Operation of Single Phasing Preventer by Creating Single Phasing Fault for a 3-ph Induction Motor with D.O.L. Starter

Experiment No.: 3

Experiment Name:

Demonstration of the operation of single phasing preventer by creating single phasing fault for a given 3-ph induction motor with D.O.L. starter.


To demonstrate the operation of single phasing preventer by creating single phasing fault for a given 3-ph induction motor with D.O.L. starter.


A 3 phase motor is connected with 3 phase supply. if there is any phase failure and still the field device is operating, then it may damage it. If one supply fails, on the occurrence of single phasing, the phase current of the healthy phases reaches up to 3 times of normal running current. The increased current causes heating and, eventually, causes motor failure.

This protection can be done by a single-phase preventer relay.

Single phasing causes unbalance in phase current. The unbalance current is the negative phase sequence current. The negative phase sequence current produces negative torque, and thus the net torque reduces. The reduction in the motor torque may cause motor stalling. The motor trips with an over-current fault or stall protection fault. But, during a single phasing period, the motor experiences heavy electrical and mechanical stress. In this case, a single-phase preventer relay is the best choice.

Therefore, a Single phase preventer relay is necessary for motor tripping to avoid unnecessary stress on the motor. The single-phase preventer trips the motor immediately when single phasing occurs.

The reasons for the single phasing of the induction motor are as follows.

  1. The one of the phases supply not coming from the supply source
  2. Single phasing at the main contactor in DOL starter
  3. The loose connections ultimately lead the loss of connection caused by heating
  4. Stator winding open

Circuit Diagram:

Observation Table:

Sl. No.Single phasing switch statusObservation
1.S1, S2, S3 ClosedMotor running normally
2.S1 Opened, S2 & S3 ClosedMotor stopped after few second
3.S2 Opened, S1 & S3 ClosedMotor stopped after few second
4.S3 Opened, S1 & S2 ClosedMotor stopped after few second
5.S1 & S2 Opened, S3 ClosedMotor stopped instantly

Apparatus Used:

Sl. No.Name of the ApparatusSpecificationQuantityMaker’s Name
1.Demostration of Single Phase Preventer of Induction Motor Study Kit415 V Input1Microline
1.3 phase Squirrel Cage Induction Motor3ph, 415 V, 0.5 HP/0.373 kW, 1440 rpm, 0.8 A, 1Triple-D
2.Direct On Line StarterPush Button Switch: 1 NO, 1 NC
Contacts: 3+1 (auxiliary) NO Contacts
Relay Voltage: 415 V
3.Single Phase Preventer415 V AC, 3P3W, 50/60 Hz,
5A ‘NO’ @ 240 V AC (Res)
3A ‘NC’ @ 240 V AC (Res)
1General Industrial Controls Pvt. Ltd.

Remarks: The operation of the single phasing preventer is observed successfully.

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