Generator OCPD

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steveve1

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Location
Tucson, AZ
Happy 4th of July!!
I've got a used 120kw/480v generator with no main ocpd. Nameplate has it @ 150a. I would like to protect with a fusible switch. The main load is a 75hp ball mill with very high starting current. What would be the largest fuse size allowed to protect the generator and accommodate the starting current at the same time?
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
It's a question I would pose to the generator manufacturer. That said, the output of 120kw would be about 144 amps, 445.13 states the conductors should be protected at 115% of the nameplate output or 172 amps. 430.52 would call for a motor SCGF of 168 amps upsized to 175 amps.
 

Cold Fusion

Senior Member
Location
way north
Happy 4th of July!!
I've got a used 120kw/480v generator with no main ocpd. Nameplate has it @ 150a. I would like to protect with a fusible switch. The main load is a 75hp ball mill with very high starting current. What would be the largest fuse size allowed to protect the generator and accommodate the starting current at the same time?
Steve -
Article 445 does not give much advice on how to design a generator protective system. It does not even tell you how to protect the gen or the conductors from the gen to the first disconnect. (gus - 445.13 just sizes the conductors, it does not protect them)

445.12 tells you to protect the gen from overload, but it does not tell you how. The NEC pretty well leaves the design up to you.

So, to answer your question, it depends on the protective devices built into the VR and Governor, the pf rating of the gen, the power output rating of the driver (engine)

So, from that point of view - we need a little more information. You definitely need the generator book. I wouldn't bet on the mfg or vendor being any help. For one thing there is no money in it for them to help you. And if they do offer anything, I would expect them to tell you the gen is too small. And they would be right.

The gen is undersized to start a 75hp motor. I'm not sure what you mean by a "ball mill with very high starting current". Is that a normal Design B with a long acceleration time? Or something differerent than a design B, or has a strange (other than G) code letter (T430.7.B)?

Say it the motor is a Design B, Code G, 96A FLA, 550A LRC. That has the gen over-loaded about 3.5X until the motor speeds up a bit. That should suck the engine right to it's knees. And sometimes the engine will struggle, cough, smoke, and snort it's way back to life - if it doesn't have a protective relay trip. Sort of a vfd starter - the frequency voltage, motor speed all struggle back to life together. That's nothing I'd want to do very often if I expected to keep the smoke in.

Here are a couple of things to look at:

Take a look at the nameplate again. I would have expected this size of a gen to be rated at a .8pf. So, for 120kw, this would put the FLA at 144A/.8 = 180A. If it is a 150FLA, .8 pf, that suggests a 100kw driver. If it is a 120kw driver and a 150FLA gen, (strange, but possible) then all you are going to get is 100kw at .8pf.

Check the nameplate close. Is there a pf given?

No CB = probably was not a package unit. Does the generator have a voltage regulator and a governor. Sometimes they do, but external control signals are required. Sometimes they do, and the voltage and freq are set from front panel knobs. If it does, are there any built-in protective functions (like OV, UV, UF OF, over-current, loss of excitation?

Let's asume the worst. The driver is120kw, the gen is 150FLA, no built-in protective devices, the customer is willing to hammer the gen into oblivion as long as the motor starts at least a few times.

You could go with 1.25 x fla on a class rk1 (Ferraz A6D), next size up = 200A. Trip time for 550LRA is about 85 seconds. So if the motor comes up to speed in 60 seconds, you should be okay.

Since the driver is capable of overloading the gen at a .8pf, maybe you would want to set the fuses a bit closer. Again a class rk1, 1.15 x FLA, next size up = 175A. Trip time at 550LRC is 55s. It probably will still stand a minute of start time.

Since you are the engineer of record, I'd suggest getting the fuse curves and layout the motor starting curve. It has to be under the fuse you select.

If you are okay with this, and think the gen will start the motor, there are a couple of other things about motor portection I can suggest.

cf
 

templdl

Senior Member
Location
Wisconsin
I have always looked at it this way:
Is the generator capable of providing enoungh over current for a long enough period of tim e in order to trip a thermal magnetic breaker thermally.
Also, is the generator capable of producing a high enough fault current in order to trip a themal magnetic breaker instantaneously.
Remember providing power with a generator is different than providing power from a utility. There is a lot more power available from a utility than from a generator.
As previously stated it is best to consult with the generator manufacturer for their recomendations. Cold fusion also has made some very good points as he points out protection is tied to the performance of the engine.
 

Cold Fusion

Senior Member
Location
way north
...Is the generator capable of providing enoungh over current for a long enough period of tim e in order to trip a thermal magnetic breaker thermally.
Also, is the generator capable of producing a high enough fault current in order to trip a themal magnetic breaker instantaneously. ...
Yes, good point.

Generators this size generally have a short circuit current in the range of 4x to 5x FLA, 600A - 750A. Sometimes it's a trick to get the OCP inside of the damage curve.

For this case, from the OP, I'm guessing the primary design criteria is getting the motor to start. And since the gen SCC is the same order as the motor LRC, long term generator health is secondary.

cf
 

templdl

Senior Member
Location
Wisconsin
If I can recall Cutler hammer has the FG-NG series breakers 15-1200a specifically for generator application. It appears the the thermal element is a standard curve but if has a lower magnetic calibration.

This should be as link to a PDF:
http://www.eaton.com/ecm/idcplg?Idc...ased&Rendition=Primary&&dDocName=985829569667

But still the fact remains that there are closer but will they get the job done considering the pwoer that a generator id capable of generating.
 
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