HZ in motor with switch turned off

[blenkhn]

I have come across something I would consider weird. Why is it that with the switch turned off and the motor still being plugged into the wall would it have a HZ reading matching the HZ of the wall plug?

 

[ScottyUK]

HZ? Obscure abbreviations are best avoided.

 

[electricpete]

How are you measuring the hz? Did you get a voltage magnitude to go along with it? Use an analog voltmeter like Simpson to load the circuit.
Could be only one leg is interrupted by the switch?

=====================================
(2B)+(2B)' ?

 

[blenkhn]

It is the Hertz of the circuit and I will check on the single leg being interupted. That makes sense to me now I will have to get a double pole switch to interrupt it completely. No voltage at all or amps to go with it.

 

[ScottyUK]

Sorry for missing the obvious, I've not seen frequency used in that context before.

 

[LionelHutz]

You mean the frequency of the circuit?

How about some details on this circuit and where you are measuring this frequency.

 

[blenkhn]

Hi, LionelHutz

Yes I was talking about the Frequency of the electrical current going into the circuit. The circuit is pretty easy, wall plug to a single pole switch to the motor. I was measuring the Hz on the motor when the motor was off. I also made the assumption that the Switch would turn off everything. But if I want the Hz to go away I would need a double pole switch? I put the switch on the black which I made sure was the live wire not the neutral.

Is there any further insight?

 

[LionelHutz]

If the motor is off then you're likely really measuring the frequency of a voltage on the circuit. You need to be clear about these things. Measuring frequency of the current would require using a meter with a current shunt or a clamp-on probe. I doubt you're doing that. I expect you are taking the meter with the leads connected to the voltage measurement terminals and just switching the function to frequency. Where are you connecting the two probes? And finally, why does it matter?

On another note. I don't think you're following the comments which are directed at the proper description for what you are doing. You measure frequency. The unit of frequency is Hz.

To answer your basic question. The reading is likely either from induced voltage or possible your neutral has a small AC voltage on it.

 

[waross]

Or the plug is reversed, or the plug is incorrectly wired.
When you are seeing 60 Hz, switch the meter to volts with the same connection and see if there is any voltage.
Try unplugging the wall plug and see if the frequency indication goes away.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[blenkhn]

I know the frequency is Hz not HZ like I did before (I didn't think it mattered too much).

When I unplug from the wall the frequency does go away. So I have set something up wrong. I thought I was being careful and making sure that the black was the hot one and the one that is being interrupted by the switch. Is it possible that the neutral has a current on it because the neutral in the house is shared?

When I switched to Volts there wasn't anything measurable... Measuring frequency is the same as measuring volts where you are hooking up the two poles to the voltmeter and when you measure amps you are hooking it up in-line.

 

[DRWeig]

I'm betting a small voltage on the neutral. It could be a few tenths of a volt and still be read by your frequency meter. I'm assuming here that the motor case is grounded.

Best to you,

Goober Dave

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[blenkhn]

Yes the motor case is grounded... Thanks all for your insight.

 

[LionelHutz]

OK, so where did you connect the 2 meter leads and did you check that the receptacle was wired correctly?

I'm thinking it could easily be a small voltage on the neutral as well. The neutral better not have any current at your motor with the motor off. However, a voltage drop on the shared part of the neutral wiring may be causing a small voltage at your motor. It doesn't take much of a signal for some meters to measure a steady frequency. I've seen a meter with the leads not connected but close to some power wiring still showing a steady 60Hz on the display.

 

[squeeky]

Strange that some people don't speak engineering. I find I learn a lot reading. My money is on live and neutral being reversed, causing the motor to stop running but it is still live, or the neutral at the point of supply is not connected properly. You would then get the frequency at a very low voltage seen on the neutral. Please post something if you find out what it is / was.