Motor controller voltage dropping to zero when under load

Question

I'm having a problem with my KBMD DC motor controller. When no motor is connected, it puts out 0-90V, depending on the speed dial. However, when I attach my 1/40hp 90V DC motor, the voltage drops to zero and the motor won't move at all. I tested the motor and it's working fine. Any ideas as to why this is happening?

---

As per Kevin's recommendation, I adjusted the CL trimpot until the LED turned off. The voltage was still zero when the motor was attached, and it did not run. I hooked up an identical motor that was allowed to spin freely, but it too did not work. Interestingly enough, the ammeter pulled a current of 6A when attached in series to the controller and motor, but the motors are rated at .36A. However, I attached each to an 18V drill battery, and they seemed to operate correctly with no bindings or stalling. I used an identical motor controller for a few months before it went out, so I think they should be compatible. Any suggestions?

Answer 1

The behavior you describe is typical for a current limit that is set too low (the voltage will be high as long as insignificant current is drawn, such as by a voltmeter, then drops under load).

According to the KBMD's manual, it has an adjustable current limit and an indicator LED showing when it is triggered, but you may need to remove the cover to see whether the LED is on (the manual does not say but it shows the LED being on the internal circuit board).

If you observe the indicator LED on when you attempt to move the motor, you need to increase the current limit trimpot setting. (Be sure to use an insulated adjustment tool and general line-voltage precautions since this is done with the power on.)

If this is not the problem (the CL LED remains off even when the output voltage is also measured to be zero), please edit your question to include the additional observations.

---

Note that your observations are also consistent with the motor being unable to turn — under that stalled condition one should expect a very low voltage and high current. You say you've tested the motor, but it might be worth also checking the motor current (using an ammeter in series) when it's hooked up to the controller. If it's zero or very low, the problem is with the controller; if it is high, the problem is with the motor.