PWM - pulse width modulation
We cant see it with our eyes... A voltmeter shows a lower or higher output voltage to the load and we just know its a dimmer switch. Hey its more than that. Its a duty cycle control, maintaining the source power constantly but changing the switching speeds and this is what drives inverters. Too fast for us to see... but not for the oscilloscope. Imagine switching both positive and negative feeds to create a sine wave , then chop the same feeds twice more at differing time intervals and you have 3 phase inverter output from one single phase DC supply. Folks ... don't beat your brains out. it is just that simple to do. You see here you are looking at one positive signal switching which we call DC. No negative signal other than 0V. But give it a negative switch and it looks like a raw AC waveform. This is the basis of phase creation from +/- DC You see an inverter gets to do both at the same time... And the world just got smaller for all of you , nothing is over complicated. Its just ON - OFF high signal, low signal positive on and negative on. Absolutely cool PTC - Compressor starter - Positive Temperature Co-efficient
Hey, simplicity works but it has its limits The most cheapest compressor starting device, and can I say it is not a relay , more a resistive switch. For very small class capillary system refrigerator, usually a bar fridge or cubic fridge enter the PTC. Now these are not high tech, and they need to have decent intervals in time before starts so they can cool down. Also the system must equalise pressure between the high and low sides to allow for a low compression start. The PTC starter is similar to the potential relay not by design but where the start winding is engaged immediately from first power up. . At start up the PTC is cool and has no opposition to current flow by a preset low resistance which is in series with the compressor start winding. The compressor starts by resistive start and induction run, as the current flows through the PTC , it heats up increasing its internal resistance until it will not allow more current to flow , disengaging the start winding. The start winding is isolated and will not engage further as a trickle of current to the start winding will maintain heat and high resistance during the operation of the compressor. When the unit stops the PTC MUST!!! cool down to allow current to flow again on next start-up. If it trys to start to early the internal overload will activate in the compressor. So it really is not designed for frequent starts and requires very low starting torque. Don't expect too much , price and capacity dictates control, yet I do like how simple these are. The PTC is small, no bigger than a 10c (Australian) coin and as about 4mm thick. Note also the compressor terminals are inverted to this design as per the Danfoss types. I have marked them. These starter types are typically only used on low capacity, low torque compressor where off cycle pressure equalisation occurs , as you see in capillary systems Now does anyone see a Star Trek logo on the PTC relay disc in the pictures above? , or am I just seeing things :D
 Check out my YouTube video on inverter testing a small Daikin Split outdoor unit
 Check out my YouTube clip
 The most important part of the system is the restriction of flow.
The compressor prevents back-flow and maintains pressure against the restriction which could be a capillary tube or expansion device orifice. It is a venturi effect..... The most amazing part of thermodynamic transformation , without which we have no separation of saturation temperatures. This is for the apprentices to review from their first week back and to step into the greater world of refrigeration we live in. I'm always fascinated by the simplicity of this design to achieve a manipulation of energy to create a heat transfer effect. |
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