Author Topic: DC Motor drawing too many Amps  (Read 5861 times)

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Offline fitz1j

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DC Motor drawing too many Amps
« on: December 10, 2014, 05:20:08 am »
I have a treadmill incline motor that is blowing its circuits fuse every time it starts to run. This is the motor that lifts the treadmill up and down.

The motor is labeled:
Output: 65W
Input 36VDC
Current: 3A
Speed: 3700 RPM
Torque: 1.7KG-CM

I tested the voltage off the board and it's correct per the manual. I have visually inspected the motor, taking the commutator out and it looks OK. I've performed the 3 tests (180 degree resistance, bar to bar resistance and bar to stack) here: http://www.groschopp.com/how-to-check-a-motor-armature/

Everything seems ok.

I did some other "cowboy" tests, which I doubt the validity:
  • I hooked the bell end (end with brushes, I think it's called bell end) to the board by itself. It's still an open circuit and sure enough the fuse stayed fine. No surprise. I left the bell end hooked up and I put the commutator in, alone without the housing.
  • With the brushes contacting the commutator the fuse immediately blew when voltage was applied.
The resistance across for the 180 degree test was 2.5 ohm. That resistance doesn't work with the watts and voltage.

Is this motor bad? Shorted out somewhere?  Or am I missing something?
 

Offline AlfBaz

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Re: DC Motor drawing too many Amps
« Reply #1 on: December 10, 2014, 06:04:04 am »
The resistance across for the 180 degree test was 2.5 ohm. That resistance doesn't work with the watts and voltage.
Most DC motors also act as a generator when running creating a back emf (or back voltage) so the current is based on the applied voltage minus the generated voltage.

Does it run no-load? If it's stalled it wont generate the back-emf.
 

Online IanB

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Re: DC Motor drawing too many Amps
« Reply #2 on: December 10, 2014, 07:37:29 am »
The start up or stall current of a motor will be many times greater than the normal running current. It is often necessary to use a soft start system to bring a motor gradually up to speed if the supply cannot cope with very large surge currents. Could it be that there is something wrong with the control circuit in the treadmill? (The motor may be fine, something else may be wrong.)
I'm not an EE--what am I doing here?
 

Offline fitz1j

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Re: DC Motor drawing too many Amps
« Reply #3 on: December 10, 2014, 02:03:41 pm »
Does it run no-load? If it's stalled it wont generate the back-emf.

The same thing happens.  Fuse blows, rather quickly.

Could it be that there is something wrong with the control circuit in the treadmill? (The motor may be fine, something else may be wrong.)

That's why I am trying to blame the motor.  But I can't yet. I tested the voltage for the motor circuit and it is correct, according to spec.
 

Offline EMUD

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Re: DC Motor drawing too many Amps
« Reply #4 on: December 10, 2014, 04:39:36 pm »
Hi fitz1,

I think the motor uses permanent magnet for its field. If it is so, do you have a regulated power supply?
If you have you should test the motor with this without mechanical load (if there is a belt please disassemble it)
To do so connect the motor to the supply and start from 0 volts (adjust the current to maximum at ther supply).
If the motor is ok it will start turning under 10V. (You may test it with a car battery, but place a 10A fuse for your own safety. The motor should run with 12V, but slower and with less power ...)

Good luck

EMUD
 

Offline Gall

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Re: DC Motor drawing too many Amps
« Reply #5 on: December 11, 2014, 04:34:03 pm »
There are three possible causes for brushed DC motor failure of that kind:

1. Permanent magnet is demagnetized (may happen due to the overheating of an Nd supermagnet). Frequent in Nd magnet motors, impossible otherwise.
2. Dirt in the collector and between brushes (carbon dust from brushes). Frequent.
3. Short-circuit inside of the winding. Rare.
The difficult we do today; the impossible takes a little longer.
 

Offline Seekonk

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Re: DC Motor drawing too many Amps
« Reply #6 on: December 11, 2014, 04:43:30 pm »
I remember being called in to look at a motor that was drawing too much current.  I pulled the plug on the gear box and golden filings came out.  No oil had ever been put in the box.  It may be an issue with what you are driving.
 

Offline mikerj

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Re: DC Motor drawing too many Amps
« Reply #7 on: December 13, 2014, 09:41:25 am »
Failure of these low voltage DC treadmill motors is not uncommon from the research I did a couple of years back.  My mothers treadmill had the same symptoms and was caused by a shorted winding in the armature.  It would sometimes not start, sometimes start but run very roughly and would frequently blow a fuse.
 

Offline Psi

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Re: DC Motor drawing too many Amps
« Reply #8 on: December 13, 2014, 12:37:06 pm »
The resistance across for the 180 degree test was 2.5 ohm. That resistance doesn't work with the watts and voltage.

Reliably measuring under a few ohms can get quite hard unless you have a good meter and good probes.
Did you check the ohms with the meter probes shorted?
Greek letter 'Psi' (not Pounds per Square Inch)
 


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