general question about MOSFETs and electric motors

[Stori]

Hello,

Let's assume you have an electric motor and the current that runs through it is controlled by 2 MOSFETs.
The motor gets old and the brackets are a bit worn out but it is still running fine.
Could that pull more load on the MOSFETs and eventually kill them (because of more heat or sth) or will the motor just spin slower?

thanks



 

[retrolefty]

Well a general partial answer might be that a DC motor will draw maximum current when stalled, sometimes called locked rotor current. But that doesn't mean that your MOSFETS are in danger or not in danger, too many details not given in such a general question.

[max_torque]

The current that is driven through the windings of an electric machine is driven by the difference between the Back EMF and the applied voltage (forward EMF).  For a fixed power switch duty cycle, and hence a fixed forward voltage, if you slow the machines rotational speed (via a higher load) then the BAck EMF must fall, the difference between them must increase, and for a fixed winding resistance, the current must increase (and assuming the machine isn't magnetically saturated, mechanical torque would also increase)

[T3sl4co1l]

I would say no, BUT, that's because I design my controller circuits with heavy protections: current monitoring, voltage clamping, EMI filtering and so on.

If the circuit is question is a naive half bridge with PWM input and no control to speak of (that is, feedback and monitoring), then lots of things can explode it, including normal every day use -- there's no guarantee that a naive circuit won't simply blow up whenever.

Tim

[Stori]

Thank you all for your fast response 
A friend of mine owns a Mercedes S-class (W140) from 1994. That car is perfectly fine except for one thing: The blower-regulator of the airconditioning system which controls the actual speed of the motor always breaks down after a short time. Those regulators cost ~300€ + taxes and he already spent a shitload of money on those.
The mechanics at Mercedes just replaced that regulator. It never came to their mind, that maybe something was wrong with the blower. Some mechanic (not at Mercedes) "tested" the blower and said it was fine. My friend bought a few cheapo chinese regulators, with those the blower works for 1-2 months till they break down.
Now he came to me because I am the "computer guy" and I am supposed to know everything about electronics 

The connector of the regulator has 4 pins:
pin 1: ground
pin 2: ~12-14V goes around the regulator directly to the blower
pin 3: connected to the head unit inside
pin 4: connected to the switchover valve unit
and the switchover valve unit is connected to the pneumatic unit

The head unit has an automatic mode, a manual mode and a "defroster mode" which blows all air to the windshield with full power.
It has a voltage range of <1V - 6V in manual mode and <1V - 4.5V in automatic mode.
I measured the voltages, it was all within spec.
Unfortunately there are no specs for the switchover valve unit in the official documentation.
It puts 12-14 volts on pin 4 when the "defroster mode" is turned on. That just tells the regulator to spin the blower at full speed.
I guess that should be OK since all documented voltages concerning the switchover valve unit were 11 - 14V.

I did not measure the current because that would have vaporized my multimeter.

Today I took a good look at all the components and found the connectors at the motor were corroded. I cleaned them up as good as I could and now
the thing is running better than ever.
It seems like all those headaches were caused by 2 corroded connectors  ...