Rerigerator compressor problem

[opengski]

Hi, we are trying to control the speed of the compressor by using an H-Bridge INVERTER CIRCUIT

The problem is that when we use this circuit on the compressor, based on our observation.. the RELAY keeps on ticking back and forth. The inverter Circuit outputs a SQUARE WAVE DC Voltage which we interchange its polarities to produce our AC VOLTAGE.

What could be done?



we just represented the MOTOR as a resistor.
T2 and T4 are the pair transistors and such as T3 and T1

our H-bridge works since we were already able to control the motor of an electric fan, but we know the compressor is different...

Hope you can help us!

 

[dick_freebird]

Try giving it full speed (voltage) for some period so the motor can start.
Then pull back the voltage to whatever running value you want?

 

[chuckey]

Domestic fridge compressors have a big thermistor surge suppressor in series with the motor. Where is this relay you talk about? As Dick says, the comp motor takes several seconds to come up to speed (thermistor heating time?). If you try and reduce the input voltage, the current falls and the thermistor goes high resistance.
Frank

 

[RCinFLA]

The thermister is a positive temp coefficient and is in series with start winding. It drops the start winding current as it gets hot. If the motor does not reach its run rpm, the thermistor gets hotter and is temp coupled to a mechnical Klixon to shut off the power to the compressor. If grid flickers or compressor is restarted before refrigerant has equilized its pressure, the compressor has a hard time to start up against head pressure and overheats.

You cannot modulate the speed on a regular compressor motor not designed to run at anything other then its constant rpm speed. You can slow it up but it will run hot, likely continuously popping the Klixon and will eventually fail.

Most conventional refrig's use a simple orifice tube for refrigerant expansion that relies on a contant volume flow rate of refrigerant. Reducing the compressor flow will just reduce the pressure in the condenser and result in poor condensation back to liquid starving the evaporator of liquid refrigerant. The cooling of the compressor also relies on return gas from evaporator.