RELUCTANCE MOTOR—
This is sort of "Something new under the sun", but not really. The reluctance type motor is similar but different from the well know induction motor and the nearly as well known PMSM, permanent magnet synchronous motor both of which are widely used in sizes ranging from 100 watt up to 500,000 watts.
Let's define reluctance as R=(amp turns)/[magnetic flux (in webers)]The R is usually in a script capital R. The only time I have ever used the reluctance formula was in the redesign of a relay armature. In the case of motors you can think of a reluctance motor as induction wound motor but with the rotor replaced with one of two types of rotors. The first is a smooth iron with strategically placed "flats" and the second is again smooth iron but with strategically placed embedded copper bars.
The upshot of this replacement produces a motor that has induction motor starting characteristic but with synchronous motor characteristics at synchronous speed. Not obvious other advantages that accrue are very high torque output and extreme rpm speed range when driven by VVVF (variable voltage variable frequency) drive. More about that later. Emerson Motor Co. call their offering SR for "switched reluctance" and build in standard NEMA frame sizes. Other companies offering reluctance motors are Swedish Emotron AB and Rocky Mountain
Technologies. SR motors don't do well direct-on-line, and are much better with an associated power converter to complete the package. Further the mechanical tolerances between the rotor and field windings are about 2 to 1 less critical than in either the PMSM or induction types of comparable size. Also the reluctance type motor can be more easily shaped into pancake or long cylinder type as end use requires.
One example is the jet engine starting motor. In this mode the motor produces very high torque for the jet rotor mass and as rpm builds and the combustion starts, the motor continues to accelerate the rpm until it goes into the regenerative mode and continues to recharge the battery source. These motors are in the 10-50KW sizes.
Another example was using a reluctance motor + gear box to replace a power stepper This resulted in a project designing a radar antenna drive for manual steering while tracking. The antenna itself was an 8' diameter parabola fabricated as a spun aluminum dish with a rolled tubular frame at the edge. It was the mass of this rolled tube that caused the inertial load. Initial try using the power stepper motor was not satisfactory. The inertia caused the stepper during acceleration to slip steps and deceleration was just as bad. The drive circuitry to the power stepper was directly useable as an 8-step per revolution to a
2- phase winding. I visited a business neighbor to where I worked who was an immigrant from Russia forced out by the 1917 Revolution. I had discussed other motor projects with him while monitoring progress on other projects and knew he was near genius where motors were concerned. Upshot he suggested a "smooth iron" rotor with machined "flats" to make it start as an induction motor but run as synchronous. Calculated pull-out torque when the rotor was in the 8 positions were adequate for our needs when viewed through the gear reduction needed to duplicate the step-per-revolution of the power stepper. He also built a prototype rotor using embedded copper bars to enhance the inductance motor
performance. The remaining improvement was a low cost constant-current sourced supply to drive the new motor. This project was done 48 years ago and eventually some 60 radars were built each using this scheme for azimuth and elevation scan control.
A more recent example of reluctance motor use is in the modern washing machine. Here the flexibility of shaft output control via the VVVF allows the elimination of several gears and clutches making the machine simpler and less costly.
Mention has been made of VVVF drives which are used by both induction and PMSM motors to widen the range of efficient performance. The converter drive for most reluctance motors is characterized best as constant-current, variable frequency drive i.e. CCVF (constant current variable frequency). The CC level establishes the torque level while the VF defines the rpm and/or the rotation direction. To achieve this mainly becomes a question of sampling the current to the reluctance motor winding The current to the motor winding does NOT have to sinusoidal.
The writer has to question whether the reluctance motors might find a market in the hybrid and Electric only car market. The reluctance motors are quite flexible in layout and quite useful for electric car layout. Reluctance motors just may have significant advantages in the BER, Brake Energy Recovery technology. And for sure this is the direction of improving efficiency of any electric final drive system.
Interesting.
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