motor speed control
motor speed control
(OP)
Hi guys,
My apologies if this is a simple question that has already been answered but I wasn't able to find anything helpful in the archives.
I'm designing a glorified dryer. The point of this machine is to tumble clothes at a specific speed over a specific time to condition them for other tests. The trick is that the spec requires a tumble speed of 5 rpm +/- 2%.
Ideally I would like to use an AC motor and hope that it stays within the right speed range and then just use belts and pulleys to ramp down to the correct speed. The torque load should be fairly consistant (my guess is ~0.2 hp +/- 5%). If I have to, I could use a DC motor, with some kind of feed back.
So my questions are:
Would an AC motor deliver a consistent enough speed over my torque range?
If not, are there any simple/good DC motor and controller packages that would work? I would like to avoid the purchase of a PLC if I can. (I found this option, which seems both appealing and confusing to me http://focus.ti.com/lit/an/sboa043/sboa043.pdf )
Thanks in advance
Charlie
My apologies if this is a simple question that has already been answered but I wasn't able to find anything helpful in the archives.
I'm designing a glorified dryer. The point of this machine is to tumble clothes at a specific speed over a specific time to condition them for other tests. The trick is that the spec requires a tumble speed of 5 rpm +/- 2%.
Ideally I would like to use an AC motor and hope that it stays within the right speed range and then just use belts and pulleys to ramp down to the correct speed. The torque load should be fairly consistant (my guess is ~0.2 hp +/- 5%). If I have to, I could use a DC motor, with some kind of feed back.
So my questions are:
Would an AC motor deliver a consistent enough speed over my torque range?
If not, are there any simple/good DC motor and controller packages that would work? I would like to avoid the purchase of a PLC if I can. (I found this option, which seems both appealing and confusing to me http://focus.ti.com/lit/an/sboa043/sboa043.pdf )
Thanks in advance
Charlie





RE: motor speed control
Here is what I recommend.
Get a small three-phase motor. 1/2hp. It won't be very big as three phase are smaller than single phase to begin with.
Acquire a VFD (Variable Frequency Drive) that will control the motor's speed.
You will want a specific VFD.
One that will take 120VAC single phase and drive a 240VAC three phase motor.
These two items are all you need to dial-a-speed with far more durability and for less money than your DC solution.
Keith Cress
kcress - http://www.flaminsystems.com
RE: motor speed control
I forgot to mention one last part. It runs on 240 hour cycles. Do VFD's have problems with sustained run times?
Also, any recommendations on VFD's?
RE: motor speed control
Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: motor speed control
I'm waiting for one of the VFD gurus to come and say whether or not you need what's called a vector drive versus the lesser scalar VFDs. The scalar drives put out a frequency that drives your motor. But all motors have slip. The power they provide is thru this slip. The slip results in speeds below the motor's synchronous speed. More load more slip. It is not very much actually so it may not be a problem for you at all. The scalar drives know not what the motor is actually doing - they just put out the appropriate frequency and the motor turns it's actual speed accordingly.
A vector VFD knows exactly how fast the motor is turning and it even knows what the precise position of the shaft is. I don't think you need that feature. I rather think that you could use a basic scalar drive and set it up to show the RPM of your drum. You then actually run your moderately loaded drum and count the actual turns it makes over some period, then calibrate the display, and you're good to go from then on.
Others who do this stuff for a living can probably say immediately that a scalar or a vector drive should be used.
After that a specific model can be tracked down. Meanwhile find a three phase motor you can work with. Check all the usual places. Grainger, etc.
Keith Cress
kcress - http://www.flaminsystems.com
RE: motor speed control
The way I understand it, an AC motor works off of the wall frequency, minus the slip. As long as this is consistant enough and stays within my 2% criteria, I was hoping to skip any additional controller.
RE: motor speed control
If you need tighter speed control, then spend the extra dollars for a VFD. Your best voltage choices are a VFD rated for 240 volt single phase or 208/240 volt three phase input, and a 200 volt or 230 volt three phase motor.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: motor speed control
Itsmoked is correct all you need is a drive and 3 phase motor.
You could use a drive with voltage doubler 120/240 input as he suggests but one with a standard input would be cheaper ie 120V 1 Ph in / 208V 3 Ph out
Usually you have to de-rate the drive slightly when you are only use a single phase supply but the drive vendor should tell you that.
A standard drive should be OK as anything you gain in having a vector drive for the motor would probably be lost in belt slip anyway. If you need accuracy perhaps you could detect the tumble speed and use that to feed back to the drive, most drives have a PID option available.
Regards
Roy
RE: motor speed control
Gunnar I did say 120VAC. You need to dial back your
charland; I didn't stop tho think that you need a fixed single speed. I saw the feedback/DC motor and suggested a better alternative. It could be that just a single phase motor geared correctly would be all you need!
Keith Cress
kcress - http://www.flaminsystems.com
RE: motor speed control
A Baldor, single phase motor;
Model L1304M
Hp 1/2, volts 115/230, 1725 RPM(nominal)
Percent load 25% 59% 75% 100%
Speed (rpm) 1780 1765 1750 1730
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: motor speed control
If you run between 25% load and 75% load, your regulation is 1780 RPM - 1750 RPM / 1780 = 1.69%
References,
http://
htt
http://www.baldor.com/pdf/literature/PR2525.pdf
If you need closer regulation, go to a 3/4 HP or 1 HP motor and use a smaller part of the load range.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: motor speed control
Keith Cress
kcress - http://www.flaminsystems.com
RE: motor speed control
I am in your debt. Thank you
Charlie
RE: motor speed control
You said +/-2% for the speed accuracy. That motor run over the 25%-100% load range is a 2.8% speed change so it could get you say +/-1.4%. At worse case running from 0%-100% load it should keep you within the +/-4% you specified.
RE: motor speed control
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: motor speed control
Cement Plant Electrician and
Instrumentation Technician
RE: motor speed control
http://w
Cement Plant Electrician and
Instrumentation Technician
RE: motor speed control
Learned something today. 115 V in 230 V out - that's a first for me.
See what you meant now, Smoked.
Can you have these in larger sizes?
Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: motor speed control
I think that type tops out about 1 or 1-1/2hp Skogs.
Keith Cress
kcress - http://www.flaminsystems.com
RE: motor speed control
http://
Hi Keith and Gunnar. I also thought that the 120 volts was a typo. Looking through some current VFD listings I found out that your info was right on. Thanks. It would be a good fit for equipment meant to run on a portable cable and a standard receptacle.
For a permanently wired installation above 1/2 HP I would probably prefer to go with a 208/240 volt input VFD so as to reduce the breaker size and conductor ampacity.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: motor speed control
Scroll to the second page - top three VFDs.
h
Almost obsoletes single phase motors all together.
Is a 3/4HP three phase motor $99 less than a 3/4HP single phase?
Keith Cress
kcress - http://www.flaminsystems.com
RE: motor speed control
At 1/4 HP grainger has about 25 single phase motors under $100.
Ignoring multi speed motors, wash down motors, brake motors and other special motors, there are are about 80 in the $100 to $200 bracket.
Most 3 ph, 1/4 HP motors are in the $100 to $200 bracket.
The prices for 3/4 hp ratchets up about $100 but there is still not that much difference between 1 ph and 3 ph.
BUT, I like the price on those drives. It won't take much of an excuse to add a VFD drive to a project at those prices.
Thanks for the info, Keith.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: motor speed control
"115 V in 230 V out", I think these have a voltage doubler on the input (2 diodes and a capacitor) to give 230 VDC at the bus. I used one recently to give me 3ph 400Hz for a tiny submersable well pump.
Regards
Roy
RE: motor speed control
Keith Cress
kcress - http://www.flaminsystems.com
RE: motor speed control
It was a Grundfos Redi-Flo2 pumping 7gpm @ 100ft head
2 HP @ 23,000 rpm (yes 23 thousand) The website is www.grundfoss.com
I have seen 400Hz power tools also, but not recently
Roy
RE: motor speed control
www.grundfos.com In the environmetal section. I used a Mitsubishi drive, not the Grundfos.
Roy
RE: motor speed control
Keith Cress
kcress - http://www.flaminsystems.com