54 Hz generator 1

[IBRCAN]

HI All,

I am working on a belt driven generator--150 kw, 4 pole, 600V/60Hz. We know this unit should be driven at 1800 RPM.
The drive sheave currently installed dictates that the generator only spins at 1650 RPM/54 Hz. It has worked this way for years. I have advised the customer on the correct pulley size to achieve 1800 RPM.

The customer would like the unit to be repaired as is for now until the correct sheave can be sourced and installed. I am replacing the voltage regulator and believe that to maintain the correct V/Hz ratio, the AVR should be adjusted for 540 VAC. My understanding is that 600V at 54Hz could be detrimental to electric motors designed for 60 Hz, due to magnetic saturation. Thoughts?

Thanks
Ian

 

[waross]

You are correct.
Good call.
But; Be aware of unintended consequences.
I have found that it is sometimes better to leave an existing system as is.
Is the customer happy with the voltage as it is presently set?
Does the customer's existing equipment work well with the voltage as it is presently set?
There may be unique factors specific to that installation of which we are not aware.
When an AVR setup for 60 Hz is operated at 54 Hz, the Under Frequency Roll Off will be continually active.
Normally the UFRO does not become active at and above 57 Hz.
At 54 Hz, the UFRO will be responding to any drop in frequency with a corresponding drop in speed.
Any motors should take + 10% voltage.
595 Volts should be safe.
Be sure that the set is running at 54 Hz when you set the AVR.
If the set is running slow when the AVR is set, the voltage will rise when the frequency comes up to 54 Hz.
You could avoid UFRO issues by setting the UFRO jumper to the 50 Hz terminal, but under the doctrine of unintended consequences I would leave the UFRO jumper the same as the original AVR setting.
When the sheave is finally replaced, be sure to reset the AVR before connecting any load to the set.
Beware of unintended consequences.


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

 

[edison123]

When you increase the frequency to 60 Hz, check the loads for their power vs speed relation. Some loads will require more than linear increase in power when the speed is increased. Will the generator take that additional load?

Muthu

http://www.edison.co.in

 

[IBRCAN]

Waross--therein lies the problem. The AVR has failed resulting in low voltage output, so I don't know where the voltage was running prior to the failure. The customer doesn't know either since there is no metering on the unit. The generator is installed on an engine driven rock crusher--as long as the motors keep turning, everyone is happy. The motors are rated 575V/60Hz.

The UFRO setpoint is adjustable on this AVR, so I should be ok in that respect. I fully agree with your advice regarding unintended consequences--been there, done that.

Edison--good point, I'll keep that in mind.

Thanks!

 

[waross]

Your choice on the UFRO setting.
The implications:
UFRO Set at 60 Hz it will become active at any frequency below 57 Hz.
That means that at 54 Hz the voltage will vary in rough proportion to the frequency.
UFRO Set at 50 Hz it will become active at any frequency below 47 Hz.
The voltage will be stable until the frequency drops below 47 Hz.

540 Volts per V/Hz ratio plus 10% = 595 Volts.
How do you feel about a voltage of 585 Volts?
That will give you some leeway both ways.
I imagine that an engine driven rock crusher will not have a rock steady speed.
Probably best to set the UFRO at 60 Hz.
With the UFRO set at 50 Hz, the AVR will hold the voltage steady until the frequency drops below 47 Hz.
That may push your motors into saturation.

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

 

[crshears]

"...an engine driven rock crusher will not have a rock steady speed."

Wow, Bill! I don't know whether to say you're rockin' it or crushin' it!

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[IBRCAN]

Thanks Bill, I set the voltage at 585. Certainly not a ROCK steady speed. Even unloaded, the cyclical inertia of crusher rotation made the generator speed slightly unsteady.

 

[waross]

Did you go 60 Hz on the UFRO jumper?

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

 

[IBRCAN]

Bill, Apologies for the delayed response.

I set the jumper to 60 hz, however this AVR has adjustable ufro (knee) point, which I set to 51.5 hz.

 

[JohnMcNutt]

Question for you guys in regards to riding the UFRO on an AVR like this:

I seem to remember reading in documentation recently that the V/Hz slope on a generator mfr.'s AVR is more than 1V/Hz once it has gone below 57 Hz.

Something like 3V/Hz.

I assume that would be because it is not intended to be used in an application like this for non-standard motor speeds, but rather to aggressively shed load so that the system can recover quickly.

Do you find that is the case more broadly or are there other AVRs set at straight 1V/Hz UFRO?

 

[waross]

A motor will typically accept 10% overvoltage safely.
So a 460 Volt, 60 Hz rated motor will accept up to 506 Volts.
That is a maximum V/Hz ratio of 8.43:1
At 480 Volts the V/Hz ratio is 8:1
At 57 Hz and 480 Volts, the V/Hz ratio is 8.42:1
That is close to the allowable limit before the onset of saturation.
So for a 480 Volt machine you want an UFRO V/Hz of at least 8:1, preferably 8.43:1
If you use a higher V/Hz, the motor voltage will be dropping below the optimum voltage for the applied frequency.
How that affects the set loading will depend to some degree on the mix of resistive loads and motor loads.
A UFRO setting of 3 V/Hz will only be suitable for a 120 Volt generator and will result in the voltage to motors being lower than optimum.
Quick chart of nominal V/Hz ratios.
480 Volts, V/Hz = 8:1
240 Volts, V/Hz = 4:1
120 Volts, V/Hz = 2:1
It's not a case of "More is better".
Too much is bad, too little is bad.

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