V/F Control Of Single Phase PSC Motors

[v1.5]

Hi,

Until last weekend i have been thinking about controlling single phase , permenant split capacitor based motors speed control for some projects. This motors are both a fan and a compressor. Although searching lots of times , i still have questions of unanswared or need to check from experts.
At the many forum pages , everyone was asking this question for fan application. As everyone described , for below 2000W of fan applications , dimmers are good choice. They are acting neary lineer (depending on load which is airflow) at speed control using dimmer as a voltage source but if the topic is compressor things are getting changes. As you know when compressor speed control done via dimmer , compressor rotor cannot spin at low voltage using dimmer because of the low torque value. At this situation , it has to be handled by V/F constant control method for constant torque value at low voltages and low frequeny especialy for soft start operation. Since the compressor require permenant split capacitor for auxilary winding , i have some worries about changing frequency value of it.
As you know , at low frequency , capacitor will blockade the current flowing thurough to aux.winding and will do vice verse when frequency increase to nominal frequency value which is 50Hz in my location. Such i was thinking to increase motor power with increase its frequency (2kW@50Hz but 2.2kW@60Hz) for some cost saving application. The drive will be an single phase unipolar inverter for such purpose. Drive will control V/F value at constant when speed of rotor needs to be change.
Changing frequency of PSC motor is critical , must not exceed the frequency limit before capacitive reactans get too low. This will cause excessive current flow to aux.winding. So when increasing its frequency 20-50Hz to someting 70Hz , how do i know if aux winding is ok or not ? Are my thoughs are ok or do you have any idea for this ? I really need some advice before dive into it .

Thank you.

 

[MaxHeadRoom78]

Many PSC motors under 1/2hp have windings which are equal to each other, making it easy to reverse using a SPDT SW.
These motors can be rpm controlled but usually only for light duty such as fans etc, otherwise they can easily drop out of run under load.
This is also why the typical VFD for 1ph motors never really succeeded due to this.
.

 

[rjenkinsgb]

IF the motor has two identical windings, you could hypothetically use a three phase output VFD and leave out the capacitor; possibly with a ballast resistor in series with the second winding instead.

That would give matched power to both windings & though the phase angle between the two is not perfect, it should give the required rotating field?

 

[v1.5]

IF the motor has two identical windings, you could hypothetically use a three phase output VFD and leave out the capacitor; possibly with a ballast resistor in series with the second winding instead.

That would give matched power to both windings & though the phase angle between the two is not perfect, it should give the required rotating field?

Two windings are not identical clearly as manufacturer datasheet says. Different winding resistance (4ohms and 2.1ohms) has been specified in technical datasheet of the compressor.
Still i have thought that trick for PSC motors but at the 3Ph VFD output will have 120 degree of phase shift so it will reduce the torque at low speeds. ABB has some special product that can drive PSC motors even it is a 3PH output device. This has been done via changing some parameter on the panel and that changes phase difference between main and aux windings typicaly to 90 degree.
Balast resistor is a good idea if i could find any . It can protect the aux winding even at out of the frequency range. But still capacitor needs for phase shift when driven via an v/f inverter. What do you think ?

 

[v1.5]

Many PSC motors under 1/2hp have windings which are equal to each other, making it easy to reverse using a SPDT SW.
These motors can be rpm controlled but usually only for light duty such as fans etc, otherwise they can easily drop out of run under load.
This is also why the typical VFD for 1ph motors never really succeeded due to this.
.

Unfortunatly two windigs has different resistance described at the datasheet. If you say so , yes , it would be great if such a case possible.
Today i have tried to control speed of compressor using dimmer as a voltage source. As expected before , at no load condition compressor speed change cannot even understandable and noticable. But at the load condition it only active at greater than 150Vac at 50Hz. Also at this trigger point , motor current reaches 4 even 5 times greater than the nominal current. I can cleary say that this cannot work on the field , i mean not for so long.
At this situation , V/F controll needed for such a speed control. Mostly i am woring about the aux winding because of at higher frequency (70Hz) because of the capacitive reactanse but after some calculation and some reverse engineering things , correct choicen capacitor will fit to my goal .

Any ideas or experience ?

Thanks.

 

[MaxHeadRoom78]

What is the reason for requiring RPM control of a compressor?

 

[Diver300]

VFDs are designed for equal current on each phase, but that doesn't mean that they won't work with unequal currents on each phase.
I have seen three-phase motors run from single phase with a capacitor. I guess that they were run at less than full power.
I would expect a single phase motor to work fine from a three phase VFD, with the start winding taking a bit less current than the main winding. The voltage on the start winding will be far better controlled than using a capacitor from a variable frequency supply.

 

[rguttenb_newb]

Having recently been experimenting with this on a PSC HVAC compressor, I can offer some input. First, if you are looking to optimize the power conversion efficiency, not only will you not change the capacitor size with frequency, but you will leave it in the circuit and power the motor with only 2 phases of your 3-phase output VFD. Obviously, this requires the ability to deactivate the phase loss detection fault of the drive.

As you reduce the frequency from baseline, as you'd expect, you'll see the auxiliary winding current drop. Due to the loss in torque generated by the auxiliary winding, you will see a bump in the current on the main winding most likely the result of a slight drop in RPM. For my motor, going from 60 to 50 Hz resulted in a 10% increase in main winding current with almost a 40% drop in auxiliary winding current. You can increase the capacitor size to increase the auxiliary winding current, but you don't see a corresponding drop in main winding current, I suspect because the resulting phase angle is incorrect. You just end up just putting more heat into the motor and drawing more power overall (and I'm talking watts, not VA), so it's really not worthwhile. Going the other way, you see the opposite trend.

If you kept the motor within roughly 10hz of its base frequency and or never loaded it near its full capacity (according to torque, not power), the change in winding current is not likely to be enough to matter. However, down to 20hz, you are likely to have some issue with heat generation in the main winding without some additional cooling. And I'm not referring to the effect of a slowly spinning cooling fan. At 20hz, based on my testing, at rated load, the main winding current would be well above its rating.

Your best bet would be to run the motor at base frequency and the highest load you can get, ideally its rated load according to its nameplate. Under these conditions, measure the current in each of the windings. Then, when you run the motor at different speeds, you can get an idea of what the relative currents are. If you enter operating windows wherein a winding's rated current based on your previous measurement is exceeded, you can either reduce the duty cycle or figure out a way to add some cooling. Keep in mind, any cooling you add must be in addition to any cooling the motor was expected to have at rated speed.

I am editing the above because it dawned on me that the test I originally ran was flawed and the discrepancy between the results and what I thought they should have been bothered me until I figured out what I did wrong. Basically, if you reduce your drive frequency, you'll want to increase the capacitance by the square of the difference in frequency assuming you are using a drive that maintains a linear V/Hz relationship. If you were operating over a small enough frequency range and needed maximum torque, you would size the capacitor for the highest frequency and hope you still had enough torque to operate at the lowest frequency. If the maximum rated torque of the motor isn't needed, the solution in the next paragraph is a viable option depending on the characteristics of the auxiliary winding.

Along the lines of not worrying about energy efficiency or requiring peak torque, it is also an option to remove the capacitor altogether and wire the motor to the VFD as though it were a 3-phase motor. As long as the motor doesn't have to start against a large load, it'll run just fine. I didn't document winding currents in this configuration, but I would expect the auxiliary winding current to be very low since the voltage across said winding when the capacitor is in play is well above line voltage. This would come with a corresponding bump in main winding current. It's also worth noting that depending on the electrical characteristics of the auxiliary winding (and in your case, the 2:1 resistance for the aux versus main windings is about the same as the motor I tested), the motor can be reversed via the VFD when wired in this configuration.