DC Motor Low RPM under load

[benhanson]

I'm trying to repair an automotive emergency tire inflator. I know this device probably isn't worth repairing, but I'm using the scenario to better understand the electronics involved.

When the unit is assembled, the motor turns, but never develops "normal" RPM. Occasionally, the relay that appears to drive the motor will click/chatter and the motor won't spin. Tapping the relay when it is clicking seems to help the relay latch. If I remove the motor from the gear assembly, it seems to turn at normal speeds.

With gear assembly attached, voltage at motor bounces between 1v and 1.5v

With gear assembly removed, voltage at motor is steady at about 7.4v.

I attempted to power the motor directly with a benchtop power supply, set voltage to 7.5, current to max, with gear assembly attached the motor still behaved the same. With gear assembly off, the motor runs to full speed, draws about 1a @ 7.5vdc.

I appreciate any feedback. My guess is that the motor is failing/failed, and that it shouldn't see such a dramatic voltage drop under load. I've just never experienced a motor failing partially, and the clicking relay made me wonder if a relay can somehow cause a downstream partial failure of a motor. (I also though a relay would either work or not work)

 

[pharaon]

i would suggest you to check the motor brushes , and change the 3 capacitors

 

[benhanson]

Or maybe a burnt resistor? I took the board off to look at back of caps, saw this which I didn't notice before.

 

[pharaon]

that one definitely must be change

 

[benhanson]

After changing that resistor, changing 1 capacitor(22uf 50v) that had dried gunk at the base, and checking the other capacitors with a Fluke, I'm still in the same boat.

Also, I'm not sure how to check the brushes and winding. I don't see any sparking at the brushes when the motor turns.

 

[Bluejets]

Motor undoubtedly driven by the relay.
It may well have cooked contacts and even cooked coil.
I would disconnect the motor wires at the motor and connect directly to a 12v car battery for a short test.( couple of seconds)
That would eliminate problems within the motor.
Motors like this do not have overly engineered internal parts and it may not even have brushes as such.
Some elcheapos just use a spring leaf contact.
If as you say there is still no sparking on the motor, then I'd go for the relay.

 

[(*steve*)]

I would suspect the contacts on the relay, or maybe a faulty solder joint. Something is reducing the voltage to the motor, and I suspect it's some additional resistance that isn't in circuit when you power the motor directly from a bench supply.

You might also check the battery? voltage when this is operating through the relay (does it have it's own batteries?) are they dropping to a much lower voltage when the device is under load?

 

[benhanson]

Sorry for the delay getting back.
Bluejets, I took your advice, desoldered the motor and took that assembly out and powered off a car battery. The motor turned at speed, pump produced a decent amount of pressure. So, I re-soldered the motor and figured I'd give it a try in the the car. Low and behold, it now works as normal. I remembered that the small 120v -> 12v automotive plug wall adapter doesn't really push much power(maybe it's 1A-2A), and I guess I assumed my bench top power supply would have had more oomph. So, it would appear that replacing the burnt resistor and possibly replacing the cap fixed it. It just took me a bit longer to realize it was repaired. Does it make sense that a weak power supply could produce similar systems as a burnt(but not totally broken) resistor?

Steve, no batteries in the unit.