2 phase VFD design for conventional split air conditioner

[aqeelkhan]

Hello,

I am a refrigeration engineer. I have studied a bit about the DC inverter air conditioner that promises a lot of energy saving. What I understood is that they require BLDC motor based compressors and dedicated electronic circuitry for operation.

I think the existing compressors in conventional split air conditioners can be used for variable speed operation as well. The motors in the convectional compressors are permanent split capacitor motor. The capacitor connected in series with the auxiliary winding creates phase difference in the voltages of main winding and auxiliary winding. This creates the rotating magnetic field and starts the motor.

My idea is that we run Permanent split capacitor motor without the capacitor. We build a 2 phase inverter that creates two voltages that have same frequency but different magnitude and phase (theoretically 90 degrees apart). The two voltages will be given to the main and auxiliary winding running the motor.

I want to know what are the theoretical limitation in implementing it? Also, what practical challenges may be encountered such as component rating, filtering requirement etc.

Regards,

 

[D.A.(Tony)Stewart]

Maybe this picture is worth a thousand words

 

[aqeelkhan]

Thanks Sunny Skyguy,

I tried to find out the details of the inverter you showed in the picture. It seems it is for 3 phase motors

**broken link removed**

I am looking for a single phase input, single phase (two unequal phases? according to my idea) output inverter.
Do you know something similar?

Regards,

Aqeel Khan

 

[chuckey]

You need two single phase VFDs, one for the main 220V, the other for the start/run winding, this will have to have a phase and amplitude that is the same as the 60 HZ value due to the capacitors and its phase and amplitude will have to be changed as the frequency changes.
Frank

 

[D.A.(Tony)Stewart]

Thanks Sunny Skyguy,

I tried to find out the details of the inverter you showed in the picture. It seems it is for 3 phase motors

**broken link removed**

I am looking for a single phase input, single phase (two unequal phases? according to my idea) output inverter.
Do you know something similar?

Regards,

Aqeel Khan

No
These 2HP rated VFD's are single phase
1.5KW 2HP 7A 220VAC SINGLE PHASE VARIABLE FREQUENCY DRIVE INVERTER VSD VFD

U Cant get 3 phase VFD's for US $105.00

 

[Warpspeed]

I think the existing compressors in conventional split air conditioners can be used for variable speed operation as well. The motors in the convectional compressors are permanent split capacitor motor.

Are you absolutely sure about that ?
There are two very different types of split phase motors.
The first type is the capacitor start and run type in which the second out of phase winding is always connected via a capacitor.
These have very limited starting torque and are usually only used to drive fans, blowers, and similar loads which can easily start and gently accelerate up to speed with loads that require minimal low rpm starting torque.

The second type is the capacitor start motor.
Here we have a massive start winding which is used to produce a starting torque often multiples of the full load rated running torque.

The start winding is disengaged with a centrifugal switch once the motor reaches full rated speed, which it will do in typically about one second under a full starting load.
These motors are used for hydraulic pumps, cranes, refrigeration compressors and similar applications, which require a huge initial starting torque.

A typical sealed refrigeration compressor will usually (?) be of the capacitor start type, and when running, only has a single winding connected. The start winding will quickly overheat and burn out if run continuously, its just not made to survive that.

You cannot soft start a refrigeration compressor, it will quickly pump up to full high side pressure and simply stall the motor. It has to hit hard to reach full rated speed very quickly, before the high side pressure can rise up high enough to stall it.

Induction motors just cannot generate high enough torque when run much below full rated speed with full mains frequency applied. So for refrigeration service you need a very violent quick start up.

You will find these motors rated for "starts per hour" because the start winding needs to cool back down. As soon as you try to speed control it, the start winding will kick in via the centrifugal switch, and the motor will not last long running like that, even in a refrigeration cooled sealed unit.

Three phase motors are ideal for speed control, they can deliver 100% of full torque right down to zero speed, provided the fan cooled motor is cooled externally by a continuously running air blower.
It should work in a sealed unit too, but I would be a bit cautions about that.
You can soft start it even for refrigeration service and it will work fine with a normal variable frequency drive.

That is the only practical way to do it. If what you are suggesting were possible, there would be a lot of single phase variable frequency drives on the market for all kinds of applications.

As it stands, split phase motors can be speed controlled, by phase control, (voltage only) but only if the load is of the fan or a blower type, where the torque requirement falls away very fast with speed reduction (inverse cubed law usually).

 

[schmitt trigger]

The real reason for the energy savings IS NOT the electronic drive but the BLDC motor, which is far more efficient than a Single Phase Induction motor.

Running a SPI motor at constant speed from an electronic drive with its associated power loses, not only is inefficient but costly because of the added drive.

 

[FvM]

I see some misunderstandings in previous posts.

Nearly all "single-phase" VFDs on the market are 1-phase-in/3-phase-out, used e.g. to drive 230 V three phase motors from a 230 V single phase power supply. The output is symmetrical 3*120 degree like regular 3-phase grid. I'm not aware of any device that can be switched to 2-phase/90 degree, although this would be possible in principle.

There are a few true single-phase inverters available like

Invertek claims that their E2 single phase device can drive permanent capacitor split phase and even shadow pole single phase motors with variable frequency.

Combining two VFDs to generate a two-phase voltage system would need phase-lock of both devices, which isn't supported by any simple VFD.

VFD compressor operation is standard with recent AC devices, using three phase motors, don't know if they are of the synchronous ("BLDC") or asynchronous type. I presume that the speed range and startup behaviour is adapted to the requirements of a refrigeration system. Most of the energy saving is achieved by operating the system continuously at reduced power. Reviewing thermodynamic theory clarifies why.

 

[schmitt trigger]

VFD compressor operation is standard with recent AC devices, using three phase motors, don't know if they are of the synchronous ("BLDC") or asynchronous type. I presume that the speed range and startup behaviour is adapted to the requirements of a refrigeration system. Most of the energy saving is achieved by operating the system continuously at reduced power. Reviewing thermodynamic theory clarifies why.

Units are manufactured employing standard 3 phase induction motors and BLDCs. Of course the BLDCs provide better efficiency, at an increased cost.

You are correct about the energy savings of running a compressor at lower speed vs. turning it on and off.
However, if the thermal load is such that the unit runs at full speed most of the time, it makes sense to invest a little more in a BLDC-equipped unit.

 

[cts_casemod]

There's a bit of confusion going on here

Most single phase air conditioner compressors have a bi-phase rotary compressor. This is a 2 phase motor, not a single phase!
The capacitor provides the second phase to the auxiliary winding, which is required for both running and starting. Should this phase be lost during operation the motor will stall.

Smaller refrigerator motors, namely the reciprocating type, are single phase. The additional starting winding creates the needed phase angle trough means of its own inductance, from the same supply as the main winding and it is only used for starting. These can be easily powered with a single phase VDF as long as the starting device is a magnetic relay and not a PTC.

I started and run many of these with a single phase output VDF's down to 15-20Hz and well into 80Hz.

The capacitor run rotary types are a bit more involved to run from a VFD. It has to be a bi-phase VDF and the amplitude (output voltage) for the second phase may be lower. The hardware is exactly the same as that of a 3 phase VFD, the only thing needed is a software change.

Microchip Application Note AN984 is a good starting point to clear things up.
https://ww1.microchip.com/downloads/en/AppNotes/AC%20Induction%20Motor%2000984a.pdf