Brushless versus Brushed efficiencies

[bbaca]

I know that there is a point at which the brushless become more efficient but I don't know where that point is. I've heard often in the 2kW range, but I have yet to see any hard numbers or proof. Does 2kW sound right? Is there more involved? We're looking at running at 24V, if that makes any difference. Is there anywhere you can suggest for me to look for proof?

Thanks,
bbaca

 

[cswilson]

I am presuming you are talking about thermodynamic efficiency here (mechanical power out divided by electric power in), not cost effectiveness. I also presume that you are talking about using a brushless motor with some kind of rotor angle feedback -- otherwise you really could not talk about substituting it for a brush DC motor.

In that case, I am hard put to come up with a scenario in which the brush motor would be more efficient than the brushless. Fundamentally, the mechanical commutation of these brush motors is quite primitive, and you will always have current in some armature windings that don't generate much useful torque due to their orientation at the time. With any decent electronic commutation scheme for a brushless motor, whether six-step or sinusoidal, phase current will be zeroed or at least greatly reduced at these times.

Other factors in favor of brushless motors: Equivalent stator windings will run cooler, and hence with lower resistance, than rotor windings because of lower thermal resistance to the outside world. Also, you don't have the mechanical friction and added electrical resistance of the brush/commutator-bar contact.

Can anybody else think of factors in favor of higher efficiency for brush motors? I can't.

Curt Wilson

 

[bbaca]

Thanks for the reply. Just a few specifics to see if it changes the answer...

We are talking about 8.85 ft-lbs for 60 seconds. This would be on about 600 watts electrical power (or less). I guess my main question is, in this particular situation, would a brushless make that much of a difference?

None of us have much experience with brushless and we plan on using its back emf when it is off and we aren't 100% on how to do that with a brushless, so we would prefer a brushed if a brushless isn't better enough (sorry for the poor english, but I just don't know how else to say it).

We'll also need hard facts, since our reviewers tend to like to see proof of reasons for our decisions. Any suggestions for where to find equations or a good description of brushless and brushed efficiencies vs. torque or power input?

Thanks,
bbaca

 

[cswilson]

I don't think the differencies in efficiency are major. I have never heard of increased efficiency as a reason to go brushless, but there are others.

If you want 600 watts at 24V, you are talking about 25A. That's a lot to put through the commutator of a small brush motor -- although automotive starter motors do more, I believe. The brushless motor will put out AC waveforms when you are using it as a generator, but a simple diode bridge will rectify it to DC (that's how your car's alternator charges the battery). The diode bridge is automatically part of any power circuitry for electronic commutation anyway.

Curt Wilson