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Switched reluctance motor - can you explain the operation of this driver circuit?

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sleeper1987

New Member
Hi, I've got a query about the circuit halfway down this page (the animated GIF):

Switched Reluctance Motor Drives Information


As I understand it - torque in the rotor is controlled by the rate of change in the field in the coil. Applying a voltage to the coil should result in a positive linear change in the coil's current, equating to a certain amount of torque.

Removing that voltage means the field will collapse very quickly, so producing an larger negative torque. (Assuming the rotor is at the optimum position, that is.)

What purpose then, does freewheeling serve? Is it to regulate the rate at which the field collapses?

-Any help would be greatly appreciated!
 

MikeMl

Well-Known Member
Most Helpful Member
...
Removing that voltage means the field will collapse very quickly, so producing an larger negative torque. (Assuming the rotor is at the optimum position, that is.)

What purpose then, does freewheeling serve? Is it to regulate the rate at which the field collapses?...
Yes, if there is a "freewheeling" circuit, usually a diode path, the voltage drive is switched off, but the current keeps flowing through the inductor in the SAME Direction and same magnitude as it was just before the drive voltage was switched off. Since torque is proportional to current, the torque continues...

There was a long thread about SNUBBER diodes on these forums within the past week. Even though it talked about relay coils, the same principles apply to motor windings.
 

sleeper1987

New Member
Thanks for the reply!
I've read the relay posts, and I think I understand:

- The diode prevents reverse EMF as the field collapses, so current continues to flow in the same direction.

- The traces would look like this:
Code:
    |	         ______
    |          |      |
V   |          |      |
    |          |      |
    |          |      |
    |          |      |
    |          |      |
    |__________|______|_____________________
               |      |      |
	   q1.q2   q1'.q2      q1'.q2'



    |
    |
    |                /\
    |               /  \
    |	            /    \
    |             /      \
I   |            /        \
    |           /          \ 
    |__________/____________\______________
               |      |      |

	   q1.q2   q1'.q2     q1'.q2'
Is that right?

If so, why is the negative gradient shallower than the positive one in simulation?
 
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