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Author Topic: How does a DC motor work?  (Read 767 times)

theThinker

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How does a DC motor work?
« on: 31/08/2016 16:31:57 »
When a current flows in a conducting wire, all energy input is dissipated as Joule energy - heating + radiation. The formula for the power dissipation is : IČ R.

DC Motor paradox: There is a close circuit when the motor is running. A current enters one lead, flows along the motor armature coil, and out of the other  lead. At any time, the current along the circuit is the same as there cannot be any built up of charge at any one point along the circuit. Now what it means is that all input power is dissipated as Joule heating loss, then where is there any surplus power for the motor to do any mechanical work.
« Last Edit: 31/08/2016 19:52:41 by chris »

chiralSPO

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« Reply #1 on: 31/08/2016 17:01:18 »
Joule heating and buildup of electrostatic field are not the only fates of electrostatic energy.

DC currents can also transfer energy to magnetic fields, kinetic energy, and chemical energy.

Bored chemist

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Re: How does a DC motor work?
« Reply #2 on: 31/08/2016 22:15:40 »
... The formula for the power dissipation is : IČ R...

That's true for a pure resistance, but not for a motor.
Part of the applied EMF goes into opposing the so called "back emf" of teh spinning motor.

Colin2B

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Re: How does a DC motor work?
« Reply #3 on: 31/08/2016 23:06:36 »
If it was constant DC through the coil then what you say would be valid. However, to make the motor work the current is regularly reversed in the coils by a commutator. For this alternating current we must consider inductance rather than dc resistance. As BC says there is also the back emf. All this combines with the kinetic energy generated as ChiralSPO says to account for the total energy balance.

theThinker

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Re: How does a DC motor work?
« Reply #4 on: 01/09/2016 05:25:41 »
If it was constant DC through the coil then what you say would be valid. However, to make the motor work the current is regularly reversed in the coils by a commutator. For this alternating current we must consider inductance rather than dc resistance. As BC says there is also the back emf. All this combines with the kinetic energy generated as ChiralSPO says to account for the total energy balance.
I think there is no AC nor inductance involved here.  The simple DC motor has the armature  surrounded by permanent magnets, N at top, S at bottom. The torque is force of magnetic field on a parallel current. The commutator is needed to change the current direction at an appropriate moment so that the sense of the torque remains the same for one full turn of the armature.

This simple DC motor works on Lorentz magnetic force alone. AC motors may be different.

theThinker

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Re: How does a DC motor work?
« Reply #5 on: 01/09/2016 05:37:42 »
... The formula for the power dissipation is : IČ R...

That's true for a pure resistance, but not for a motor.
Part of the applied EMF goes into opposing the so called "back emf" of teh spinning motor.
I think it is not a really simple problem to some.

I vaguely know about the back emf, etc. But how do we put real figures into the problem:
Say the battery is 1.5 V battery. When the motor is running, let current entering the leads be I. Power input would be P = IV. Now current within the running coil remains as I.

Question: What is the real or effective voltage across the coil resistor R.
Answer: My guess, it is (V - back-emf). So power loss to Joule heating is less then IV.

But not easy to understand why just going into the armature immediately reduces the voltage by amount back-emf. It is like when we travel along the coil slowly, the "point" potential changes suddenly.

Colin2B

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Re: How does a DC motor work?
« Reply #6 on: 01/09/2016 09:03:04 »

I think there is no AC nor inductance involved here.  .... The commutator is needed to change the current direction at an appropriate moment so that the sense of the torque remains the same for one full turn of the armature.  .
So, the current direction changes in a wire, is that AC or DC?
When the direction changes the max current will not be instantaneous but limited by the L/R time constant. This inductance L is a result of the induced emf.
Remember that even with a simple commutator the current direction is changing twice each revolution.
« Last Edit: 01/09/2016 09:41:05 by Colin2B »

theThinker

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Re: How does a DC motor work?
« Reply #7 on: 01/09/2016 17:28:21 »

I think there is no AC nor inductance involved here.  .... The commutator is needed to change the current direction at an appropriate moment so that the sense of the torque remains the same for one full turn of the armature.  .
So, the current direction changes in a wire, is that AC or DC?
When the direction changes the max current will not be instantaneous but limited by the L/R time constant. This inductance L is a result of the induced emf.
Remember that even with a simple commutator the current direction is changing twice each revolution.
I think you should be correct. I don't know any circuit theory.

When the motor is running with a load, at any instant, the input power= iV, V= battery voltage. The Joule loss is generally still iČR, R= resistance of  armature winding. Because of inductance and back emf, we have :
W = iV - iČR > 0.  W is the amount of input power available for useful mechanical work.

A good motor design should aim for a high W/i ratio to reduce Joule loss.

So there is no motor paradox.

Bored chemist

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Re: How does a DC motor work?
« Reply #8 on: 02/09/2016 18:50:52 »

I think there is no AC nor inductance involved here.
The motor doesn't know or care what you think.

vhfpmr

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Re: How does a DC motor work?
« Reply #9 on: 03/09/2016 00:27:01 »
When you apply the power, a current flows in the armature I = V/R where R is the armature resistance. The field produces a torque proportional to the current and the motor starts to accelerate. Once the armature moves, the motion and field induce an EMF in the armature which opposes the supply voltage and hence the current, so the current is now I = (V-E)/R. The power dissipated as heat, P = RI^2, and the power used in accelerating the armature, P = EI.

As the back EMF is proportional to the speed of rotation, the current will diminish as the speed rises, and the acceleration diminishes with it. Eventually you will reach a point at which E is almost equal to the supply voltage, and EI is equal to the frictional losses in the motor. The armature will then stop accelerating. If you now apply a load torque to the motor it will start to decelerate, and as it does so the EMF will reduce, the current will increase, and the torque delivered by the motor will increase. Eventually the torque delivered balances the torque from the load, and the armature will stabilise at a new speed lower than it ran with no load. The power delivered to the load will now be EI minus the frictional losses previously mentioned.

The above is true for permanent magnet and shunt wound motors, but series wound motors are slightly different, they will continue accelerating indefinitely in the absence of a load because as the current decreases, so does the field.

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Re: How does a DC motor work?
« Reply #9 on: 03/09/2016 00:27:01 »