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