Sync Motor PF Trip 7

[T247]

We have a sync motor that is tripping on power factor shortly after it starts. The motor comes up to speed and the trips on power factor after the delay expires. They were able to watch the PF on the relay display during the start event. The PF is initially at 0.3 lag, but then decreases to 0.04 lag by the time it tripped (the PF trip is set at 0.8 lag for 12 seconds). The waveforms show the currents are lagging the voltage by about 90 degrees. What could be causing this to happen? The relay was changed and work was done to the compressor (not the motor) but the same thing was done to a sister motor without any issues.

 

[edison123]

Rotor not getting the juice. Faulty diodes, wiring, exciter. Check all of them.

Muthu

http://www.edison.co.in

 

[waross]

Agreed. Check for loss of excitation.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[davidbeach]

I'll third that, it's an excitation problem.

When one this sentence into the German to translate wanted, would one the fact exploit, that the word order and the punctuation already with the German conventions agree.

-- Douglas Hofstadter, Jan 1982

 

[T247]

Thanks, I'll have this checked out. For a brushless sync motor, what happens if the field leads are reversed?

 

[Kiribanda]

Do you have a Loss of Field relay?

 

[T247]

No, just power factor trip

 

[crshears]

For a brushless sync motor, what happens if the field leads are reversed?

I don't believe this would have any appreciable effect, unless there is sufficient remanent magnetism in the field poles that the excitation system can't generate enough field strength to overcome it, in which case the result would be that the magnetic field on the rotor could be minimal or perhaps even zero, rendering normal motor operation impossible.

 

[edison123]

No, reversal of current in DC exciter field (brushed), AC exciter field (brushless) and the main rotor field does not change anything including rotation direction in synch motor or phase sequence in synch generator.

Muthu

http://www.edison.co.in

 

[panter]

T247
Can you measure resistance of the rotor windings .

 

[wcaseyharman]

What is the motor voltage doing during the event? What is the field current doing during the event? What does the VAR meter show?

How are you monitoring power factor? Is it with the power factor relay that was just replaced?
Although I agree it sounds like a loss of excitation, it’s also suspicious that the relay was changed out. If it was wired improperly that could also explain the problem.

If it is a true loss of field event voltage will be exceptionally low, lagging VARs will be very high and field current will be low (or zero).

If you don’t see low voltage, low field current and high lagging vars, check the wiring of your relay.

 

[waross]

and field current will be low (or zero)

Checking the actual field current is often difficult or impossible.
With failed rotating diodes, the field current will be low (or zero), while the AVR output current will be high.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[T247]

panter - no, we don't have access to the motor to measure the resistance of the rotor windings at this point.

wcaseyharman
Motor voltage dipped to 3815 V during the start, but was back to 4120 V within 3 seconds. I don't have the field current readings during the initial start, but right before the relay tripped, it was at 8 A and the field voltage was 66 V. There's a note on the drawings that say the exciter field is rated for 66 V, 8.6 A so the field seems to be normal.

During the starting event, the power factor was monitored on the front display of the relay that was just replaced.

 

[wcaseyharman]

I’d verify the wiring of the relay. That doesn’t quite sound like the loss of field events I’ve experienced.

Not sure how the relay is wired but to me it sounds like the phase angles are off - maybe LN voltage is being used instead of LL or something like that.

In my company we’d use our relay department to inject voltage and current to verify the wiring and metering.

 

[Gr8blu]

Per the OP, we have a sync motor (presumably on brushless excitation, given the subsequent V-A rating for the field) that trips on low PF.

Rereading the post chain to date it appears everyone has assumed that the motor actually synchronized. This may or may not be the case, particularly since it appears the power factor never reached unity or leading. If the motor did not synch it is a failure within the: a) exciter, b) rotating rectifier assembly, c) driven load / inertia is too high - which means some back pressure or moisture in the compressor itself, or d) controlling relays have wiring issues.

If it DID synchronize and STILL failed to get to a unity or leading power factor, then I would suspect that the field discharge resistor circuit never got taken off line (i.e. a failure in the rotating rectifier controller) OR the relay wiring is wrong.

Converting energy to motion for more than half a century

 

[T247]

Gr8blu - I believe you are correct. The doesn't seem like the motor ever synchronizes. It is a brushless motor and the field is applied as soon as the motor starts. The following information is what we have for the start. The initial current is ~1600 A (motor FLA is 428 A), the current drops to ~450 A after about 3 seconds and it stays between 435-450 A until it trips about 15.5 seconds after the initial start event. The time delay for the power factor trip is set at 12 seconds but is blocked during motor starting.

 

[Kiribanda]

When the field (excitation) is lost to a syn. motor it behaves like an induction machine. As a result it draws a lot of MVARs from the connected system
at a very low power factor until it is disconnected from the system. In my opinion that is what has happened to your sync machine. But the machine will not fail instantaneously due to the heat generated.
It takes sometime to heat the stator.
Therefore, going forward,
1) You may have to thoroughly check the excitation system.
2) It is better to incorporate a Loss of field relay (ANSI#40).

 

[waross]

the field is applied as soon as the motor starts

The brushless exciter field is applied as soon as the motor starts.
The main field is not applied until the rotating "Polarized Field Frequency Relay" or electronic equivalent completes the circuit from the rotating diodes to the main field.
The rotating Polarized Field Frequency circuit may have failed.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[edison123]

OP


It looks like you start this synch motor via a rotor damper winding.

You close the field circuit only when it is near synch speed for it to pull to into synchronism, not right from the start.


Bill

Polarized Field Frequency Relay???

Muthu

http://www.edison.co.in

 

[LionelHutz]

I'm doubtful the motor would run inductive at 0 power factor. It's more likely to be running around 0 leading if it's synchronized and not loaded. Some relays will detect a very leading power factor as lagging.