Switched Reluctance Motors For Ebikes?

[safe]

Switched Reluctance Motors For Ebikes?

http://machinedesign.com/article/the-switch-to-switched-reluctance-1211

A quest for energy efficiency has fostered development of switched reluctance motors that are inherently simple and can supply variable speeds.

They can be more efficient than induction motors and work at variable speeds. They are also inherently simple. For a time, they powered Maytag Neptune washing machines.

Switched-reluctance (SR) motors were developed in the 1800s but, apart from a few embedded-drive applications, they have not been widely applied. Their optimum operation depends on relatively sophisticated switching control, something not economical until the advent of compact but powerful solid-state power devices and ICs. Now, with a new emphasis on energy efficiency, switched-reluctance motors may be ready to take a more prominent role in appliances, industrial equipment, and even off-road gear.


http://www.mlittle.com/appliance/neptune/neptune2.htm

Motors are a vital part of most appliances. An important consideration of the Neptune is that the drive motor be capable of high torque, high speed, soft starts, rapid reversal, and tight control. Criteria such as this can be expensive to obtain, driving up the cost of the appliance. Maytag enlisted Emerson Electric to help with the design of a motor and controller which could meet these requirements at a low cost. They selected a switched reluctance motor because of the control available, and the low manufacturing cost of the motor. Since the rotating component of the motor (rotor) has only vanes and no magnets or windings, it is cheaper than other motor designs. Unfortunately, this type of motor requires more sophistocated electronics for control; however, the electronics could be developed more cheaply than another type of motor which could perform under the same criteria. Another issue is the need for position feedback from the motor to the controller. This motor uses a tachometer device to inform the motor control board of the current rotor position, so that the controller may energize the proper phases at the proper times.

 

[safe]

http://www.memagazine.org/backissues/membersonly/february98/features/risevsr/risevsr.html

A four-phase, 1/2-horsepower variable-switched-reluctance motor, used in applications such as electric motor scooters and industrial fans, can run at more than 90-percent efficiency.

A VSR motor is generally used as a stepper motor and, if properly controlled, can be made to behave like a servomotor. Basically, the motor is a rotor and stator with a coil winding in the stator. The rotor, which consists of a laminated permeable material with teeth, is a passive device with no coil winding or permanent magnets. The stator typically consists of slots containing a series of coil windings, the energization of which is electronically switched to generate a moving field. For the most part, only a single coil set is activated at any one time.

When one stator coil set is on, a magnetic flux path is generated around the coil and the rotor. The rotor experiences a torque and moves the rotor in line with the energized coils, minimizing the flux path. With the appropriate switching and energization of the stator coils, the rotor can be encouraged to rotate at any desired speed and torque.

This setup offers better performance than many other types of motors. A VSR motor does not require sinusoidal exciting waveforms for efficient operation, so it can maintain higher torque and efficiency over broader speed ranges than is possible with other advanced variable-speed systems. The optimal waveforms needed to excite a VSR motor have a high natural harmonic content, and are typically the result of a fixed voltage applied to the motor coils at predetermined rotor angles. Such waveforms can be achieved at virtually any speed.

In addition, as long as the commutation can be accurately controlled with respect to the rotor angle, the motor will operate at its predicted high efficiency. "With VSR technology," Holling said, "it is possible to design a low-cost motor with over 90-percent system efficiency and variable speed at a good price."

VSR motors also provide other benefits. They can be programmed to precisely match the loads they serve, and their simple, rugged construction has no expensive magnets or squirrel cages like the ac induction motor. With no internal excitation or permanent magnet, the motor is inherently resistant to overload and immune to single-point failure. Finally, once in high-volume production, they are likely to be less expensive than competing systems.

 

[safe]

94% Peak Efficiency

Probably expensive... but this one is ready to be plugged in and used because the voltage is in the right range. (24 volts or 48 volts)

 

[johnrobholmes]

10lbs for a one horse peak motor. No thanks! Where are you going to get a four phase controller?

 

[arceeguy]

You guys are making my head hurt with all the electric motor types, controllers and batteries!

What type of motor do they use in hybrid cars? Maybe this is what would be good for bicycles.

Maybe a Prius or Insight motor would make a good electric motorcycle.

 

[johnrobholmes]

Hybrids generally use induction, separately excited (sepex), or permanent magnet motor. I think the sepex may win out in the long run for larger vehicles as long as the controller prices continue to drop. PM motors have the highest power to weight ratio, whereas induction are cheap to produce and simple to control. On my smaller vehicles I will stick with PM motors, but if I had something that was a few hundred pounds I would consider a sepex.

 

[safe]

Understanding Efficiency

Efficiency is not a simple thing to know.

With the permanent magnet motor the efficiency is only good when there is an exact match between the motor rpm and the current being supplied and there are other factors that make getting the better efficiency hard to achieve unless you are content with just sort of cruising riding.

In the "real world" of ebikes for people that are racing them around at full speed all the time the permanent magnet motor needs to use gears in order to utilize the motor effectively.

And then there's the laws... most places limit the motor to 750 watts or less, so all these motors that exceed the 750 watt limit are motors that are not legal anyplace.

-----------------------------

The reason for Induction type motors is that given the world that ebikes exist in it would be "nice" to be able to get the full legal limit of power from 0 mph all the way up to the top speed. But many states limit speeds too, so that's another issue that Induction motors bring up that isn't a concern with a permanent magnet motor.

Permanent magnet motors have "speed limits" built into them...

Weight is something that you can go down on later with refinement of the motors. The main thing at this point it to (somehow) make that gigantic leap from the permanent magnet world to the Induction or Reluctance world.

--------------------------------

However... in the short term one might just slap a Brushed or Brushless DC motor onto an ebike and go out and ride. If you just want a quick and easy answer then that's what you do.

I like the idea of the Switched Reluctance motor because it could in theory provide the "ideal" flat powerband that is most efficient under load while being cheap to produce.

94% claimed efficiency might be worth it because that means you get more from your batteries.

---------------------------------

There's no hurry to get anything done either (in my opinion) because this is supposed to be a hobby. It takes whatever time it wants to take. I've already done 6,500 miles on DC motors, so I pretty much have already learned what they are capable of doing.

 

[safe]

Very Cheap To Buy... But Hard To Run...

I should have posted this earlier:

http://cgi.ebay.com/Maytag-Neptune-...in_0?hash=item5d25bc5818&_trksid=p3286.c0.m14

...these Switched Reluctance motors that were used on the Maytag Neptune washing machine are being sold for as little as $25. The weight appears to be about 14 lbs. (which is a few pounds more than a Unite motor of similiar power, but not by much)

The real problem with such a radical departure from the brushed motor (and even the RC sensorless brushless motor) is that the Switched Reluctance motor is hard to control. So this becomes a "software problem" and not a hardware problem.

Remember... Switched Reluctance motors are like Induction motors in that they can be driven very hard and they have no permanent magnets that limit the powerband. This means that if it's possible to pull this off it would be pretty good even with one gear.

A hard challenge... but no one can complain:

"It's too expensive to try."

(building a controller is cheap as far as the parts, but the time involved would be significant)

Permanent Magnets Have Powerband Limitations

The simple fact is that while it's possible to increase the power of a permanent magnet motor by using stronger magnets and a better rotor you are always trapped by the powerband shape which "peaks late".

The great thing about an Induction motor or Reluctance motor is that the powerband can have high torque while having high efficiency from zero rpm upwards. The limit of these motors can be really high.

It's really a change in mindset AWAY from the "arms race" of increasing power (which cannot end well from a legal standpoint) to a focus on QUALITY of power within a constrained power limit. For the goal of improving the quality this makes sense... otherwise it doesn't.

(on a power to weight basis you might be better off with the basics)

 

[safe]

Some Advantages of SRM's

One thing that I've read that is good to realize is that unlike the AC motors there is no need for negative voltages in a Switched Reluctance motor. This means that you only need to deal with MOSFET's that handle positive voltage. (makes things a little easier)

Induction motors rely on "slip" to determine behavior.

SRM's need very precise timing of the pulses to get it right. (it has to be perfect)

Also, the rotors in a SRM are made of iron and there is no IR^2 losses and that means no heat... so all you have to worry about is cooling the stator which is on the outside.

Did someone say "Ice Cooling"?

 

[johnrobholmes]

Negative voltages allow 6 step commutation for 3 phase motors, which greatly smooths torque and power delivery. Reluctance motors have power limitations as well, oversaturation occurs just as with any other motor type. They also have speed limits too, but it is tied with commutation frequency instead of voltage. Assuming same efficiency and power handling, a switched reluctance motor will not have any speed advantage over a PM motor IF we consider variable gearing with the PM motor. Having a fixed gear ratio certainly favors the SR motor if very high motor RPMs are desired. Of course there are still rpm limits to the rotor.


I do like the concept of the SR motor though, I am hitting up my controller manufacturer to see if we can get some programming for SR motor control. Maybe have sensor input for low speed control and better commutation sync.