synchronous motor

I think that if you don't know the answer to this question, you should conduct the experiment yourself and learn first-hand what happens to a running 3 phase motor when it looses 1 phase. Hint: Good design dictates installing phase loss protection or warning systems.

P.S. - Forgot to mention to keep a fire extinguisher rated for electrical fires handy.

You will let all the smoke out of the motor.

Actually it's even worse with a synchronous motor. The 58% loss of torque would allow the motor to fall out of synchronism, which could result in massive current pulses in the armature and major mechanical damage as well.

If the motor is loaded and left running long enough, 2 phases will burn out & the disconnected phase will not - until the fire develops in which case all phases will burn out!

Greg L

Good answers from all concerned!

If the motor is left running the load of the two remaining phases will grow and will warm them up. The torque of the motor will fall, the load angle will grow and swing. In addition there will be a tangential swing on the rotor because the load remains continuous but the supply will not. As a result, current will be inducated in the rotor and will warm it up too.

I too have a question about this subject, If you have an open-circuit coil motoring within a magnetic field will the open circuit coil enerate high voltages such as arcing etc?

In an induction motor, loss of supply on one phase will allow the motor to continue to run and the magnetic field will in fact generate a voltage on the unused leg. In fact, this is the thing that often fools voltage-based phase protection relays into not noticing the phase loss. But the voltage created on the "phantom" phase will be lower than the other two. This b y the way is the basic principle behind a "rotary phase converter" (with the addition of a capacitor to boost the phantom phase voltage).

However, all of the other issues will also be true, i.e. the motor torque will drop so severely that almost any load connected to the motor will pull the speed down, which will drastically increase slip and thus current on the 2 real phases, which will smoke the motor. That is why, on rotary phase converters, the 3 phase motor used to create the 3rd phase for other smaller motors down stream is not connected to anything and is referred to as an "idler" motor.

I must point out once again however to all of the other posters who discussed single phasing and loading, that the ORIGINAL post was about a SYNCHRONOUS motor, not an induction motor! Losing a phase on a synch. motor is a recipe for near instant disaster and protection systems must be in place to take it off line immediately if a phase is lost.

What is the cheapest and fastest way of detecting phase loss on 3 phase loads?

I am primarily Instrument not electricaly biased but-

I have developed a passive sensor technology that can detect an imbalance of 0.25 Amps in any phase. The sensor is based on a current transformer that generates an LED light signal plus an isolated volt-free output to the motor control system and woud cost a mere 20$ for the electronics with the aditional cost of a suitable CT.

The CT has to generate a voltage output >0.35 Volts at 3milliamp for it to give an output, 0.25 A is based on a single pass Primary through the CT. Response is < 10 mSecs

No other power supply is required, it is powered by the line current alone.

Its patented and Im looking for a manufacturer wishing to profit while affording a reasonable licence deal.