PSC motor with single phase output VFD
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    Default PSC motor with single phase output VFD

    Hi everybody, (and excuse me for my English)

    First timer, I recently got a single phase output (so said the seller) VFD in order to upgrade (a bit) my big wood lathe whose motor is a 1.8 Hp PSC motor.
    I have been playing around with that, and I would like to share with you my experience.

    First option offered is to simply connect two VFD terminal with motor.
    Result:
    1. Many vibration out of the range in between 40-80 hz
    2. Below 20 Hz it stop itself do no torque available
    3. Reverse from VFD is not possible; it will stop and start again in the same way.

    Second option offered is to remove capacitor and supply its winding with a second Phase supplied from the third output terminal. So you are working with a true 2 Phase motor

    A bit better!
    1. Few vibration from 10 to 100 Hz
    2. Not so bad torque between 30-75 Hz
    3. But anyway the slippage under load is too big to be honest.

    Outstanding (a least for me)! I discover that this VFD can be set to give a 3 Phase output also!
    So I tested with a little smaller 3 Phase motor and I discovered, to say that, the warm water!
    As everybody says in forums you can’t use a 1 (or even 2) Phase motor with VFD and get good performance, not too far from 3 Phases option.
    Everybody say that, but nobody apparently knows why, or at least they did’t say!
    Ok, if you keep the capacitor and change frequency you will lose the correct angle between the 2 Phases and with so, performance.
    But, if you remove capacitor and you supply the motor with 2 Phases at 90° straight from the VFD which one are the deep technical reasons you lose so much torque, even at standard frequency, and you get so much slippage compared with connection straight from plug?
    Are anybody able to give me an explanation?

    Thank a lot and Best regards
    Osvaldo

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    The reason you dont get the torque you need is because the second phase of the motor is almost certainly wound for a higher voltage than the line.

    For example i have a 1/4 hp (0.2kw) 120vac motor with a 10uf run capacitor. At no load there is 250vac across the capacitor and across the aux winding. This ratio is not set in stone and is different for each motor.

    So i would need a transformer between the vfd and the winding after removing the cap.

    Also you cant get a 90 degree phase relationship from a 3p vfd so you need a scott t transformer to drive the motor properly, this is the second reason your motor didnt produce as much torque

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    The biggest immediate problem with standard single phase motors on VFD is the stat switch, that re-engages if there is much slow down. That is not an issue with a PSC motor.

    The PSC can be slowed down, but the problem is that the capacitor winding loses current as BOTH the voltage and frequency decrease, as is normal with a V/Hz based VFD. So torque decreases more rapidly than expected, because the frequency change reduces the current through the capacitor on top of the reduction due to the voltage change. The VFD coes not control the voltage on the auxiliary winding, the capacitor does. However, for many loads it works.

    A voltage reduction can do more , as long as there is a load that changes with speed such as a fan. The PSC motor is still an induction motor and has a natural no-load speed determined by frequency. So reducing voltage only slows the motor by increasing the slip under load. At no load it would still go back to a near-normal speed.

    You need to do the frequency change also in order to get a true speed reduction, and then you lose torque.

    Reversing is "mechanical" in the PSC motor, the direction is set by the placement of windings. Often there are two identical windings that are simply swapped to reverse the motor, which changes thephase sequence of the windings so that the motor reverses.

    It would be possible to use a 3 phase inverter to operate the PSC, but the 3rd output would have to be changed in phase, from 120 deg to something close to 90 deg. The PSC is a form of 2 phase motor internally, with the added phase created by the capacitor, which cannot give more than a 90 deg phase change by itself. One could also do something with the capacitor winding such as not changing voltage as much, with the capacitor still in place.

    Finally, the PSC must be started at full voltage and frequency, or something reasonably close. Single phase VFDs therefore do that, slowing to the set speed after the motor is started..

    The 3 phase VFD sets the rotation by the electrical order of phases in time, so it can run a 3 phase motor either direction. the motor windings are all set 120 deg apart. Also, because the VFD is in control of voltage, phase sequence and frequency, there is torque at all voltages. Torque is due to the phase sequence being controlled directly and providing a rotating magnetic field, not by a "trick" using a capacitor to create another phase..

    Quote Originally Posted by johansen View Post
    The reason you dont get the torque you need is because the second phase of the motor is almost certainly wound for a higher voltage than the line.

    For example i have a 1/4 hp (0.2kw) 120vac motor with a 10uf run capacitor. At no load there is 250vac across the capacitor and across the aux winding. This ratio is not set in stone and is different for each motor.

    So i would need a transformer between the vfd and the winding after removing the cap.

    Also you cant get a 90 degree phase relationship from a 3p vfd so you need a scott t transformer to drive the motor properly, this is the second reason your motor didnt produce as much torque
    Many reversible PSC motors have identical windings for run and aux, which are swapped, with run becoming aux, and vice-versa. The capacitor is chosen to act at 60 (50) Hz, and will not work correctly at another frequency with that winding.

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    If you had a VFD with a "3rd output terminal", you did not have a true single phase output VFD, you had a 3 phase output VFD and someone told you it would work on a single phase PSC motor. It would, but not well as you found out. The phase angles are all wrong for that motor.

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    Quote Originally Posted by Jraef View Post
    If you had a VFD with a "3rd output terminal", you did not have a true single phase output VFD, you had a 3 phase output VFD and someone told you it would work on a single phase PSC motor. It would, but not well as you found out. The phase angles are all wrong for that motor.
    I suppose you are prety right.
    The seller sell it as a single fase output VFD, but it's probally not.
    We are talking about 2.jpg, a V/Hz type.
    From the software manual at point P92 you can select (0), called three phase or (2), called three single phase line.
    From the seller the VFD at P92 was set at (1) which does not exist in the manual; anyway at (1) it act pretty the same as at (2).
    Set at (1) or (2) i was thinking i was working with neutral and one phase with the option of a second phase oriented at 90° to be used removing capacitor, but i am now convinced it's not like this.

    Thanks

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    Quote Originally Posted by johansen View Post
    The reason you dont get the torque you need is because the second phase of the motor is almost certainly wound for a higher voltage than the line.

    For example i have a 1/4 hp (0.2kw) 120vac motor with a 10uf run capacitor. At no load there is 250vac across the capacitor and across the aux winding. This ratio is not set in stone and is different for each motor.

    So i would need a transformer between the vfd and the winding after removing the cap.

    Also you cant get a 90 degree phase relationship from a 3p vfd so you need a scott t transformer to drive the motor properly, this is the second reason your motor didnt produce as much torque
    I confirm you that, even of the motor is a reversible type, windings are different.
    The one which was connected in series with capacitor is 0.8 Ohm and the other 0.2 Ohm.
    Strangely the motor works better using the 0.8 one as main winding.

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    Hi JST,

    I hoped it was like that 'Often there are two identical windings that are simply swapped to reverse the motor, which changes the phase sequence of the windings so that the motor reverses', but instead I can confirm you that windings are different.
    The one which was connected in series with capacitor is 0.8 Ohm and the other 0.2 Ohm.
    Strangely the motor works better using the 0.8 one as main winding.

    One could also do something with the capacitor winding such as not changing voltage as much, with the capacitor still in place.

    Playing with low frequency boost, but without capacitor, I got a not so bad torque, but noise and vibration was very big.

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    Quote Originally Posted by Jraef View Post
    If you had a VFD with a "3rd output terminal", you did not have a true single phase output VFD, you had a 3 phase output VFD and someone told you it would work on a single phase PSC motor. It would, but not well as you found out. The phase angles are all wrong for that motor.
    Hi Jraef,

    I suppose you are pretty right.
    The seller declare 1 phase input/1 phase output but I believe it's something else.
    We are talking about this:2.jpg
    I saw the scheme and I thought at the output as a neutral/phase with an optional second phase oriented 90° in order to remove capacitor and avoid the related problem with variable frequency use, but must be something else.
    Should a PSC with such a provision work as well as with a VDF/three-phase motor?
    From the software manual at point P92 you should choose to set 0 (called 3 phase) or 2 (called 3 single phases).
    From the seller the VDF come set to 1! which does not exist in the manual!
    To 1 the system perform as it will with 2, means in the way i already described.
    Setting at 0 (3 phase) is running, you got plenty torque, but unbelievable noise and vibration.
    In conclusion, am I wrong if I say that the usual poor performance using PSC motor with VFD are not from the motor itself, but from the fact that available VFD are inadequate?

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    Hi Jraef,

    I suppose you are pretty right.
    The seller declare 1 phase input/1 phase output but I believe it's something else.
    I saw the scheme and I thought at the output as a neutral + phase with an optional second phase oriented 90° in order to remove capacitor and avoid the related problem with variable frequency use, but must be something else.
    Should a PSC with such a provision work as well as with a VDF/three-phase motor?
    From the software manual at point P92 you should choose to set 0 (called 3 phase) or 2 (called 3 single phases).
    From the seller the VDF come set to 1! which does not exist in the manual!
    To 1 the system perform as it will with 2, means in the way i already described.
    Setting at 0 (3 phase) is running, you got plenty torque, but unbelievable noise and vibration.
    In conclusion, am I wrong if I say that the usual poor performance using PSC motor with VFD are not from the motor itself, but from the fact that available VFD are inadequate?

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    If the motor is a dual voltage motor 120/240, you can connect the aux winding from the midpoint of the run coils like so: |-

    The motor's main run coils are then utilized as half of the scott-t transformer.


    you need to verify the auxiliary winding is up for the task. the ratio of 4:1 for the resistance ratio is correct if the coils are of equal volume of copper but the aux winding is twice as many turns of smaller gauge wire. but frequently it isn't equal volume of copper and the wire is significantly smaller cross section so it can't handle the amps, and has less surface area available to dump the heat into the stator core.

    Quote Originally Posted by osvaldo1961 View Post
    In conclusion, am I wrong if I say that the usual poor performance using PSC motor with VFD are not from the motor itself, but from the fact that available VFD are inadequate?
    both the vfd and the motor are not compatable.

    the motor needs to be driven with say, 240vac on one phase, and somewhere between 240 and 480 volts at 90 degree phase shift on the other phase.

    the vfd is delivering 240vac to both phases, with a 120degree phase shift. if the aux winding is of enough turns.. it may be pushing power back into the vfd, rather than contributing torque to the shaft.

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    Hi johansen,

    tanks a lot for your explanation.

    Osvaldo


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