10 HP DC Motor acceleration resistor.

[BigAl56]

I hope that maybe someone can help me with this. I have a 10 HP Shunt wound DC motor which will be running an emergency oil pump. This motor will be ran off a 120VDC battery pack. The Armature Amps are 73.8 and the Armature volts are 120VDC. I know that I need to use an acceleration resistor for this application, with a total of two steps and three speeds. I have searched the web looking for references to size this resistor, but have come up empty handed.
One article I read stated that the correct resistance could be found by dividing 120 by the Armature Amps multiplied by 125%. This would give me 120 divided by 92.25, which is 1.30 Ohms. I am wondering if this is correct, and if so what the second step resistance should be, and how long to wait between the steps.
If someone could point me to a good reference I would really appreciate it.

 

[Skogsgurra]

Resistors are usually chosen so that you get a reasonable starting current up to half speed, and then the same current up to 3/4 speed and then you just short out the resistor. 120 V and 74 A would give you 1.6 ohms for rated current. You can use 1,6 times rated current for most motors, but if you do not need much torque you can use 1.6 ohms. Next step, you have about 60 V, which calculates to 0.8 ohms. And finally, you just short the resistor.

The resistor values depend a lot on the load, but those values are about right and probably safe to use.

Gunnar Englund

http://www.gke.org

 

[ScottyUK]

An almost identical application exists on our gas turbines, except the pumps is slightly larger at 15kW. We sized our starting resistor to limit armature current to <4x FLC. The initial resistance in series when the motor is at standstill is easy to calculate because the back EMF is zero. The trickier resistance value to calculate is the intermediate value because you need some idea of the back EMF from the motor at the point the transistion occurs. With a (centrifugal) pump application you can take a rough guess based on the quadratic relationship between speed and pump output, so at 50% power the pump will reach 70% speed, etc.

I don't have the calculation to hand, today being Saturday, but from memory the ratio was roughly 5:1 between the two stages, with the larger 'half' being the first to be shorted out. A different load would have a very different ratio.


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