Originally Posted by Danny Barber
A DC motor would benefit .. not sure why you would put one on an AC single phase motor ..
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A DC motor would benefit .. not sure why you would put one on an AC single phase motor ..
Contributor
As you drop the Hz in an induction motor (3 phase or single phase) the impedance of the motor decreases. If everything was held constant, you would get excess current through the motor. Because of that, the VFDs decrease the voltage into the motor so that the current is maintained at rated current (at full load). Below rated Hz, the motor maintains constant torque. Since HP is torque times RPMs times a constant, the HP of the motor falls as you slow it down (but the torque stays constant).
There's no difference between a 3 phase motor and a single phase motor in this regard.
So to answer your question, it's exactly the same as when you control a 3 phase motor with a VFD.
Mike
[The primary difference between a 3 phase motor and a single phase motor is how it starts. On a single phase motor, you create a second phase with the starting capacitor and starting coil, which gets the motor turning in one direction. Once it's running, the starting coil is usually (but not in all motors) removed from the circuit. There would be nothing wrong with leaving the starting coil in the circuit except that most motors are not designed for that.]
Last edited by Mike Henderson; 07-28-2016 at 12:54 AM.
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No. Assuming you can find one and the motor used matches the drive manufacturer's specification. And assuming of course, the user understands that specification and it's impact on their process, tools, and health.
Not at all.
Not at all. Assuming the user realizes that the process may create a speed condition that opens and closes the switch - potentially rapidly and repeatedly. And again, assuming the user understands the impact of this condition on the cap, switch, and drive.
Melting:: I believe in your linked thread someone (...you maybe?) mentioned touching the cap to make sure it wasn't hot. 'Hot' leads to 'melting'. Pardon my embellishment.
Welding:: See answer to #3. Every time a switch opens or closes under load there is some arcing. Enough load and they can 'weld'; enough times and they can carbonize and quit. I've no idea which might occur in this case, but I think it would be a shame if it happened on the first use. Embellishing again. My bad!
Admittedly, none of the above is a problem if you have enough money. In this specific case, I'd get a 3-phase motor, sleep at night, and be comfortable recommending it to others.
...Don't agree? Please ignore all above, build what meets your needs and budget, and let me know how it turns out. Never too old to learn; I might give it a whirl myself!
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I don't think I ever mentioned touching the capacitor to see if it was hot. I did mention touching the motor to see if it was getting hot.No. Assuming you can find one and the motor used matches the drive manufacturer's specification. And assuming of course, the user understands that specification and it's impact on their process, tools, and health.
Not at all.
Not at all. Assuming the user realizes that the process may create a speed condition that opens and closes the switch - potentially rapidly and repeatedly. And again, assuming the user understands the impact of this condition on the cap, switch, and drive.
Melting:: I believe in your linked thread someone (...you maybe?) mentioned touching the cap to make sure it wasn't hot. 'Hot' leads to 'melting'. Pardon my embellishment.
Welding:: See answer to #3. Every time a switch opens or closes under load there is some arcing. Enough load and they can 'weld'; enough times and they can carbonize and quit. I've no idea which might occur in this case, but I think it would be a shame if it happened on the first use. Embellishing again. My bad!
Admittedly, none of the above is a problem if you have enough money. In this specific case, I'd get a 3-phase motor, sleep at night, and be comfortable recommending it to others.
...Don't agree? Please ignore all above, build what meets your needs and budget, and let me know how it turns out. Never too old to learn; I might give it a whirl myself!
It's unlikely that the contacts on the centrifugal switch would "weld" because the current in them would be less than the current during the normal startup. That is, as the Hz are decreased, the impedance of the starting capacitor would increase and less current would flow through the contacts. As a guess, it would likely be somewhere around half the current through the starting coil on rated Hz startup (depending on the Hz where the centrifugal switch cut in.)
My experience with contacts which are operated within their rated current - and wear out - is that they open (no longer conduct) rather than welding shut. Welding shut (in my experience) only occurs when the contacts are operated at a much larger current than they are spec'ed for. In that case, the resistance of the contacts generates a lot of heat and the contact surfaces melt and stick together. That doesn't happen when the contacts are operated at half normal operating current, and normal operating current is usually significantly below maximum allowed current. I suspect many readers here have cleaned contacts that "opened" because of dirt and/or corrosion and put their equipment back in service. A lot more than found the contacts "welded" shut.
Yes, anyone who hooks up a VFD to an induction motor should know what they are doing and should know all the caveats you list. And many, many people have successfully installed and operated VFDs on induction motors. Running a single phase induction motor from a VFD is no more complex than running a 3 phase motor from a VFD, providing you always start it at rated Hz. If you can provide any specific issues or problems in operating a single phase motor from a VFD, other than the ones I listed, please do so. Most of what you said above is just standard jargon, applicable to anyone doing almost anything.
Mike
Last edited by Mike Henderson; 07-28-2016 at 2:01 AM.
Go into the world and do well. But more importantly, go into the world and do good.
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Please pardon my embellishments, my generalities, my standard jargon, and my lack of specific issues. I trust the OP will find joy and fulfillment with your setup.
I can only hope you'll forgive me if I stick with a cheap 3-phase motor on a VFD.
Contributor
In fact, there's a way to eliminate the issue of the centrifugal switch and the starting circuit completely. And that's to remove the starting circuit electrically from the system except when you're starting up the motor. This idea is based on a comment posted by Rod Sheridan in a separate thread.
Disconnect one of the wires connected to the starting capacitor - doesn't matter which one. Run that wire to a switch - either a momentary switch or a regular toggle switch. You'll have to splice wire to the existing wire to do this. Then run a wire from the switch back to the starting capacitor. If you use a toggle switch, label the switch in the "on" position as "Start" and in the other position as "Run".
But let's say you're using the momentary switch. When you go to start up the motor, press the switch and then turn on the power from the VFD at rated Hz. The starting capacitor will be in the circuit electrically and the motor will start normally. Once the motor is up to speed let go of the momentary switch.
As you slow down the motor, you'll reach to point where the centrifugal switch cuts in. But now the starting capacitor is out of the circuit so the closing of the centrifugal switch will not do anything electrically - no current will flow through the centrifugal switch and the starting circuit will not be engaged. You will be able to operate the single phase motor exactly the same as you would a three phase motor on a VFD.
The biggest problem I see with operating a single phase motor on a VFD is finding a VFD that will operate with only one phase of the output connected. If you can find a VFD that operates that way. I do not see any problems with using it on a single phase motor.
Mike
[This brings up the question of where would you use VFD control of a single phase motor in a woodworking shop. The tool I'd modify first is the drill press. Many drill presses change speed with belt changes, and because of the difficulty of doing that, many people just set the press to one speed and use it for all size bits. With VFD control of the motor, you could easily fine tune the RPMS for different size bits. And if you needed a big change in speed, you could change the belts and get a different speed range for fine tuning.]
Last edited by Mike Henderson; 07-28-2016 at 10:28 AM.
Go into the world and do well. But more importantly, go into the world and do good.
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That would certainly work. You could also automate that. The VFDs I've looked at all include a rudimentary scripting language and some logic level I/O ports. Change your switch to a relay connected to an output port and connect the centrifugal switch to an input. Write some code that starts the VFD at 60Hz and disconnects the capacitor when the centrifugal switch opens but won't reconnect without a restart. Or let the VFD monitor current and disconnect the capacitor at the appropriate level.
Beranek's Law:
It has been remarked that if one selects his own components, builds his own enclosure, and is convinced he has made a wise choice of design, then his own loudspeaker sounds better to him than does anyone else's loudspeaker. In this case, the frequency response of the loudspeaker seems to play only a minor part in forming a person's opinion.
L.L. Beranek, Acoustics (McGraw-Hill, New York, 1954), p.208.
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Some more generalities of 1-ph output units I found surfing at lunch:
Invertek.JPGBardak.JPG
Neither indicates available scripting language. Both have single DO (1 has no info on it in User Guide; other has limited parameter set to map to it). Generally, the DI is used to map the Start/Run/Stop functionality. Specifically, potential users might review the motor types spec'd. Hope this helps.
(I found and read thru User Guide for both.)
Last edited by Malcolm McLeod; 07-28-2016 at 2:26 PM.
Contributor
I've seen those also. Last time we discussed VFD control of single phase motors, I researched VFDs with single phase output and found that they only recommend using them on motors that do not have a centrifugal switch. I understand why they make such a recommendation. The discussion here - and in the previous thread - was to examine whether it would be possible to operate a single phase motor with a centrifugal switch from such a VFD. I believe if you do the "cut out switch" I discussed above that these VFDs will function on a regular capacitor start single phase motor with a centrifugal switch.
Let me discuss why in a bit more detail.
The problem with leaving the starting capacitor in the system (so that the starting coil will be energized when the centrifugal switch closes - when you slow down the motor) is that it causes a step change in the current into the motor when the switch "contacts". The VFD will likely see that as a load change on the motor and attempt to make changes to the input voltage and/or current.
Even if the VFD does not (say you used an inverter instead of a VFD), the extra current going into the motor will likely reduce the rotor slip, which will cause an increase in the RPMs. If you hit just the right spot, the motor can oscillate between two RPMs as the centrifugal switch cuts in and out.
By removing the starting circuit (electrically) from the system after you get the motor up to speed, the motor will function like the motors specified in the documentation. There will not be any step changes to the current as you slow the motor down.
Mike
Last edited by Mike Henderson; 07-28-2016 at 5:20 PM.
Go into the world and do well. But more importantly, go into the world and do good.
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Mike did you ever get a setup actually working in your shop?
Glad its my shop I am responsible for - I only have to make me happy.
Contributor
No, I got sidetracked with other projects. Maybe this fall I'll hook up my drill press.
[If I already had a setup running, I would have mentioned it as proof that the system works. I can't see why it won't, but the proof is in successful operation.]
Go into the world and do well. But more importantly, go into the world and do good.
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Cool bro. Be careful and good luck. Would be neat to see but not if you ruin a working motor or worse a new VFD. Would be a reason to just add a 3ph motor though.
I am still a fan of just going with a decent VFD (prices are pretty low on VFDs these days) and a purpose purchased 3ph motor - tons of used motors out there.
I never learned the OPs tool he wanted to power but a lot of DIY small cnc mill guys (G0704) go the 2h DC treadmill motor route.
Glad its my shop I am responsible for - I only have to make me happy.
Member
I just set up a VFD for the first time on a grain auger application. Used a 3hp Super E Baldor motor and slowed it down to 500 RPM. Cool stuff. Oh and I bought if from Wolf Automation, super guys to deal with!
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