Strange voltage reading

[Nspacheco]

I'm replacing a 12-Volt DC tilt motor on a 48-Volt Minn Kota Edrive.

In diagnosing a problem, I unplugged the two wires running into the motor and tested the voltage from the engine's circuit board into the motor, and saw 48-Volts on the DC setting of my multimeter. Not wanting to burn out a replacement motor, I contacted Minn Kota, who confirmed it was a 12-Volt tilt motor, and although the circuit board operates from a 48-Volt source, it uses a duty cycle so the motor sees a lower voltage. I understand that concept but just to make sure my circuit board isn't shot I got a multimeter that also measures duty cycle, and got strange numbers.

On the DC setting I see 48-Volts, but if I go to the AC setting I see 12-Volts. On the duty cycle setting I see 0. So either the multimeter is interpreting the duty cycle as an AC source and reporting 12-Volt equivalent, or it really is 48-Volt DC and the multimeter AC setting reading doesn't count.

And it bothers me that the duty cycle reading is 0 but maybe my meter isn't sensitive enough. So I'm confused and am afraid of burning out a replacement motor.

I talked with my local mechanic and he says he's seen strange voltages on this engine design. I will give him the job if I can't figure it out but prefer to do it myself yo save some money. Any insights in the strange voltages?

[jimh]

If the 48-Volts DC is being converted to 12-Volts DC by using a 0.25-duty-cycle, I would expect that in order for that conversion to work there has to be a load on the circuit. If there is no load, and you connect a DC voltmeter to the circuit, the voltmeter may read the peak voltage, 48-Volts.

If the source voltage is toggling between 0-VDC and 48-VDC with the 48-Volts being there 25-percent of the time and 0-Volts being there 75-percent of the time, I am not sure what an AC Voltmeter might read. It could decide that the peak-to-peak Voltage was 48-Volts, then convert that to the usual RMS voltage associated with a sine-wave AC waveform, and show you RMS, which would be

RMS = 0.707 x (Peak-to-Peak)/2

For this case that would be RMS = 17-Volts

The exact relationship between an AC voltage's peal-to-peak voltage and its RMS voltage is based on the waveform being sinusoidal. Clearly the waveform from a pulse-width DC convertor is not a sinusoidal waveform; it is more likely a square wave with asymmetry. Therefore, exactly what an AC voltmeter will read on a pulsating DC voltage with a particular duty cycle is hard to predict, and most likely the reading will depend on the voltmeter's circuitry and how it handles this kind of voltage fluctuation.

[Nspacheco]

If the 48-Volts DC is being converted to 12-Volts DC by using a 0.25-duty-cycle, I would expect that in order for that conversion to work there has to be a load on the circuit. If there is no load, and you connect a DC voltmeter to the circuit, the voltmeter may read the peak voltage, 48-Volts.

Thanks Jim that may well be the answer. There was no load on the circuit when I tested it and saw 48V DC. I didn't know that conversion was load dependent. That motor has a largely varying load. When it's lowering the engine it has a very light load. When raising it, the load goes from light at first to heavy as it gets horizontal then lightens again.

I'm guessing like you said, since it's not true AC there's no telling what the meter on AC setting setting will read. I assume from your load comment that with no load, or just the miniscule load from the meter, that the duty cycle may get close to 100% until it calculates the load which, being light, remains close to 100%. So the DC setting would see 48-Volts but the AC setting may see some drop at first and do some averaging.

Bottom line I think there's enough info here that I can risk ordering the part and trying it out.

Thanks again.

[jimh]

Usually those pulse-width voltage conversion devices depend on an output capacitor to smooth the voltage and store current when there is no current from the 48-Volt source during the off portion of the pulse-width. With no load, the capacitor just sits at 48-Volts waiting for a load to be applied--or that's all I can come up with to explain your measurements. Sometimes the "load" is a 12-Volt battery, which provides much improved voltage regulation.

I'd call the manufacturer of your device to get better insight into what a proper measurement of the output voltage of the 48-to-12-Volt convertor should be without any load attached, other than the one-milliAmpere or less that a voltmeter will draw.

[Nspacheco]

I ordered a new Minn Kota 12-Volt motor lifting assembly and installed it--worked like a charm. So the 48-Volts I had measured was just the peak of a 25-percent duty cycle: one cycle at 48-Volts and three cycles at 0-Volts equals an average of 12-Volts. The cycle rate has to be fast enough to not have the peak voltage burn the motor.

[jimh]

An accurately reading DVM is a great help in any sort of analysis of electrical voltage readings.