# How does supplying a DC motor with lower than rated voltage affect current draw?

I have a DC motor with the following characteristics:

• Voltage: 24V
• Rated Current: 14A,
• Speed: 2750RPM
• Power: 250W

I am using a 12V 7Ah battery as a supply for this circuit.

The motor is controlled with 12V 6A relays, which I can hear furiously clicking on and off (and getting slightly warm) as I start the motor, but once it gets some momentum, they settle down and things run smoothly.

I can't check the actual current draw at the moment as my multimeter can only measure in the mA range.

Questions: Am I correct in guessing that the motor is attempting to pull too much startup current through the relays causing them to turn off, but because they are getting the required voltage on the coils, they turn on again (and repeating until current draw lowers/evens out). Based on P = V x I, does that mean that running the motor at a lower voltage will increase the current draw? E.g. 250W/12V = 20.83A.

So I should be looking for relays with a rated current of about 25A?

• Assuming the 12V coils of the relays are powered from the same battery as the motor, then I'd say that the battery voltage is dropping drastically when the motor starts. In other words: The battery is too weak. You can thank your weak battery for not delivering enough current to kill your relay.
– JRE
Commented May 5, 2021 at 11:58
• Your assumption is correct, they are powered off the same battery. So much for keeping things simple I guess. Back to the drawing board...
– Cody
Commented May 5, 2021 at 12:14