Brushed DC motors generally have only a maximum rated voltage. Anything below that is fine, as long as it does what you want and doesn't stall. The motor should last quite a bit longer at lower voltages, too.
You do need to take into account the load applied to the motor. If your motor still spins well at the lower voltage, there is probably no problem.
If you load the motor, it draws more current due to reduced back EMF from the motor spinning. If the load is rated for running at 6v and you run it at 3v, the current draw will be higher and can overheat the motor.
Weedpharma
Industrial motors depend on rotation to keep them cool. There's a fan attached to the shaft. You can run a lower voltage but then it's slower and you can easily get to the point where the lower cooling is insufficient. Smoke, flames and a big insurance claim are the inevitable result.
For a small 6V motor that doesn't have a fan on the back, I think you are OK. If it stops turning because you turned the voltage down too far, then it may overheat.
weedpharma:
If the load is rated for running at 6v and you run it at 3v, the current draw will be higher and can overheat the motor.
Would it be better to PWM for an average 3V rather, while still going to 6V maximum?
Caveat: I am not an engineer!
I think it also depends on the load. If it is rated at 100W at full load and voltage, and you feed it at 1/2 voltage, while maintaining the same load, you would damage the motor. If using PWM I would expect the same problem.
When the motor is not under load, it can still spin freely and not need much current to work.
Weedpharma
JimboZA:
Would it be better to PWM for an average 3V rather, while still going to 6V maximum?
No.
Why? Because PWM means you are in this case, feeding twice as much current for half the time.
Power dissipation in a resistance is proportional to the square of the current (or voltage), so whilst 50% PWM may have the same effect in terms of torque as 50% voltage, it will double the power dissipation.
That is considering the motor to behave as a resistive load. It's worse than that, Jim!
The motor has a back EMF proportional to its speed, and the work actually done is proportional to the current drawn times the back EMF - ignoring some losses. In effect, the resistance of the windings is in series with that back EMF and counts for the difference between the back EMF and the actual voltage across the motor. If you feed double voltage with 50% duty cycle, then for the 50% time the power is on, the back EMF is the same as it would be for the lower voltage, but the voltage across the winding resistance is now much greater and it thus draws substantially more current - more than double the current and the power dissipation is therefore more than four times for 50% of the time, thus more than double overall (compared to continuous application of the lower voltage).
In short, PWM is not good for motors.
Paul__B:
In short, PWM is not good for motors.
Very interesting and clear explanation, thanks Paul.
(PWM has the advantage of being easy to do in a digital world though I suppose (as in, analogWrite() in our little world of Arduino) and I guess that's a plus.)
Paul__B:
thus more than double overall (compared to continuous application of the lower voltage).In short, PWM is not good for motors.
but if I've understood you correctly, it's still close to half of the rated current of the motor at its rated voltage of 6V, so can you explain why this is bad please.
yaafm:
but if I've understood you correctly, it's still close to half of the rated current of the motor at its rated voltage of 6V, so can you explain why this is bad please.
It may be within spec for your motor - but that doesn't change the fact that it's stressing the motor more and using more power to do the same thing (as powering it off the voltage you want it to would), hence, you probably don't want to do that.
DrAzzy:
It may be within spec for your motor - but that doesn't change the fact that it's stressing the motor more and using more power to do the same thing (as powering it off the voltage you want it to would), hence, you probably don't want to do that.
Erm... it's not my motor DrAzzy - just an interested bystander.
I understand exactly what's being said 3V 100% - better than 6V 50% duty cycle from a power point of view and possibly from a stress point of view.
Whatever that stress is though it's still clearly WELL within spec so I don't see that the statement "PWM is not good for motors" has yest been qualified.
Doesn't matter though - don't want to cause any grief - just thought I might learn something.
Paul__B:
No.
Why? Because PWM means you are in this case, feeding twice as much current for half the time.
Power dissipation in a resistance is proportional to the square of the current (or voltage), so whilst 50% PWM may have the same effect in terms of torque as 50% voltage, it will double the power dissipation.
That is considering the motor to behave as a resistive load. It's worse than that, Jim!
The motor has a back EMF proportional to its speed, and the work actually done is proportional to the current drawn times the back EMF - ignoring some losses. In effect, the resistance of the windings is in series with that back EMF and counts for the difference between the back EMF and the actual voltage across the motor. If you feed double voltage with 50% duty cycle, then for the 50% time the power is on, the back EMF is the same as it would be for the lower voltage, but the voltage across the winding resistance is now much greater and it thus draws substantially more current - more than double the current and the power dissipation is therefore more than four times for 50% of the time, thus more than double overall (compared to continuous application of the lower voltage).
In short, PWM is not good for motors.
A good explanation but I think you have missed something.
If the PWM frequency is high enough the winding inductance will smooth out the current variation.
If you drive a frictional load at half the speed you are loading the motor with half the torque so the current will be halved (in the case of dc drive or sufficiently smoothed PWM).
Yes, the efficiency of the motor may be lower than when driving it from dc but it will not be likely to overheat. The ultimate criterion will be to check that the motor doesn't get too hot.
Russell.
I'd just run the motor less often.
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russellz:
If the PWM frequency is high enough the winding inductance will smooth out the current variation.
That may be true to some extent for a simple three pole rotor - but this is not an inductor - it is a motor!
It is operating on a different principle.
Thanks for warning me. Good job you have done.
Paul__B:
That may be true to some extent for a simple three pole rotor - but this is not an inductor - it is a motor!It is operating on a different principle.
I didn't say it is an inductor but it does have inductance. If you don't believe me perhaps you believe Wikipedia:
"In a circuit known as a chopper, the average voltage applied to the motor is varied by switching the supply voltage very rapidly. As the "on" to "off" ratio is varied to alter the average applied voltage, the speed of the motor varies. The percentage "on" time multiplied by the supply voltage gives the average voltage applied to the motor. Therefore, with a 100 V supply and a 25% "on" time, the average voltage at the motor will be 25 V. During the "off" time, the armature's inductance causes the current to continue through a diode called a "flyback diode", in parallel with the motor."
Russell
russellz:
I didn't say it is an inductor but it does have inductance. If you don't believe me perhaps you believe Wikipedia:
Seriously - are you pulling my leg? 
Paul__B:
Seriously - are you pulling my leg?
Thanks for that link Paul. It made me chuckle. The original authors must be pleased with their joke.
Seriously, if you still doubt that a permanent magnet dc motor has sufficient armature inductance to make PWM drive a practical proposition perhaps you would like to read a more authoritative paper:
Calculation of inductance in permanent-magnet DC motors, by T. Miller et al. IEE Procedings, Electric Power Applications, March 1999.
Russell.
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