Three-phase power supply (imbalance)

[embeddedlover]

Hello guys,

I was working with one technician in getting one motor repaired. Was discussing some 3-phase related things with him.

Came across one case, where, he was mentioning to use a single phase preventer. Was wondering what it does. He was explaining but couldn't get clarity. I have browsed through several products but couldn't get the clarity what they were doing in practical.

I have following questions regarding this preventer:

1. When i am feeding my motor with 3-phase, what does the "single phase prevention" mean?
2. As per my academic knowledge, a 3-phase system voltages are like 440V phase-phase, 220V Phase-line. Am i right? If so, what will i measure across line-phase, phase-phase if a "single phase" condition occurs.

 

[chuckey]

"Single phasing" occurs when one wire to a 3 phase motor is broken. The motor is then running on two phase wires which is like a single phase supply ( L&N). Depending on the load and design of the motor it can draw an excessive current and burn out.
1. Its a device that make sure that all the phases are present before allowing the supply to come on. N.B. you can not measure the voltage on the phases of a 3 phase motor because it will generate the missing phase (at the correct angle and > 90% of V phase to phase). The only valid way is to summing the phase currents by passing the motor leads through a toroid core and having a detection coil also on the toroid. If all phases are present then there is no magnetic field , so no voltage in the detection coil. If a phase goes missing you get a high error current, because the generated phase is SUPPLYING current. hence the detection coil generates a voltage.
2. As i said the voltage phase to phase is more or less the same.
Frank

 

[FvM]

The usual way to protect a standalone motor or a machine with few drives against phase failure is a motor circuit breaker of correct current rating. Larger machines might use a phase failure relays that monitors the supply voltage.

 

[embeddedlover]

The usual way to protect a standalone motor or a machine with few drives against phase failure is a motor circuit breaker of correct current rating. Larger machines might use a phase failure relays that monitors the supply voltage.

I guess both the cases must use a relay anyway.. Any machine, must use a detection mechanism and break the circuit in case of discrepency, that circuit breaker must be a relay.. Hope, i am right..

you can not measure the voltage on the phases of a 3 phase motor because it will generate the missing phase (at the correct angle and > 90% of V phase to phase)

As per this statement, can we come to a conlcusion that voltage sensing is not a correct mechanism in finding out the missing phase?

The only valid way is to summing the phase currents by passing the motor leads through a toroid core and having a detection coil also on the toroid. If all phases are present then there is no magnetic field , so no voltage in the detection coil. If a phase goes missing you get a high error current, because the generated phase is SUPPLYING current. hence the detection coil generates a voltage.

Do you have a basic circuit for this kind of detection for understanding more clearly?

 

[chuckey]

Step back a minute, your 3 phase will be brought to your site on a metering/switching panel, this is a valid point to check the phase presence and raise an alarm if one goes low or missing - so you can phone up your supplier and complain. After this main distribution board there will be sub boards until you get down to a piece of equipment or a motor.
The way to measure if all the phases are present at the actual motor is to measure the currents, this is what the motor overloads do. basically the motor currennt goes through a heater/phase. if the heaters get to hot they mechanically break the supply to a contactor feeding the motor, by operating a microswitch. These are pretty unreliable, certainly less reliable the the motor they mean't to be protecting. A more modern system is to embed thermistors within the motor windings which change their resistance if the motor gets hot and an electronic unit trips the feeding contactor. Unforunately the embedded thermistor are subject to undue mechanical stress and also tend to fail.
So you can check the phases by voltage at the input to the board, but not at a motor, you use current. if your kit is non motor, i.e. heaters,power supplies then you can use voltage right at the kit, because there is nothing that going to generate any voltage.
if you pass all the motor leads through a current transformer, there will be no output from it when all phase are there and balanced. There will be an output if a phase goes missing. An ultra sensitive detector would be a three phase RCD. These trip the supply if the phase currents are outof balance by a small amount. For protecting people, this is 30mA, if its to protect the equiment, you can get 100 and 500 mA trip range and the internal switch and wiring is rated for, 60, 100. . . Amps. These might trip if one phase goes a bit low, if you are dealing with a huge motor that is running at , say 50 A/phase, then a unbalance of 1% in the phase voltage would cause the switch to open.
Frank

 

[embeddedlover]

Thanks, chuckey.. For such a detailed explanation.

if you pass all the motor leads through a current transformer, there will be no output from it when all phase are there and balanced.

Didnt get this point though. You mean to say use 3 current transformers, for 3 different phases? Because, we may not know, which phase might be off?

 

[FvM]

The RCD idea won't work. Current sum in a motor cable will be always zero as long as no ground fault exists, wheter the supply has all phases up or not.

You could check the balance of RMS or rectified current values. But I'm not aware of this option being used regularly.

To refer to a previously stated point:

you can not measure the voltage on the phases of a 3 phase motor because it will generate the missing phase.

It depends. If the motor is rotating at regular speed, it will generate the missing phase. But that's a rare case, I think. It can e.g. happen if the phase is disconnected after motor start.
Usually the motor won't start with a missing phase, it will neither generate a rotating field nor a respective terminal voltage. The current through the remaining phases will be high and trip the motor circuit breaker within a few seconds.