Synchronous motor no load caused fluctuating current & PF
Synchronous motor no load caused fluctuating current & PF
(OP)
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





RE: Synchronous motor no load caused fluctuating current & PF
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.
RE: Synchronous motor no load caused fluctuating current & PF
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.
RE: Synchronous motor no load caused fluctuating current & PF
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
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"Why not the best?"
Jimmy Carter
RE: Synchronous motor no load caused fluctuating current & PF
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?
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If we learn from our mistakes I'm getting a great education!
RE: Synchronous motor no load caused fluctuating current & PF
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.
RE: Synchronous motor no load caused fluctuating current & PF
This is a brushless synch motor and the exciter currents posted here are the field currents of the ac exciter.
Muthu
www.edison.co.in
RE: Synchronous motor no load caused fluctuating current & PF
RE: Synchronous motor no load caused fluctuating current & PF
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?
RE: Synchronous motor no load caused fluctuating current & PF
Muthu
www.edison.co.in
RE: Synchronous motor no load caused fluctuating current & PF
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.
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If we learn from our mistakes I'm getting a great education!