Reversible single phase motor question

[dave154304]

Hello,

I've been asked a question about a reversible single phase motor setup that I can't answer. I've attached a picture to help describe what I'm working with. In this system, we turn 120V onto wire 22231 for the motor to go one way, and 120V on wire 22241 for it to go the other way. Wire 10091 is a 120V common. The design has been in place for many years and historically, the duty cycle on this motor was very low - the motor might make an adjustment every day or two. If it reversed, it was human controlled, so there were seconds between CW and CCW commands. There were never any complaints about the operation of this system.

Now, a new requirement has been added and new software developed where we are making this motor more active. With this change, the motor can be making moves 2-3X (in different directions) in 10 seconds (i.e. go CW for 2 seconds, CCW for 6 seconds, and CW for 2 seconds). It is possible that the moves are close to each other, for instance one move could stop and 50 msec later the reverse move start. The two signals are never on at the same time. The question that was posed to me is that sometimes the software tells the motor to go CCW, but the motor actually turns CW. This doesn't happen often, but regularly enough that bugs in the software have been eliminated.

Can anybody explain the details of how the CAP and RES change this circuit to make the reverse happen? I'm not an expert in this system...it has been in place for 25+ years and just gets carried forward.

If the motor hasn't stopped moving in one direction before the reverse signal fires, could reversing it so quickly possible make it not reverse?

Would strange things potentially happen if the motor was being back driven a little when the signal to turn in a direction happened? I'm thinking as the motor is working like a generator that something strange is happening with the command voltage?

Are there any other theories about anything that could cause this?

Thanks in advance everybody.

 

[Ingenieur]

Think of it as a 2 phase motor
each switched line goes to a fwd/rev coil, the third is the common
the cap and resist cause a phase shift
when you put power to 1 coil input the other sees it also but with some degrees of phase offset
150 - j212 is 55 deg or so

are you sure it's not 240 vac line-line?

the motor is designed to stop and reverse instantly

 

[Jraef]

...(i.e. go CW for 2 seconds, CCW for 6 seconds, and CW for 2 seconds)...

How often does this cycle take place? About the best any standard motor can handle is about 40 starts per hour. 3 starts in 10 seconds with no run time in between for the motor to cool down is going to take a severe toll on this motor if it was not designed (i.e. de-rated) to handle that kind of duty cycle. And that fast of a reversing cycle with a capacitor start motor might be problematic because of the centrifugal switch. it could easily be that the switch is not even disengaging the start winding / cap before you tell it to reverse again.

If it were me, I would take this change as an opportunity to do swap it out with a 3 phase motor and use a properly selected VFD to run it from your single phase source and do the reversing electronically.

 

[Ingenieur]

It is a synchronous step motor
starting, locked rotor and running current are the same
instant start/stop/reverse
made for positioning and machine tool use

the cap is not for starting (well kind of)
When applying voltage to the coil, the other sees v with a ph shift, producing torque in one direction
do that to other and the ph delta is reversed as is the torque

 

[GoldDigger]

The symmetric setup of the forward and reverse windings make it unlikely that there is a start winding and a centrifugal switch.
Instead the motor keeps both windings energized, with a lower starting torque and less than ideal phase angle.
Starting from there, and the lower current associated with whichever winding is acting as the start winding, I would not be surprised if the main winding of the moment would not be able to keep the motor rotating in the same direction if it does not get below a specific speed before reverse power is applied.
I agree that a VFD might be a better solution, except that I am not sure a standard VFD could be programmed to work with this type of motor. So replacing the motor would be necessary.

As for the duty cycle, there will be no extra power dissipation from a start winding, but there will be higher current through the main winding until the motor gets up to speed. That could cause much higher than normal heating in this fast cycle use.

mobile

 

[Ingenieur]

The motor is rated for continuous stall
starting/lr/running current all the same
0-100 rated speed 5-25 ms depending on load
reversing time is the same
no vfd required

a common machine tool application
also precise positioning in mfg process

I'll find a cut sheet
page 5
http://mcsupplyco.com/uploads/images/drawings/pdf1/SYNCGUIDE.pdf

Virtually instant starting and stopping characteristics are among the principaladvantages of a SLO-SYN motor. Generally, the motor will start within 1-1/2 cyclesof the applied frequency and will stop within 5 mechanical degrees. Figure 7 showsa typical starting curve for a 72 rpm SLO-SYN motor. The motor will start and reachits full synchronous speed within 5 to 25 milliseconds. The unusually short stoppingdistance of a SLO-SYN motor is obtained by simply deenergizing the motor. Nomechanical or electrical braking is necessary. The quick stopping is the result of theslow rotor speed and the presence of a no-load reluctance torque produced by thepermanent magnet and the tooth construction of the stator and rotor.


Because of the nature of the permanent magnet inductor motor, there is no highinrush current when power is applied. The windings are excited by the alternatingcurrent, with no current being conducted through the rotor or through brushes.Because energization of the SLO-SYN motor merely involves energizing thewindings, the starting, running and stall currents are, for all practical purposes,identical. Therefore the engineer designing a system need not be concerned abouthigh inrush currents with the SLO-SYN motor. Consult the motor catalog for currentrequirements of the various SLO-SYN motor models

Because of the characteristics described in Section 2.6, a SLO-SYN motor does notdraw excessive current when the motor is stalled. Since the windings are merelybeing energized by the alternating current, it doesn’t matter whether the rotor is inmotion or at a standstill. Also, no detrimental overheating will take place. Therefore,if this motor were used in an application in which it was operating a remotelycontrolled valve, and the motor stalled, there would be no possibility of systemdamage due to overheating of the motor, etc. One precaution must be noted: in thisstalled condition, the motor will oscillate severely, eventually causing bearingfailure



It can probably start/stop/reverse in 5-6 cycles

 

[GoldDigger]

I am not convinced that the OP's motor is a SLO-SYN type, but it seems to be the right tool for the job, and the drawing does say "synchronous".
What explains the failure of the motor to reverse occasionally?

mobile

 

[Ingenieur]

I am not convinced that the OP's motor is a SLO-SYN type, but it seems to be the right tool for the job.

mobile

Judging from the application description
drawing note
RC network
wiring schematic

I would be surprised if not
then again, won't be the first, nor the last time

 

[GoldDigger]

...
It can probably start/stop/reverse in 5-6 cycles....

At 60Hz, 5-6 cycles would be about 83-100 milliseconds.
The OP states that the delay between direction contact closures could be as little as 50 msec.
Maybe a problem there, maybe not?
And probably dependent on the inertial characteristics of the load.

 

[Ingenieur]

At 60Hz, 5-6 cycles would be about 83-100 milliseconds.
The OP states that the delay between direction contact closures could be as little as 50 msec.
Maybe a problem there, maybe not?
And probably dependent on the inertial characteristics of the load.

He says it may stop, then reverse 50 ms later
that should be no problem
it stops in less than 20 ms (depending on speed)

I was conservative
starts moving in 1-1/2 cyc
full load 5-25 ms
if sized properly close to 10 ms or less
stopping 5 deg or less
assume a slow speed of 60 rpm or 2x15 ms (60-0 or avg 30 rpm) to stop in 5 deg (or less)
more like <30 ms start/stop/reverse
obviously they will need to tune it or possibly a larger motor
in the doc they spec techniques to make them respond even faster!
they do state it will randomly start in the wrong direction then reverse
the say it is inconsequential

how fast can a vfd start/stop/reverse an induction motor?
and position it with a few degrees

 

[gar]

170224-2046 EST

Way back in the 50s Superior Electric started making stepping motors, others did as well. SoloSyn was Superior's trademark for their motor. These were made both as a low voltage stepper with with transistor switching of of DC to the two coils. I believe the rotor has alternating permanent magnets. Thus it is an AC synchronous motor.

Typically 200 full step positions. Typically 4 wires, two different phases.

When used as a DC stepper driven motor current limiting is provided by resistors. Using higher resistance and high source voltage shortened the LR time constant allowing higher stepping speeds.

In the units designed for AC operation more turns were used on the stator coils increasing the operating voltage and designed for 120 V 60 Hz. Only three wires went to the two sets of coils. My memory was that for this AC sine wave motor mode the capacitor, without the series resistor, was connected between the two windings. This capacitor provided the phase shift to get two phases.

When winding A was directly driven the rotating magnetic field would go in one direction, and when winding B was directly driven the rotor would go in the opposite direction because the rotating magnetic field changed direction.

This kind of operation should not allow incorrect direction of rotation, but with poles so closely spaced is there some way it could get confused? I haven't tried to analyze if confusion is possible. I have worked with these motors and never had a problem.

In the application here I would suggest a short time delay be added between direction change.

.

 

[dave154304]

Thanks for all the good feedback everyone. I will try to offer some more clarification based on comments made.

Yes, this is a SLO-SYN motor.

I think Golddigger made the comment that if the motor wasn't stopped before reversing that strange things could possible happen. In another way to think about this, if the motor were being back driven (yes, I know you can lock it, but for sake of discussion, let's go with this), then it would be a generator. Would it be possible that the voltage generated by the motor could mess up the phase shift required to start the rotor turning in a specific direction on a commanded start? I'm not sure this is possible within the realm of physics, but it is my best guess for what could cause this.

I'm doing this as a test in a lab environment, so my perspective on this is more to learn what might be happening than to design the final solution. If this was machine in production, I would have a very different approach to this problem .

I appreciate the feedback and hope I can help others in the future.