Generator Sizing and Precautions

[TimWilborne]

I have an application I'm working on that will be generator powered. It will have 4 motors on it and a limited amount of electronics on it. 5hp 230 Volts 3 phase 3/4hp 230 Volts 1 phase 3/4hp 230 Volts 1 phase 1/2hp 120 Volts 1 phase I will stagger the starting of the motors so that the generator will only have to deal with the inrush of one motor at a time and worst case senerio only two motors will run at the same time, the 5hp motor and one of the 3/4hp motors. So my biggest simultaneous load will be 12.4 Amps 3 phase and 5.8 Amps off of two legs. As an alternative, if it is cost effective to get a larger single phase generator I will change the 5hp motor to single phase which will make my total load 19.5 Amps for the 5hp and the additional 5.8 Amps for the 3/4 hp Any suggestions as far as sizing the generator, particular brands liked, and precautionary measures that should be taken to protect the electronics? Thanks TW

[paulengr]

If there are electronics involved, it is best if you have a "power supply" in between, even a simple "power conditioner" to eliminate "brown-out" type problems, or a DC power supply for 24VDC devices. If you can stand to run on 12VDC, you can also tap into the generator's alternator. Check out some of the power "cleaning" products from www.defender.com for ideas of how to protect the electronics from the DC alternator noise. Make sure the generator has a good frequency and voltage control on it. If not and the voltage droops on you, the motors will start pulling high amps and burn out prematurely. Electronics will blink and cause you all kinds of problems. If you are going to be looking at drives anyways, you can also run that 5 HP 3 phase motor directly on single phase power if you use a VFD in between. The VFD converts input power to DC anyways so they don't mind doing this 3-phase conversion for you. The voltage droop problem is another reason to not be afraid to "oversize" the generator by a good safety margin. I would have added all the motor loads together and THEN taken into account the starting current of the 5 HP one. Don't underestimate that inrush either. Remember...up to 17 times FLA at dead stop, even though 6 times FLA is the usual margin for locked rotor torque. If you don't plan on 17 times FLA, it can and will blow the regulator fuses in the generator. I can vouch for that from experience. In fact you will probably find that you can get by with a smaller generator if you put that 5 HP motor on a soft start, or a VFD. As to brands, and this might surprise you...depending on your application, do NOT buy a "generator". Buy a welder that has "auxiliary power" capability. Most of the better ones (Lincoln, Miller) do at up to 5-10 kW. Interestingly enough, they are frequently the same price or even cheaper than equivalent sized generators! Although welders are generally "noisy" in terms of power, if you take the advice to use a good quality power conditioner, everything should ride through just fine. Outside of those, at the size you're talking about, I've had good luck with Kohler, Honda, SOME Generac's, and Caterpillar. I've had much better luck with diesel generators when it's not a welder than gasoline engines because the engines are generally cast and built to run for a long, long time. The gasoline ones tend to be for "home/emergency" use. They target a market that intends on running it perhaps 2-3 times per year at MOST. The moment you buy a diesel generator, you put yourself in the "contractor/construction" business at the very least where the expectation is that the generator will be running 8-16 hours per day, day after day, with little to no maintenance. In terms of pricing, I have found that the "generatorjoe" web site prices are online and they tend to be pretty close to what I've gotten for actual quotes at least for the sizes that I've bought in the past (50 kW, 350 kW). Edited March 17, 2008 by paulengr

[TimWilborne]

Boy, I missed one obvious thing. I need to figure out the duty cycle. I'll try to figure out that tomorrow. Thanks for your advice

[Clay B.]

Since you can stagger the start of the motors I would suggest you use an RPM sensor on your motors and generator. Instead of having to figure how long to delay the start of the next motor in the series you can start based on the previous motor's RPM and the Generator's RPM. This very helpful if the loads on the motors vary and your generator get some age on it. Also to protect your electronics if your Gen's RPM starts to really crash then you can stop the motor start and let the Gen speed back up. This will also help with protecting your electronics from brown out.

[TimWilborne]

Good thought Clay. I'm waiting on something back on the duty cycle