Help Needed Phase Balancing a Rotary Converter

Hello all...

Recently built a 4kW rotary 3 phase converter of the transformerless design for use on the domestic UK supply of 230v single phase. The unit works/starts and runs unloaded and will start another unloaded 2.2kW motor and remain reasonably in phase balance. The problems arise when the 2.2kW motor is loaded on start via connection to an air compressor. The motor starts but takes around 10s to build up to speed and when operating at full speed the phases become somewhat unbalanced.

Thought of detecting the low voltages in relation to the incoming supply of 230v and powering a donkey motor connected to the idler motor for a short duration to help bring the rotary converter idler motor back up to speed then disconnecting the donkey motor.

What are your thoughts?

Problem is the amp draw on the compressor on initial startup. Rule of thumb is 2 1/2 times the motor for the startup of a compressor, the hardest thing to get going. So for 10HP compressor, need a 25HP roto phase, Balanced or not.

Thanks to OffRoadXX for your comments but really need an an automatic way of electrically balancing the phases so as not to cause excessive amp draw on each phase. The mechanical way suggested is not really practical and that just leaves the process of switching in and out additional capacitors. Has anyone a circuit for doing this? Would be very please to hear about such a circuit!

Plus a loaded 3 phase motor can draw up to 5 times is rated full load current on startup. A single phase motor can draw up to 10 times its rated full load current at startup.

Any other way of balancing 3 phases on a rotary converter would be welcomed. Any ideas?

Kindest regards, Tayloned

I am no expert, just have a few ideas that have worked for me...

Any change in current draw will effect the phase balances. Your converter should be balanced under the normal operating load of the compressor.

For starting a heavy load, switch in a large motor start capacitor.

I use a microcontroller to activate a solid state relay. The SSR kicks in a mechanical contactor, which connects the start capacitor to the circuit.

The capacitor is only active for a fraction of a second (the microcontroller sees to that), just long enough for the motor to start.

I've been using this setup for about a year with no problems.

My 'load' is an 80 year old Van Norman #10 knee mill.

Hi, the big problem with switching capacitors in and out as the load fluctuates is that you need to do the switching at the zero crossing point when the AC wave form is zero volts, or you'll get an amp surge and ripple on the line, plus capacitors don't like to be connected and disconnected when there's a lot of current present.

I recently built a 2HP rotary phase converter, and was aware of this problem from the start.

If you have a fairly even load such as a lathe or milling machine you can set it up for the average current draw, and live with the temporary load imbalance at start or when a big cut goes on.

An air compressor should start up unloaded anyway, and if it isn't unloaded the motor won't last too long.

I eventually lashed out and bought a 4 HP converter for A$2000, that electronically senses the load and switches capacitors in and out as the need arises using SCR's.

The sensing circuit is pretty complex and is really beyond the capability of the average DIY man.
Without it, it's like driving a car with only one gear in use.
Ian.

The Idea given by Eurisko is quite practical for DIY man.
Some changes or fine tuning the circuit to suit your requirement are needed.

Compressor applications demand current continuosly, which may cause, motor or the convertor to burn. Therefore, controlling the current supplied to the motor, so that the motor takes 3 to 5 secs. to run up to full speed is O.K.

This is my version.
It self starts most of the time, or just kick the shaft.
Loads up to 1.5HP before grumbling a bit.
A good size flywheel would improve it greatly.
The capacitor is shown connected between T3 and T8.
Try it between T9 and T2, and use whichever gives the best startup.
This requires a DUAL VOLTAGE STAR CONNECTED MOTOR for 380V output.
I use it for rapid feed motors and suds pump.
Make sure the capacitor is big and rugged. Mine was 450vAC 4uF.
Probably work better with a larger capacitor. Never tried it though.

Quote:

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Originally Posted by handlewanker

Hi, the big problem with switching capacitors in and out as the load fluctuates is that you need to do the switching at the zero crossing point when the AC wave form is zero volts, or you'll get an amp surge and ripple on the line, plus capacitors don't like to be connected and disconnected when there's a lot of current present.

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Ian is quite right. I should have mentioned that I use a zero crossing SSR in my application.

Make sure also to use a bleed resistor across the motor start capacitor.

One of my not-so-bright ideas was to use a bright blue LED as a power indicator. When I press the button to start the circuit, the motor jumps to life, the old mill (belt driven) roars, and that darn blue LED flashes like a lightning strike...

:D

I was going to build a rotary converter.
Ended up finding a 25Kw 3ph generator and a diesel engine, Listeroid, now I get to enjoy the slow thump, thump while running my 3ph stuff.
No problems at all with load imbalances.

Edit:

Should maybe finish my thoughts.

Just saying that if you needed better 3ph then you can get with a simple rotary convertor don't overlook the posibility of going to a full motor generator and get real 3ph. Those 1800RPM generators from China are not very expensive.