mastervolt inverter

[neilsty]

I have a mastervolt mass sine 24/2500 inverter that stopped working. The LED shows that it is on, and there is no flash from the failure LED. The inverter is controlled remotely from a mastervolt micc - that shows that the inverter is on and has no error codes. My batteries are fully charged - so it is not a battery problem. I have also checked the fuses on the DC side. The wiring diagram does not show an AC side fuse. The AC output is wired to a selector switch on my AC panel - I checked the output wires on this switch and was seeing no power, nor does my panel voltmeter show any AC, but the light on my AC panel that shows the inverter is present glows when the micc has the inverter turned on. The inverter was working with no problems. This issue happened when I turned it on but had the breakers for two air cond units turned on. I thought that the inverter was meant to shut off when it was overloaded instead of burning out. Does anyone have a suggestion on what I can test, or do I need a new unit. Are there any user serviceable or testable parts inside the inverter? It is 10 years old so I am guessing it may be better to buy a new inverter instead of sending this away for service - any thoughts?.

[pbiJim]

I don't know that inverter in particular. Most inverters that I have repaired were not fused on the AC output side. They instead relied on internal current sensing/limiting circuits. A few inverters had board mounted fast blow fuses to kill power to the output thyristors in the event of a massive overload. This is most common on very old inverters that use an SCR for the output thyristor. An SCR tends to fail as a short circuit so the fuse is needed to prevent unpleasantly exciting events.

About 90% of the inverters that I have repaired only needed to have the output thyristors replaced. Some older ones also needed the main capacitor banks replaced. A few had problems in the firing control circuit. Most of those I scrapped.

My best source for replacement parts was Galco industrial electronics. They carry a pretty full line of SCRs, IGBTs, rectifier bridges, etc. They can be found on the web.

You really need someone with a strong electronics background to go poking around inside one of those things. Also be aware that many of those will have high voltages stored in capacitors for quite some time after external power is removed.

As a general statement, when a motor load (such as an air conditioner) is run off of an inverter, it beats on the output thyristors in the inverter. An electric motor will normally draw about 6 times it's rated full load amperage when starting from a dead stop. This is called locked rotor current. These short duration current spikes peck away at the P/N junctions inside of a semiconductor like a woodpecker pecking at a tree. The effect is cumulative & eventually enough junctions will be gone that the semiconductor will fail. If you are going to run motor loads off of an inverter, it is usually best to oversize the inverter by a good margin to compensate for this issue. If the locked rotor current is within the capacity of the inverter, then there should be no problems. If the locked rotor current is outside the inverter's rating by a moderate amount, then trouble should take a while to manifest itself.

[Andina Marie]

On the subject of failed output devices, SCR or thyristors. I blew out 3 boards on my inverter that were replaced under warranty. It was on the third time that I discovered what was happening.

I have a SSB transmitter with an automatic antenna tuner. When the tuner was tuning, the transmitted energy was apparently triggering the driver circuits to the output boards and blowing them up.

After putting a sign on the transmitter "ALWAYS TURN OFF THE INVERTER BEFORE USING THE TRANSMITTER" it worked OK for 10 years.

More modern inverters may have better noise protection and filtering but better not to risk it. I would also suggest turning off your inverter if there is lightning in the vicinity.

[neilsty]

Thanks for the responses. I don't run my AC from the inverter - it is usually just for the coffee pot and a hair dryer (not together). But i sometimes have the AC breaker on when the inverter is turned on, which is what I did when it stopped working. So I am guessing I have damaged the inverter. Based on pbiJim's response i will not attempt to trace the fault myself, and send it off for repair instead. thanks for the responses.

[pbiJim]

The hairdryer & the coffee pot are both ohmic heating loads which fall under the classification of incandescent loads for the purpose of load & sizing calculation. If they dump the full load across the line all at once, the inrush can be as high as 10 times the full load amps. This is worse than a motor load in terms of current inrush, but this load type does not also have the flyback voltage issue that inductive loads, like motor loads, have.

Some modern coffee pots have soft start technology in them that eliminates this issue. Hair dryers generally do not. With the hair dryer, if you turn it on low, then a few seconds later go to medium, then a few seconds later go to high, you will greatly reduce the maximum inrush current draw compared to just flicking it on high straight away. This will prolong the life of a marginally sized inverter.