RPMs increase when microwave is on

On a recent trip I noticed that the starboard engine would increase by 15-20 RPMs when the microwave (powered by the inverter) was being used. It would go back to normal when the microwave was off. Both engines have their own 8D starting batteries. The inverter is powered by a separate bank of 6v Lifelines. What would cause this?

First question is the rpm gauge mechanical or electrical?
Is the engine rpm actually increasing or is the gauge just jumping up that much?

I would think its got to be electrical and the RPM's are not increasing on the engine. Just doesn't seem possible.

folivier said:

First question is the rpm gauge mechanical or electrical?
Is the engine rpm actually increasing or is the gauge just jumping up that much?

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The rpm gauge is the electronic gauge that connects to the engine via the wiring harness. I noticed the jump in rpm because the engines stopped being in sync. It is not the gauge, the engine RPMs actually change. The engines are electronically controlled JD4045s, but this only affects one. I haven't noticed any rpm change when a DC motor, like the water pump is in use.

That makes more sense knowing about the electronic controlled engine. Low tech guy like me finds it hard to fathom how a micro wave could affect the engine. I would expect a drop in RPM if anything from power draw.

Thankfully, my JD 4045 NA isn't electronic. Shifting electric loads have no effect on engine performance.

A microwave pulls a lot of power; something to do with signal noise or a response to a change in the load on the alternator?

Can you hear the rpm change? On a twin you should hear a big change in beat frequency with 15rpm change.

High tech electronic weirdness. I would not worry about it. Maybe not a good idea to stand too close to the 'wave.

Ski in NC said:

Can you hear the rpm change? On a twin you should hear a big change in beat frequency with 15rpm change.

High tech electronic weirdness. I would not worry about it. Maybe not a good idea to stand too close to the 'wave.

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Yes, the rpm change is very apparent. That was how I first became aware of the surge.

It seems the issue may not be indicative of a major problem based on responses. The simple solution is not to use the microwave while underway.

How old is the microwave? What happens if you move it to the fwd vee berth area? Is there another similar load you can plug into the inverter because it might be the inverter sending out the interference. If the surge goes away; microwave, if not then inverter suspect. Inverter Modified SW or pure?

High Wire said:

How old is the microwave? What happens if you move it to the fwd vee berth area? Is there another similar load you can plug into the inverter because it might be the inverter sending out the interference. If the surge goes away; microwave, if not then inverter suspect. Inverter Modified SW or pure?

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The mw is 2010 and is a convection oven as well. The Magnum inverter is pure sine wave. I'll need to do more troubleshooting.

Maybe the wave is off to one side of the boat, closer to the affected engine. Maybe if you put it centerline it will affect both engines the same.

J/K....

Trying to get a bit more technical: If you set wave at partial power where magnetron cycles on and off, does engine rpm follow that? Or does engine respond to unit simply powering up?

Also try a load similar to wave, like a space heater or hair dryer. Both probably 1000-1500W. Maybe the wave is the biggest thing you run on the inv, and it is not unique to the wave but to the inv at that load.

Does it do it when wave is run on gennie?

Can you isolate house bank from engine alternator and see if it still does it?

Some basic troubleshooting steps that popped into mind while waiting on miserably unreliable construction crews. Never anticipated this, but the painters are the most dependable and reliable and professional crew so far.

If your boat is wired like my 2003 OA, this might explain the issue:

My starboard engine alternator output is wired to a battery isolator (combiner?), which in turn links to the house bank and the engine's start battery. The Inverter draws current from the house bank, putting more resistance on the alternator, which in effect slows down the engine. The other engine is not effected because it only charges its own start battery. Depending on the microwave and state of charge of the house bank, the alternator could be lugged down to the tune of 1000- 1500 watts.

Gordon

Gordon J said:

If your boat is wired like my 2003 OA, this might explain the issue:

My starboard engine alternator output is wired to a battery isolator (combiner?), which in turn links to the house bank and the engine's start battery. The Inverter draws current from the house bank, putting more resistance on the alternator, which in effect slows down the engine. The other engine is not effected because it only charges its own start battery. Depending on the microwave and state of charge of the house bank, the alternator could be lugged down to the tune of 1000- 1500 watts.

Gordon

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I thought that was the answer, but the starboard engine RPMs increase when the microwave is on, not decrease.

So it could be the port engine slowing down instead of the stbd engine speeding up?

Makes no sense to me, especially if you've a diesel engine. An increased electrical load one would think would decrease engine speed because of the extra alternator load. But if your engine is like mine, the "throttle" controls the RPMs, not the fuel supply. So regardless of the engine load, the engine runs at a constant speed for a given setting.

Governor adjust fuel, but add load and rpm will drop a touch. But these are electronic engines, so there should be almost zero droop, and there is usually some sort of synchronizer with the electronics.