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motor speed control 2

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charland

charland

Mechanical
Apr 12, 2006
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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 )

Thanks in advance

Charlie
 
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Wow a lot of effort for a tumbler.

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 -
 
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Thanks Keith,

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?
 
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No problem. 24/7/365

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 -
 
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Using a scalar drive VFD: wouldn't this be similar to picking an off the shelf, AC motor and just selecting the correct pulley ratios to get the speed I need? I can get a custom pulley machined if I need something between the commercial sizes.

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.
 
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Look at the speed/torque curves of some standard single phase induction motors. The speed regulation is quite good. A capacitor start motor may do the job for you. The speed will be close to 1750 (1725, 1740, 1750). The motor nameplate will give the rated speed. Consider using a variable pitch pulley to fine tune the speed.
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
 
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Just a few additional points,
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
 
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Man it drives me crazy when you post a response. Go back and look at it in the chain of the thread, then an hour later when you come back there is a new response ahead of your last one.

Gunnar I did say 120VAC. You need to dial back your
521ff3k.gif
's as they are moving too fast. [smile]


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 -
 
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Try this on for size;
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
 
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To continue after inadvertently hitting the Submit Post button!

If you run between 25% load and 75% load, your regulation is 1780 RPM - 1750 RPM / 1780 = 1.69%
References,
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
 
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Waross has the right answer. Just use a single phase motor and gear/pulley reduce the speed.

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.

 
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No need to use that specific motor. I chose it at random to demonstrate typical motor specs. I would find a motor first and then check the speed regulation. Good luck.

Bill
--------------------
"Why not the best?"
Jimmy Carter
 
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I just wired up a baldor 1/3 hp motor using a small baldor vfd that converted 120v up to 240v three phase. The drive is compact and does an excellent job of regulating the speed. The whole setup ran about $700. I can't remember the model number of the drive off hand, I think it was a 13J or something like that. The drive was capable of powering a 1hp motor.

Cement Plant Electrician and
Instrumentation Technician
 
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Aha!?

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
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
 
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As an example rather than a recommendation; here is a 1/4 HP baldor motor that should meet your requirements without the added expense of a VFD.
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
 
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