Q about delayed shutdown of overloaded motor.

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pjdd 19 years ago

I'm designing a motor speed control circuit as part of a larger system for a one-off project. Specs are :

The motor is an existing PM brush type with an integrated reduction gear, and is also coupled to an optical tachometer. The name plate has faded until it's no longer legible, but it was originally powered from a 24V, 90VA transformer via a controller card. DC resistance is about 1 ohm.

It drives a timed process that lasts a few minutes per session. Load is fairly constant. Speed regulation has to be fairly precise.
The original (damaged) controller card has the current limit set at 3.5A. I have not yet measured the current under normal load.

If the motor is stalled or significantly slowed down, the process is ruined and has to be repeated. So, instead of just limiting the current, I thought I'd have the motor shut off until it's restarted. I intend to introduce a time delay so that the motor won't shut down during normal start-up.

My question is : How long should the delay be ? It's probably not possible to give an accurate answer without knowing details about the motor and the load. But can anyone give a ball-park figure - something like 3 secs, 5 secs ... ? TIA for any input.

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Tim Wescott 19 years ago

The delay should be as long as it has to be. Sorry. If you were trying to start up something with a lot of flywheel action it may take close to a minute (or an hour, if it's a lab application); if you're mainly limited by the inertia of the motor it could take as little as 10 or 100 milliseconds.

I would current limit for startup, or make sure (by calculation, knowing how long it'll take to start up) that starting up at full voltage wouldn't cause the motor too much grief.

Tim Wescott Wescott Design Services http://www.wescottdesign.com Posting from Google? See http://cfaj.freeshell.org/google/ "Applied Control Theory for Embedded Systems" came out in April. See details at http://www.wescottdesign.com/actfes/actfes.html

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Homer J Simpson 19 years ago

You should ideally measure the temperature of the motor windings and wait until they are cool enough. Usually 130 deg C.

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bill.sloman 19 years ago

The copper windngs of the motor can be used as their own reasonably accurate resistance thermometers - resistance is roughly proportional to absolute temperature (about +0.25% per degree Centrigrade/Celcius/Kelvin). There are gotcha's, particulary if the wire has been wound under heavy tension, but it can be a useful approach.

Bill Sloman, Nijmegen

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pjdd 19 years ago

Thanks. Given the sketchy info provided, I guess I couldn't really expect a more precise answer. But it's given me some more food for thought. I think I'll rig up a temporary drive circuit, observe the behaviour under load, and make modifications and set cut-off points as indicated.

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Luhan 19 years ago

I have designed just such controllers for an automated soldering machine. The motors have a tachometer output used by the controller to measure speed. If the pulses come out too slowly, the controller ramps up the power. A number is stored in the controller representing the maximum power to be attemped for a specific speed. If that number is exceeded, the motor is stopped immediately.

Under normal operation, with PID control of the motor, a normal ramp-up is expected on startup. The internal cutout value is set at the factory for the expected physical load on the motor.

Been working like gangbusters for over 10 years now.

Luhan

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Q about delayed shutdown of overloaded motor.

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