DC Motor Starting

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eanton22

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Hello all,

I am trying to find out the starting time of a DC motor. Basically, its a backup motor and the project requires that the motor starts within 2 seconds. I wanted to verify if I size my starting resistor accordingly so I get enough starting current so I can start it within their limits.

I tried using ETAP unfortunately but I only know how to do motor acceleration in AC mode not DC mode.

If anybody can provide me formulas or let me know how to do it in ETAP i would greatly appreciate it.

Thank you
 
Well, the manufacturer recomemends that the initial current be around 200% of FLA. It is a backup motor(7.5 hp) for lube oil pumps. The contractor requires that the motor be up in running in two seconds. I just dont know how to figure if the motor starts within this time.

Not a high inertia 3.23 lb-ft2
 
That's gonna be hard to hit outta the gate.

Where are you getting the DC supply?

Will this motor be operating at a fixed speed?

What is the motor voltage?

Have you considered a "package" drive ?
They have built in adjustable control for acceleration and torque.

steve
 
080827-1317EST

eanton22:

See http://www.reliance.com/mtr/flaclcmn.htm for high inertia load equation.

If full speed is 1800 RPM it looks like you need 10 #-ft of torque if the inertia that you inticated is the total load.

Assume 7.5 HP at 1800 RPM is the motor rating, then torque at this HP is 5252*7.5/1800 = 21.8 #-ft.

Given that your total load is as specified, and you ran the motor at full rated current, then you would reach speed in about 1 second.

Assume 80% efficiency, then full load current at 200 V would be about:
I = 7.5*746/(0.8*200) = 35 A.

Maximum possible resistance at 0 RPM to get full normal torque at 0 RPM is:
R = 200/35 = 5.7 ohms.
Most likely a 1 ohm would be a good choice and short it out part way to full speed.

This is not a good way to achieve your goal. Note 200 V and 1 ohm is 40000 W. You might want to use a several hundred watt resistor. 1 ohm at 35 V is 1225 W. Therefore, you might select something in this range in case your shorting mechanism failed.

A much better approach is a motor control with current limiting.

.
 
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