VFD with small synchronous AC motor 5

[cokeguy]

I need to run a small 120VAC/60Hz/72rpm single phase synchronous motor at one third or one quarter of its nominal speed. The obvious solution that comes to my mind is using a 220 VAC VFD (one of its output phases and neutral) to run the motor, and adjust output voltage and frequency accordingly, lets say, to 40 VAC at 20 Hz. I don´t have much choices, since I can't change gearing and cannot get a slower motor. My question is, can I do it with no ill-effects other than reduced torque?

The motor is a slo-syn SS241-1003 that is part of a Honeywell Herculine series actuator. The motor is located in a tricky location, so my main concern is not damaging the motor. Thanks for your comments.

 

[jraef]

In a nutshell, no. You cannot do the "one phase and a neutral" with a 3 phase VFD, at least not for more than a second or so before the magic smoke comes out. There are a few single phase VFDs, but you would have to check with those manufacturers to see the appropriateness of using them on PM Synchronous motors. VFDs are optimized to work on circuits with the magnetic and inductive characteristics of induction motors. Synchronous motors are different.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[Skogsgurra]

You can put a transformer between VFD and motor to solve the neutral/phase problem. Some VFDs can tolerate unsymmetrical loads.

Slo-Syn motors are stepper motors and not at all like standard induction motors. There are special drives to use with them.

They are probably discontinued, but occasionally available on eBay. Or produced by someone else than the original manufacturer.

How was it run before? Isn't it possible to sdjust the frequency setpoint of existing drive?

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[waross]

Your motor is a three phase motor. An external RC package is used to provide a phase shift for one winding when running on single phase. The SS-241 is a 120 Volt model. The SS-242 is the 208/230 Volt model of the same motor.
Change your motor for the model SS-242.
The Bronco controller from Seco is specifically recommended to drive the SS-242

http://www.electrosales.com/warner/pdf/seco/Catalog_BrnAcNew.pdf

If you prefer a mechanical solution, gear boxes are available for the SS-241.
These are available with a range of gearboxes including 3:1, 4:1, 5:1, 9:1.

http://www.inoteh.si/downloads/SynchronousMotors_181206_ENG.pdf

Page 20.
Yahoooo

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[cokeguy]

Thanks for your answers. The motor is currently run directly from a fixed 110VAC source, which we switch to the appropriate motor terminal depending on whether we need CW or CCW movement. No speed controller used.

What I need right now is to test the concept of slowing down the motor to see if it helps with a particular process problem we are dealing with related to the actuator´s speed. If it works and an optimum speed is determined, then I'll try too find a suitable slower motor to run the actuators or even change the actuators altogether.

In the meantime, I'll check my available VFDs (Powerflex 4s and Saftronics GP10s is what I use) to see if they can handle unbalanced loads (anybody out there knows beforehand?), and use a transformer to step down and isolate the output voltage. Now, assuming the test works out fine, do you think it will be OK to leave the VFD running the motor for a few weeks or months while we change the motors? My concern would be if those types of motors "like" the switched/reduced voltage from a VFD, although I guess the output transformer should somewhat smooth out the switched output voltage. The duty cycle of the motor is about 5 seconds CW, 5 seconds CCW, 5 seconds OFF, 24/7. The actuator is in a very tricky and hot place, and it is not nice to change it in the night shift for example , that's why I´m concerned about reliabilty.

Thanks again in advance for your comments.

 

[cokeguy]

I forgot, regarding waross comment, the actuator indeed includes an RC package for the single phase SS241 motor. Any concern with this RC package if I run the motor at reduced voltage/fequency? I´d guess not, but just want to be sure also. Thanks again.

 

[Skogsgurra]

Yes, the impedance of the C part will be three times higher at 20 Hz. So you may need to make the capacitance three times higher to get the same running conditions.

(So, they are not stepper motors? They usually are)

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[cokeguy]

Skogs, I also imagined it was a stepper when I first saw it and only "SLO-SYN" was visible, but it is indeed a AC motor (PSC I suppose, from what I've leaned so far about single phase motors; normally I only deal with 3-phase motors).

JRAEF, I saw one of your posts in a similar disussion where you mention that none of the current line of AB 3-phase drives can run a single phase motor. There is no way then to switch off the phase unbalance protection on those drives? If that is the case I'll try first a Saftronics GP10 drive, is it possible on those? Have't measured it but motor current draw should be pretty small, so maybe I could get away with it?

Now I am also concerned about Skogs comment, since cap reactance at 20 Hz will be 3 times higher I don't what it will affect most, starting or running torque (or non of the above hopefully?)

If I really need to change the cap to make the intended test it complicates things quite a bit, because then I'd have to take out the actuator and change the cap, if I only add the VFD I can do it all externally and pretty easily with minimal process disruption.

thanks again guys

 

[waross]

Read my lips.
The manufacturer describes this as a three phase motor.
The RC network is added to run it on single phase.
This motor only uses two windings on three phase.
The currents will be unbalanced.
The manufacturer builds suitable gear boxes.
You should disconnect the capacitor for three phase operation, either by VFD or across the line.
Mistakenly describing a three phase motor as a single phase motor and then trying to solve the problems of running a single phase motor on a three phase source can be confusing.
220763-004 may be a suitable reduction gear.
CLICK the LINKS.
RTFM

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[cokeguy]

Thanks again waross, I guess I wasn't very clear, but I actually intend to run it single phase for the test. For 3 phase I would require a 3-phase 440:110 or 220:110 transformer or a 110 VAC drive, since I wouldn't feed direct 220 VAC PWM or higher to the little motor, cause even if I program the proper volts/hz curve the switched voltage spikes would still be there.

Also, for 3-phase operation I would have to remove the actuator from the process to access and remove the capacitor, which is something I definitely need to avoid at this stage.

So, back to the question, do you think it will be OK to run it as it is at 20Hz, assuming the VFD doesn't trip for unbalanced load. What would the actual effect on motor performance if I don't change the cap?

Thanks again everyone.

 

[Mobius44]

I know you're interested in using a VFD, but another method to effectively slow down this type of motor is to slow down it's control signals.

For an example see the FFP interface board from Advanced Control Tech:

http://www.act-solutions.com/ICC/pdfs/ffp/ffp_spec.pdf

Good Luck,
Sean

 

[LionelHutz]

I doubt you will find a 3-phase VFD that will run with a single phase load.

Even if you do find a suitable 1-phase VFD, that RC phase shifting circuit will most likely just cause you trouble.

So, remove the RC circuit and run the motor on 3-phase. It should not be hard to find a transformer or even 3 x 1-phase transformers wired correctly.

 

[Skogsgurra]

Mobius

If we forget the 'interesting' use of Floating Point, how would that device work with an induction motor?

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[waross]

If you want to use it in place, build a switching circuit that will cycle on about 15 cycles out of 60.
The motor is capable of being stalled so the duty cycle should not be a problem.
There is a VFD recommended for the 230 Volt version of this motor. If you go with a VFD how about buying the proper VFD and using 480:240V transformers at 50% voltage to drop the VFD output down to 120v. The transformers should isolate the motor from the voltage spikes, but if the 120V model is wound with the same insulation as the 240V model, PWM should not be a problem.

To anyone interested, this is not a stepper motor. This is a three phase synchronous motor using multiple permanent magnets in the rotor. The fields run on 60Hz AC. The motor uses an RC network to create a phase shift for single phase applications. Sort of a miniature rotary phase converter.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[cokeguy]

Thans again everybody. I actually made some tests today on a spare actuator, it ran Ok down to about 25 hz when it started vibrating and stalling (but with no load). I´ll try it with another motor later on, to discard possible motor damage as a cause of the vibrations.

Waross, about your suggestion of varying duty cycle as an alternative, I discarded initially because I'm actually monitoring actuator position via slidewire feedback and need the movement to be pretty smooth rather than step like, but what if I go to the extreme of switching maybe 3 cycles out of 12, for example, to achieve a smoother movement. Do you think I would get more torque at low speed that way than with a VFD? Right now I´m concerned that maybe I won´t get enough torque with the VFD solution, so I may need alternatives.

 

[waross]

Quick and dirty, try adding two more RC networks in parallel. You can do that on the external leads.
The torque should be good with a VFD.
The Bronco VFD is recommended for this motor. With a two winding, three phase motor, unbalance will be inherent. The safe way is to go with the recommended VFD. If you have a VFD on hand, try it and see. Feeding a small delta/delta transformer may help with the balance, isolate the motor somewhat from the PWM pulses and get you 120V from a 240V VFD.
There is a gearbox option if your actuator has room for the extra length.
Duty cycle at 3:12, try it and let us know.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Skogsgurra]

Dear. With relays!? And inductive load? I wouldn't even think of it. From the data sheet:
Rating 2 amp maximum resistive @ 24V
Electrical Life 100,000 operations

Two amps resistive at 24 V. Nothing said about inductive load and higher voltage. Not to mention the 'bumpy ride' that kind of control will produce. Also, the switching will probably not be synchronized with the grid cycles. No way I would try such a thing.

If the device is to be used to reduce number of cycles from 60 to 20 per second, it will switch on/off 20 times per second. That is rather tough. And life at 24 V resistive will be around a little more than 1 hour. If not loaded at all (mechanical life is 1 million cycles) it will be just under 14 hours.

Isn't this thread getting rather impractical? The main reason for all this seems to be a problem to get acess to the motor and its terminals. How difficult is it anyhow? Most machines I see need a few covers to be removed - at most. Are these covers welded in place?

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[waross]

Hi Gunnar. I was thinking possibly a pair of timers and SCR switching. A zero crossing circuit would be nice.
But I agree with you.
There are both VFDs specifically approved for this motor and gear reduction options available for a mechanical solution.
"Fix the motor without touching the motor" is becoming tedious.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[cokeguy]

thanks skogs and waross, I know the thread is becoming pretty tedious but very helpful for my cause. I'll work on it and let you know, thanks everyone. And it is really helpful for me not to touch the motor because it implies 2 or 3 hours of assembly/disassembly and (more important) of manually controlling an important part of the process, so anything that helps me not touching the motor is very helpful. thanks again...

 

[waross]

This is way out in left field and kludgy but it got me out of a jam once.
I had a proprietary analog computer that wasn't doing its job. I suspect that it never did it's job. Actually, times four, there were four identical lines.
The computer controlled motor operated valves. The valve position was fed back to the computer. The actuator motors may have been similar to yours but memory fails.
We dumped the computers and went with a pair of economy model PID controllers. The PID controllers had no trouble monitoring the process and outputting control signals but the problem was interfacing with the existing valves.
I put the PID controllers on time proportional control. I set the on-time to open the valves about 5% or 10%. When we were below setpoint, each pulse would progressively open the valve. Eventually the valve would be 100% open. When the PV went a few percent above set point, an alarm contact would drive the valve to the off position. When the PV dropped, the PID would slowly start nudging the valve open. The system was more stable than it sounds. There was a long response time to the process and the valve would hang in a close to optimum position for quite awhile. Then it would eventually go above set point and be driven closed. It would then start opening by increments and again hang at close to optimum position for awhile.
You may be able to do something similar and not open the valve all the way and so slow the cycling down and get closer control at the same time.
To the control gurus;
If I came across such a system without knowing the story behind it, my scornful reaction would be about the same as yours but here's the back story.
I intended to install control valves that were compatible with the PID controllers.
With the new system the batch time which had been running over 28 days was reduced to 7 or 8 days. We thought we could get down to 4 or 5 days but the owner declined.We had more than enough throughput and a bottleneck had been eliminated. The owner did not want to take a chance on degrading product by pushing it too fast and we were producing enough product by far.
The new controls were so successful that a $50,000 plus expansion (on which the majority of the money had been spent) was canceled permanently.
The owner was offered %2000 each for the 4 old computers if he would trade them in on the new improved version computers. He declined to consider any more computer controls. (The new controls packages had probably cost about half that.)
I tried to buy proper control valves so I could use normal proportional control. The owner declined. We were getting all the good quality product that we needed. Why spend more money. The temporary kludge had worked well enough that it became permanent.



Bill
--------------------
"Why not the best?"
Jimmy Carter