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Motor question

Motor question

Motor question

What exactly creates the torque with in a motor? Does the rotor or armature diameter and length have any affect on it? How about the number of windings? All with one specific supply voltage.

RE: Motor question

It's the magic smoke. Keep it in the motor and in produces torque. Let that smoke escape though, there's no more torque.

What's your Intro to Electric Machines text have to say about it?

I’ll see your silver lining and raise you two black clouds. - Protection Operations

RE: Motor question


What exactly creates the torque with in a motor?

The controlled interaction of [electro]magnetic forces.


Does the rotor or armature diameter and length have any affect on it?


Have you tried an internet search? There's a virtual fecal matter-tonne of information on line about this . . .


"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

RE: Motor question

So then if the magnetic forces are stronger and larger area of stator and rotor, then more HP or torque?
Now how to get a stronger electro magnetic force using the same voltage? More windings?

I studied electric motors years ago.
I just feel we all need to relearn the basics of electricity and magnetism, the reason for my basic questions.

RE: Motor question

Strength of magnet is proportional to the number of ampere-turns (for an electro-magnet design). More turns and/or more current means more magnet. More magnet means more torque.

Current and voltage are related through impedance (Volt / Amp = Impedance in ohm).

So ... to get more magnet from a fixed voltage supply, either winding impedance has to go down (to allow more current to flow) OR the number of turns has to go up (without increasing the impedance).

Other factors also come into play: thermal stress on windings, geometry for positioning the requisite number of poles involved to handle either the current (DC machine) or the speed (AC machine), desired performance (i.e., operating efficiency), etc. These factors also tend to have influence on the diameter/length of the active material (rotor and stator core iron).

Converting energy to motion for more than half a century

RE: Motor question


What exactly creates the torque with in a motor? Does the rotor or armature diameter and length have any affect on it? How about the number of windings? All with one specific supply voltage.

As a first cut, the steady state torque for an induction motor is determined by the load (the motor will deliver whatever torque is required to keep the load running at the operating speed which varies over a narrow range and might be considered constant)

.... from that viewpoint, torque doesn't depend directly on any motor parameters. But of course not every motor can sustain the specified torque in steady state without overheating (or exceeding breakdown torque, different subject let's stick with overheating). There are a number of interrelated factors that determine what load torque the motor can sustain in steady state without overheating. These do indeed include core length (longer can generate more torque all other things equal), core diameter (larger can generate more toruqe without overheating all other things equal) and winding configuration, and more.


What exactly creates the torque with in a motor?

If you weren't confused already, I'll ask you to consider how does a motor generate torque. Is it simple Lorentz force as a sum of force on conductors F=q (V x B) = length (i x B)? That's a good principle to get started but it's not actually what's going on if the conductors are embedded in slots. EDIT See my attachments (short version and long version) at this thread: thread237-450172: Motors: Lorentz or Magnetic Repulsion

Sorry op, that last linked thread will probably confuse you more than helping you (but it may be of interest to other members). This is after all a site for engineering professionals. I hope it doesn't sound condescending but I would suggest you search and read and come back with a specific link and question if something you read in your links that doesn't make sense to you.

(2B)+(2B)' ?

RE: Motor question

So if we have a steel or iron rod, with say 50 turns on the coil around it and 1.5 volts DC applied to the coil.
Then another same size and material rod with 300 turns on the coil, and same 1.5 volts DC.
I forgot the wire size I used 40 years ago doing this, maybe it was 27 ?

Question which of the above coils will have more magnetic strength?
And which of the above coils uses less power?

Some times engineering professionals need to look back to the basics.

RE: Motor question

Even if the wire cross section were equal, the one with 300 turns would have more resistance and thus draw less current. If it were the amount of copper held constant then this would be even more pronounced.

Finding the optimum, is why you pay the specialists the big bucks.

RE: Motor question

The torque of the DC motor or ac motor is proportional to the magnetic flux and current drawn by the motor. The current produces the magnetic flux which interacts with the main flux and produces the torque.

Fort more details you may refer to this article.

RE: Motor question

And now we see that it is possible to get more magnetic strength with less electric power.

RE: Motor question

What about special magnetic materials used as the core, will that increase field strength?

RE: Motor question

Electromagnetic torque is produced at the interface between field and armature fluxes or, considering that at least the armature is an electromagnet, as the product of electrical and magnetic loading. Torque estimates from geometry can be made, proportional to D2L, as an estimation of the airgap shear stress. With either buried conductors in the rotor (induction machine) or interior permanent magnet machines, the increased permeability of the rotor iron directs the field flux out through rotor poles into the airgap. It is the (attractive) interaction of this rotor field flux with the rotating armature flux that produces torque. Since the magnetic circuit of the field flux is held and guided by the rotor iron, I see how you could say that the armature flux acts on the rotor iron instead of directly on the conductors. I would technically agree with that (in the same way that the reaction on the stator side is primarily through the teeth, since they effectively carry the flux) but I don't know if it really gets at the heart of the matter. There is also reluctance torque to consider, which I'm not directly addressing here.

So then, with this thinking, it's easy to see that increasing either the field or armature flux in the airgap would lead to an increase in torque. For permanent magnet machines there are a few obvious ways to do this (more magnet material, stronger magnets, etc). The armature flux is dependent on current and the number of turns. Increasing either will earn you more torque. Any material changes that lead to increased flux in the airgap will lead to more torque such as, for instance, steels with higher permeability.

There is of course no free lunch. Higher flux densities, besides when they approach demagnetization limits and thermal issues, will lead to higher BEMFs - so that it will take more voltage (and therefore more power) to operate.

RE: Motor question

BEMF, more wasted energy that should be recovered.

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