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Old September 1st, 2004, 11:38 AM   #1
bwheat
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Synchronous AC Drive

In the mill I work we have a 700 hp synchronous AC motor being run by a synchronous AC drive. What is the purpose of using a synchrounous motor here? Power factor correction? We have several other motors in the mill that are this size and most of them are synchronous but they are run by starters. This drive was made by Milltronics in 1986 and parts are very hard to get. Don't most VFD's now days do power factor correction? If this is true would it be worth changing over to a standard induction motor and VFD? Just had a failure last night (bad solder joint on main control board) and I know the clock is ticking... Just dont know when the alarm will go off.

PS. I posted this on rsdoran's site yesterday. I corrected a few things here. The drive is a 700 not a 2000 hp and it is a 1986 model. I made the changes here but not on rsdoran's site.
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Old September 1st, 2004, 12:14 PM   #2
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AC technology in 1986 had not evolved. It is just a matter of time before drives break-down and parts become impossible to find.
It is best to upgrade to IGBT drives of today. If you are interested in getting information on a Drive & Motor package, please e-mail me. Present Drives & Motors should last you aleast for the next 20 years.
Running high HP drives on accross the line starters are a big drain on power, You could get Reduced Voltage Starters that have a quick payback on initial investment by conserving power and helping power factor.
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Old September 1st, 2004, 12:17 PM   #3
Gerry M
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What type of load is the synchronous motor driving? Please provide some detail on the whole control system and what it controls. This may help to explain why the motor is synchronous.
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Old September 1st, 2004, 12:29 PM   #4
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The drive is running a water pump that maintains water pressure to the whole mill at about 35psi. I dont understand the need to use a synchronous motor on a water pump other than power factor correction. We generate about half of the 90 Megawatts we use here and one of our 20MW generators is basically a VAR generator. Anyway, the most of our equipment was in a corrosive atmosphere until we put air filtration on our MCC rooms about 5 years ago. While corrosion is still a problem the life span of the equipment due to corrosion is much better. Of course this drive had to live in the corrosive environment for a long time before the room air was filtered. Mainly I wanted to know if a VFD will correct power factor?
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Old September 1st, 2004, 12:53 PM   #5
DonsDaMan
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A drive will not actually "correct" the power factor, unless it has been specifically designed to do so.

However, an IGBT dual converter with a diode front end will typically run at a power factor of 0.9 to 0.95 at full load. Since the majority of your loads are significantly below this (0.7 being about average, I think), the overall power factor will improve with the use of the IGBT dual converter drive.

I was working on drives back in the 1980's. Do you know the configuration of the drive itself? If it is an SCR current source dual converter, then its power factor would probably be 0.8 at a maximum. At lower loads, its power factor will dip to pitiful lows, but since the load is smaller, so is the current draw, and therefore, so is its effect on your overall power factor.

You will need some type of capacitive load on your system to improve power factor. Synchronous motors can be used to simulate capacitors, but the drive isolates the motor from the line, so that is probably NOT what is happening here.

In short (is it possible to have a short answer in such a long post?), I agree that today's drives are the way to go. See about getting an IGBT voltage source for your load. I see no reason why you shouldn't use an induction motor for this load, unless there is something more to its use than you've outlined.

Be careful of which AC drive you get. I can name names, but won't... There are unscrupulous manufacturers out there that see their customers as nothing more than a source of funds. One quote I heard from a manufacturer with exorbitant service fees, "There downtime is a lot more expensive than our rates. They have to pay them."

HTH,
Don
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Old September 1st, 2004, 12:59 PM   #6
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Power factor!

Well, yes and no. The input to most AC drives are rectifiers and capacitor banks that form the DC bus from which the "new" frequency of the output is derived. So, if you have a highly inductive power factor, and most industrial plants do, then the front end of the drive looks capacitive to the power line which will correct the inductive effect to some degree. This is offset sometimes due to the addition of line reactors in the input side of drives so that may negate the PF correction. Dick DV can probably shed some better light on this subject than I can. The motor you are quoting does not need to be synchronous unless the speed of that motor has to be way more precise than its induction motor replacement would be. The drive can still maintain the required speed +/- the slip and if that is a problem, just put a tach feed back on it and that should solve your problems of speed.

p.s. This is just my opinion, and I am not an expert in the power field!
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Old September 1st, 2004, 03:57 PM   #7
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Maybe the drive is controlling rotor excitation?
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Old September 1st, 2004, 04:12 PM   #8
Tom Jenkins
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First off, as I recall 1986, VFDs were a pretty rare and untrusted item in large horsepowers. They were also generally Current Source Inverters and many had issues with reliability. The norm for large horsepower AC motors where you had to vary the speed was a synchronous motor with some kind of control, often a variable resistance kind of a gizmo called a ReactoSpeed. This combination was very common for pumping - is this what you have? The purpose for using this type of unit was energy conservation, since reducing speed on a pump can be significantly lower in power than throttling.
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Old September 2nd, 2004, 04:43 AM   #9
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Induction motors????

Hi,

I'm still a trainee at college but my questions are:

1 Are Induction motors and Asynchronus motors the same thing?

2 Is there a good basic book that explains the theory of motor
control that explains why IGBT are better then Thyristors
power factors and drive types etc.


Thanks
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Old September 2nd, 2004, 06:59 AM   #10
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Dont know if this a reactospeed type or not but looking at the schematics we have a six scr bridge that rectifies the ac and then there is a large reator between another six scr bridge that creates the new waveform. Does not have a big capacitor bank for DC bus (like I would expect to see in every other drive I have worked on) just the reactor. There is an encoder on the back of the motor that according the experienced guys here "tells the scrs" when to fire.
Were synchronous motors "typical" on large horsepower VFD's back in 1986? My main question here was why did they use a synchronous motor?
What is the benefit in a water pump applicaiton while using a VFD. I dont see any real need to have very tight speed control. The back up for this is a 50 year old steam driven pump.
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Old September 2nd, 2004, 08:24 AM   #11
Tom Jenkins
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I can't answer all of your questions, but it wasn't uncommon to use synchronous motors in the old days.

The main reason for variable speed wasn't for flow control, it was to save energy on systems that didn't have constant demand. With variable speed you can get lower flow when the system demand dropped for whatever reason, at significant power savings. Pump flow generally drops linearly with speed, discharge pressure capability drops with the square of the speed, and power drops with the cube of the speed. Consequently, on a system with mostly friction loss creating the head at 90% flow you only use 73% of te power - not bad!
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Old September 2nd, 2004, 09:11 AM   #12
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OK, it sounds like you have a current source dual converter there. It is very similar to the drives I did back in the 1980's.

The large inductor is called the DC Link inductor or DC Link reactor. Often there are two of them (one on the positive bus, and one on the negative bus). The capacitors are probably there on the DC link for smoothing, but more importantly to provide the energy to commutate (switch the current flow from one SCR to another) the inverter.

The reason you have a synchronous motor is so you can more easily commutate the inverter. With a synchronous motor, you get more counter EMF (here we go again) from the motor than you would with an induction motor (same thing as an asynchronous motor, guest). In other words, the motor is being used to help you turn off the inverter SCR's. It actually has to do with the beta angle of the load, which gets into more detail than we need for this discussion. Suffice it to say that when the load appears capacitive to the inverter, the inverter SCR's will tend to turn off. An induction motor will pretty much never appear capacitive, except during rapid deceleration.

The inverter SCR's/thyristors are on a constant polarity DC bus. SCR's won't turn off in this case. The motor is generating CEMF (sometimes called back EMF) which is what causes the SCR's to turn off.

Of course the above is just speculation.

As far as being typical, I worked on more asynchronous/induction motors than the synchronous type. We made both types of drives, and the only time I thought a synchronous motor was required was for very high inertia loads, which a pump is NOT. Induction motor drives needed an additional circuit to commutate the inverter SCR's. This circuit was called, you guessed it, the "commutator."

You probably have an older design than we made, though, so that design needed the CEMF from the synchronous motor to help it commutate. Our design did not, but it was "revolutionary" 20+ years ago.

I reiterate my original advice: Get an induction motor and an IGBT voltage source drive when you replace this ancient beast.

I'm up to 10 cents now, I think....

Guest, there are about ten books that you'll need to really answer everything you're asking. It depends on your present level of knowledge which books I'd recommend. The power converter handbook is the final say on most matters concerning motor drives. That's about $120, the last time I checked. Probably $150 by now.

If you want a non-mathematical introduction to motors and drives, there are many good ones out there. The training manager for ABB wrote one that's fairly complete. The name of the book is "Motors and Drives: A Practical Technology Guide" As of this writing, it costs $85 and there are only 3 left at Amazon.

A good "beginner's" book on electricity is "Electricity 1 - 7" It used to be seven little booklets, but is now a single book. It's been updated recently so now it has pretty color pictures. Completely non-mathematical approach to things electrical. I'd say any fairly intelligent 9th grader could probably read it and understand it. Harry Mileaf is the author.

Regards,
Don
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Old September 2nd, 2004, 11:57 AM   #13
bwheat
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To summarize a long but informative thread.
My drive is real old and they used a synchronous motor because it was easier to do so back then. No commutator circuit etc. Probably another thing that contributed was we had several motors like this in the plant at the time.
The final answer here is to tape a quarter to the drive and motor, throw them off in the bushes and buy a induction motor and a new VFD.
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Old September 2nd, 2004, 12:21 PM   #14
Tom Jenkins
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Instead of just throwing that quarter away, I'd suggest sending it to Don. That was a great explanation!
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