Inverter without load connected

[2PAC Mafia]

Hi guys,

sometimes when I had to repair some device based in inverter design, 300V or 400V PFC DC power changed to 220 or 380 Vac power, always at the end stage we find a power IGBT module. For troubleshooting can be necessary to disconnect the load (as a Vac electric motor) but I´ve seen usually at devices or documentation an advice saying "Don´t connect main power without load, IGBT can be damaged".

I would like to test the output and see the waveform without load and I would like to get an explanation why IGBT can be damaged if it works without load connected. The signal at gate should be the same and without load connected IGBT shouldn´t drain amps...

This is to test that IGBT works properly isolating the load stage.

Thanks

 

[crutschow]

Without seeing the circuit schematic that can't be answered in detail, but it's apparent that without a load the internal voltages on the IGBT will rise due to the apparent loss of control. The circuit design requires some minimum load and you need to supply that to avoid zapping the circuit.

 

[2PAC Mafia]

Thanks Crutschow,

I don´t have any special schematic or design, it´s a doubt I have because I had to repair systems like that, marine stabilizator system, air conditioning... probably I´ll see them also in future so I would like to know if it makes typical Vac at output why we can not leave it on without load as for example 220Vac at home.

This is a typical schematic:
**broken link removed**

 

[crutschow]

Some designs cannot operate no-load that are normally always connected to a load. The design of such a inverter may be somewhat simpler than one that can operate with no-load. It's design specific.

 

[2PAC Mafia]

Understood, thanks

 

[audioguru]

An inverter that is used to power an important refrigerator during a power failure or on a boat is useless when it zaps itself when the refrigerator normally turns off. Was the inverter designed by an idiot?

 

[2PAC Mafia]

Hi Audioguru, many years since my last post, I hope everything runs very well!!!

Of course, in your example, when you switch off the refrigerator the processor stops sending signal to IGBT module so it protects the output. My doubt is what happen when everything is running and you disconnect the compressor to test Vac output, in theory processor stops signal to IGBT module because there is no feedback and as I said if you give power to unit with no load connected you can damage IGBT module (message from manufacturer).

 

[audioguru]

Hi 2PAC,
I hope you do not switch off your refrigerator, it should cycle on and off by itself without telling the processor. Then the output of the inverter gets zapped.

 

[dr pepper]

Just a guess but I'd say somewhere in the system theres a switching supply that has no or limited feedback, and without a load will increase voltage beyond that of some of the power devices.
Either that or the circuitry uses the output current to regulate, obvious loss of control could then happen if theres no safeguards which on cheap designs there often isnt.