Cordless drill motor.

I'm guessing but something in the region of 6ohmm -50/+100%

What is more important is that the resistance is independant of position, subject to the segment to segment change under the commutator. Assuming brushed of course.

Is this stripped-down and measuring just the windings?

That'd only be 1.5 to 6A, so 27 to 108W, you can see 18V cordless drills specced as taking 30A so over 500W, some who have re-purposed drill motors for use in robots claim nearer 900W

Yes, my estimate is rather low.

This replacement motor has a surprising spec:

I can tell you with certainty it will be less than an ohm

Exactly,. at at least 60% efficiency so:

Input current = 500w/18v= 27A...40% losses 200W, so resistance

Even a poxy little 20W brushed motor us still under an ohm

Its very hard on low voltage motors to tell if there are shorted turns as we are talking fractions of an ohm here, I can tell you from my model boat days, such a tiny short makes the motor unusable and eats the battery of course. If we had an idea of your problem? Brian

They would do that, certainly some of the small model makers drill control circuits stop stalling from frying the motor. Brian

A few questions and comments...

What wire would be used in that motor, and what would be the length of it?

According to

That table for the motor also says the no-load current is only 2.6 amps, so there is almost a factor of 100 between stall and no-load. Is that usual?

The better the motor the bigger it is. The ideal motor has infinite stall current and zero resistance.

And also zero no load current.

power is lost in electric motors in several ways.

(i) Through the rapid magnetisation and reverse magnetisation of the armature. hysteresis means means you don't get back what goes in.

(ii) Eddy current losses. The radically changing magnetic fields introduce currents in the iron which are subject to resistive losses. That is why armatures are laminated.,

(iii) mechanical friction and bearing loss.

These three are all relatively constant at a given RPM and are independent of the load, and are lumped into 'idle current' when measuring the motor. Also known as 'iron losses'

(iv) heat lost due to the resistance of the motor windings (ideally zero) known as 'copper loss'.

(v) energy lost as mechanical energy to drive the actual load.

It can be shown that peak efficiency occurs when the copper losses and iron losses are equal.

This model of an electric motor is not perfect, but works well enough to predict performance quite accurately. Those of us who have measured motor performance know that idle current varies with RPM quite a lot, so it's important to measure it at the normal working voltage.

The theory of the motor is that It will spin up until a notional 'back emf' marches the applied voltage. The ratio of rotational speed to back emf is expressed as the Kv or 'RPM/V'. The resistance of the motor is deemed to appear 'in series' with the motor and therefore reduces the applied voltage as 'seen' by the motor in proportion to the current being drawn. Thus, in a less than perfect motor, the RPM will drop as load is applied, and extra current is drawn in an attempt to match back emf to applied emf.

Using longer thinner wires to wind the armature of a permanent magnet motor simply means that its best efficiency working voltage is increased. And the current is decreased in the same ratio. In the end the power is determined - as it is in a transformer - by the peak flux density that can be sustained in the armature, times the operating frequency - or RPM.

So, the important thing to remember is that the resistance of the motor has *almost no part to play* in determining what current it will draw. That is far more determined by the *load that is applied*. In short the motor will draw whatever current it needs to do, to supply the power to the load, yea - even unto letting the magic smoke out in the attempt.

The only way to get more power out, is to increase the RPM. This we found early in in making model planes fly better. Gearboxes to reduce shaft RPM to sane levels allowed us to take armature speeds towards the mechanical limits of the motors = either brush bounce, or mechanical disintegration. By increasing the applied voltage we gor more power whilst keeping armature current - the killer - in reasonable limits. The price was RPM up towards 100,000 , whereas a typical propellor operated best below 10,000.

Very interesting. Thanks for spending the time putting this together. It's been saved for future reference.

I agree with you up to this point. Can you cite any reference as to why this might be the case?

I'm surprised you haven't moved into solid state, wound stators and spinning magnets with no wires to part with the armature?.

Unless you are doing the adiabatic calculation (for a given speed of trip), you can only speak of fusing current in the context of its heat loss to the environment. Which is a lot less in the middle of a winding than in free air.

So your post was unjustifiable twaddle, as per usual?