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Faunsdale, AL | Generator will produce power that has less "hash" in it than a VSD, so should be easier on a motor. But less efficient. Drive will be in the 90% efficient range or higher. Generator is in the 20-30% range. Then you couple that with the motor efficiency to get the total efficiency of the system. Not really much use, it's not what you write the check for, have to know fuel and electricity cost to see what it'll cost to run each one.
Running VSD's on generators can be a problem if the generator isn't spec-ed for it. Can cause problems with the voltage regulator. So if you have the generator, probably best not to use the VSD unless you need the soft start or the variable speed. Starting a large motor on a generator is somewhat of a soft start anyway. When the load hits it, it drags the engine speed down for a second and voltage usually drops proportionally to the speed, so you can get a pretty big droop in speed, voltage and torque until either the load starts moving or it drags the engine down enough that the generator voltage decays completely.
Generator kW = to total motor volts x amps x 1.732 x 2 divided by 1000 is not too far off, especially if there is more than one motor and they don't all start at once. This is because starting the largest motor first allows the generator to work only on the amperage surge it requires without any other "overhead". When starting the second (hopefully smaller motor), The generator has to meet that starting requirement in addition to the existing load. However, the running load can have a bit of a flywheel effect during the starting surge droop if it's motors.
Starting surge droop is not usually a problem for the motors, but if it's too severe, it can cause the starters on the motors to drop out which allows the generator to recover and pull them in again, which drops the voltage again and they drop out. This chatter can destroy a starter in short order, so it cannot be tolerated. Somehow the starters have to have enough voltage to ride on through the droop. Also may have HID lights (sodium vapor or metal halide for example) blink off and of course they don't restart immediately when hot. Can be a problem.
Like I recommended earlier, get a quote from a couple generator dealers and use as a guide. They should also be able to give you some fuel consumption data that will make your decision making a lot more accurate.
Edited by ccjersey 8/11/2012 16:19
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single to 3 phase vfd's
