Microwave electrical and generator

[Helmsman]

Ok, this is becoming an ongoing issue. As mentioned in other posts, I installed an Promariner 2000 watt PS inverter last year with a new battery bank. We run the outlets on the boat through the inverter, which includes the circuit for the microwave. That may or may not have anything to do with the problem.

We are having a problem with what I would call an amperage surge with the microwave. When we have the microwave on for more than a couple of minutes, the amps surge from 14 amps to over 30 and if it is on shore power, the microwave breaker cuts it off. When we were on generator (Kohler 7.3e) power for the 4th, we had the two AC’s (original equipment) on and started heating food up to serve. Two minutes into cooking, the generator started laboring, the microwave light dimmed, and shortly after the generator quit. We waited twenty minutes, tried again with one AC on and started heating food again, and the generator was surging (laboring) again, so we cut it off.

I have noticed when at anchor, when I start the gennie, the amperage on the battery charger side of the panel goes up to close to 20 amps until the new bank is “assessed” and then drops down gradually as the batteries receive the first step charge. This may have happened on the second generator start try. The first time the generator had been running for about three hours when it started struggling with the AC and microwave load.

I said all of that to ask this. Based upon the description, do you guys think it is the wiring or outlet to the microwave, or the microwave itself? The microwave is supposed to pull 14 amps and is the old Kenmore with the side coffee pot (which we don’t use). It is a 1200 watt generator that is original to 2006. Any help or thoughts would really be appreciated!

[Viper]

Was this the first time you ran these set of circumstances since the inverter install or have you run this configuration before with no problem?

How is the inverter hooked up to the ship's main panel? In other words, what do you have to do to switch from ship (generator), or shore, to inverter? Are you using an selector switch or breaker to accomplish this or did you wire the inverter to switch automatically? If you did, are both the shore and generator sources hooked up to the inverter's automatic switching option?

You may have more than one issue going on and the inverter install is just a coincidence. Could be a generator issue and a microwave issue. The magnetrons in microwave ovens degrade over time. This tends to draw more current and generate more heat. The exterior walls start getting hotter than usual. It would be interesting to see if the microwave does the same thing on another outlet or someone else's boat without an inverter. Try turning on other high load items on that outlet like an electric kettle and see if it you get the same symptoms. Leave the oven off and try turning other high load equipment like the water heater, AC, etc. and see if that affects generator operation Of course you can eliminate the possibility of an inverter issue or it's wiring by disconnecting it and try to recreate the fault.

[Midnightsun]

Disconnect micro and try a countertop unit in the same conditions and same outlet the micro is presently plugged into. Sure sound like a magnetron on its way out.

[Helmsman]

Was this the first time you ran these set of circumstances since the inverter install or have you run this configuration before with no problem?

How is the inverter hooked up to the ship's main panel? In other words, what do you have to do to switch from ship (generator), or shore, to inverter? Are you using an selector switch or breaker to accomplish this or did you wire the inverter to switch automatically? If you did, are both the shore and generator sources hooked up to the inverter's automatic switching option?

You may have more than one issue going on and the inverter install is just a coincidence. Could be a generator issue and a microwave issue. The magnetrons in microwave ovens degrade over time. This tends to draw more current and generate more heat. The exterior walls start getting hotter than usual. It would be interesting to see if the microwave does the same thing on another outlet or someone else's boat without an inverter. Try turning on other high load items on that outlet like an electric kettle and see if it you get the same symptoms. Leave the oven off and try turning other high load equipment like the water heater, AC, etc. and see if that affects generator operation Of course you can eliminate the possibility of an inverter issue or it's wiring by disconnecting it and try to recreate the fault.

The inverter has a breaker on the AC panel. It automatically switches to “shore power” when the boat connects to a pedestal or we start the generator. The VictronConnect battery monitor shows the voltage going to 14.4 (AGM) and the amps on the panel go to about 15 amps when either are connected. If the batteries are close to fully charged it “settles back down” pretty quickly. If I switch off the inverter the amp reading drifts back close to zero. That is important, because running other equipment on it adds to the amp demand while a short wait eliminates that demand.

If I disconnect the inverter the outlets won’t work. I didn’t follow your advice of having a switch installed to provide power to them if I disconnect the inverter, unfortunately. However, I can still use the high load appliances below to check how the gennie handles them. And can add a high load appliance to that outlet with the inverter on.

I will pull the microwave and try it on the cockpit outlet to see if I have the same problem. If it does, that eliminates the circuit, I guess. I will also try turning on just the water heater and AC together with the generator running. If that works, I will add the stove to it and see what we get.

I will also check the amp draw with a multimeter to see what we get. Hoping it is just the microwave!

[Helmsman]

Source of the post Disconnect micro and try a countertop unit in the same conditions and same outlet the micro is presently plugged into. Sure sound like a magnetron on its way out.

Don’t have a countertop unit, but will try some high load draw on it, as mentioned in the reply to Viper. Thanks!

[bud37]

I can't speak to the inverter....but your 7.3KW genny is capable of providing 30 amps each to two separate sides.
Try to consider what are the total load numbers when this shows up.....if the gen quit there must have been a reason ....would total load not have just opened a breaker first.....

[km1125]

Source of the post
...It is a 1200 watt generator....

^^^ that seems like a typo.. is it?

Sounds like you have too much load on one side of the generator. Is anything else on like the water heater on while you're doing these tests?

Is it ONLY the microwave on when you're seeing 14A and then while it's still on it jumps to over 30?

A microwave pretty much takes up it's own 15A circuit. Anything else on that circuit that is used at the same time will usually cause the breaker to blow.

[Helmsman]

Source of the post

Source of the post
...It is a 1200 watt generator....

^^^ that seems like a typo.. is it?

Sounds like you have too much load on one side of the generator. Is anything else on like the water heater on while you're doing these tests?

Is it ONLY the microwave on when you're seeing 14A and then while it's still on it jumps to over 30?

A microwave pretty much takes up it's own 15A circuit. Anything else on that circuit that is used at the same time will usually cause the breaker to blow.

It was a typo, should have been 1200 watt microwave, sorry. The microwave is on a dedicated circuit. When this happened with the generator running, that 30 amp side had the outlets on (nothing plugged in) through the inverter, of course the microwave, the refrigerator, the two Charles chargers (10 amp and 60 amp) though batteries were fully charged. Nothing else was on.

On the other 30 amp side, the two AC’s were running. In the manual for the microwave, the rated power consumption should be 1650 watts, the output is 1200 watts, and the oven should pull 13.8 amps. It would pull 14.5 amps if we were using the coffee maker, which we weren’t.

Again, the microwave ran for over two minutes before the generator started surging.

Now, a couple of weeks ago, when we took the first long trip after the inverter install (another thread), the inverter batteries were really low. That was due to two charge wires coming off the isolator being “switched”. When that happened, and we docked at a marina and hooked up to shore power, my wife started heating food and we blew the breaker on one side of the incoming power. We reset and waited a while. After the batteries had recharged for a bit, and the amperage usage on the meter settled to under 5 amps, she tried again. This time the microwave ran for over two minutes. I watched the amp meter at the panel suddenly it surged up past 30 amps and the microwave breaker popped.

With nothing else on, other than the chargers and inverter running, I attributed it to the microwave and the batteries being so low then. A little later she used the microwave with no issues, but I don’t think she ran it for over two minutes.

This latest “episode” on the 4th has me pretty convinced it is the microwave. I will run the suggested “tests” with the generator running, adding other loads, then take the microwave to another outlet to see if I get the same problem. That should tell me where the true issue is.

[km1125]

You also want to look at the voltage on the line when the microwave is running.

If you have a kill-o-watt to plug in where the microwave is, that would be great. It would show the voltage and can show the current draw as well. If the voltage is dropping (perhaps due to a bad or corroded connection) then the microwave will surge current to make up the total power. That will not happen if you plug in a comparably-powered resistive load like a space heater. You will lose some overall heat from the space heater and the current will actually drop a bit, whereas the microwave current will go up.

[Helmsman]

Source of the post You also want to look at the voltage on the line when the microwave is running.

If you have a kill-o-watt to plug in where the microwave is, that would be great. It would show the voltage and can show the current draw as well. If the voltage is dropping (perhaps due to a bad or corroded connection) then the microwave will surge current to make up the total power. That will not happen if you plug in a comparably-powered resistive load like a space heater. You will lose some overall heat from the space heater and the current will actually drop a bit, whereas the microwave current will go up.

I will check that also. Thanks! Nate