Synchronous motor no load caused fluctuating current & PF

[power2engineer]

In our plant we have 4.16KV 5500HP, 0.8PF leading Brushless synchronous motor with manual exciter regulator for reciprocating compressor drive. When motor was started at noload after upgrading the relay protection system I observed strange thing. The motor rated field Amps is 5.8 Amps. We started the motor with 2.5A field current applied it sychronized but the stator amps was flucting between 70A-130A and Power factor was also fulctuating between 0.3-0.5 Leading. Varying the field current at 2.0A and 3.5A we observed similar results. After applying 25% load the stator current and Power factor stabilized can anybody explain this Phenomena? Thanks

 

[dpc]

Was the motor in power factor mode? With no kW load to speak of, any variation in system voltage will cause big percentage swings in the motor current as the controller tries to maintain whatever power factor you have demanded. If you were in voltage control mode, it's a similar problem.

If you have an on-load tap changer or automatically switched cap bank, these could have been fighting the motor's excitation control.

When you have some real kW load, it won't be as noticeable in the motor current.

 

[power2engineer]

Thanks Dpc for your reply. Let me clarify on your assumptions The synchronous motor is in manual field voltage (excitation amps) control once manually set it is not changed, it does not have any close loop control.

The following info I don't know if it is relevant. The motor is 22 poles, 327 RPM. The motor is connected to large utility substation at 13.8KV with a dedicated stepdown transformer to 4.16KV.There is no voltage control or fluctuation of supply voltage. Yes although the reciprocating compressor is connected to motor without any load.

 

[waross]

With a weak field you may have mechanical oscillations. The fly wheel speed may not be stable with a light load. Also, with no load, your output is mostly VARs. Power factor changes with the load and with no load, PF will be very low.
I suspect that with rated field current, the current will be more stable but the power factor will be low until some load is applied. That shouldn't be a problem. A synchronous condenser is a motor with no load and almost 0% PF.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ScottyUK]

Remember that PF is a ratio measurement: if the actual values of MW and MVAr are very small then you can get wild variation in PF for small changes in either MW or MVAr. Here's a couple of (vaguely) plausible explanations:

The grid voltage and frequency isn't perfectly stable. Your machine under no load is virtually floating on the system, and any small frequency variation will result in your machine either dumping energy stored in the rotating mass into the system or absorbing energy from the system. You are seeing this effect.

When you say 'manual' field control, just how manual is your field controller? I expect you have a manual setpoint but there are almost certainly a number of control loops still in circuit unless you have something as basic as a big resistor and a DC source: 'manual' normally has a fair amount of 'automatic' still active, you just can't see it. It is possible that you have a slightly lively AVR with tuning optimised for operation under load: small voltage or frequency fluctuations may be enough just to provoke a response from it which shows up as a large PF fluctuation. The active power involved is so small that it only takes a small movement in MVAr to cause a big change in PF.

What are you seeing on the MW and MVAr meters when the PF measurement is unstable?


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[panter]

Hello, everyone !
I agree that ,in no load work of synchronous motor, small variation in field Amps cause big fluctuating in stator curent and power factor meter but only 5,8 Amps rated field Amps is ,for me, too small for motor of 5500HP .In my factory we have couple motor between 1,5 and 2 MW and their field current are in range 200 - 400Amps.

 

[edison123]

panter

This is a brushless synch motor and the exciter currents posted here are the field currents of the ac exciter.

Muthu

http://www.edison.co.in

 

[LionelHutz]

Could there be an irregularity in the rotating exciter which causes the current to oscillate as it rotates?

 

[power2engineer]

Edison - answered my question, the field current I am referring in my question is exciter field current. Panter you are right the motor field current is in order of 200A.
There is no AVR the exciter field is supplied by a 1 Phase bridge rectifier the output of it is controller by a potentiometer to control exciter field voltage and therefore the current. Do anybody have answer with this clarification?

 

[edison123]

I would suspect a defective pot, which is not stable at low current (high resistance) but is apparently ok at higher current (low resistance). Try replacing the pot. My 2 cents.

Muthu

http://www.edison.co.in

 

[ScottyUK]

Is the field ammeter stable? Put a clampmeter or a clamp-on scope probe over the field connection. A 'scope will probably show high frequency commutation noise but the DC level should be fairly steady.

My best guess is the MVAr load is actually fairly stable and the fluctuating power factor is a direct result of small changes in the active power as the machine floats on the system with virtually no load.


----------------------------------

If we learn from our mistakes I'm getting a great education!