A 12V motor drawing excesive current

Thread Starter

q12x

Joined Sep 25, 2015
909
A quick question for a problem.
I discovered a small motor in my box of motors that had a plastic fan on it good to use as a blower or turbine. It must be from a hair drier or something similar. The motor does not have any label on it so I have to guess its for 3, or 6 or 12V. I test it already and my best guess is for a 12V from the sparks on its pins and how fast it is turning.
This is how is looking:
1652001244153.png
I test it all this time only at 5V but no matter what voltage I use, 2V, or 5V or 12V, it is always drawing 3A - yes 3A!!! I am using my variable PSU that goes up to 5A maximum and it is showing me on its electronic screen the consume of the load I plug into.
I open up the motor, I looked inside, I clean a little bit of dust, but nothing really shows me any kind of visual damage.
So it either must be some kind of electrical trick I don't know, or it is broken beyond repair. The motor spins fine and happy though. But is drawing so much current that its contact are getting very hot and I can smell the hot metal. I test it for a few seconds and I can tell all of these little details and observations.
- If you encounter such a problem in the past... please tell me what must be the cause.
- I will do everything you will tell me to check or to build !
Also, it had a rectifying bridge made from discrete diodes 1N4004 originally on its contacts, but I remove it. I tested with them and without them and they dont really change anything. They are there from it's original construction (probably), to rectify a 12VAC (most probably). It didnt had any filter capacitor on its pins though... It looks like is made in 1980 the whole construction and assembly.
Thank you !
 
Last edited:

Alec_t

Joined Sep 17, 2013
12,808
If the motor is from a hairdryer then the resistance of the heating element might well have been used to limit the motor current.
 

Thread Starter

q12x

Joined Sep 25, 2015
909
If the motor is from a hairdryer then the resistance of the heating element might well have been used to limit the motor current.
yes... you might be right... I dont know what is from exactly, it is my best guess is from a hair drier because of the bigger plastic fan that I somewhat recognize but not completely sure.
And what is the solution? How to lower down this excessive current is drawing, other than a wasteful heat dissipation in a resistive element?
 

KeithWalker

Joined Jul 10, 2017
2,354
That looks like a very low voltage motor (1.5 to 3V ?). Without the fan, it will run very fast and should draw very little current. If the current is still high then it probably has shorted turns on the armature.
 

MaxHeadRoom

Joined Jul 18, 2013
25,211
I test it all this time only at 5V but no matter what voltage I use, 2V, or 5V or 12V, it is always drawing 3A - yes 3A!!! I am using my variable PSU that goes up to 5A maximum and it is showing me on its electronic screen the consume of the load I plug into.
That is perfectly normal for a DC motor off load.
I did some definitive tests on a 90vdc T.M. motor using a 0-120vdc variable supply.
The current remained the same all the way through the RPM range.
Reason. The motor also generates a voltage with RPM.
This opposes the applied voltage, hence no increase in current.
 

Thread Starter

q12x

Joined Sep 25, 2015
909
Put the fan back on and try it again. Some motors do not like running with no load. Bob
This motor is a scrap find and I can not put it in its original case, whatever it may had be. I have it in my box for motors for >20 years I think, when I was young and energetic and I was gathering whatever I could put my hands on.
So I will have to rethink it and find a solution.
If possible. If not possible, then it is equally ok. It is more as a challenge.
 

Thread Starter

q12x

Joined Sep 25, 2015
909
That looks like a very low voltage motor (1.5 to 3V ?). Without the fan, it will run very fast and should draw very little current. If the current is still high then it probably has shorted turns on the armature.
I test it at 12V and it appears it is ok. ( I test it at lower voltages when I was unsure)
The plastic fan weights probably 100 milligrams being made from light plastic, which is not really that much of a drag or force for the motor spindle. It consumes the same current with or without the plastic fan.
I open it and I don't (after a simple visually inspection) see any possible short with the armature.

- But I listen to your suggestion and I actually checked with a continuity tester the armature and its pins.
And Bingo, it is shorting with the armature, both pins are shorted each other and are shorted to the armature, both of them.
Oau... why didn't I think of this myself? Such a simple test to do.That's why is good to have friends in high places like you here.
- Now what? How do I SEE the short? And repair it? I strongly believe the coils inside are making a short at some point with the metal axis that is in turn, making contact to the case... is my wild guess.
The rotor is looking pretty much like this: (this is a picture from internet)
1652021412568.png
 

MaxHeadRoom

Joined Jul 18, 2013
25,211
I have a brand new motor that looks very similar to the one posted, I ran a current/voltage test on it and what do you know, it showed pretty much spot on 5AMPs through the speed range when varying the DC. :cool:
 

Thread Starter

q12x

Joined Sep 25, 2015
909
Ok- subject closed.
Because I open it up, I really tried to un-wire a single coil and everything was stick, I could barely could re-wind a couple of wires without ripping them off. But I ripped and cut a lot from the same winding and everything was stick extremely hard.
I know this thing from long time ago actually. The enamel melted due to high temperatures at some point inside the rotor, and the wires themselves made short with each other.
My attempt was to number the wires turns and I reach 44 and almost half remained extremely glued and stuck. I guess they may be around 80 or 100 turns per each (of the 3) bobbin.
It should have been re-wind more easily !!! This is sign the motor was busted...
 

Thread Starter

q12x

Joined Sep 25, 2015
909
I have a brand new motor that looks very similar to the one posted, I ran a current/voltage test on it and what do you know, it showed pretty much spot on 5AMPs through the speed range when varying the DC. :cool:
If you are using a variable UPS then that 5A is the maximum of that UPS. Try to use a higher amperage transformer, if you have this option. No such motors should go this high in amperage, in my opinion. It should stay in a couple of houndreds mA. 100 to 500 probably. From what I know. Also check the terminals of your motor, and see how hot they are after 5 or 10 seconds at 3A at least. They got to way beyond 100 dgr C in my case.
 

Thread Starter

q12x

Joined Sep 25, 2015
909
Thank you for the involvement, to everyone.
You know me already, I will get a result, one way or another. Haha.
 

MaxHeadRoom

Joined Jul 18, 2013
25,211
If you are using a variable UPS then that 5A is the maximum of that UPS. Try to use a higher amperage transformer, if you have this option. No such motors should go this high in amperage, in my opinion.
I do not use SMPS if I can at all help it.! Especially for motor duty :confused:
I have a heavy duty Industrial variable power supply that is intended for Lab duty.
The terminals do not heat up, if a good connection is made.
 

Thread Starter

q12x

Joined Sep 25, 2015
909
Ok... I managed to strip one 1 bobbin ( from 3 it has) and I managed to number the windings, even after braking off a lot of them.
I got 65 turns of copper wire that is 0.25 diameter. (measured with a micrometer)
The plastic armature right at the end was visible melted and it also cracked by itself when I reach the last wires. This telling me that high temperatures were involved in these coils.
The 3 coils this motor has , is actually a single wire wrapped around all 3 of them in 65 turns per coil. Good to know.
 

Thread Starter

q12x

Joined Sep 25, 2015
909
How many commutator segments does it have?
Three (3) .
And I managed to melt the plastic that was holding them and they fly away. But I have them and they can be glued back with poxipol, at least that is my plan if I will be crazy to rewind the coils, which im not that inclined to do it, since is a lot of copper wire waste and is very expensive. I am on hold at the moment for the re-wiring part.
Under the comutator segments or the brush contacts, is a strange brown isolator, definetly not plastic because it didnt melt that easy. I believe it may be a fiberglass element... im not completely sure.
 

Thread Starter

q12x

Joined Sep 25, 2015
909
The rotor steel plates are connected directly to the AX of the motor. The copper coils are around these steel plates.
Thus the path of continuity from the pins of the coils to the outside metal case.
They didnt insert isolation foil/tape between the windings.... to make economy of space most surely.
Not as you expect it to be, huh.
 
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