Generators and AC induction Motors (Capacitor Sizing?)

[defisher40]

I own a Northstar 15KW generator. I am running an HTC 500 (7.5Hp)planetary grinder(240VAC)on a VFD. It runs fine when no other loads are connected. I can have 110VAC angle grinders (2) with Shop vacs running together. As the angle grinders are turned on and off, it sets a fault on the VFD. The amp in rush is a bit much. Is it possible to add capacitor(s) to cushion this current inrush?

 

[waross]

It may help a little if you share with us the nature of the fault on the VFD.
Does the fault cause the VFD to shut down?
How do you measure the current inrush?

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

 

[defisher40]

I believe it is a Toshiba AC Drive. The VFD is in a Nema Enclosure so the fault is not readily accessible. I believe I have recorded "Low DC Bus Voltage" fault. It stops the VFD and we press a reset button and the machine is restarted. The 110VAC hand grinders start and stop frequently. So the problem occurs frequently. Again, I am hoping to add a capacitor to buffer the amp rush during start up? How would you size something like that? There should be Caps on the generator now? right? Could/should I put larger ones on?

 

[ScottyUK]

You might be able to add capacitance to the DC link of the drive which would increase its ride-through, provided that the DC link is brought out to terminals. Normally you need to be careful not to make the capacitance too large because the peak current can damage the rectifiers but on a low capacity generator supply this is unlikely to be a problem.

Please be very careful - the DC link caps will operate at about 340V DC or so and can deliver more than enough current to kill you ten times over. As for how much capacitance you need to add, it's difficult to say. Look on Ebay for something around 2200uF and at least 400V rating and and try it. It will be a big capacitor - beer can / pint glass sized.

 

[defisher40]

An interesting perspective....... Why wouldn't we build capacitance on the AC side. The VFD is a complicated and delicate device. The generator is a brutal POS. If the Voltage could be held closer to constant during the amp rush demands, we might not see the DC Bus drop enough to register a fault? This is my theory for now? Are there any capacitors on the generator? Could they be upsized safely?

 

[ScottyUK]

Capacitors on generator outputs are rarely a good idea. If the set has an AVR then it will probably not respond well because capacitance on the terminals pushes the machine into the leading area of its capability chart. This is where you don't want to be operating if you can avoid it. On a small generator the performance of the AVR isn't usually all that good (in comparison to the multi-MW AVRs I'm more familiar with) and you're likely just stuck with the voltage fluctutation as the grinders start. You could probably use a more powerful AVR to give you some degree of field forcing, i.e. the ability overdrive the field beyond its continuous rating to produce a faster response to a disturbance on the output, but that's getting a bit serious for a 15kW machine.

Adding caps to the DC link essentially increases the energy stored in the drive to the point where it can ride through the momentary dip without dropping below the trip threshold.

 

[waross]

Caps on the generator may interact badly with the Automatic Voltage Regulator.
Your controls may need tweeking.
Your no load frequency should be 61.8 Hz.
You may want to check the stability setting.


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

 

[itsmoked]

Caps on the AC side do not store energy the way you think they might. Caps on the AC side would not help your issue. However adding the cap to the VFD's DC link does exactly what you're trying to do. Follow waross's directing and be done with it.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

Don't forget Scotty's contribution.
A 15 KVA machine probably has a conventional AVR. A lot of smaller economy model machines don't have an AVR. On some smaller machines the excitation is supplied by capacitors. Adding capacitors to these machines will drive the voltage up.
If you have an AVR then you may have too much stability dialled in. This will cause slow voltage response to load changes.

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

 

[wayne440]

IMHO, a larger, somewhat higher quality generator set would probably be the most straight forward solution.

 

[ScottyUK]

Cost might be an issue though.

Big electrolytic cap: ~£100.
New larger generator: ~£10,000?

 

[defisher40]

OK! There is a concensus. Adding Caps to the AC side will raise operating voltage. Adding caps to the DC side of the VFD should /might cushion the voltage variations. What is my next step? Will I have to desolder and solder new caps into an IC board? Are the caps on a 7.5HP VFD large and remote mounted? I guess I need to pull the thing apart?

 

[defisher40]

On another note? Could I raise the RPM of the motor to elevate the operating voltage? Could this alleviate the problem? How much? What happens when the frequency deviates from 60Hz?

 

[waross]

Years ago I was out of town for awhile. When I came back one of my customers was just about to send his standby generator out for a rewind. The voltage was surging and someone diagnosed it as a fault in the windings. He called me for a second opinion. I gave the stability adjustment about 1/16 of a turn and the problem went away.
You just might want to check the stability and the no load Hz before spending money.
If your frequency is a little low, the UFRO feature of the AVR may drop the voltage in response to sudden loads.
A 15 KVA set should not have issues with an angle grinder if it is set properly.

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

 

[wayne440]

The only Northstar set(#165607) of that size which I could conveniently find a manual for appears to be a "capacitor regulated" unit. My experience has been that motor starting ability is not a strong point of that design.

 

[waross]

Thanks for the "Heads Up" Wayne.
Check your voltage.
Check your frequency. If you can find an old, old electric clock with a synchronous motor drive it can be used to check the frequency.
The specs show a speed of 3600 RPM. The industry standard world wide would be 3600 RPM plus 3%, or 3708 RPM. The voltage is speed dependant. At 3708 RPM the voltage should be about 124 volts. At full load the speed should drop to 3600 RPM and the voltage drop to 120 Volts.
With capacitor voltage control you won't have a stability adjustment or UFRO.
Speed is probably the issue, If the voltage is low when the speed is correct, let us know.

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

 

[ScottyUK]

Find out what thew drive model is and we might be able to help. Some manufacturers make it easy to augment the DC link capacitance, others make it outright difficult.

 

[itsmoked]

defisher40; On any 7.5HP drive the capacitor is LARGE as was already mentioned and not soldered in but hooked up via screw terminals.

Keep in mind none of us are suggesting you open up the drive to add capacitance. On a 7.5HP drive there should be DC link screw terminals at its field wiring terminal blocks.

Please keep Scotty's warnings firmly in mind..

Keith Cress
kcress -

http://www.flaminsystems.com