Inefficiency of a widely used electric motor

[martin007]

A new electric motor has about 90% efficiency
With the use and the passage of time electric motors are becoming less and less efficient.
.
What exactly causes an electric motor to lose efficiency?
Physical wear of the rotor?

[Burgerman]

A typical DC Brushed wheelchair motor is at its extreme best, at a single load and speed only around 80% efficient. At all other speeds and loads efficiency is much less.

At stall its taking massive current, zero RPMs and making zero power (power = torque x rpm). So it is 0% efficient here. The efficiency rises as the rpm rises in a curve that peaks around 90% max rpm and maching load. At higher speeds efficiency falls again.

Below is a standardised efficiency, power and torque curve for any theoretical DC motor. Only the current, volts, rpm scales change.

Then as they age the effiency drops not only in the motors (magnets become more demagnetized over time due to the opposite pull from the armature or warmth in the motors, bearings, vibrations, brushes and commutator wear/burns and so resistance etc) but in the gearboxes too. As the gears wear they develop larger flatter areas that slide against each other. And the gearboxes are at least half of the motor losses.

In the hobby world with brushless motors and ultra powerful n52 rare earth magnets we have super high efficiency motors. Where a tiny egg sized motor is sold as 700 watts, and we typically run these at 1500 watts continuously in flight. Nothing gets too hot due to high efficiency. These are 94% efficient typically. And they dont die from brushes burning up as there are non.

[martin007]

Brushes and bearings can be replaced.

[Burgerman]

So can the magnets in theory, and gearbox innards, and all can be rebalanced. But the cost, availbility, and time mean its seldom worthwhile.

[martin007]

OK.
Thanks.

[Burgerman]

There are places that "recondition" motors. The reality is that they dont actually do any such thing. They strip, clean, examine all parts carefully. Generally then they will fit new (usually pattern not OEM but can be both) brushes, bearings and rubber motor couplings. They do the same in the gearboxes but dont fit new gears and shafts etc. Just bearings. They MAY remachine the commutator if its badly scored. They likely wouldnt have any way to replace magnets or gearbox parts. They likely use pattern parts in the brakes too.

So the reconditioned motors are still using most of the old existing internals. Just cleaned, painted to look good. With bearings/brushes etc. So whilst cheaper than new ones, its not often cheaper "enough" compared to buying a set of brand new ones. Depending where you live and which motors specificall it may not even be cheaper. And at least the new ones are as efficient, quiet and unworn as it gets. Motors/gearboxes/brakes generally al wear at the same sort of rate. So stripping and rebuilding just some parts of it is seldom a good idea. And thats coming from someone that has on several occasions.

[martin007]

I understand.

[Burgerman]

Are you looking for a new chair, new motors?

[martin007]

I'm just trying to learn the why of things.
I see that everything is very complex...
...too complex.

[Burgerman]

Thats because it just is.

Those that dont understand things fully, think they do. Those people see most things as much simpler than they really are. But the more you learn, the more you understand, the more complex everything seems to get. And its an ever deepening spiral. I wish I had known as much about the world, and how it works when I was much much younger.