15 replies to this topic

[Duckers]

This question may not be exactly relevant to motorsport engineering in general, however I think this is the best forum for the question:

Does anyone know how to work out how much torque an electric motor has?
From the way I see it, it would be the magnetic force created by the magnet/field windings multiplied by the distance from the centre of the axle to the edge of the armature/coil.
If this is true, how could I find how to calculate the magnetic force applied?

Thanks

Duckers:)

[fuzzy bigpants]

Well well well duckers, what a rubbish question, i would ask people not to answer his question he already has the answer from his father mr Cooney.

But if u wanna know duckers, its gravity, timjesmagnetic fied strenght times the length of the axel

[Duckers]

You never learn do you Fuzzy Bigpants. Stop trying to be funny when you reply to every post I do.
I would like to apologise for my friend's immaturity in him ruining this tread.

Duckers

[desmo]

Obviously our knowledge of electric motors here is seriously lacking. Symptom of being a petrolhead I'd say.

[moog101]

It's all to do with lines of flux

and theta

and, and, and, magnets and wires.

I'll go to bed and post an appropraite formula in the morning, when my bonce is working at at least 75%

[Christiaan]

I am very rusty on my physics but I think the formula for calculating the force a magnet exerts on a coil with current running throu it (effectively the armeture) was

F = BInL*sin(theta) where

F = Force exerted perpendicula to the magnetic field
B = Magnetic field strength
I = current in coil
n = number or turns of the coil
L = length of coil cutting the magnetic field
theta = the angle that the length L makes with the B-field

So for an electric motor with permanent magnets the strength of the B-Field must be known. Unfortunately I forgot how to calculate magnetic flux for a motor with electromagnets (its been about 7 years since I had to do this)

After calulating the force on the coil the torque will be easy to find. Since a motor is circular theta will always be 90^ so sin90=1

I might be wrong

[jdowns]

Christiaan,
I've just started my electomagnetivity subject at uni, but haven't got to electric motors yet. For the case of a single wire in an electric field, force is also proportional to the length of the wire in the magnetic field. More like F=B.I.l.sin(theta). For many wires, I'm guessing it'd be F=B.I.n.l.sin(theta), where everything is the same as your forumula, except l is the length of wire perpendicular to the magnetic field.

I had to refer back to my notes from this week to get that formula - hate to think how I'll be after 7 years

B has always been supplied to us in questions, but from the definition, you calculate B from the force produced in a wire of known length carrying a known current (Tesla = kg/C.s - kilograms per coulum second). Kinda like the dog chasing it's tail here. I think normally you'd be given B by the manufacturer of the magnets involved.

I, too, may be wrong ;)

Jordan.

PS. Just looked it up, and without taking the time to understand it fully, the torque in electric motors = IABn = current * area * magnetic field * # turns, where area is the area contained by the wires in the coil (length * radius * 2) and n is the # of turns in the coil. Again, the greatest unknown is B, which is about .01 for fridge magnets, .1-.01 for 'normal' bar magnets, 2-4 for 'large' magnets, and only 100 for the largest 'pulse' magnetic field ever created in a lab. Apart from answering questions in discussion forums, will I ever need to know this stuff again?!?!

[StephenJK]

Practically speaking, the working man's formula for electric motor torque/horsepower is:

Torque (lb.ft.)=Horsepower X 5250/RPM

Like a lot of electrical/mechanical formula, the number 5250 is a constant derived from I don't know where. I could look it up in an old text book, but I'm really not that interested.

At the end of the day, an electric motor will have less torque/horsepower than a fuelled engine of comparable size. That's to be expected, the supply of electric energy doesn't have the same oomph as a fuel supply.

On the other hand, a fuelled engine can't run 24 hours a day, 7 days a week, with a small shot of bearing grease once a year.

[Ursus]

If one wants to get technical there are several types of el.motors, DC-motor, synchrone-motor, inductance-motor, reluctance-motor and a few others. That's as far as I want to go, I have learnt to fear and hate the inner workings of an electric motors.;) I only learnt the bare minimum to pass my exam and then promptly forgot it all.

[Croaky]

When I was at high school they showed us how to work out the torque of a motor by measuring the force it produced at the output shaft. Why use formulas when you can actually play with machines that buzz and whirr?

[jdowns]

Another complication (I think this applies to all electric motors) is that torque reduces with revs. As the revs increase, the same mechanism that creates the torque in the first place (magnets in electo-magnetic fields) produces back-emf (a voltage of an opposite polarity to the voltage driving the motor in the first place). This is what limits the speed of an electric motor. At 0rpm, torque is at a max, but as hp = torque * revs / 5252, you have no horsepower. The maximum horsepower rating is at a fairly high rpm (relative to it's maximum rpm), so torque is much less than at startup (I heard it can be ;) )

PS. Petrol engines sound way cooler But then again, the environment.... "Everything enjoyable in life is either illegal, immoral, fattening, or depletes the ozone layer" [, or is carconegenic, upsets the neighbors, causes divorce, makes you go blind ... the modern challenge is to see how many categories you can fill with the one act ;) ] Please do NOT reply with sugestions ;)

Duckers, does that help?

[Duckers]

Thanks for all of those who have answered my question. JDowns I gather that where the torque is highest when the motor is at 0 rpm is what the stall torque is, quoted a few times from a book I was reading about automobile electrics.

Thanks

Duckers

[Melbourne Park]

You blokes seem quite critical of electrical engines. but then you are reading this thread.

they are quite handy in the mornings though, especially just after you put the key in ...

[jdowns]

Don't know if I'm critical, or just sceptical. Despite the 2-3 seconds my starter motor runs compared to the many hours my engine does, and the comparitive work the two do, I'm sure I've gone through more starter motors than petrol ones ;)

[Melbourne Park]

Originally posted by jdowns
Don't know if I'm critical, or just sceptical. Despite the 2-3 seconds my starter motor runs compared to the many hours my engine does, and the comparitive work the two do, I'm sure I've gone through more starter motors than petrol ones ;)

And I shouldn't mention the alternator ...

Reality is though accountants have more influence in such items compared to engines. While alternators were once a sales issue, at least their capacity was, I don't ever recall stater motors having that status, at least post '30s.

I do recall once that whether a car had Lucas electronics or not was a sales issue though.;)

[palmas]

[QUOTE]Originally posted by jdowns
[B]Another complication (I think this applies to all electric motors) is that torque reduces with revs. As the revs increase, the same mechanism that creates the torque in the first place (magnets in electo-magnetic fields) produces back-emf (a voltage of an opposite polarity to the voltage driving the motor in the first place). This is what limits the speed of an electric motor. At 0rpm, torque is at a max, but as hp = torque * revs / 5252, you have no horsepower. The maximum horsepower rating is at a fairly high rpm (relative to it's maximum rpm), so torque is much less than at startup (I heard it can be < 1/4 the torque at 0 rpm).

Christiaan is quite right.

Torque IS practicly constant from 0 to nominal speed. After nominal speed (i.e. nominal power) it will reduce linearly to keep nominal power.
But do not forget there are many kind of electrical engines and there are a fewwith separated excitation, where you can adjust torque as you please (even making torque propostional to speed, as used for ventilators)..
So can you please specify what type of engine would you want info on?