Hey everyone - I'm a Mech Eng co-op student who has been asked to find a backup power supply to power a 50hp 3-phase 460V motor for 1 minute. Since there can't be a time lag in the event of a power outage, I was thinking about using a battery pack instead of a generator. However, I'm not particularily circuit savvy, so I am not quite sure how to figure out what voltage and current the battery (or batteries) should supply. I've tried doing my own calculations, but I'm not great a transient problems and I'm not sure whether or not I have all of the required information. Please help me solve this problem and let me know if I'm missing any information.

Thanks

 

At 760 watts/hp, that's 38,000 watts. If you are constrained to using 12 volt batteries (more like 13.2 volts actually measured), that will be about 35 batteries in series to reach that voltage. The current draw will be on the order of 30 amps/phase. Plus you will need a simplified VFD circuit to actually drive the motor.

You might want to look for a deal on an old submarine battery. In any case, the charger will be a nightmare.

For an idea, think about a flywheel coupled to the motor. Perhaps with a normally open electromagnetic clutch that would spin up a big three phase alternator and its driver in the event of power failure. Don't know about a small diesel coming up that quick, but a spark engine should perform just fine. It could use propane or natural gas. Gotta have enough mass in the flywheel to start things up without slowing too much.

 

the current is large(75A)
so at starting it will be (3-5)*75A
so i think the batteries is not right idea.
then for 1min,is the motor need the supply?the inertia of it will drive it continously,
the power disconnect and active the alternate power will take mor than 1min,
when we use ATS panels which have main supply from the Elc.company and the alternative generator we set the phase failer which confirmed the disconnection and that take a time.

 

Assuming that your internal resistance is low enough to supply the necessary current, then you may apply a very basic equation to solve your problem. You will need 38000/460 Amperes of current, assuming a 100% efficient inverter. You will need enough (82.6 ampere hour batteries)/60 in order to supply current for one minute. You should probably be generous and overrate the batteries at the very most 200% to do this. This is due to inverter inefficiency and the fact that an ampere-hour rating isn't derived from drawing 1 ampere per hour.. More like 10mA / several hours kind of thing

Steve

hd is right, you won't necessarily be drawing 38000 watts continuously, you probably need to evaluate exactly how much power is needed for one minute of operation. This would require knowledge of if the load is already spinning or not.

 

You should call up some companies the make uninterruptable power supplies (UPS).
If you give them your specification they will present you with options.

Your specification is a motor load of 50hp at 460V 3 phase for 1 minute and you
need zero switchover time.

I have worked with APC (www.apc.com) and Best (which is now www.powerware.com).
Either of these companies should have a power supply that will meet your requirements.

(* jcl *)