Induction motors 1

[Sharkbiteattack]

I've been looking at used industrial motors that could work to power an electric car and it seems like 90% of the motors used in industry are induction type, why is this? My guess is simplicity, but aren't other types of motors more efficient or have better torque curves? I would think permanent magnet motors would be more efficient. I had heard that synchronus motors produce instant torque from a stall. Also there's the issue that the power factor gets all screwed up on an induction machine so the real power is less than apparent power right? Finally are induction motors the only motors that use frequency to vary rpm?

 

[cswilson]

Your question indicates that you know far too little about motors to attempt such a project. Still, your (very basic) questions may be appropriate for less experienced readers here.

There are several reasons why induction motors are the most common industrial motors. First, as asynchronous AC motors, they can start and run directly off the 50 or 60 Hz lines without any intervening electronics. Second, they are cheap and rugged. Third, they behave well (enough) over a wide range of loads, and can recover well if momentarily loaded past their normal operating range. (I welcome further reasons.)

In a vehicle, the trade-offs are very different, especially since the motor will be operating off an electronic inverter. Permanent magnet AC synchronous ("brushless") motors are more compact and more efficient because they use permanent magnets to create the rotor field. They are more commonly used in hybrid electric/combustion engine vehicles like the Prius. As synchronous AC motors, their RPMs must be directly proportional to the AC frequency.

Induction motors are more commonly used in pure-electric vehicles like the Tesla. The biggest reason for this, I believe, is that it is easy to vary the rotor field strength electronically over a wide range, since there are no permanent magnets on the rotor. Changing the rotor field strength has the same effect on the torque vs speed tradeoff as changing gears in a mechanical transmission, so this permits these vehicles to do without a mechanical transmission. This can more than compensate for the larger size and weight, and lower efficiency of the AC induction motor. And when operating from an inverter, power factor isn't really an issue.

Curt Wilson
Delta Tau Data Systems

 

[rhatcher]

good (lps) answer cswilson.

 

[jraef]

Minor additions:
Induction motors are significantly less expensive than PM motors and once you get over a certain size threshold (maybe 15HP?) the cost of PM motors becomes ridiculous. The magnets in Permanent Magnet motors are rare-earth materials. Rare = $$$

DC (non-PM) and synchronous motors need to get power to the rotors, which means brushes and commutators (DC) or slip rings (AC synch). That means wear parts that need regular maintenance. There are PM AC Synch motors, but again, rare-earth materials.

"Will work for (the memory of) salami"

 

[waross]

And as for synchronous motors developing torque at low speeds, they have a squirrel cage winding and run as induction motors until they reach a speed where the field may be energized so that the motor may pull into synch. They typically accelerate up to very close to synch speed before the field is energized and they transition to synchronous mode.

Bill
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"Why not the best?"
Jimmy Carter

 

[electricpete]

are induction motors the only motors that use frequency to vary rpm?

No.

I hate it when the power factor gets all screwed up.

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(2B)+(2B)' ?

 

[mikekilroy]

It is neat to know that power factor is IDENTICAL for either design when used on a drive, as it will be in an EV, so no difference in PF between the two.

It is also neat to know such things like my two Rinehart drives on my 132kw PM synchronous Delphi motors on my 100% electric Equinox conversion will do torque angle control OVER MORE THAN 400% RANGE! So the 'advantage' of induction motor's field weakening like this is a thing of the past too.... my motors have a 'base speed' (where they hit max 350v peak around 2500rpm yet will torque angle control upto 12,000rpm! E und A switerland also makes PM motors to do similar - it is also VERY good to know that if the drive dies while running at these speeds, the motor back emf will instantly resort to physics and try to be this same 4x higher than 350v! Curt, this is an extension of the ServoStar buried magnet patent you are familiar with from our drives; I can send anyone who wants it the Delphi speed,volts,amps,efficiency, torque angle data in a spreadsheet....

Also since PM just make an electric field, and so far as published, do not make energy, the only difference between induction and PM ac motor inefficiencies is a bit of I2R losses to make Isq, and with tweaking, the induction motor now can be about the same 90-95% IIRC efficient as the PM motor. Hopefully some of the research being done with magnets will become fruitful and actually add to the energy output of a "motor" one day, similar to today's heatpumps... I know one client we are doing the controls for on just such a patent project believes they are there already. google the likes of PM space boots or alternative energy with magnets and see what has and is being investigated...

Normally a PM motor's 3-6x lower inertia helps them be much more responsive for accel/decel, but since the reflected inertia of the EV mass is so much higher than even the induction motor, this advantage goes away also.

Yep, the induction motor probably weighs 2-3x more than the PM motor, so this seems like a clear advantage for the PM model; but with liquid cooling, the induction motor can be much lighter also, so the weight advantage may be minimized in time too, to allow the lower cost to be the advantage.

I suspect the induction motor will gain market share in EV faster than PM synchronous motors for these reasons.

 

[waross]

Add a squirrel cage winding and a rotating solid state Polarized Field Frequency Relay to a brushless exciter generator and you have a brushless synchronous motor.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Sharkbiteattack]

Wow a lot of information here that's above my understanding. I have a lot of learning to do! Mikekilroy that setup with twin 132kw motors sounds wicked! Where can I find more info on it?

 

[Skogsgurra]

ElectricPete
I have been thinking about it for a couple of years now. My answer is: 1. Definitely.

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[mikekilroy]

Sharkbiteattack:

http://ampelectricvehicles.com/

not much engineering details left as they shift focus into UPS/FedEx/box trucks instead of cars now. I would be happy to send motor & drive details if you want (mike-at-KilroyWasHere-dot-com)

 

[electricpete]

Gunnar - I'm used to subtle entertaining comments from you, but that one is over my head. Can you explain.

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(2B)+(2B)' ?