12 Volt DC Motor and Carbon Fiber housing

[Mino]

Hello all.

I hope Im in the right place to ask help for a problem that I have.
I am engineer and not electrician and I do not have any special knowledge on this subject.

I am producing a tool which is mainly made of aluminium and plastics and Its purpose is to harvest olive trees.
Low weight of this tool is a key factor for high productivity and easy handling
by the user.

Anyway, I am now replacing aluminium parts with Carbon Fiber and one of them is the housing of the electric motor of this tool. This part is made of iron and replacing it with CF saved me 200gr which is a great gain for the tool as a whole.

But here start the unexpected ( for me) trouble...
I knew before changing it, that the housing width has a role to the magnetic field of the motor. Anyway I made the CF to have exactly the same inner and outer diameter as the iron. But when I placed it on and started the motor it seemed to me that it was running faster. I used my digital rpm counter and it showed me with exactly the same voltage and no changes unless the housing an increase of about 650 rpm, almost the double!!! With iron its 850 and CF 1500 . I also noticed that motor lost its capability to maintain rpm when stressed. I cant use it with so many rpm for the safety of the motor but also for reducing rpm issues.
Does anyone have an idea of what is going on??

Thank you in advance.

 

[simonatlondon]

sounds like the cf has increased the rpm of the motor at the expense of torque. what are you using as a supply for the tool ? ni-cd, ni-mh, li-ion etc etc,

are you using any capacitors on the motor?

how many windings on the motor?

how many magnets?

 

[Mino]

Thank you for your reply simonatlondon.

Well I suspected that I would have a change in rpm but I was shocked with such a difference in rpm.

Well the power source for this kind of tools are car alternators powered by 4 stroke engine of 3-6hp.
I used this type in my test and also a transformer 220 to 12 Volts I do have.
The rpm I mentioned in my first message are from the transformer testing, cause the alternator makes the motor to run in higher speed.

This motor has no capacitor and it has 2 magnets. It is a Magnetti Marelli which I think it is used as windshield wiper motor in cars.

I have no idea how many the windings are, but if you think a change to this could fix my problem I could try to count them and let you know. It would be bad news though If I would have to increase them and add more weight.