

2.5

2

1.5

1

0.5

0 0.25 0.50 0.75 1

Torque in delta

(dire

ct)

Torque insta

r

Load torque

Torque in delta

(dire

ct)

Torque insta

r

Load torque

21

43

65 1 3 5

1 3 5

1 3 5

2 4 6

2 4 6 2 4 6 2 4 6

2 4 6

1/L

1

3/L

2

5/L

3

U1

V1

W1

U2

V2

W2

-KM2 -KM3 -KM1

-F2

-Q1

M13

Star-delta starters

Primary resistance

starters

10 Star-delta starters are particularly suited to machines

which do not present a high load torque at start-up, or

which normally start off-load. It is also important to

note that, during the star-to-delta transition, a high

transient current is generated. If a magnetic short-

circuit protective device is to be used in the starter,

this transient must be taken into account in the

selection of the device, in order to prevent nuisance

tripping.

Although the transient produced at the star-delta

transition is very brief, the current can be quite large

and, particularly for larger motors, some form of

current limiting may be necessary. One solution is to

introduce a delay of 1 to 2 seconds during the star-to-

delta transition. To avoid too large a speed drop

during the transition, however, this method can only be

used with low-inertia loads.

Primary resistance starters

Starters of this type start the motor at reduced voltage

by connecting a resistance bank in series with the

motor windings. Once the motor has run up and its

speed has stabilised, the resistance bank is shorted

out, and the motor becomes direct-connected. This

changeover is normally controlled by an adjustable

timer within the starter. Unlike star-delta starters,

primary resistance starters do not require access to

both ends of the stator windings.

Values of starting current and torque are determined

by the values of the resistors used. Typically, however,

the peak starting current will be around 4.5 times

nominal full-load current, and peak starting torque will

be around 0.75 times nominal operating torque.

Primaryresistance

starters

11

Torque7

6

5

4

3

2

1

0 0.25 0.50 0.75 1

Speed

Current on 2nd. step without resistance(dire

Current on 2nd. step without resistance(dire

Current on 1st. step with resistance

Current on 1st. step with resistance

21

43

65

2 4 6

1 3 5

2U

4V

2W

-F1

-KM1

-KM11

R2

R4

R6

RU

RV

R6

R1

R3

R5

M3

21

43

65

1L

1

3L

2

5L

3

-Q1

Torque

2.5

2

1.5

1

0.5

0 0.25 0.50 0.75 1

Speed

Torque on 1st. step with resistance

Torque on 1st. step with resistance

Torque on 2nd. step

without resistance (direct)

Torque on 2nd. step

without resistance (direct)

Load TorqueLoad Torque

Primary resistance starters are

especially suitable for applications,

such as ventilator fans, where the

load torque increases with speed.

A possible disadvantage is the high

peak current at the instant of

starting, but this can be reduced by

increasing the resistor values. Care

must be taken, however, since this

also reduces starting torque.

Primary resistance starter

Primary resistance startingcurrent/speed characteristics

Primary resistancestarting torque/speed

characteristics

Auto-transformerstarters

12

In auto-transformer starters, the motor is startedat reduced voltage which is supplied from anauto-transformer. The starting sequence hasthree stages.

Auto-transformer starters

Auto-transformer starting is particularly used for largemotors (above 100kW), but tends to be an expensivesolution, largely because of the cost of the auto-transformer itself. These starters may also produce a

current peak atthe instant when the motor is switched directly to thesupply. This peak can be minimised by careful designof the auto-transformer, but only at the expense ofincreasing the peak current at the commencement ofthe first stage of the starting sequence.

Torque

2.5

2

1.5

1

0.5

0 0.25 0.50 0.75 1

Load Torque

Torque on 1st. step

Torque on 2nd. step

Load Torque

Torque on 1st. step

Torque on 2nd. step

During the first stage, the auto-transformer is star-connected, and the line contactor is closed. Thisstarts the motor with a reduced voltage, the value ofwhich depends upon the ratio selected for thetransformer. Auto-transformers are normallyprovided with taps to allow the best ratio to bechosen during commissioning.

In the second stage, the star connection is opened,and the auto-transformer acts as an inductorconnected in series with the motor. This transitionis normally timed to occur when the motor speedhas stabilised at the end of the run-up period. Thethird stage then follows almost immediately, andinvolves shunting the transformer completely, sothat the motor is direct-connected to the supply.

The starting current and torque are reduced as afunction of the reduced starting and run-up voltages(Usupply/Ustarting)2. Typical values for peak startingcurrent are 1.7 to 4 times nominal full-load currentand, for peak starting torque, 0.5 to 0.85 timesnominal operating torque.

21

43

65

2 4 6

11

/L1

3/L

2

5/L

3

3 5 1 3 5

2U

4V

2 22

U1

U2W

-KM3

-F1

-Q1

-KM2

-T1

-KM1M3

24

V1

V2

26

1 3 5W

1W

2

U3

V3

W3

Current

0

1

2

3

4

5

6

7

0.25 0.50 0.75 1

Speed

Current without auto-transformer (direct)

Current on 1st. step

Current on 1st. step

Current without auto-transformer (direct)

Current on 2nd. step

Current on 1st. step

Auto-transformer starting current/speedcharacteristics

Auto-transformer startingtorque/speed characteristics

Auto-transformer starter

13

Electronic soft starters

Electronic soft starters

This relatively recently introduced

form of starter is rapidly growing in

popularity. Soft starters operate by

gradually increasing the voltage

applied to the motor, so as to

produce steady, smooth

acceleration. This technique

eliminates sudden changes in

voltage which could produce peaks

in both starting current and torque.

The steadily increasing supply

voltage for the motor during

starting is produced by a thyristor

bridge which, in each phase, has

two thyristors connected back-to-

back. By varying the firing angle of

each set of thyristors, it is possible

to control the starting voltage and,

hence, the starting current. Note

that, unlike inverter drives, soft

starters do not vary the frequency

of the supply to the motor.

The detailed design of soft starters

varies from manufacturer to

manufacturer, but a representative

unit is the Telemecanique Altistart 46.

This is fitted with a six-thyristor

power-switching bridge which

allows complete control over the

starting and stopping of a three-

phase squirrel-cage motor. It

provides:

• control of the acceleration and

deceleration ramps of the motor in

such a way as to keep within all

required limits on current and torque

• thermal overload protection for itself,

and for the motor which it is

controlling

• mechanical protection for the

machine being driven, by eliminating

sudden changes in current - and,

therefore, torque - during starting

and stopping.

M3

Note: = firing angle ofthymistors

5

2

2.5

4

ATS

2

0 0.25 0.50 0.75 VN 1.25

0

2

1TdB

2

3

2

1

1

1

0.25 0.50 0.75 VN 1.25

TC

TdCTdB

TBTB

TdA

TN

TA

1A

IN

lB

lC

Electronic “soft starter”

Electronic “soft starter”current/speedandtorque/speedcharacteristics

Electronicsoft starters

14

This type of starter may be used with any

asynchronous motor. The Telemecanique Altistart may

be bypassed by a contactor at the end of the

acceleration ramp, the contactor being controlled by a

contact provided for this purpose. This will avoid

thyristor heating and losses which occur during normal

running. Even with the starter bypassed, however, the

protective devices of units rated at 18.5kW/415V and

above remain operational, thus protecting both the

starter and the motor. For smaller units, a separate

thermal overload is required. Other features which can

be provided by soft starters include controlled

deceleration, and braking to a complete stop.

The peak starting current may be adjustedbetween 2 and 5 times nominal full-load current,corresponding to a range of starting torquesfrom 0.1 to 0.7 times the starting torque whichwould be produced if the motor were startedwith a DOL starter.

Rotor resistancestarters

15

Starters of this type can only be used with motors

having a wound rotor to which external connections

can be made, usually via slip rings. This type of motor

cannot be started direct on line because the peak

starting current at the instant that the supply is

connected would be far too high. Instead, the motor

is started with a resistance bank connected in series

with the rotor windings (NOT the stator windings, as in

primary resistance starters).

The starter is designed so that, at start-up, there is

maximum resistance in the rotor circuit. Various

sections of the resistance bank are then shorted out

progressively until, during normal running, no

resistance remains and the rotor windings are simply

star-connected.

Rotor resistance starters

21

43

65

2 4 6

U V W

K L M

-F1

-KM1

-R2A

M3

21

43

65

1/L

1

3/L

2

5/L

3

-Q1

-R2B

R2C

-R1A

-R1B

-R1C

A2B2

C2

-KM12

1 3 5

2 4 6

A1B1

C1

-KM11

1 3 5

2 4 6

Rotor resistance starter

Rotor resistancestarting

current/speedcharacteristics

Rotor resistancestartingtorque/speedcharacteristics

Current

Speed

1

2

3

4

5

6

7

0 0.25 0.50 0.75 1

Current on 3rd. step (no

resistors, direct)

Current on 2nd. step (some resistors)Current on 1st. step (all resistors)

Current on 3rd. step (no

resistors, direct)

Current on 2nd. step (some resistors)Current on 1st. step (all resistors)

Torque

2.5

2

1.5

1

0.5

0 0.25 0.50 0.75 1

Speed

Torque on 1st. step (all resistors)

Torque on 2nd. step(som

eresistors)

Torque without resisors)

Torque on 1st. step (all resistors)

Torque on 2nd. step(som

eresistors)

Torque without resisors)

Rotor resistance

starters

16

SummaryThe principle objective of all methods of motor starting is to matchthe torque characteristics to those of the mechanical load, whileensuring that the peak current requirements do not exceed thecapacity of the supply. Many starting methods are available, each of which has slightly different characteristics. The following tablesummarises the main characteristics for the most popular forms of starter.

For this type of motor, the torque is

virtually proportional to motor

current. A starting torque of twice

normal full-load current, therefore,

produces a starting torque which is

twice the nominal operating torque.

This is much better than a DOL

starter, where 6 x full-load current

produces only 1.5 x nominal torque

during starting. Slip-ring motors

with rotor resistance starters are,

therefore, ideal for high-inertia

loads which need to be started on-

load, but where the peak current

taken from the supply must be

limited. Further, the values of the

resistances and the number of

stages can be calculated so as to

match the motor characteristics to

those of the application.

Selecting a starter

17Selecting a starter

• The power for the machine

installation will normally be

supplied by the Regional Electricity

Company, and the user will need

to comply with any local

regulations. The Regional

Electricity Company will normally

limit DOL starting to a maximum

motor rating. If the motor is below

the DOL starting limit, determine

the peak starting current which

it would draw if started

direct-on-line.

• Check that this peak starting

current is within the capacity of

the supply.

• The installation will normally be fed

from a stepdown power

transformer. Check that the peak

starting current will not initiate a

circuit breaker trip on the high-

voltage (primary) side of the

transformer.

• Check that the supply line will not

introduce unacceptable voltage

drops when the peak current is

taken. If this is a problem, the

choice lies between installing

larger cables or selecting a

starting method other than DOL.

• If the above conditions are all

satisfied, DOL starting will provide

an economical solution, provided

that the mechanical load can

handle the peak starting torque

produced.

• If any of the conditions are not

satisfied, use the table to choose

an alternative method of starting.

Be particularly careful to ensure

that the starting torque produced

by the method of starting chosen

is adequate for the application.

When choosing a starter for a particular application,

the following procedure should be used:

Speedregulation ofasynchronous

motors

18 Speed regulation ofasynchronous motors

While speed regulation, strictly, goes a little beyond

motor starting, the two subjects are so closely related

that a brief discussion of speed regulation is included

here for the sake of completeness.

For many years, the scope for varying the running

speed of asynchronous motors was rather restricted.

Only motors with pole-changing facilities, and those

with separate windings, were popular for applications

requiring multi-speed operation, but even these types

could only operate at one of a number of fixed speeds.

This situation changed dramatically with the

introduction of frequency inverters which allow the

running speeds of standard motors to be accurately

controlled over a wide range. Inverter technology is so

successful that AC inverter drives are now being

adopted for many applications where, in the past, only

DC machines, with their inherent ease of speed

control, would have been suitable.

While various methods of speed control are possible,

which use only conventional components such as

contactors and resistors, these methods are fast

becoming obsolete as they are replaced by inverter

systems. This supplement will, therefore, deal

principally with speed control by inverter.

Speedregulation ofasynchronous

motors

19The frequency inverter drive

Inverter drive operation

This type of drive is intended mainly for use with three-phase squirrel

cage motors. It operates by using a technique called pulse-width

modulation (PWM) to synthesise a sinusoidal waveform, the frequency

of which can be varied, that is used to supply the motor. By varying the

frequency of the supply to the motor, the stepless motor speed

variation is possible over a wide range. Since the synthesised supply

waveform is very close to sinusoidal, smooth motor rotation is achieved

even at low speeds.

The AC supply (single or three phase) to the inverter is rectified by a

full-wave diode bridge, and is used to charge the main reservoir capacitors.

This provides the system with a high-voltage DC source which is then

switched by the output power bridge to produce a pulse train made up

of precisely controlled long and short pulses. The train of pulses

produces a sinusoidal current in the motor, the voltage and frequency

of which can be accurately controlled. By retaining the correct

voltage/frequency ratio in the supply to the motor, its torque can be

maintained over a wide speed range.

W

V

U

M3

rectifierBridge

Reservoircapacitor

Transistor outputpower bridge

Main circuit of afrequency inverter

Summary of characteristics of various starting methodsSquirrel cage motors

Advantages • Simple starter• Low cost• High starting

torque

• Simple, economicstarter

• Good startingtorque/currentperformance

• Possibility ofadjusting startingparameters

• No break in supplyto motor duringstarting

• Good reduction inpeak transientcurrents

Disadvantages • Very high startingcurrent and torque

• Supply mustwithstand peakcurrent

• Mechanically harshstarting sequence

• Low startingtorque

• Non-adjustablestartingparameters

• Break in supply tomotor leads tosevere transientpeak current

• Small reduction inpeak current

• Resistance bankrequired

Typicalapplications

• Small machinesmay often bestarted on full-load

• Machines startingon no-load (smallcentrifugal pumps,fans, etc.)

• High inertiamachines withnormal startingcurrent/torquecharacteristics

Run-up time 2 to 3 seconds 3 to 7 seconds 7 to 12 seconds

Direct-on-line Star-delta Primary resistance starting starting starting

Peak starting 4 to 8 In 1.3 to 2.6 In 4.5 InCurrent

Peak starting 0.6 to 1.5 Tn 0.2 to 0.5 Tn 0.6 to 0.85 Tntorque

Control On or off On or off 1 fixed step

Economic and rugged squirrel cage motor

20

Slip-ring motors

7 to 12 seconds Adjustable, 1 to 60seconds

0.1 to 999 seconds • 3-step : 2.5s• 4 and 5 step : 5s

• Good startingtorque/currentperformance

• Possibility ofadjusting startingparameters

• No break in supplyto motor duringstarting

• Parameters arefully adjustedduringcommissioning

• Compact• Solid state• Easily adapted to

the application

• Parameters arefully adjustedduringcommissioning

• Compact• Solid state• Easily adapted to

the application• Infinitely variable

speed• In-built motor

protection• Low starting

current

• Good startingtorque/currentperformance

• Possibility ofadjusting startingparameters

• No break in supplyto motor duringstarting

• Expensive auto-transformerrequired

• Not tolerant tosupply linetransients

• Can causeinterference on thesupply duringstarting andstopping

• Can causeinterference on thesupply

• Relativelyexpensivecompared todirect-on-line

• Expensive slip-ringmotor required

• Resistance bankrequired

• High inertiamachines where areduction ofstartingcurrent/torque isrequired

• Machines requiringvery smoothstarting (centrifugalpumps and fans,conveyors, etc.)

• All machineswhere speedneeds to be variedto improveproduction andreduce mechanicalwear

• Machines whereenergy can besaved by reducingspeed (centrifugalpumps, fans, etc.)

• Machines startingon-load, wheresmooth run-up isrequired, etc.

Auto-transformer Electronic Variable speed Rotor resistancestarting “soft-starting” drives starting

1.7 to 4 In Adjustable, 2 In 1.8 In for 200 ms <2.5 Into 5 In

0.4 to 0.85 Tn Adjustable, 0.1 1.7 Tn <2.5 Tnto 0.7 Tn

3 fixed step Gradual Variable 1 to 5 fixed steps

21

22

Inverter driveapplications

Frequency inverter drives are very easy to use with

standard squirrel cage motors. Their torque

capabilities allow their use with all types of load,

including those requiring very high torques. For

applications where overhauling loads may be

encountered (hoists, mechanical handling, etc.) drives

are available for four-quadrant operation. These can

control both forward and reverse (hoist and lower)

operations, and they often include a braking facility.

Inverter drives almost invariably incorporate electronic

protection against thermal overloads and short

circuits. This protects both the motor and the drive.

Many drives also incorporate communications

capabilities which facilitate their integration into

automated systems.

Inverter drive applications

Variable voltagecontrollers

Variable-voltage controllers

An alternative to inverter drives, these units offer

another method of achieving motor speed control

electronically. As they are much less versatile than

frequency inverters, however, they are now declining in

popularity.

The principle of operation in this type of controller is to

vary only the voltage applied to the motor. The torque

produced by an asynchronous motor is proportional to

the square of the supply voltage. This type of drive

operates by regulating the voltage such that the torque

produced just balances the load torque at the speed

required. The motor supply voltage is usually

controlled by varying the firing angle of a pair of back-

to-back thyristors in each phase of the supply.

The use of variable-voltage controllers is limited by the

high losses in the rotor, which occur when

asynchronous motors are operated under high-

slip/low-speed conditions. These drives are most

suitable for motors with ratings of 3kW or less.

Summary

The availability of inverter drives has made

variable speed operation for asynchronous

motors increasingly popular. While other

methods of speed control are available, none

offers the versatility and performance of

frequency inverters.

23

24

Starters by design

Starters by design

This supplement has dealt with the general principles

of motor starting, and it is intended as an aid to

choosing the best starting technique for a particular

application. With the starter type decided, the next

step is either to select an off-the-shelf starter, if it is a

simple standard type or, for more complex

applications, to design a suitable starter.

Design guidelines for popular starter types are readily

found in the literature available from control gear

suppliers, but designers are also encouraged to talk to

their suppliers. Products and methods are constantly

evolving and, perhaps even more important, new

standards and regulations are imposing new duties

and responsibilities on designers. There is no better

way to keep up-to-date than to talk to an expert

supplier which has a strong focus in the control gear

market.

Furtherinformation

Further information

In this short supplement, is has not been possible to

do more than discuss briefly the most popular

methods of starting and motor speed control. Further

information is, however, readily available.

Telemecanique, a brand of the Schneider group, offers

two invaluable publications which are particularly

relevant.

Power control and protection

components

This contains technical details and

characteristics of motor starting components

necessary for the starting methods

described in this supplement.

Practical Aspects of Industrial

Control Technology

This comprehensive and up-to-date 290-page

hardback publication is available for purchase

from Telemecanique. It provides proven

design and application information covering

both electric and electronic products for

industry, and it has substantial sections

dealing with motor starting and control.

These publications are available from:

Telemecanique

University of Warwick Science Park

Sir William Lyons Road

Coventry CV4 7EZ

Tel: (01203) 416255

25

GSUK 0244 MAR 98

Schneider Limited University of Warwick Science Park Sir William Lyons Road Coventry CV4 7EZ Tel: 01203 416255 Fax: 01203 690209

Internet address: http://www.schneider.co.uk