starting dc motors

[wired1]

I have a hydralic system to test which is powered by a 20hp dc motor. 125volt bus feeds everything - armature, field, and discrete (relay) controls. Motor FLA is 140amps. Due to the odd power supply requirements, I'm looking at leasing a test rig. The rig is basically the front end of a ups which can be configured for 130VDC 175 amps. A battery bank will handle the starting amps. If there is a decent load on this motor does anybody know if it will start if my bus is lacking in amps? I know from the mtr mfr that the arm resistance is .05. Does that mean starting amps are 140/.05=2500? Must I ensure that the batteries can deliver the full amps? For the motor people: how come starting amps are not published for dc motors?

 

[gordonl]

A DC motor typically isn't started with full voltage, a resistor is inserted to limit the current to the motor, and switched out to increase speed. I would suggest you do the same. Without a resistor you have predicted the zero speed current properly.

Your batteries will be limited by the amps they can supply for a certain duration, I suggest you ask the battery supplier what the amp-hour rating is, and what the peak amps is.

 

[dydt]

hi wired..i have this experience before and might help you directly or indirectly...

i used to work in a shop that builds UPS, battery chargers and indutrial rectifiers..one part of the test we do for the UPS is to start the inverter with load connected using the battery ..the loads are couple of motors and lamp banks..in this particular case we are using 10 of 12volt, 100 A-H battery..in some occasions 2 volts cell for a total of 48VDC...once the load is running via the battery we switch the rectifier to recharge the battery...

the Ampere-Hour rating of the battery will play a crucial role when doing this test...we noticed that the battery voltage drops momentarily then gradually picks up...

just a thought..
good luck

dydt

 

[wired1]

Thanks for your inputs. I like reducing the starting amps. But getting practical, a .05 resistor (of massive wattage) leaves half the bus available for the motor. If we want more voltage for the motor, the resistor is correspondingly smaller. It sounds like this would be hard to get.

gordonl: do you think the motor will start with a reduced bus rather than stall? I don't know from experience. If it's true, then maybe my test setup will work even if it has less than 2500 "cranking amps." correct?

 

[wired1]

thanks again for you inputs. I've since learned that across the line start is a bad idea. working on a "step-start" circuit now.