Ill-defined question (possible OT) about PMDC motor efficiency and voltage.

Please re-forum (or delete) this if it is too far off-topic here.

If you have a PMDC motor that develops maximium power at 36V and 400RPM, that spends a large proportion of its life running at (say) 133rpm, but needs to be capable of excursions upto 500rpm; would it be more efficient to run it at (say) 12V when running at 133rpm, and use something like a boost converter to increase the voltage when higher speeds are required?

I'm looking at this "typical" power/efficiency curve as the basis of my thought processes:


Best efficiency typically comes when the motor is running close to its maximum speed. Lower speeds are typically achieved by varying the duty cycle of the PWM driving the 3-phase bridge inverter.

Within limits, the maximum speed of a PMDC motor is proportional to the voltage supplied.

The question is, if you reduce the supply voltage to the inverter & motor in order to run it at a lower speed, does that move the regime closer to the peak efficiency?

Buk said:

The question is, if you reduce the supply voltage to the inverter & motor in order to run it at a lower speed, does that move the regime closer to the peak efficiency?

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Yes. Those curves are for the rated voltage. At 1/3 voltage, the no-load speed and the stall torque will both be about 1/3 as much as at rated voltage.

The biggest losses come from the current, and the current is proportional to the torque, so if the torque at low speeds is low, the current will be low and there won't be large losses.

Thanks for the reply.

The problem I have in my head about this is:

When running at 36V input, at one third speed and (for now) constant & low torque requirement, the PWM will be providing 1/3rd duty cycle 36V pulses to the motor coils; but the reactance/reluctance of those coils will tend to smooth or damp the pulses out, so the voltage acting on the impedance of the coils will be lower than 36V, thus the current drawn is less than it would be if the full 36V were flowing.

Effectively, the PWM and coils are acting as a buck converter (I think).

So, the question then becomes, are there any efficiency gains to be had by keeping the inverter duty cycle at or close to 100% by varying the input voltage?
(Whether an inverter rated at 36V would operate correctly at 12V is another question that I'm ignoring for now.)

My thought is, that the vast majority of the losses in the motor are due to eddy currents induced in the cores as a result of the changing voltage across the coils -- 0v...36V...0V * 133/minute * 3 phases.

If you can reduce the maximum voltage, you reduce the rate of change, thus the induce eddy currents will be smaller. (I theorise.)

Of course, the boost converter of the required rating will also include a L component, but in a well designed circuit this will be chosen for efficiency.

I don't know if I am completely out of my tree in this thinking; but the idea has been floating around my head for a while now, so I thought I'd ask for opinions here.

Buk said:

So, the question then becomes, are there any efficiency gains to be had by keeping the inverter duty cycle at or close to 100% by varying the input voltage?

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No, I don't think you would gain anything.

The question is a bit confusing though - you start out saying "PMDC" - "Permanent Magnet DC" motor in the original question, then refer to three phase PWM...

Also, in a full PWM bridge, H or three phase, the diodes across (or internal to) each power device allow some energy recovery or regeneration back to the power circuit reservoir capacitors, improving overall power efficiency.

rjenkinsgb said:

The question is a bit confusing though - you start out saying "PMDC" - "Permanent Magnet DC" motor in the original question, then refer to three phase PWM...

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to electronically commutate the windings on the stator; as used in cordless power tools, Tesla 3s, Chevy Volts, etc.

rjenkinsgb said:

Also, in a full PWM bridge, H or three phase, the diodes across (or internal to) each power device allow some energy recovery or regeneration back to the power circuit reservoir capacitors, improving overall power efficiency.

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The amount of energy recovered through regeneration is fairly minimal most of the time. Under anything but the lightest of decelleration, power is regenerated far faster than the battery packs can accept it. That's why F1 cars and some Ferrari, Porsche and other hypercars use Supercapacitors as short term storage for regened power.

I also think it would not be too difficult to bypass the boost converter to feed whatever regen is available back to the batteries.

PMDC motors are not BLDC; two very different things!

PMDC motors are conventional brush/commutator types, with permanent magnet field as opposed to wound field.
eg. Take it from a motor manufacture:
**broken link removed**


Re. energy recovery, it's not just about braking - the motor current circulates through the diodes [and back to the reservoir caps] when the PWM switches off in each cycle, allowing a continuous or at least extended motor current.

That means the motor current can be higher than the supply current even in steady state conditions; as you mention, in effect it works in a similar way to a buck regulator, giving higher current at lower voltage compared to the drive supply.

Okay. Let this thread die, cos its going nowhere now.