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Electric Car Motor Question: RPM

by Ed
(Delpho, Ohio)

I have an AC motor that I would like to use in an electric car. The motor is a 20 HP, AC 3 phase 1170 rpm. Is that going to work, or do I need a higher rpm motor?




Hi, Ed!
I'm going to send your question over to Dan.
He says:
Yes it could work. Any motor can run at well over 60 Hz on a VFD. Running motor at 120 Hz will give you about 2340 RPM which depending on gearing may work. 20 HP is not larger enough for highway speed for the average size car ie 1800 to 2000 lb.
- Dan

Good luck, Ed! Keep us posted on your progress.

Regards,
Lynne

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Thanks for the response Dan Bentler!
by: Anonymous

Ah ha! That makes sense. What does not is why do micro surgery when you can buy it already done. So, the one drive system on the AC propulsion site on 1-5-09 does it all with 67 HP out? (No allowance for transmission losses. . . Sounds like VW beetle power! Just the thing for a dune buggy. Perhaps we should move south and enjoy the fresh air. 8-) Sounds good right now.

There were no prices visible. Relays and logic circuits are about as far as we go with enginering right now. Now ask me about protecting underground structures from corrosion. . .

No Micro Surgery for me.

Tommy

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Dan's answer
by: Anonymous

1. When in regen,what does the inverter do with the changing frequency of the voltage and current from the motor?
ANSWER The IGBTs (output transisters ie switches) that convert the DC on the DC bus to 3 phase outp are dual purpose. The generated AC from the motor when overdriven (overhauling load) is rectified when "backfed" thru the IGBTs and fed to DC bus. On industrial VFD a braking resistor is used to dump the load and keep DC bus from going too high. ON an EV the DC bus is tied direct to battery so the generated power from motor would feed to battery.

2. When the car slows down, the frequency changes, right?
ANSWER IF you reduced the speed demand signal to the VFD yes the output frequency would drop. IF the VFD is not told to change the output freq would stay the same, motor speed would drop (ie more slip on motor) to the point that VFD would detect motor overload and either trip on overcurrent or if set up for it reduce speed to reduce motor speed to reduce load.

3. Does the inverter simply rectify the AC?
From the motor yes WHEN motor is being overhauled ie backdriven and thus a generator

Here comes the big one...

4. Could the inverter be used to charge the batteries from the electric service (mains in Brit.)?
4.a.Perhaps it would be necessary to add some programming to protect the batteries, but does the concept have merit?
ANSWER in theory yes. YOu could disconnect the motor connect the VFD output to an AC line and feed back. You would definitely have to have some programming to feed back thru IGBTs.

ANOTHER way use an industrial VFD feed thru the normal AC and charge the DC bus thus the battry. Again you would have to come up with some kind of control algorythm.

5. Since so much of the cost of the AC system is the smart inverter with Variable Frequency Drive, why spend more on a smart charger too?
ANSWER In short use the right tool for the right job. I suppose if you are a good engineer type and REALLY know about VFDS you could do it. The other problem is that with VFDs to do this kind of modification would be like micro surgery - they really got the stuff crammed in the box and access would be quite hard.
Take a look at AC propulsion they have it all in one box VFD, charger, DC to DC converter - but OH BOY the price tag.

Dan Bentler

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Inverter/motor frequency and regen questions
by: Tommy

1. When in regen,what does the inverter do with the changing frequency of the voltage and current from the motor?
2. When the car slows down, the frequency changes, right?
3. Does the inverter simply rectify the AC?

Here comes the big one...

4. Could the inverter be used to charge the batteries from the electric service (mains in Brit.)? 4.a.Perhaps it would be necessary to add some programming to protect the batteries, but does the concept have merit?

5. Since so much of the cost of the AC system is the smart inverter with Variable Frequency Drive, why spend more on a smart charger too?


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