Why Electric motors are inefficient at low revs?

[Alan B]

flathill put it very well, though rather than saying "inefficient" I would say "less efficient". They are still hugely efficient compared to say ICE engines or Human engines, even in most of these "less efficient" ranges.

 

[Miles]

Of course, I was trying to be provocative because so often we hear that efficiency depends on motor speed......

Take a typical hub motor and, keeping the peak efficiency load for 50 rpm, speed it up to 250 rpm. The difference in efficiency might be around 5% or so.... Of course, a slower motor is a less powerful motor

 

[avada]

Both are true:

1) Motors are inefficient when the load is too low or too high

2) Motors are inefficient when the rpm is too low or too high

In both cases you hit zero percent efficiency at either end

for any given rpm>0
start with 0 load and you have 0% efficiency (do no work)
increase the load and the efficiency increases
keep increasing the load and you hit maximum efficiency
increase the load pass this point and efficiency declines
keep increasing the load and now you stall the motor and are back at 0% efficiency

for any given load >0 and <stall
start at 0 rpm and efficiency is 0%
increase the rpm and efficiency increases to a maximum while power output keeps increasing
increase the rpm and efficiency begins to drop while power output keeps increasing
keep increasing the rpm until you hit peak power (somewhere around 50% efficiency)
keep increasing the rpm and now more power input results in less output trending back towards 0% efficiency

I still don't get why the high efficiency area is so small, and why can't it be broader. Or why is this great dependance on RPM but not polecount.

(PS: I totally forgot to check back, and I didn't get notifications)

 

[panandtilt]

Would it matter if the motor was rewound for a much lower rpm?

Like taking a 100rpm per volt motor and rewinding to 3rpm per volt. Assuming the copper fill was the same would the motor be as efficient if they were both fed 100V with one hitting 10,000rpm vs the other only 300?

BTW, more closely to the topic:
Couldn't you simply maximize efficiency by increasing the number of poles to the maximum possible. That's essentially the same as going faster with a motor that has fewer poles. And since (by the diagrams) the most efficient area is near the max RPM it would it would improve efficiency all through the RPM range.

Bluefang and avada's suggestion seems to be what I would think would also increase efficiency at lower rpms, which I suppose isn't true?

If this is the case then, what would a motor designed for peak efficiency at low rpms look one? One that is only designed to go say 400rpm such as for a typical 700cc wheel

 

[Miles]

Look at your drive train from a loss perspective and the shape of the efficiency curve will stop seeming so mysterious.