AC Motor Control

hlreed · Post by **hlreed** » Thu Jan 13, 2005 12:36 pm

I know how to control DC motors. How is the best way to learn to control AC motors. I am interested in power as in electric cars.
Books, references, any help would be appreciated.

Chris Smith · Post by **Chris Smith** » Thu Jan 13, 2005 1:03 pm

Electric cars run on DC for obvious reasons. Battery charged, battery driven. AC would have to be produced from DC, but still stored as DC, then converted at a loss,...back again to AC?AC Motor controls are called choppers, and work like a light dimmer, only a big bank of SCRs or Fets do the chopping.

josmith · Post by **josmith** » Thu Jan 13, 2005 3:36 pm

All motors run on A.C..

Sterling Martin · Post by **Sterling Martin** » Thu Jan 13, 2005 4:03 pm

----All motors run on A.C.. ---- Huh????? I too think that it would be neat to run a small car with an AC motor. I did check into it some and have reason to think that all you would need is a commercial VFD. Connect your 12 V. batteries in series to get about 300 Volt DC. Feed that into the DC bus of the variable frequency drive and you should be set! I'm about positive ( 90% ) that most VFD's feed the control circuit off of the DC bus. If you want a really cool setup, get a Vector drive with regenerative braking and feed that back into your battery bank instead of a resistor bank like they are designed for! Vector drives also have an amazing amount of torque at low RPM's, to where you might even get by without a transmission!? Good luck!

Chris Smith · Post by **Chris Smith** » Thu Jan 13, 2005 5:09 pm

"All motors run on A.C.. " .....but all [electric] cars run on DC, as far as I know?

hlreed · Post by **hlreed** » Fri Jan 14, 2005 8:12 am

I am buying a Toyota Prius. Its electric motors run on 500 volt AC. Its battery pack is 206 volts
The electric motor also charges the battery pack when coasting.
I thought it was time to go Tesla and learn AC motors.
Now we have IGBTs to make inverters, but I don't know how to use them. I don't know how speed control is done on AC motors.

Sterling Martin · Post by **Sterling Martin** » Fri Jan 14, 2005 9:00 am

I don't have a phd on this stuff, but I work with it every day. VFD's are just pulse width modulated IGFET's. They work excatly like a DC PWM drive, except for the fact that they are making a facsimile of a sine wave instead of DC. Basically, you start out with a carrier frequency of usually between 2 and 8 kHz. The lower frequency is easier on the motor, but tends to make more noise, where the higher frequency is starting to get out of the human ear's audible range although it feeds more and harder "spikes" to the motor. For the positive half of the sine wave, the pulse width starts out very short, gains length all the way to about steady on for the peak of the wave, and then tapers back off to represent the zero point of the sine wave. The same process is repeated for the negative half of the sine wave after the DC polarity is switched in order to represent a negitive voltage.
To change the speed of the motor, each half cycle of the sine wave facsimile is shortened or lengthened to represent a sine wave of variable frequency.
The voltage that the motor "sees" is very noisy and looks rather terrible, but the current very close to a sine wave, with suprisingly little "noise". The current is really the only thing to be concerned about, provided the voltage spikes don't punch through your motor windings, as it is the factor that causes heating and other unwanted losses. There is no reason why an AC motor running in such a fashion is any less efficient than a DC motor operating on PWM.

Sterling Martin · Post by **Sterling Martin** » Fri Jan 14, 2005 9:17 am

Harold, here is a very good primer on VFD's http://www.baldor.com/pdf/manuals/780-1002.pdf .
Also check out "The Cowern Papers". http://www.baldor.com/support/literature_manuals.asp
Again, good luck!

Chris Smith · Post by **Chris Smith** » Fri Jan 14, 2005 12:09 pm

Making Ac uses a "stair case" generator. Instead of a actual smooth sine wave, it gently goes up in a stair case fashion, usually 256 steps for a sine wave and the chokes and coils level out the mini steps even further.

Sterling Martin · Post by **Sterling Martin** » Fri Jan 14, 2005 2:26 pm

Chris,
I am sure that you know what you are talking about, but as far as industrial variable frequency drives go, they are all of the pulse width modulated variety.

Chris Smith · Post by **Chris Smith** » Fri Jan 14, 2005 4:21 pm

Pulse width, says "width, not voltage height. The height is achieved by a stair case design. If you dump the whole load into a FET type of device, you need a coil/transformer to produce a sine wave. First on, and then a negative voltage is produced when it is turned off or the field Collapses. The way to synthesize a sine wave it to dump a series of increasing voltages over a single cycle, first up and then down. First above the arbitary ground, then below it. This looks like a stair case on a Oscilloscope mimicking a sine wave, with tiny increments. Any thing else will be DC pulsating and not AC.Industrial power controlers start with AC, not DC, and then Chop the circuit, timed by the incoming AC cycles or frequency. SCRs and Fet devices can do this. I have repaired many 120, 240, and 440 volt units as well as the DC types for industry and electric vehicles.[ January 14, 2005: Message edited by: Chris Smith ]

Sterling Martin · Post by **Sterling Martin** » Fri Jan 14, 2005 7:35 pm

Chris, like I said I do believe that you know what your talking about, but I would have to believe that that was for older controllers and maybe also for low power control systems. The new controllers are of the PWM variety. They take the AC power right into a rectifier and convert everything over to DC. There is no AC signals or anything, all DC. Then they take the full DC voltage and PWM it out to the motor. Yes, the motor is always seeing full voltage, it is never stepped. Hence all the latest concern over spike resistant motors. How excatly would you get a voltage spike with the type controller like what your talking about? Again, I know what your saying and believe you, but I feel that those were probably old machines that you were working with. If you have a moment, please check this page out. http://www.baldor.com/pdf/manuals/780-1002.pdf

Chris Smith · Post by **Chris Smith** » Fri Jan 14, 2005 8:33 pm

I was clarifing harold first post....And Yes, for the DC feeds to a motor. “Woods controllers.” I have done those for production line assemblies that require a variable feed. But the above was speaking of Running a car, off a battery, using AC motors. This usually requires a loss in most cases converting anything to AC, when DC motors are the most efficient current, to power a motor. *****************************
Harold wrote ......I know how to control DC motors. How is the best way to learn to control AC motors. I am interested in power as in electric cars.I am buying a Toyota Prius. Its electric motors run on 500 volt AC. Its battery pack is 206 volts
The electric motor also charges the battery pack when coasting.
I thought it was time to go Tesla and learn AC motors.
Now we have IGBTs to make inverters, but I don't know how to use them. I don't know how speed control is done on AC motors.
********************************************
Electric cars run on DC for obvious reasons.
Battery charged, battery driven.
AC would have to be produced from DC, but still stored as DC, then converted at a loss,...back again to AC?

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