driving a single phase capacitor start/run motor from three phase vfd or power grid.

So I have yet to make a serious attempt at determining the effectiveness of what I'm about to describe, but I think some here may find some interesting applications for it.

You are probably already familiar with the Scott T transform for three phase to two phase conversion.

Well single phase motors with a capacitor start or cap run are two phase motors.
If you have a motor with aluminium wire for the start coil but without capacitor then you may not have a good candidate for this application but there are exceptions.


From dozens of cursory experiments with small motors I have found significant energy efficiency improvements can be made to single phase cap start motors, even if they have an aluminium winding, by finding the optimal capacitor to permanently install across the start switch. Or in the case of a positive coefficient resistor used as a start switch (as most refrigeration compressors have), in many cases removing the ptc and installing a capacitor can provide sufficient starting torque. Or leave the ptc in circuit, usually its just a couple watts of wasted power.

Anyhow so usually you cannot run a single phase cap run motor from a VFD and get satisfactory low speed torque because the capacitor value is a compromise between full load power factor control, starting torque, and cost. But with a scott T transformer you can. The difficulty is finding the correct voltage ratio for the "teaser" transformer to power the start winding, and determining a safe amount of current that can be sent through it may take some careful measurements.

The second point I want to make is in the case of a dual voltage motor, 120/240, 220/440 motor, you do not need two transformers to drive the motor, you only need one.
You can simply use the motor itself as a center tapped transformer to connect the teaser transformer to power the start coil.--this detail may offer the most motivation for performing this science experiment....

Here is the usual circuit diagram:
http://electrical-engineering-portal...to-2-phase.gif

you can clearly see that any case where voltage isolation is not required between the two phase output and the three phase input, you can do away with one of the transformers (replacing it with an autotransformer)

To drive a single phase capacitor run motor directly from a VFD, you can connect the run winding directly to L1 and L2 of your vfd. (H1,H2,H3 in the diagram)
Find a transformer with an appropriate voltage ratio and connect the primary between L3 of your vfd and the midpoint of the motor's winding.* Connect the start winding of the motor to the output of your transformer. The capacitor is no longer needed.
I suggest additionally connecting a variable transformer to the output (or input) of the teaser transformer (Y1 and Y2 in the diagram) because you will need to experiment to find a safe operating point for the start winding.
Measuring the voltage across the start coil when run from the grid is a good place to start, but keep in mind it is often difficult to measure the output voltage of a VFD due to the square wave pwm output.

*this will only work on dual voltage motors configured for high voltage)
Some capacitor run motors have identical windings for both start and run coils, these motors are the best candidates for this operation, but they are unlikely to be dual voltage and will require two transformers for the conversion.

So the end result here is you will have a VFD powered "single" phase motor with the same starting and running torque you would expect from a three phase motor (provided the start winding is up to the task) and the motor will reverse direction just as any three phase motor will.

Yes I understand you can just go buy a three phase motor...

I understand that using the motor's windings as an autotransformer is also not optimal, however, the end result is that the current flowing through the "start" winding is now forced to be 90 degrees out of phase, and its voltage will be set by the transformer ratio and the VFD, it will not vary with the load, so you should see a net gain across most all operating conditions.
In most cases the power shoved through the "capacitor run" winding is less than 50% of the main winding, so you should not find a significant overload of the main winding. (I would expect total running losses to be reduced at nominal loading)

I forget the KVA requirement of an auto transformer for T1 in the diagram but it is pretty minimal...a 200 watt 120:120 isolation transformer connected as a centertapped auto transformer would be sufficient for a 1 hp 220 volt motor.
The teaser transformer will need to be rated for the full current and voltage flowing through the start windings.. so 300 watts perhaps for a 1 HP capacitor Run single phase motor.
Its voltage ratio will be somewhere in the range of 1:1.5 to 1:3.

Thanks for sharing this detail with us. It is looking a very helpful discussion for the single phase motor working. I want that you must describe me something about the start and run capacitor. I mean why they are necessary and if they both are necessary to connect? Also i want to know the working of the VFD?

Eldon_Joh

Nothing practical that you are doing, that has not already been done, life's too short to be messing with something like this with only marginal results, & a big chance of lots of smoke

Just buy the right drive for the job, there are lots of options