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What happens if You Connect a 3-Φ Induction Motor to 1-Phase Supply?

What will happen to the 3-Φ (400V) Induction Motor If Connected to 1-Phase (230V) Supply?

As the name suggests, a 3-phase induction motor must be connected to the three phases (lines) for proper operation where each of the phase acts as a return path for the current out of three phases. But what if we need to operate a three phase motor on a single phase supply? Well, The answer is 1: No: You can’t and 2: Yes! You can.

What will happen to the 3-Φ (400V) Induction Motor If Connected to 1-Phase (230V) Supply

If you directly connect a single phase supply to the three phase induction motor, following are the results with different scenarios:

If the Motor is Stopped:

The motor won’t start at all and sit there as there is no uniform torque and RMF (Rotating Magnetic Field) in the single phase supply system. That’s why single phase motors are not self started. In this case, the motor may start to smoke and burn due to the large amount of current in the short circuited winding slots of the rotor. If there is proper protection circuit, the thermal relay, fuse or circuit breaker may trip to disconnect the motor from the supply.

If the Motor is Running:

If the motor is started and run due to the phase shifter, VFD or starting/running capacitors in one of the coils, the motor will still run but with less output power and efficiency as well as humming noise and vibration. In addition, the speed of the motor will be lower as single phase voltage is less than that of three phase voltage where the speed of the motor depends on the supply voltage.

If You still Want to Run the Motor:

You can do it by using a phase shifter capacitor (according to the rated value) in one of the series winding (mostly in Delta connection), or rotary phase converter or most recent and reliable VFD’s (Variable Frequency Drive). Manually, you can twist the motor (after connecting the 3-phase 400V motor to the single phase 230V) and it will run in that direction but with less efficiency, power and toque. It is only possible if the motor is designed for both single phase and three phase supply (Star / Delta) e.g. 400V/230V supply system.

How to Run a Three Phase Motor on Single Phase Supply

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In conclusion, you can’t run a 3-Φ motor on a 1-Φ motor without any phase shifter like capacitor. If so, the motor will damage due to excessive amounts of current in absese of protection, else, it will trip the protective cicuit breakers. If you still want to drive the 3-phase motor on a single phase supply, you must use a VFD, phase shifter / converter or simply a capacitor which is used to make the phase difference (a capacitor is used for the same purpose like in a ceiling fan).

Mostly, the small motors are in Star “Y” connection. In case of delta connection, the capacitor can be connected in the free leg as the phase is connected to the first terminal, Neutral top second terminal. If you want to change the direction or rotation of the motor, you may simply change the Phase to another terminal and connect the capacitor to the new free leg of delta configuration.

The static capacitor method is used for small range motors. In the capacitor start capacitor run method, the starting capacitor is connected in parallel to the free leg of delta connection (Phase and Neutral is connected to three terminals, so the third one is free). When the motor is started, the starting capacitor is disconnected while the running capacitor is still connected (Note that the running capacitor is optional).

Keep in mind that the above method is not suitable for permanent operation of the motor as it will reduce the life span of the motor because only two phases (out of three) are active in this kind of configuration. In addition, the efficiency of the motor may be reduced 1/3rd to 2/3rd). For example, the efficiency and output power of three phase 5kW (6.7 Hp) motor will be reduced to 3.33kW (4.46 Hp) to 1.66kW (2.22 Hp). In short, the operation of a 3-phase motor on a single phase supply with the help of a static capacitor is only applicable for temporary applications.

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Rating of the Phase Shifter Capacitor:

The value of starting capacitor as phase shifter can be calculated by using the following formula.

C = 1950 x (Ie / Ue ) x CosΦ


C = 50 x Hp x (220/V)2 x 50/f 

C = 32 x P x (220/V)2 x 50/f 


  • C = Capacitor value in µF
  • Ie = Rated Current
  • Ue = Rated Voltage
  • V = 1-Φ Voltage
  • CosΦ = Power factor
  • Hp = Motor horspower
  • P = Power in kW
  • f = Supply Frequency

As a rule of thumb

  • The Value of Phase Shifter for Motor as Starting Capacitor = Start capacitor = 50-100µF per hp or 746W.
  • The Value of Phase Shifter for Motor as Running Capacitor = 12-16µF per Hp or 746W.

It shows the value of the running capacitor in micro farad is less than 4 to 5 times than the value of the starting capacitor. Keep in mind you may only use non-polairized capaictor designed for AC and the rating should be 5-20% higher than the supply voltage to the motor.

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  1. when three phase induction motor is connected on single phase supply , its rotor doesn’t rotate because magnetic field in single phase supply is pulsating.

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