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switched reluctance motors/drives - the new thing in vsd? 6

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electricpete

electricpete

Electrical
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A few years ago when touring a motor manufacturing facility, I heard confident precictions that switched reluctance motors and drives were going to take the place of induction motors in many vsd applications in the near future. I haven't heard much since.

Has anyone seen these installed? What are the benefits?

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I've been hearing these predictions since the late 80s, and actually less recently. While they have some intriguing properties, and they are finding some niches, there are several things holding them back:

1. Their dynamics are highly non-linear in many respects (e.g. torque is fundamentally proportional to the square of current), so they are very hard to employ for general-purpose use. It's easy to forget how easy linear system dynamics make things until you lose it. With SRMs, you have to do detailed performance characterization and compensation all over your performance zone. You are starting to see a lot of SRMs in washing machines, where this is feasible, but that's not a general-purpose application.

2. For VSDs, they have the disadvantage of being synchronous motors, which makes starting and stalling big problems without rotor position feedback. Algorithms for estimating rotor position from drive electrical states are mind-numbingly complex due to the above-mentioned non-linearities.

3. Several of the early proponents of SRMs have backed off. One big problem is very high magnetic losses in the rotor at high speeds due to the high frequencies the rotor "sees". These guys have concluded that induction motors still are generally better.

Curt Wilson
Delta Tau Data Systems
 
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Sometimes it proves too difficult to improve upon simplicity.

Thanks for that excellent recap Curt, I had wondered myself. Somewhere around 1990 I attended a demo of a SR motor and controller prototype near you (Corvallis OR). The proponents predicted boldly that by year 2000, squirrel cage induction motors and VFDs for them would be in museums. At least they did find a niche or two, it seemed like an intriguing concept.

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I think that the main attraction was the predictable speed without needing an encoder or tach. They are, as Curt pointed out, synchronous machines and as such don't need speed feed-back to keep accuracy up.

But, also as Curt said, they proved to need an encoder for other reasons. So nothing could be gained there.

I have lived quite close to SRMs and I have seen the problems: High reactive (magnetizing) currents needed, an awful lot of noise because of the varying magnetic path. They sound like fishing boats at low speed and like machine guns at higher speed. Also, the fabrication is not at all as cheap and simple as the proponents thought. At least not so much simpler or cheaper than ASMs that they can hope to replace them.

Gunnar Englund
 
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Fishing boats at low speed and machine guns at higher speed! Mercy!

What can technology do for us next?!!
 
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Ah, the old memories come flooding back!

One of the hard tradeoffs in SRM design is that the torque is (almost) inversely proportional to the aligned airgap between rotor and stator. This is because the torque is proportional to the change in inductance between unaligned and aligned states, and because virtually all of the magnetic reluctance is in the airgap, cutting the airgap in half cuts the net aligned reluctance almost in half, which therefore almost doubles the aligned inductance, and hence the torque.

However, two bad things happen when you try to reduce the airgap too much, both of which Skogs alluded to. First, just stamping out rotor and stator pieces from lamination is no longer accurate enough, which starts eliminating the manufacturing cost advantages. Second, as the rotor's salient pole whips by the stator's salient pole, a virtual shock wave is generated in the air, making them very noisy.

Still, I am kind of disappointed that they haven't taken off. About 15 years ago, I did some playing with an SRM built by Warner Electric for the last generation of HP's vector pen plotters. These motors were a little smaller than a Coke can (~500W IIRC). I could run them to 30,000 rpm, and it was the controller that gave out, not the motor. I could also reverse the motor from +9000 rpm to -9000 rpm in 20 msec (unloaded). These were done under servo control with an optical encoder on the back.

jraef: Corvallis, OR is near me (LA, CA)? Only about an 18-hour drive. It's all relative, I guess...

Curt Wilson
Delta Tau Data Systems
 
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Wow. That is a fast change.

I can see where an induction motor is at a disadvantage for rapid speed change due to the low power factor when slip is high.

But with that rapid change of speed there is also very high energy input required to accelerate the rotor inertia. Is the SRM rotor inertia significantly lower than an induction motor rotor inertia?

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Another place that SRM's have found a home is in power tools. Variable speed, low cost, high speed, long life (no brushes as in universal motors) and noise is not a major concern.
 
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I thought PM synchronous motors were dominating that market now. SRM? Is that so?

BTW, I reversed an ASM from +1500 RPM to -1500 RPM in 7 ms. Not as fast as +9000 to -9000 in 20 ms, but still quite fast. It was a 1.5 kW motor and an oversized Sinus drive.

Gunnar Englund
 
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Curt,
Sorry, I was mistaking Delta Tau with an add I recently saw for Delta Computer Systems in Vancouver WA, thinking maybe you guys had moved and changed your name. They are also in the motion control biz. I was familiar with your products from when Anderson Electric was using your SBCs as the brains onto gutted Eaton VFDs to make them into full vector systems. Are they still doing that?
 
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sreid:

I share skogs' doubt about whether SRMs are appropriate for power tools. First, they absolutely have to be electronically controlled (if only to half-wave rectify and generate multiple phases). Second, as synchronous motors, they need either position feedback sensors or complex "sensorless vector" algorithms -- neither of which seem appropriate for power tools.

jraef:

Delta Computer is a separate company, doing mostly hydraulic motion control. We still are doing vector-controlled drive systems with a variety of integrators.

Curt Wilson
Delta Tau Data Systems
 
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Dyson (the vacuum cleaner people) are developing the DDM, Dyson Digital Motor, according to an article in this month's Torque magazine from RS components.

" Our new digital motor for example, is run by a chip instead of using mechanical continuous currents through brushes. It's a switched reluctance motor that has two phases, the phases being switched on and off using a chip. So it can go much faster, it's more reliable and you don't get all those problems associated with mechanical motors such as commutators breaking up and maitenance issues.
Basically, all the problems you get with ordinary motors go away and you're left with a much smaller, lighter, faster motor because of the chip. So that's a good example of how it has changed things...
The DDM has a power to weight ratio equal to that of a Ferrari sports engine! It is the first ultra-high speed sitched reluctance motor to incorporate turbocharger aerodynamics and aerospace materials and is up to 3 times faster (and much smaller) than a typical vacuum cleaner motor"

Hmmm...chips haven't made me smaller lighter or faster....

"I love deadlines. I love the whooshing noise they make as they go past." Douglas Adams
 
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Rob:

Thanks for that update. I was familiar with SRMs in washing machines and scooters, but this was a new one on me. It does make a lot of sense for this application.

I'm not sure what "to incorporate turbocharger aerodynamics" means. At first I thought they might have skewed the rotor and stator stacks so that the spinning rotor could serve as a turbine (!), but from what little I could find on the web, that looks not to be the case. From one picture I saw, it looked like they filled in the gaps between the salient rotor poles, probably with a non-permeable metal, to reduce windage drag. But "turbocharger aerodynamics sounds sexier...

Curt Wilson
Delta Tau Data Systems
 
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Yeah, and comparing to a Ferrari engine is sexy as well. Never mind that it's not difficult for an electric motor to have a better power/weight ratio than an internal combustion engine, and it would be far more useful to compare it to other types of motors!

I also liked the bit about mechanical continuous current and mechanical motors. We know they're talking about the mechanical aspect of slip rings but like Keith said... it gives me the creeps.

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The marketing men's input makes me grimace: 'aerospace materials'? Such as... aluminium? Steel? Plastic? That aside, I admire what Mr. Dyson has achieved. I imagine it is a good company to be an engineer with, if manufacturing and product design is your thing.

The article Rob mentioned is available online for those who are interested:
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image.php
I don't suffer from insanity. I enjoy it...
 
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