Mosfet Question

Thread Starter

Ceejay101

Joined Jan 19, 2021
3
Good afternoon, I have an exam question I'd like to get assistance. Here is the question:
A MOSFET with an RDS(on)=5milliohm is used to vary the speed of a dc-motor. The motor draws an average current of 12A from a 13V supply if the duty cycle of the switch is 30%. Calculate the average voltage on the motor terminals to one decimal.
 

MrChips

Joined Oct 2, 2009
23,520
This is not a question about MOSFET.
You are required to calculate the average voltage of a 30% rectangular waveform.
 

crutschow

Joined Mar 14, 2008
27,198
The reason for showing the on-resistance of the MOSFET is for you to determine whether that significantly affects the motor voltage at its current draw.
 

crutschow

Joined Mar 14, 2008
27,198
To solve the problem you need know how the average voltage varies versus the PWM pulse duty-cycle, and Ohm's law.
Do you know either of those?
 

djsfantasi

Joined Apr 11, 2010
7,689
The rules here do not allow us to give you an answer. But we can guide you to getting it on your own. You have to show us what you’ve done first. You have been given guidance. What have you done with it? Show us you’re working on the problem, not just asking for an answer.

The forum’s title is Homework Help, not Homework Answers.
 

DickCappels

Joined Aug 21, 2008
7,427
I think this may be a trick question -it needs one more step to find the solution. Why should the voltage across the terminals of a DC motor drop very much during the 70% of the time the battery is not connected?

I think we need some additional information about the motor.
 

BobTPH

Joined Jun 5, 2013
3,308
I think we need some additional information about the motor.
I think the intent is to ignore the effect of inductance, since it was not given.

You know the voltage, you know the % of the time it is on. So:

1: Can you calculate the average voltage based on that?

2: Is there another consideration that would change this value?

Bob
 

crutschow

Joined Mar 14, 2008
27,198
Why should the voltage across the terminals of a DC motor drop very much during the 70% of the time the battery is not connected?
Why not?
When the PWM signal is low (off), so will the motor voltage (likely -0.7V from the motor inductance with the usual free-wheeling diode across the motor terminals).
The motor current, of course, may stay relatively constant.

But the question is to find the average voltage.
 

DickCappels

Joined Aug 21, 2008
7,427
DC motors tend to put out the same voltage with which they were driven. If the motor does not slow down significantly during the 70% off time the motor would put out about then same voltage as the original driving voltage, provided the rotational speed did not change much. DC motors are also generators, so they tend to fill in any voltage their drivers stop providing. Right?
 
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