Hello, I wonder if anyone could help me with this please. I have a 12V DC motor with commutator (it's a Denso starter motor with four brushes) which works fine when it's cold, but runs poorly when it gets to about 40 degrees Celsius or above.

So I thought I'd take it apart and inspect and do some basic checks. With a DVM, I checked the resistance between the +ve and -ve terminals when the motor is cold and an average of 10 readings gave a result of 5 Ohms. I then put the motor into the kitchen oven to heat it to 40 deg. Celsius (bread proving temp !) and then did the same resistance readings. The average this time was 50 Ohms.

I appreciate that the resistance of metallic stuff generally increases with Temp, but I think a) The cold resistance of the motor is probably too high and b) The resistance when warm is way too high. Am I right? Also, I am lost as to why the temperature goes up so much when it's heated. Just using basic Ohm's law tells me that if the resistance goes up five fold, the current goes down in the same proportion which probably causes the poor performance when hot.

A general inspection of the armature, commutator, brushes and springs shows them to be in good order.

I've only got the one motor so can't compare stuff.

Any help much appreciated.
Paul

 

Hello, I wonder if anyone could help me with this please. I have a 12V DC motor with commutator (it's a Denso starter motor with four brushes) which works fine when it's cold, but runs poorly when it gets to about 40 degrees Celsius or above.

So I thought I'd take it apart and inspect and do some basic checks. With a DVM, I checked the resistance between the +ve and -ve terminals when the motor is cold and an average of 10 readings gave a result of 5 Ohms. I then put the motor into the kitchen oven to heat it to 40 deg. Celsius (bread proving temp !) and then did the same resistance readings. The average this time was 50 Ohms.

I appreciate that the resistance of metallic stuff generally increases with Temp, but I think a) The cold resistance of the motor is probably too high and b) The resistance when warm is way too high. Am I right? Also, I am lost as to why the temperature goes up so much when it's heated. Just using basic Ohm's law tells me that if the resistance goes up five fold, the current goes down in the same proportion which probably causes the poor performance when hot.

A general inspection of the armature, commutator, brushes and springs shows them to be in good order.

I've only got the one motor so can't compare stuff.

Any help much appreciated.
Paul

I'd try changing the brushes. There may not be enough spring pressure to get a good contact.

 

The only reliable method to obtain the armature resistance is to lock the rotor and and feed with a LV DC supply of a known value and read the current,
Check at a few points on the rotor and take the lowest resistance value.
Is this a PM version, or an older wound field?

 

The only reliable method to obtain the armature resistance is to lock the rotor and and feed with a LV DC supply of a known value and read the current,
Check at a few points on the rotor and take the lowest resistance value.
Is this a PM version, or an older wound field?

Thanks for replies, its a PM motor. I've got a bench power supply, so could set it to say 2v, lock the armature and read the current with a clamp meter. I'll try that and post back. I just thought it was odd that the resistance changed so much with a Temp rise from 18 degrees to 40. I appreciate as you say though that my readings weren't reliable. I'll take some with it at room temp and at 40 again to see what happens.

I looked at the brushes and there's plenty of length - 11mm of brush and the springs are firm, but again I appreciate my idea of firm may not be the same as the manufacturer. However, the motor works really well when cold and starts the vehicle crisply, no probs. Then once hot the motor is really sluggish to turn.

 

Have you noticed any increase in effort to turn manually when hot?

 

Have you noticed any increase in effort to turn manually when hot?

I'll check the turning force when hot. Unfortunately my bench PSU isn't up to the task, I set it to 2 v and the motor drew 10 amps which is the max my PSU can deliver.

 

Check for worn bushings. Expansion can make the armature rub inside.

 

i managed with the psu set at 1 volt. The current draw was 4.5A, so roughly 0.20 Ohms when cold, when heated to 55 deg. C., the resistance measured in this way goes down to 0.15 ohm. Interesting, so probably no problem with the motor.....

 

i managed with the psu set at 1 volt. The current draw was 4.5A, so roughly 0.20 Ohms when cold, when heated to 55 deg. C., the resistance measured in this way goes down to 0.15 ohm. Interesting, so probably no problem with the motor.....

Interesting, I kept on changing brushes but all in vain. Would try this out, hopefully it will work for me as well.

 

Just as an update to this. The motor was on a Triumph Street Triple Motorcycle. All the earth connections were checked, cleaned and retightened. We bought a new brush set - there are four on this one. Since reassembly the problem has gone away. So its a bit unsatisfactory as we didn't find a problem. Checking the brush lengths old vs new, there was about 10% wear only.
thanks for help,
Paul

 

Thanks for the update brother, I have high RPM bldc motors and hoping it would work with it as well.

Moderators note : removed commercial link

 

If the motor has sleeve bearings , know that they can wear out much sooner than is reasonable if they are not made of the right materials. I have had that happen on two vehicles, a worn out starter bearing before 10,000 miles. Not covered by the warranty because it is an engine accessory, like the fan was. And as the armature heated just a bit it grew just a bit and rubbed hard on the stator. Close tolerances plus junk sleeve bearings add up to warm failure.
And brushes can be funny because they may not be uniform material. One material to wear and something different to back it up. so cleaning the commutator is often a short term fix.

 

If the motor has sleeve bearings , know that they can wear out much sooner than is reasonable if they are not made of the right materials. I have had that happen on two vehicles, a worn out starter bearing before 10,000 miles. Not covered by the warranty because it is an engine accessory, like the fan was. And as the armature heated just a bit it grew just a bit and rubbed hard on the stator. Close tolerances plus junk sleeve bearings add up to warm failure.
And brushes can be funny because they may not be uniform material. One material to wear and something different to back it up. so cleaning the commutator is often a short term fix.

thanks for this, I forgot that we did clean the commutator. The bike is in daily use now and the starter motor is still working properly hot and cold.