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Old December 20th, 2005, 09:22 AM   #1
hashem
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control methods for AC motor drives

I am looking for documents that describe the control methods for AC motor drives such as V/F control, Vector Control....
Any idea will be appreciated.
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Old December 20th, 2005, 09:31 AM   #2
Tom Jenkins
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Go to the Cutler Hammer Site, http://www.eatonelectrical.com/NASAp...eSearchResults , and do a search for TD.08H.17.T.E
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Old December 20th, 2005, 09:34 AM   #3
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Try this site. This is a member's site. It has a good "laymans" explaination.


http://www.patchn.com
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Old December 20th, 2005, 04:37 PM   #4
hashem
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I wonder why most VFD support output frequency more than 100Hz, although most AC motors have the rated of 50 or 60 Hz?
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Old December 20th, 2005, 05:11 PM   #5
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Probably because VFD's can be used to control devices other than AC motors. I have used bowl Feeders controlled by VFD,s.
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Old December 20th, 2005, 05:30 PM   #6
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Quote:
Originally Posted by hashem
I wonder why most VFD support output frequency more than 100Hz, although most AC motors have the rated of 50 or 60 Hz?
Many 4 pole motors have the same barings and mechanical components used in 2 pole motors, allowing them to run at the same speed as a 2 pole motor. 2*60hz = 120hz, which is a common maximum for many brands of VFD.

Aircraft AC power runs at 400hz, although I don't think anyone would use a standard VFD in an aircraft, or use aircraft components on the ground, but you never know.
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Old December 21st, 2005, 01:25 AM   #7
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I've found these two manuals from AB to be very useful:

http://literature.rockwellautomation...p003_-en-p.pdf

http://literature.rockwellautomation...p002_-en-p.pdf
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Old December 21st, 2005, 03:18 PM   #8
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While they are not exactly common, there are applications for induction motors that involve speeds up into the 20,000+ rpm range. A two pole motor at that speed requires frequencies over 300Hz.

These motors would be build special for the application, of course. They are often water cooled, not for cooling efficiency but to suppress the horrendous noise levels issuing from a rotor turning at those speeds.
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Old December 21st, 2005, 03:53 PM   #9
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Here is an example of the very high speed units Dick talks about:

http://www.absgroup.com/absgroup/def...?iLanguageID=1


Search for High Speed Turbocompressor

These blowers hit 25,000 rpm. Scary, huh?

Last edited by Tom Jenkins; December 21st, 2005 at 03:57 PM.
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Old December 21st, 2005, 05:48 PM   #10
hashem
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Tom,
With reference to TD.08H.17.T.E document, it is quoted that "Operation at rated torque at twice speed means that the motor will be operating at twice its rated horsepower".
Is that achieved for all AC motor, or there are some restrictions? When we can use this method?
Any example?
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Old December 21st, 2005, 08:16 PM   #11
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When you operate a motor over its base speed, the torque goes down in inverse proportion to the overspeed. As a result, the horsepower holds constant into a limited amount of overspeed. Above that point, the torque starts falling faster than the speed increase so the horsepower starts to fall off. Eventually, if the motor doesn't fly apart, it will not have enough torque to even run itself so it stops rotating.
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Old December 22nd, 2005, 01:01 AM   #12
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hashem
Quote:
With reference to TD.08H.17.T.E document, it is quoted that "Operation at rated torque at twice speed means that the motor will be operating at twice its rated horsepower".
This situation, as described in the document, is for special applications where a 240 volt motor is run from a 480 volt drive and the motor is run at 480 volts, 120 Hz. This keeps the V/Hz ratio constant to 120 Hz and the motor will develop twice its rated horsepower.

I have heard of this being done but have never seen it myself. The main reason for doing this, as I understand it, is to get more HP from a small motor where space restrictions prevent using a larger motor. The document lists the considerations and restrictions to be followed when running a motor above its rated speed.
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Old December 22nd, 2005, 03:26 AM   #13
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Quote:
Originally Posted by hashem
Tom,
With reference to TD.08H.17.T.E document, it is quoted that "Operation at rated torque at twice speed means that the motor will be operating at twice its rated horsepower".
Is that achieved for all AC motor, or there are some restrictions? When we can use this method?
Any example?
I think one restriction is the maximum insulation-voltage.
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Old December 22nd, 2005, 08:52 AM   #14
hesham
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hashem
please send your email i can provide some good documents
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Old December 22nd, 2005, 09:18 AM   #15
Tom Jenkins
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Quote:
Originally Posted by hashem
Tom,
With reference to TD.08H.17.T.E document, it is quoted that "Operation at rated torque at twice speed means that the motor will be operating at twice its rated horsepower".
Is that achieved for all AC motor, or there are some restrictions? When we can use this method?
Any example?
This has nothing to do with it's being an AC motor. This is just the laws of physics. hp = Tlbft x rpm / 5252. It doesn't matter if it is an AC motor or a hamster wheel - twice the speed at the same torque load means twice the horsepower load.

It is important to keep separate the load requirements, the load capabilities of the engine or motor, and the capacity of the energy delivery system.

The load power is dictated by the rate energy is needed to do the intended job. I could theoretically raise the Washington Monument with a hamster wheel - it would take a lot of gear reduction, reallly good bearings, and a very long time.

On the other hand, if I actually tried this I would probably exceed the thermal limitations of the hamster. After a few minutes the little bugger would die of heat stroke. Many of the limitations on AC motors are similar - sustained operation above rated power causes the thermal capabilities to be exceeded and the motor will fail from insulation burnout or whatever. You rarely see a motor fail from structural failure (twisted shaft, for example) due to overload.

If I could get a water cooled hamster (you figure out where to put the hose!) I might not have thermal problems. However, the ability of the energy delivery system would probably result in failure. The hamster's system simply couldn't provide enough energy transport in the blood stream to sustain muscular activity for the required duration. He'd stall out.

Similarly, the VFD cannot provide twice the rated power to the motor. The phenomenon Dick is talking about has to do with the limits of the VFD's ability to deliver current to the motor, and the ability of the motor to convert that current to rotating energy. The drive can't produce output voltge higher than what it sees at the input terminals. It can't produce current in excess of the thermal capacity of the internal components. That limits the power it can provide and the current it can provide. These in turn limit how much power the motor can produce and how much torque the motor can create.
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