Switched Reluctance Motor (SRM) Construction
Last Updated: 6/15/07
This motor was made from two single phase induction motors I got from salvage. They were chopped up and machined to make the stator stack which was then put into the aluminum housing. The rotor was constructed using a microwave oven transformer core. I cut out a section of the core and had the material drilled and pressed onto the shaft and then machined to form the rotor stack. Since this motor is fixed in size and topology, I have built the motor parts with close tolerances to try and compensate for the size and material quality limitations.
The stator, end plates, and rotor are shown here and the motor is disassembled to show the parts that make up the SRM motor. This is an earlier picture before the stator was wound. The outside diameter of the motor is 3.5 inches and the rotor diameter is 1.78 inches with an air gap of 0.005” between each rotor and stator pole. The shaft is grooved for “C” clips which keep the rotor centered in the motor. I will probably need to use a beefier bearing on the sprocket side of the motor when I put it on the kart.
The red colored areas are insulation coating using transformer insulation varnish. I applied additional insulation around the stator poles using Kapton tape. The stator stack and housing are electrically connected for grounding and the resistance from stator pole face to housing is 0.4 ohms. The “sticks” on the stator poles are there to help hold the windings in place and are permanently attached to the poles.
Below is a picture of the wound stator. It is wound using the 22GA magnet wire and I adjusted the number of turns to get an inductance of 28mH with rotor in aligned position and 5mH in the unaligned position. The resistance of the windings is 1.5 ohms.
I first wound the stator using 20GA magnet wire to get reduced winding resistance. I got the first stator pole half wound when the “sticks” broke and I had to remove and replace them with the larger and stronger wood posts epoxied in place. After getting as much 20GA wire on both stator poles as possible, the aligned inductance was only 3mH. This was too low and I re-wound the stator with the 22GA wire.
Here is the motor assembled. The rotor turns freely with no binding even with the relatively tight tolerances.The two rubber grommets on top are were the high voltage leads come out of the motor. I made the rotor shaft long on both ends so that either side can be used. The centrifugal clutch will be mounted on the side opposite the high voltage leads.
Using the equations for magnetic circuits and switched reluctance motor design to determine number of turns per pole and flux density yielded results that varied by a factor of 10. I believe the reason the design equations do not give expected results is that they factor out or assume small non-linearities in the energizing and de-energizing of the phase windings and my system works mostly in the non-linear region (di/dt and dB/dt) during the power stroke.