Microwave oven doesn't sound right, sagging

Dan_
Dan_ Solar Expert Posts: 61 ✭✭✭✭
in Off Grid Solar & Battery Systems #1
I have a Sharp "1100W" microwave that I've previously used grid connected and am familiar with its normal operating sound. When run from inverter, it sounds normal for a second then sags at the load, hum gets deeper in pitch and strained like if the voltage dropped excessively. Starting voltage measures 120v and with the microwave load it drops to 110v which seems OK although not great. Connected via 75ft of 14ga. Meter reads about 1700w draw (not sure what normal draw for this microwave is)
Symptoms seem to point to wiring but could it be something else?
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
    Re: Microwave oven doesn't sound right, sagging

    Fun with math time!

    1700 Watt draw? 14.6 Amps @ 120 VAC - near the limit for a standard 15 Amp circuit.
    75 ft of of 14 AWG wire with 14.6 Amps on it: looks like almost 5% V-drop, Voltage going down to 114 by calculation so it could well sag to 110.

    So ... measure Voltage at inverter output when microwave is running to see if the drop is being caused at the inverter (1700 Watts is slightly more than half what a 3048 can manage non-surge).

    Also check the DC input to the inverter under load and see if it is sagging. 1700 Watts would be around 35 Amps @ 48 VDC, a pretty hefty draw for 208 Amp hours.

    And try running the microwave nearer the inverter to see if the 75 ft. 14 AWG wire run is the culprit.
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  • Dan_
    Dan_ Solar Expert Posts: 61 ✭✭✭✭
    #3
    Re: Microwave oven doesn't sound right, sagging

    Tested it plugged directly into the load center and doesn't seem to sag. Should have got off my butt and tested that sooner. Guess the 75ft run is no good

    With it directly connected measures 1800w. Starting dc voltage 51.1 with load dropped to 48v. Sticker on the back of the microwave says 13A, its definitely drawing more
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  • waynefromnscanada
    waynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    #4
    Re: Microwave oven doesn't sound right, sagging

    And the reason there's a one or two second delay before the sag, is it takes that long for the heater in the magnetron to bring the cathode up to operating temperature so it can begin producing microwaves. It's only then that the real load is applied to the supply. Before that time, it's only the fan etc drawing a relatively very small amount of power.
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