1. Is there an electric motor that does not produce back emf.
2. Is it possible to make one that does not produce back emf.
3. What are the demerits of designing a motor that does not produce back emf.
4. What are the merits of having a motor that generate back emf.
5. Suppose a 9v dc source is connected to a motor, due to back emf that is generated by the motor the effective volts is less than 9v. Hypothetically, if the motor does not produce back emf, does this mean that the effective volts would be 9v and would this make the motor rotate faster.

 

This is a wonderful link one member provided to us the other day - http://zone.ni.com/devzone/cda/ph/p/id/46

Any question about DC motors is contained therein, including back EMF and why it happens.

 

I would think that large salient pole synchronous motors with ac excited poles via slip rings, whch can be run as synchronous capacitors, might be able to run in a no back emf mode.

 

Given that back EMF is what makes a motor effecient (resists the flow of electricity except that which is used for work), why would you want to eliminate back EMF?

In my former hobby (combat robotics) the surge current for large motors was 500A, which went down to less than an amp once the motor started turning. It was the back EMF that dropped this current, and the lack of back EMF that caused the surge (motor was getting current, but hadn't started turning yet).

 

. Is it possible to make one that does not produce back emf.

Yes if you used a non inductive system.

I can imagine generating gas by electrolysis and either directly using the pressure generated or burning it to create motion.
That would be an 'electric' motor with no back emf.
It would however be much less efficient than using an inductive system.

Electrostatic loudspeakers are a real life example of electic motors with no back emf. Their low efficiency compared to voice coil designs is legendary.

 

Consider that when 'work' is performed, your applying a force against a resistance over some distance. EMF can be thought of as your resistance. Without it, you are not performing work.

 

Technically, changing the value of a capacitor (by changing the plate spacing or the dielectric constant) will vary the voltage for a given charge - I don't know if this counts as Back EMF though..

I can think of conditions where motors are not generating back emf, such as an induction motor coasting down after the power is removed - if there is no residual magentism in the metalwork, there would be no voltages on the windings.

Also, a torque motor holding against a static load - it's taking power but not revolving, so no back EMF.

I can't think of any cases with moving, powered electromagnetic motors where there is zero back EMF.