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Application Guide for Reduced Voltage Autotransformer Starter

Application Guide for Reduced Voltage Autotransformer Starter
Updated: | 6 min read

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Background

With the introduction of M-Line range of controlgear products and F-Line range of SDFs a need for a comprehensive selection chart for autotransformer motor feeder was felt necessary. Also, we have been receiving queries from various branches for an application guide on the same.

Brief Description

An autotransformer starter reduces inrush current by using a transformer in the line just ahead of the Motor to step down the voltage applied to the motor terminals. By reducing the voltage, the current drawn from the line is reduced during start-up.


Starting with reduced voltage decreases the full load current at the motor terminals in proportion to the voltage reduction while the full load torque is reduced by the square of the voltage reduction.

Recommended wiring diagram equation

Operation

In autotransformer starters, the motor is started at reduced voltage, which is supplied from an autotransformer. The starting sequence has three stages.


During the first stage, the autotransformer is star connected, and the line contactor is closed. This starts the motor with a reduced voltage, the value of which depends upon the ratio selected for the transformer. Autotransformers are normally provided with taps to allow the best ratio to be chosen during commissioning.


In the second stage, the star connection is opened, and the autotransformer acts as an inductor connected in series with the motor. This transition is normally timed to occur when the motor speed has stabilized at the end of the run-up period.


The third stage then follows almost immediately, and involves shunting the transformer completely, so that the motor is directly connected to the supply.


As shown in the wiring diagram

  • Star connection of the autotransformer is made by KM 1, then contactor KM2 closes and the motor starts under reduced voltage.
  • The neutral point is opened by KM 1; part of the autotransformer winding is switched into each phase for a short moment, constituting a stator starting inductance.
  • KM3 switches the motor to full mains voltage and causes the autotransformer to be shunted out of circuit by KM2.

Contactor Switching Sequence

ContactorStartTransition (initial)Transition (final)ON
KM1 (Star) Close Open Open Open
KM2 (Step) Close Close Close Open
KM3 (Main) Open Open Open Close

When the motor is directly switched to lines, the motor current is generally 6 times the full load current.


Istart = 6In = V/Z (3)1/3

Istart = Starting motor current

In = Full load current

V = Line voltage

In case of autotransformer if a tapping of transformation ratio K is used, then Vph across motor is KV/(3)1/3


Motor Current during start; I = kV/Z (3)1/3 = kistart = k*6In


The current taken by the autotransformer is Ki2.

= K2 *6In = K2 Istart


Hence, though the motor current is reduced by only K times the direct switching current, the current taken by the line is reduced by K2 times.


Similarly for starting torque,

T18 (V/313)2

T1 = torque during direct starting.


With an autotransformer,

T18 (V/313)2

T1 = torque during direct starting.


Hance, T2/T1 = K1

Starting torque with autotransformer = K2 * Starting torque with direct on-line starting.


Thus it provides maximum starting torque with minimal line current. Due to transformer action, the line current will be 25%, 42% or 64% of full voltage values for the 50%, 65% or 80% taps respectively.

Control Transformer Equation

The autotransformer motor starter selection chart is based on the closed transition which never disconnects the motor from the power source, and transient phenomena are eliminated. This is Also known as 'Korndorfer' method.


The transition from reduced voltage to full voltage on motor starters can be based on current or time. The over current relay monitors the motor current. When the motor current drops below the preset value, the relay signals the motor starter to switch to full voltage. Or when the setting time on the timer has expired, the autotransformer is bypassed.


Typically autotransformer has three taps, which provide 50%, 65% and 80% of full line voltage. The autotransformer starter can be used for any squirrel-cage motor. Typically autotransformer has three taps, which provide 50%, 65% and 80% of full line voltage. The autotransformer starter can be used for any squirrel-cage motor.

Conclusion

This chart provides aready reckoner for selection of components for an autotransformer motor feeder.

Motor Rating: 3Ø, 415V, 50 Hz Contactor Relay
HP kW In (A) MaIn (KM3) Step (KM2) Star (KM1) Type Range (A) Fuse Range (A) SDF
50% 65% 80%
7.5 5.5 11.2 MO 12 MO 9 MO 9 MO 9 MO 9 RTO - 1 8.5-12.5 HF 32 FN 32
10 7.5 14.8 MO 18 MO 9 MO 9 MO 12 MO 9 RTO - 1 12.5-18.5 HF 32 FN 32
12.5 9.3 19 MO 25 MO 9 MO 9 MO 18 MO 9 RTO - 1 17-25.5 HF 50 FN 63
15 11 22 MO 25 MO 9 MO 12 MO 18 MO 12 RTO - 1 17-25.5 HF 63 FN 63
17.5 13 24 MO 32 MO 9 MO 12 MO 18 MO 12 RTO - 1 17-25.5 HF 63 FN 63
20 15 29 MO 32 MO 9 MO 18 MO 25 MO 18 RTO - 1 25-37 HF 63 FN 63
25 18.6 35 MO 40 MO 9 MO 18 MO 25 MO 18 RTO - 1 25-37 HN, 000 80 FN 100
30 22.5 40 MO 45 MO 12 MO 18 MO 32 MO 18 RTO - 1 35-45 HN, 000 80 FN 100
35 26 47 MO 50 MO 12 MO 25 MO 32 MO 25 RTO - 2 40-57 HN, 000 100 FN 100
40 30 55 MO 70 MO 18 MO 25 MO 40 MO 25 RTO - 2 40-57 HN, 000 100 FN 100
45 33.5 60 MO 70 MO 18 MO 32 MO 40 MO 32 RTO - 2 50-75 HN, 000 100 FN 100
50 37 66 MO 80 MO 18 MO 32 MO 45 MO 32 RTO - 2 50-75 HN, 00 125 FN 125
60 45 80 MO 95 MO 25 MO 40 MO 70 MO 40 RTO - 3 75-110 HN, 00 125 FN 125
75 55 100 MO 110 MO 32 MO 45 MO 70 MO 45 RTO - 3 75-110 HN, 00 160 FN 160
90 67.5 120 MO 140 MO 32 MO 70 MO 80 MO 70 RTO - 4 105-156 HN, 0 200 FN 200
100 75 135 MO 140 MO 40 MO 70 MO 95 MO 70 RTO - 4 105-156 HN, 0 200 FN 200
110 80 139 MO 185 MO 40 MO 70 MO 95 MO 70 RTO - 4 138-201 HN, 0 200 FN 200
125 90 165 MO 185 MO 45 MO 95 MO 110 MO 95 RTO - 4 138-201 HN, 1 250 FN 250
150 110 200 MO 225 MO 70 MO 95 MO 140 MO 95 RTO - 4 138-201 HN, 1 250 FN 250
175 130 230 MO 250 MO 70 MO 110 MO 185 MO 110 RTO - 4 201-291 HN, 1 315 FN 315
197 147 260 MO 300 MO 70 MO 140 MO 185 MO 140 RTO - 4 201-291 HN, 2 400 FN 400
200 150 275 MO 300 MO 70 MO 140 MO 185 MO 140 RTO - 4 201-291 HN, 2 400 FN 400
215 160 280 MO 300 MO 80 MO 140 MO 185 MO 140 RTO - 4 201-291 HN, 2 400 FN 400
225 168 300 MO 300 MO 80 MO 140 MO 225 MO 140 RTO - 4 255-375 HN, 2 400 FN 400
245 180 320 MNX 400 MO 95 MO 140 MO 225 MO 140 RTO - 4 255-375 HN, 2 400 FN 400
270 200 340 MNX 400 MO 95 MO 185 MO 225 MO 185 RTO - 4 255-375 HN, 3 400 FN 630
300 225 385 MNX 550 MO 110 MO 185 MO 250 MO 185 MN 12 340-570 HN, 3 500 FN 630
335 250 425 MNX 550 MO 110 MO 185 MO 250 MO 185 MN 12 340-570 HN, 3 500 FN 630
400 300 500 MNX 550 MO 140 MO 225 MO 300 MO 225 MN 12 340-570 HN, 3 630 FN 630
430 315 535 MNX 550 MO 140 MO 300 MNX 400 MO 250 MN 12 340-570 HN, 3 500 FN 630
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About the Author

author

Sandeep Pawar,

General Manager - Product Management

Electrical Engineer with over 20 years of experience in Product Management and Application Engineering for Protection Relays, Power Quality Solutions, and Intelligent Motor Control Centres (MCCs). Possesses strong expertise in LV and MV power system protection schemes, motor protection, monitoring, and control applications. Well-versed in both conventional and intelligent motor management systems, with extensive knowledge of protection, control, and condition monitoring solutions.

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