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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?

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    \$\begingroup\$ 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. \$\endgroup\$
    – JRE
    Commented May 5, 2021 at 11:58
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    \$\begingroup\$ Your assumption is correct, they are powered off the same battery. So much for keeping things simple I guess. Back to the drawing board... \$\endgroup\$
    – Cody
    Commented May 5, 2021 at 12:14

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

You are currently running a 14A motor through 6A relays. They aren't going to last very long, especially since a motor is an inductive load. You need relays rated for at least 14A inductive load.

In practice a motor can draw a much larger current when starting up, than when running. So don't even assume that the maximum current draw is 14A. Your little 7Ah battery isn't going to last very long either.

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    \$\begingroup\$ Idiocy on my part, still very inexperienced wrt electronics. I assumed that halving the voltage would have a similar effect on current as it does on RPM. \$\endgroup\$
    – Cody
    Commented May 5, 2021 at 13:43
  • \$\begingroup\$ In the worst case, under-volting a motor causes it to stall and draw a lot more current. Even if it's not stalled, the current may be higher than expected if the motor is labouring. \$\endgroup\$
    – Simon B
    Commented May 5, 2021 at 18:04
  • \$\begingroup\$ Motors are capacitive loads, still not good for relay though, \$\endgroup\$ Commented May 7, 2021 at 13:09

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