How to find max-current?

Hi, I have two big old DC motors I got from a swap meet. The only sticker on them says "Nominal 30vdc." They only have two terminals so no wire size to go by. They work fine, no load, using a 30vdc 15A Buck/Boost speed controller.

They are about 4" diameter and 8" long. No idea what they came from but size matters. It has a 5/8" diameter shaft with 3/16" key-way, so guessing it is made for some serious torque.

I'd like to use one of them as a variable-speed fan motor for our swamp cooler which currently has a two-speed 1/3-hp AC motor.

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I there some rule of thumb by which I can guesstimate the max current I can **safely** put into them to get 1/3-hp or 1/2-hp? This swamp came optioned with 1/3-hp or 1/2-hp but the 1/3-hp was all they had in stock at that time.

Or, do I wait for smoke and then back it off a few Amps? Joking? Too nice motors to destroy.


Reply to
Dave, I can't do that
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monitor the winding temperature with a small thermister

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Know anyone with a FLIR camera/thermal imager by any chance?

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** Can you determine how many poles the commutator has?

IME its the comm on a DC brush motor that sets the voltage and current limits.

.... Phil

Reply to
Phil Allison

Spend some time looking for similar frame-size motors (DC) and see what they are rated at (in therms of fractional HP).

Note that you may not have the torque that you want (need) at the speed that you want.

[And, are you really sure you need variable speed on a cooler? Hi and Lo seemed to be the only ones we ever needed: exchange the house air QUICKLY (to get a big delta-T in a short time) or exchange the house air more slowly (to eliminate the blower noise and "breeze" effect)] [[Also note that moving to too *high* a seed can cause your pads to dry out and be counterproductive]]

Motor is typically most efficient at a tiny fraction of its stall torque. Motor is typically about 70-90% efficient (depends on lots of things).

1 HP(E) ~= 750W

You can get a ball-park idea of how much power you will need to put *into* the motor and, from that, how much will be dissipated as waste (heat) -- to get an idea of temperature rise.

I'd be more concerned with the operating environment. Here, ambient can easily be 40+C and humidity 90%. Will the brushes corrode if they get sprayed with hard water, etc.?

Reply to
Don Y

Can you measure the RPM? Do you have a way of loading them?

Reply to
John S

Hmm, I think you'd be able to establish the "small signal" parameters in two steps:

  1. Measure no-load RPM at rated voltage.
  2. Measure RPM when the winding is shorted, and a weight is attached via simple winch setup. (Take the measurement at terminal velocity.)

If the design is "coggy", the required torque may be relatively large.

If it's very smooth, you probably have a pair of servo motors, which are wonderful. It would be a waste to use them on something as vulgar as a swamp cooler. Add an encoder wheel and use them for precision CNC actuators!

Here's a pair I removed from a tape drive (and the capstan motor),

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Reply to
Tim Williams

Thanks Tim, that looks very much like them but with solid shafts.

As to waste, I am planning to use the swamp in Winter time to circulated solar-warmed air and for that two speeds might be a bit too fierce. :)

Still a bit of a waste for nice motors, but they only cost me 12-bucks each and sitting in the garage for 12 years, it might be time for waste.

Reply to
Dave, I can't do that

At 90% we would have the AC on. :)

The motors appear to be fully sealed, not sure if they are IP65 or IP68 though. The AC motor does not appear to get much water spray on it.

Reply to
Dave, I can't do that

Thanks Mark, I have a K-Type here I can strap to the side and crank it up. No real way to load it though. I guess I will have to make the mounting brackets and see if it has the torque without melting anything.

Reply to
Dave, I can't do that


The current limit is determined by the heat it generates. Motors fall into classes according to the heat rise that they can tolerate. You might be able to guess the class of your motor, or you could assume some middle-of-the-ground. I used 100C for a motor that I tested. I measured the temperature by measuring the resistance of the windings. Or, to put it more accurately, I measured the change in temperature by measuring the change in resistance. I did this while applying the max current that I had to the motor with the rotor locked.


Reply to
Bob Engelhardt

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