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DC-motor replacement for stress test

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Pax Writer

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Hi experts

I need to do some stress-testing of a DC-motor control circuit, but without the motor - Problem is, I'm not sure how to make this the most clever (and cheap) way.
I need to run 30-40A through my circuit at 11-16V (more of both is better), but how to go about it? Should I connect a bunch of power-resistors in parallel to the circuit, or is there a better way?
I guess I need a load of 0.4-0.8 Ohms...
What would you suggest?

All advice is greatly appreciated.

/Pax
 
THey'd have to be huge resistors (bigger than the motor is!). I'd just use the motor since it burns off the energy as something other than heat and so can be smaller.

Plus resistors do not adequately simulate the stress that an INDUCTIVE motor has on a circuit. THat inductiveness will probably cause you more problems than anything else so stress testing without it is not wise.

YOu need a total of 640W dissipation it would seem. In my university motors lab, the most expensive piece of equipment by far in the lab was the 1500W resistor box.

Why do you not want to use the motor anyways? What happens if you dunk the whole resistor setup in a tub of water? Maybe it could be smaller that way.
 
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The problem about using the original motor is, that its sitting in a bb-gun, and using it would require that it shoot pellets in order no to put too much strain on the gearbox, as the piston slams back and forth (the compression dampens the impact of the piston head on the cylinder backend). The gearboxes tend to crack when "dry-fired". So either I risk a gearbox, or I use a billion pellets stress testing the circuit for a few hours...
I see your point about the inductive load, though... Darn....
So maybe there's no way around field-testing?
 
YOu could just buy some crap surplus motor with an artificial load I guess. That's what I would do. I wouldn't use the actual (probably expensive, high quality, or nearly irreplaceable) application motor for testing a circuit's functionality.
 
Hi Nigel.
I guess you mean in parallel? (silly question, I know, but just to be sure).
How many you think are necessary?
 
Pax Writer said:
Hi Nigel.
I guess you mean in parallel? (silly question, I know, but just to be sure).
How many you think are necessary?

Yes in parallel, a 60W bulb will take 5A at 12V, for 30A you would need 6 and for 40A you would need 8 - it's really VERY simple.
 
That's what I thought. I just wanted to make sure, you had nothing special in mind, or knew something which might have escaped me.
Thanks for the very simple advice - It will be big help.
 
Pax Writer said:
That's what I thought. I just wanted to make sure, you had nothing special in mind, or knew something which might have escaped me.
Thanks for the very simple advice - It will be big help.

This covers the resistive part of the problem. How about the inductive part as already mentioned by dknguyen?
 
How about using a starter motor of a junk car to cover both, resistive and inductive tests? Connect a relatively low mechanical load (adjustable electric brake) to the motor shaft, and cycle the motor on and off repeatedly. Check with an oscilloscope which spikes you have to deal with.

The normal starting load current (in a car) will be about 130 to 150A which you don't need for testing. Without a combustion engine to start the current flow will most probably only be a few Amperes.

For the electric brake I suggest to use a disc (mounted onto the motor shaft) of the car-brakes and adjust braking force with a solenoid.

This motor should have about the same behavior as the one you want to use.

Regards

Hans

P.S. Start the motor only with no load (very high starting current) and apply the brake when it's running. Then quickly cycle off and and on.

P.P.S. Having a proper idea solving a problem is not a matter of intelligence. My IQ is approx. 0.5 :D
 
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