Guide to motors and starting
2
Introduction Electric motors are deservedly the most popular prime movers for industry and commerce. Compared with other sources of mechanical power, they're inexpensive, compact, reliable reliable and versatile. As a result of this popularity, millions of motors are installed each year and, except for the very tiniest, every one needs a starter starter.. The manufacture of these starters is the basis on which the control gear industry was built, and motor starters are still at the core of almost every control gear supplier's business.
Introduction 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, asynchron asynchronous ous 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 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.
The scope of this publication
3
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 means covers all aspects of motor starting but, nevertheless, it deals with the vast majority of applications likely to be encountered in industry and and commerce. commerce. For those requiring further information, a short list of sources is included at the end of the supplement.
Scope Motors – a few basics Principle of operation
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. field. It is this principle of operation which which gives rise to the commonly used term "induction motor."
M o t o r s – a few ba bas s ics The rotating magnetic field 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 120° arrangement produces produces a rotating rotating magnetic 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 the supply current. current. Motor speed is, therefore, therefore, directly related to the supply frequency (f in cycles per second or Hz), and the number of pole pairs (p) which the motor motor uses. The motor's so-called so-called synchronous synchronou s speed is given by:
Ns (in revolutions per minute) = 60f / p The majority of motors in use are four-pole machines (2 pairs), which have synchronous speeds of 1500 rpm at 50Hz and 1800 rpm at 60Hz.
4
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. 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 rotates at synchronous synchronous speed. This is why why the motor is described as asynchronous.
Slip
The difference between the synchronous speed (Ns) and the actual nominal nominal rotor speed (Nn) is called the slip. Slip is always expressed as a percentage of the synchronous speed:
slip = 100(Ns - Nn)/ Ns
Motor construction A three-phase asynchronous motor comprises two main components, 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 of laminated silicon steel sections. The laminations are slotted so as to accommodate the stator windings which create the rotating rotating magnetic field. Each of the main windings, windings, of which there are three in a three-phas three-phase e motor, 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.
Motor construction 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. shaft. There are two principle types of of rotor - squirrel cage and wound.
Motor construction
5
Squirrel cage rotors are, by far, the most common. They have straight conductors set into slots around around the periphery of the rotor. 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 assembly gets its name. A popular variation is the doublecage, which has two concentric cages and offers a higher starting torque than single-cage single-cage versions. No external electrical electrical connections can be made to any type of squirrel cage rotor. Wound rotors have windings Wound windings similar to those used in the stator. stator. One end of each winding is connected to a common (star) point, and the other ends are connected connected to slip rings. rings. External connections to the the rotor windings are made via these slip rings, allowing additional resistance to be added to the rotor rotor circuit during starting. starting. This enables the motor's starting current and torque to be controlled.
Controlling speed Varying the supply voltage alone is a comparatively ineffective way of controlling the speed speed of of an induction inducti on 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.
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.
Controlling speed
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
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 contactor-based starters, however, there is a problem - reduced starting current means reduced starting torque which may, in some applications, be unacceptable. unacceptable. This limitation is examined in more detail in later sections which describe particular starter types. 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.
Starting current
Summary Three-phase asynchronous asynchronous induction motors are the most commonly used type in industry. industry. Their speed is largely determined by supply frequency frequency,, with voltage variations having comparatively little effect. Connected directly directly to the supply, supply, these motors have typical starting currents of 5 to 8 x FLC. Often, starting currents currents need to be reduced, and various forms of starter have been developed 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 threephase mains supply. supply. The motor starts and accelerates in a way determined by its own characteristics. character istics. Typicall ypically y, 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.
1 L / 1
Starters Motor current
2 4 6 1 3 5
-KM1 2 4 6 1 3 5
-F1 2
U
5
2.5 2 o t o
4
r c
1.5
u r r e n
3
u e
q q o r r T
o t o M o
t
1 2 u e q q o r T d L o a
0.5
1
Speed
Speed 0
0.25
0.50
0.75 0.
1
Direct-on-line current/speed characteristics
0
0.25
6
W
Direct-on-line starter diagram
I n ns
m
V
4
M 3
7
t a n t a n e o u s
3 L / 5
-Q1
Torque
6
2 L / 3
0.50
0.75
Direct-on-line torque/speed curve
1
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 low-power motor motors s are are being being used used,, and the supply capacity is high, so that the starting current surge does not adversely affect other equipment using the same supply
Starters
• the equipm equipment ent driven driven by the the motor motor is fitted with a gearbox or some other device which will soften the mechanical shock produced by the high starting torque • a high high starting starting torqu torque e is need needed ed 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 start starting ing curre current nt would would result in a serious voltage drop on the supply system • the equipm equipment ent being being drive driven n cannot cannot tolerate the effects of very high peak torque loadings • the safe safety ty or comf comfort ort of thos those e using the equipment may be compromised by sudden starting as, for example, with escalators and lifts
Star-delta starters This type of starter may only be used where access is possible to both both ends of all three stator windings. In addition, the windings must be rated to withstand the full supply voltage when delta-connected. With stardelta starting, the peak starting current current is typically 7 between 1.5 and 6 C u r r e n t 2.6 times the i n d e l t a 5 c o normal full-load n n e c current, and the t i o 4 n ( d peak starting i r e c 3 t ) torque is between Cu r re n t t i n n s t 2 0.2 and 0.5 times t a ar r c o on n ne c t t i i o n the motor's 1 nominal operating Speed torque. 0 0.25 0.50 0. 0.75 1 r
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, de-energise d, and the delta contactor energised to deltaconnect the stator stator windings. windings. Each winding is now fed with the full supply voltage, and the motor adopts its normal operating 1 3 5 1 3 5 characteristics. -KM1
Torque
9
2.5
2 1.5
)
c t
a t a e l u e u e i n d
T o r q q
1 0.5
0
T o r r q u e
n i
1 L / 1
2 L / 3
3 L / 5
-Q1 2 4 6
1 3
5
-KM2
-KM3 2 2
4 4
1 1 1 U V W
M1 3 2 2 2 U V W
6
2
4
6
1
3
5
6
-F2 2
4
6
2
4
6
a r a t s
d L o a
0.25
u e
r q
t o
0 .50 0.
Speed 0 .7 5
1
Star-delta starting torque/speed characteristics
Star-delta starting current/speed characteristics
The run-up time with the windings star-connected star-c onnected 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 star-de lta 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. contactor. This allows time for any switching arcs to be extinguished.
Star-delta starters Star-delta starter
r e i d (
10
Star-delta starters are particularly suited to machines which do not present a high load torque at start-up, or which normally normally start off-load. off-load. It is also important to note that, during the star-to-delta transition, a high transient current current is generated. If a magnetic magnetic shortcircuit protective device is to be used in the starter, this transient must be taken into account in the selection of the the device, in order to prevent nuisance nuisance tripping.
Star-delta starters 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 be necessary. necessary. One solution is to introduce a delay of 1 to 2 seconds during the star-todelta transition. transiti on. 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 motor has run up up and its speed has stabilised, the resistance bank is shorted out, and the motor becomes direct-connected direct-connected.. This changeover is normally controlled by an adjustable timer within the starter starter.. Unlike star-delta star-delta starters, primary resistance starters do not require access to both ends of the stator windings.
Primary resistance starters Values of starting current and torque are determined Values by the values of the resistors used. Typically, however, 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.
1 L 1
2 L 3
Primary resistance starting current/speed characteristics current/speed
3 L 5
Torque 7
-Q1 2 4 6 1 3 5
6
C u r r
e n
-KM1
1
4
3
n d
. s t e
p
2 4 2
u t r
n t o
n 1
s t . s t e
e s i
s t a n
p
6
5
w i t h o
C u r r e
4
-F1 2
t o n 2
5
2 4 6 1 3 5
-KM11
11
3 1 R
3 R
5 R
U V R R
6 R
2 4 R R
6 R
w i t h
r e s i s t
a n c
c e ( d i r e
e
2 1
Speed 0
0.25
0.50
0.75
1
U V W
M 3
Primary resistance starter
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 resistor values. values. Care must be taken, however, since this also reduces starting torque.
Torque
2.5
2
1.5
u t
T o r q
d
u e o n 2 n
Primary resistance starters 1
T o r q q
u e
o n
p t e .s
o h t i w
t h i t
w e p t e s . t
e ( d s tanc e i s
r e s
i r e
c
t )
ce r e s istan c
1 s
0.5
u e
r q Loa d T o
0
0.25
Speed 0.50
0.75
1
Primary resistance starting torque/speed characteristics
Current
Auto-transformer starters
12
In auto-transformer starters, the motor is started at reduced voltage which is supplied from an auto-transformer. auto-tra nsformer. The startin starting g sequence has three stages.
7 C u r r r r e en t
6
w i t h o u t a
u t
5 C u u r r r re n e n
t o n t n
4 3
During the first stage, the auto-transformer is starconnected, and the line contactor contactor is closed. closed. This starts the motor with a reduced voltage, the value of which depends upon the ratio selected for the transformer.. Auto-transformers are normally transformer provided with taps to allow the best ratio to be chosen during commissioning.
o -
t r a
n s
f o r
2 n d d.
m e r
s t t e e p
Current o n n 1
( d i r e c t )
s t t. s
tt e e p
2 1 Speed 0
0.25
0.50
0.75
1
Auto-transformer starters In the second stage, the star connection is opened, and the auto-transformer acts as an inductor connected in series with the motor motor.. This transition is normally timed to occur when the motor speed has stabilised at the end of the run-up period. The third stage then follows almost immediately, and involves shunting the transformer t ransformer completely, completely, so that the motor is direct-connected to the supply.
The starting current and torque are reduced as a function of the reduced starting and run-up voltages (Usupply/Ustarting)2. Typical values for peak starti starting ng current are 1.7 to 4 times nominal full-load current and, for peak starting torque, 0.5 to 0.85 times nominal operating torque.
Auto-transformerr starting curre Auto-transforme current/speed nt/speed characteristics
1 L / 1
2 L / 3
3 L / 5
-Q1
2 4 6 1 3 5
-F1 2
4
1
-KM3 2 4
6
3
5
1
3
5
-KM2 22 2
1 1 1 U V W
2
-T1 U3 V3 W3
U V W
2 2 2 U V W 2 4 6
Auto-transformer starting is particularly used for large M -KM1 motors (above 100kW), but tends to be an expensive 3 1 3 5 solution, largely because of the cost of the autotransformer itself. These starters may also produce a Auto-transformer Auto-transfo rmer starter current peak at the instant when the motor is switched directly to the Torque supply. This peak can be minimi minimised sed by careful design of the auto-transformer, but only at the expense of 2.5 increasing the peak current at the commencement of 2 the first stage of the starting sequence. 1.5
1
T o r q
u e
o n
2 n d.
T o r q u
p
t e s
p
e o n 1 s t. s t e e
0.5
0
Lo ad T o r q u e
0.25
0 .50 0.
0.75
1
Auto-transformer starting Auto-transformer torque/speed torque/s peed characteristics
Electronic soft starters
13
This relatively recently introduced form of starter is rapidly growing in popularity.. Soft starters operate by popularity 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. torque.
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 power -switching bridge which allows complete control over the starting and stopping of a threephase squirrelsquirrel-cage cage motor. It provides:
Electronic s o f t s t a r t e r s 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-toback. By varying varying the firing angle of Electronic “soft starter” current/speed and torque/speed characteristics
2.5
2
TdB 1
TdA
TdC
TC
TB
TA
3
0
0.25
0.50
0.75
VN
1.25
lB
5 lC
1 4
2
2
IN
0
0.25
0.50
• thermal thermal overloa overload d protectio protection n for itself, itself, and for the motor which it is controlling • mechan mechanica icall protect protection ion for for the machine being driven, by eliminating sudden changes in current - and, therefore, torque - during starting and stopping.
1A
1 ATS
Note: = firing angle of thymistors
• control control of of the acce accelerat leration ion and deceleration ramps of the motor in such a way as to keep within all required limits on current and torque
TN
2
M 3
0.75
VN
1.25
Electronic “soft starter”
14
The peak starting current may be adjusted between 2 and 5 times nominal full-load current, corresponding correspondi ng to a range of starting torques from 0.1 to 0.7 times the starting torque which would be produced if the motor were started with a DOL starter. This type of starter may be used with any asynchronous motor. motor. The Telemecani elemecanique que Altistart may be bypassed by a contactor at the end of the acceleration ramp, the contactor being controlled by a contact provided provided for this purpose. purpose. This will avoid thyristor heating and losses which occur during normal running. Even with the starter bypassed, bypassed, however, however, the protective devices of units rated at 18.5kW/415V and above remain operational, thus protecting both the starter and the motor motor.. For smaller units, units, a separate thermal overload is required. required. Other features which can be provided by soft starters include controlled deceleration, and braking to a complete stop.
Electronic
s o f t s t a r t e r s
Rotor resistance starters
15
Starters of this type can only be used with motors having a wound rotor to which external connections can be made, made, usually via via slip rings. This type of of motor cannot be started direct on line because the peak starting current at the instant that the supply is connected would would be far too high. Instead, the motor 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 resistance in the rotor circuit. circuit. Va Various rious 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.
Ro t o r r es esis istt a nc nce e starters Rotor resistance starting current/speed characteristics
Rotor resistance starter
Current 7 C u r
6 1 L / 1
2 L / 3
3 L / 5
t o n 3 r d .
5
-Q1 1 3 5
o
C u ur r r
ee n n t t o
U
V
4
c
t )
e r e s i s t
C ur r r e en t n t
o r s )
o n n 1s t t
. s t t e e p ( a a
ll l l r e e s
i s st o or r s s
)
6
W
r s , d i r e
( s o m
1 2
i s t o
t e p
2
-F1
r e s
n 2 n d . s
3
2 4 6 1 3 5
s t e p
( n
4
2 4 6
-KM1
r e n
0
0.25
0.50
0.75 0.
Speed 1
M 3 K
L
M
A2 B2 C2
A 2 R -
B 2 C 2 R R -
-KM12
1
3
5
2
4
6
que o
n 2 n
2
q ue w i t h o
o r
d T
. s
u
t
t e p ( s
r
e s
i
s
o
o
m e
C1 B C 1 1 R R -
2.5 T o r
A1 B1
A 1 R -
Rotor resistance starting torque/speed characteristics
Torque
1
-KM11
3
5
1.5
T o r r q q
4
6
s
i s t o r s
uu e e o n 1
s t . s
t e p
2
r
)
r e s
)
( a l l
r e s i s
1
t o r s )
0.5 Speed 0
0.25
0.50
0.75
1
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
16
with rotor resistance starters are, therefore, ideal for high-inertia loads which need to be started onload, but where the peak current taken from the supply must be limited. Further Further,, the values values of the resistances and the number of stages can be calculated so as to match the motor characteristics to those of the application.
Rotor resistance
starters
Summary The principle objective of all methods of motor starting is to match the torque characteristics to those of the mechanical load, while ensuring that the peak current requirements do not exceed the capacity of the supply supply.. Many starting methods are available, each each of which has slightly different different characteristics. The following table table summarises the main characteristics for the most popular forms of starter.
Selecting a starter
17
When choosing a starter for a particular application, the following procedure should be used:
• The pow power er for for the mac machin hine e • Chec Check k that that the supp supply ly line line will will not installation will normally be introduce unacceptable voltage supplied by the Regional Electricity drops when the peak current is Company, and the user will need taken. If this is a problem, the to comply with any local choice lies between installing regulations. The Regional larger cables or selecting a Electricity Company will normally starting method other than DOL. limit DOL starting to a maximum motor rating. If the motor is below • If the abov above e condit condition ions s are all all the DOL starting limit, determine satisfied, DOL starting will provide the peak starting current which an economical solution, provided it would draw if started that the mechanical load can direct-on-line. handle the peak starting torque produced. • Che Check ck that that this this peak peak start starting ing current is within the capacity of • If any any of the condit conditions ions are not not the supply. satisfied, use the table to choose an alternative method of starting. • The insta installatio llation n will norma normally lly be be fed Be particularly careful to ensure from a stepdown power that the starting torque produced transformer. transfo rmer. Check that the peak by the method of starting chosen starting current will not initiate a is adequate for the application. circuit breaker trip on the highvoltage (primary) side of the transformer.
Selecting
a st s t a r t e r
18
Speed regulation of asynchronous 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 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.
Speed
regulation o f
asynchronous motors 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 supplement will, therefore, therefore, deal principally with speed control by inverter.
The frequency inverter drive
19
This type of drive is intended mainly for use with three-phase squirrel cage motors. It operates by using a technique technique called pulse-width modulation (PWM) to synthesise a sinusoidal waveform, the frequency of which can be varied, that is used to supply supply the motor. 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 synthesised supply waveform is very close to sinusoidal, smooth motor rotation is achieved even at low speeds.
Speed
regulation o f
asynchronous motors Inverter drive operation
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 controlled long and and short pulses. The train of pulses produces a sinusoidal current in the motor, the voltage and frequency of which can be accurately controlled. controlled. By retaining the correct voltage/frequency ratio in the supply to the motor, its torque can be maintained over a wide speed range.
Reservoir capacitor
W V U
Bridge rectifier
M 3
Transistor output power bridge
Main circuit of a frequency inverter
20
Summary of characteristics of various starting methods Squirrel cage motors Direct-on-line starting
Star-delta starting
Primary resistance starting
Peak starting Current
4 to 8 I n
1.3 to 2.6 In
4.5 In
Peak starting torque
0.6 to 1.5 Tn
0.2 to 0.5 Tn
0.6 to 0.85 Tn
Control
On or off
On or off
1 fixed step
Economic and rugged squirrel cage motor Advantages
• Simple st starter • Low cost • High starting torque
• Sim impl ple e, ec econ onom omic ic starter • Good starting torque/current performance
• Possibility of of adjusting starting parameters • No br brea eak k in in su supp pply ly to motor during starting • Goo ood d re redu duct ctio ion n in in peak transient currents
Disadvantages
• Ver ery y hi high gh st star arti ting ng current and torque • Supply must withstand peak current • Me Mech chan anic ical ally ly ha hars rsh h starting sequence
• Low st starting torque • No Nonn-ad adju jus sta tab ble starting parameters • Break in su sup ppl ply y to to motor leads to severe transient peak current
• Small red educ ucti tio on in in peak current • Res esis ista tan nce ban ank k required
Run-up time
2 to 3 seconds
3 to 7 seconds
7 to 12 seconds
Typical applications
• Small ma machines may often be started on full-load
• Mac achi hin nes st star arti ting ng on no-load (small centrifugal pumps, fans, etc.)
• High inertia machines with normal starting current/torque characteristics
21
Slip-ring motors Auto-transformer starting
Electronic “soft-starting”
Variable speed drives
Rotor resistance starting
1.7 to 4 In
Adjustable, 2 In to 5 In
1.8 In for 200 ms
< 2.5 In
0.4 to 0.85 Tn
Adjustable, 0.1 to 0.7 Tn
1.7 Tn
<2.5 Tn
3 fixed step
Gradual
Variable
1 to 5 fixed steps
• Good starting torque/current performance • Possibility of of adjusting starting parameters • No br bre eak in su sup ppl ply y to motor during starting
• Para ram met ete ers ar are e fully adjusted during commissioning • Compact • Solid st state • Easi sily ly ada dapt pte ed to to the application
• Par aram amet eter ers s ar are e fully adjusted during commissioning • Compact • Solid st state • Eas asil ily y ad adap apte ted d to to the application • In Infi fini nite tely ly va vari riab able le speed • In-built mo motor protection • Low st starting current
• Good starting torque/current performance • Possibility of of adjusting starting parameters • No br bre eak in su sup ppl ply y to motor during starting
• Exp xpen ensi sive ve au auto to-transformer required • Not to tolerant to to supply line transients
• Can cause interference on the supply during starting and stopping
• Can cause interference interferen ce on the supply • Relatively expensive compared to direct-on-line
• Expe Expens nsiv ive e sl slip ip-r -rin ing g motor required • Res esis ista tan nce ban ank k required
7 to 12 seconds
Adjustable, 1 to 60 seconds
0.1 to 999 seconds
• 3-step : 2.5s • 4 and 5 step : 5s
• High inertia machines where a reduction of starting current/torque is required
• Mach Machin ines es req equi uiri ring ng very smooth starting (centrifugal pumps and fans, conveyors, etc.)
• All ma machines where speed needs to be varied to improve production and reduce mechanical wear • Machines wh where energy can be saved by reducing speed (centrifugal pumps, fans, etc.)
• Mach chin ines es sta starrti tin ng on-load, where smooth run-up is required, etc.
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Inverter drive application applications s Frequency inverter drives are very easy to use with standard squirrel cage cage motors. Their torque torque capabilities allow their use with all types of load, including those those requiring requiring very high torques. torques. For applications where overhauling loads may be encountered (hoists, mechanical handling, etc.) drives are available available for four-quadrant four-quadrant operation. operation. These can control both forward and reverse (hoist and lower) operations, and they often include a braking facility.
I nv nvert ert e r dr iv ive e
applications Inverter drives almost invariably incorporate electronic protection against thermal overloads and short circuits. This protects protects both the motor and the drive. Many drives also incorporate communications capabilities which facilitate their integration into automated systems.
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Variable-voltage controllers An alternative to inverter drives, these units offer another method of achieving motor speed control electronically. electroni cally. As they are much less versati versatile le than frequency inverters, however, however, they are now declining in popularity.
Var i ab abll e vol v oltt age The principle of operation in this type of controller is to vary only the voltage voltage applied to the motor motor. The torque produced by an asynchronous motor is proportional to the square of the supply 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 voltage is usually controlled by varying the firing angle of a pair of backto-back thyristors in each phase of the supply.
controllers
The use of variable-voltage controllers is limited by the high losses in the rotor, which occur when asynchronous asynchron ous motors are operated under highslip/low-speed conditions. These drives drives are most suitable for motors with ratings of 3kW or less.
Summary The availability of inverter drives has made variable speed operation for asynchron asynchronous ous motors increasingl increasingly y popular. While other methods of speed control are available, none offers the versatility and performance of frequency inverters.
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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.
St a r t er ers s by design 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 constantly evolving and, perhaps even more important, new standards and regulations are imposing new duties and responsibilities responsibilities on designers. 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.
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Further information
Further information In this short supplement, is has not been possible to do more than discuss briefly the most popular methods of starting starting and motor motor speed control. control. Further information is, however 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 Telemecani elemecanique. que. It provides proven design and application information covering both electric and electronic products for industry,, and it has substantial sections industry 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
Schneider Limited University of Warwick Science Park Sir William Lyons Lyons Road Coventry CV4 7EZ Tel: 01203 416255 Fax: 01203 690209 Internet address: http://www.schneider.co.uk GSUK 0244
MAR 98
