The Variable Field Brushless Motor thread

Conventional permanent magnet motors can apply high output torques up to an rpm limit called the base speed. The base speed rpm is governed by the phenomena of permanent magnet motors building up “back-emf” electrical potentials as rotational speeds increase. The back-emf is governed by the magnetic gap flux density, number of winding turns, and rotational speed. As the rotational speed of a permanent magnet motor increases, the back-emf will build up until it equals the supplied voltage. Once the back-emf equals the supplied voltage, permanent magnet motors will not operate any faster. This back-emf rpm limiting characteristic protects permanent magnet motors from the over speed damage that is common with series wound electrical motors used in vehicle applications. The back-emf base speed characteristic that protects permanent magnet motors also tends to limit the dynamic rpm range.

In order to accelerate from rest or from low speeds, many electric vehicles have a fixed reduction drive ratio that is set for high torque. While such configurations provide the necessary high torque to overcome inertia, it results in a low base speed and a limited top speed. In addition to a low speed, constant torque operation, it is desirable for many motor vehicles to also have an upper range of constant power, where speed can increase with decreased torque requirements.

There are methods by which to operate a brushless permanent magnet motor or other motor type beyond the base speed. These methods can be broadly classified as either those using electrical means or those using mechanical means.

Methods of electrically enhancing speed or varying magnet flux include high current switching of additional phase coils or switching the way the phase coils are connected. The costs of such contactors and their contact wear tend to negate the advantages of a high durability brushless motor. Supplemental flux weakening coils have also been used to reduce stator flux and increase speed. This latter approach typically requires contactors and increases heating effects in the stator. Other methods can achieve higher speed operation by varying the waveform shape and pulse angle of the applied driving current or voltage.

Other known methods include the use of DC/DC amplifier circuitry to boost the supply voltage in order to achieve a higher motor speed. This method increases system costs and decreases reliability and efficiency. Such electrical approaches to increasing a motor's base speed are exemplified in U.S. Pat. Nos. 5,677,605 to Cambier et al., 5,739,664 to Deng et al. and 4,546,293 to Peterson et al.

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michaelplogue said:

I was thinking about this a while back when I first saw the first posted video.

Rather than sliding the stator out, I was wondering if you could make the magnets move away from the stator radially. Mount the magnets on some sort of spring-like assembly. So that the faster the wheel was turning, the further away the magnets got from the stator - thus weakening the field. The tough part would be to make the magnets move uniformly - maybe some sort of spring-loaded iris setup.

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U.S. Pat. No. 6,194,802 to Rao discloses a pancake type motor that uses a fixed axial air gap. In this type of motor the axial gap is functionally equivalent to the radial gap in an internal cylindrical rotor motor design with a radial air gap. The individual magnet sectors in the rotor are mounted on spring loaded radial tracks. When the rotor rpm increases, centrifugal force causes the magnet sectors to extend radially, reducing the active area of magnet aligned with the stator coil and reducing the back-emf. This causes the motor to run faster than the base speed. Rao is similar to Masuzawa et al. and Holden et al. mentioned above in the centrifugal method of activation. The design of Rao requires extensive machining of the radial magnet tracks which increases costs and adds to the complexity. In addition, maintaining a sufficient level of balance of this magnet rotor is complicated by several factors. Even after the rotor is balanced with the magnets at their inboard position, as speed increases the position of the individual magnets is affected by difference in mass of the magnets, spring constants/rates, and sliding friction of the magnets along the tracks. Small variations in the resultant in the individual magnet positions would have a disastrous effect on the balance at high rotor speeds. These factors would necessarily adversely effect the ability to reduce back-emf of the motor and operate above the base speed.

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Doctorbass said:

I think that this torque vs speed ration is somewere like for the delta wye.. more torque less speed.. and vice versa... same power...

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johnrobholmes said:

Doctorbass said:

I think that this torque vs speed ration is somewere like for the delta wye.. more torque less speed.. and vice versa... same power...

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It would not work the same as just a wind or termination switch. Let us assume we have a 30mm wide stator. If the stator/magnet overlap is changed to 50%, we would have the speed of a 15mm wide stator, the copper losses of a 30mm wide stator, and only the power of a 15mm wide stator. If the battery and motor is big enough to handle these losses, then the system may be effective enough to use. I would rather have a two speed gearbox however.


There may be much larger losses in the wire without magnet coverage as well, but I do not have a simulator to see how the localized amp draw is affected (if at all).

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The Owen Magnetic was a brand of luxury automobile manufactured between 1915 and 1922, and was notable for its use of an electromagnetic transmission. The manufacture of the car was sponsored by R.M. Owen & Company of New York, New York. The car was built in New York City in 1915, Cleveland, Ohio between 1916 and 1919 and finally in Wilkes-Barre, Pennsylvania in 1920 and 1921.

While the cars were powered by a six-cylinder engine, power for the wheels was based upon the same electromagnetic principle that turned the propeller of the U.S.S. Battleship New Mexico.

Automobile author Henry B. Lent described the drive mechanism thus:

The drive mechanism had no direct connection between the engine and the rear wheels. Instead of a flywheel, a generator and a horseshoe shaped magnet were attached to the rear of the engine's crank shaft. On the forward end of the car's drive shaft, was an electric motor with an armature fitted into an air space inside the whirling magnet. Electrical current, transmitted by the engine's generator and magnet attached to the armature of the electrical motor, providing the energy to turn the drive shaft and propel the engine's rear wheels. Speed for the car was controlled by a small lever adjacent to the steering wheel.
The first Owen Magnetic was introduced at the 1915 New York auto show when Justus B. Entz's electric transmission was fitted to the Owen automobile. Walter C. Baker, of Cleveland Ohio, owned the patents on the Entz transmission thus each of the 250 Owen Magnetic automobile produced in New York were built under license.

The car became as famous as the company's clientele which included Enrico Caruso and John McCormack. Owen Magnetics were advertised as "The Car of a Thousand Speeds"

In December 1915, the concern was moved to Cleveland when the R.M. Owen Company joined Walter Baker (of Baker Motor Vehicle) and the Rauch and Lang concern. The Baker Electric Car company would produce the car, Rauch and Lang would build the coachwork. Because of the combined resources, the 1916 Owen Magnetic increased its model range for 1916 model year, with prices in the $3,000 to $6,000 dollar range. Production continued through 1918 when Baker shifted its focus to War goods manufacturing.

The company reorganized as the Owen Magnetic Motor Car Company of Wilkes-Barre Pennsylvania and resumed production, this time with an order for 500 vehicles from Crown Limited of Great Britain. Under the terms of the agreement, the cars were named Crown Magnetic, however before the order could be fulfilled, Owen Magnetic filed for receivership.

The Woods Dual Power car manufactured by the Woods Motor Company in Chicago also used the Entz transmission. The Woods car was similar in many ways to today’s hybrids. It used both a gasoline engine and electric motors to propel the wheels and utilized braking to recharge the batteries.

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