[SOLVED] AC monophase electrical motor rotation detection.

[meshgate]

I have to solve the following problem with a proper electronic solution.
I have an AC monophase motor that I cannot reach and I must detect whether it is rotating when the 220V ac main is connected. It is not possible to reach rotating parts because it is completely mechanically screaned. It not possible to detect tha current flowing since it will flow also if the rotor is not moving.
I had an idea to apply a microphon on the motor surface and amplify the turns vibration (2300 t/min = 38 Hz) and consequently actvate a LED etc....
Sambody else has a better idea?
Thanks for the suggestions.
Meshgate.

 

[FvM]

You most likely can derive the "run" signal from the motor current (only magnitude or possibly phase and magnitude). What's the motor type? Shadow pole? Two-phase capacitor?

It not possible to detect tha current flowing since it will flow also if the rotor is not moving.

Right if you just check for current "yes/no". Rather ignorant when considering an electronic solution.

 

[meshgate]

Thanks FvM,
the motor is really a circulator pump for hot water, it has a 2.5uF capacitor and the data are 230V AC, 2300 t/min, 112W and 0.49A. From electrical point of view the main and the capacitor pins are useful...
I do not have an idea what could be the current drain when the motor is not rotating...probably it would be higher than 0.49A, so it is possible to set a threshold circuit.

 

[FvM]

Yes, the stalled current will be surely higher. It's usually clearly above the rated motor current, so that a correctly adjusted motor circuit breaker trips after a minute or two. A current threshold switch should be able to detect the stalled state.

 

[BradtheRad]

I had an idea to apply a microphon on the motor surface and amplify the turns vibration (2300 t/min = 38 Hz) and consequently actvate a LED etc....

Besides the mic idea, you might try mounting a small coil of wire on the motor to detect the magnetic field.
Somewhere where there is not so much shielding.

The coil might be 50 turns of AWG 36. Amplify the signal coming out of it, to activate the led.

You may have to find a threshold between signal strength when the motor is turning, versus when it is stalled.

 

[meshgate]

Thanks for suggestions, a breaker switch my be suitable since it will introduce minimum paerts. The winding solution is still interesting, I had it in mind, but I did not guess the possible amount of pick up voltage...
All the troubles started when I discovered that , when the controller gives power to the motor, it started only in the 50% of times. I have also to chech if the capacitor is fault...

 

[FvM]

Sounds like the problem is about repairing/replacing a defective pump rather than designing an electronic monitor.

If you expect the rotor to be blocked by foreign objects, a monitor might be reasonable. If it's a level controlled pump, an indirect monitoring can be considered.

 

[BradtheRad]

My family had a house heated by circulating hot water. One day I heard clicking and tapping sounds coming from the pump. This lasted a few minutes.

After that the house got cool because there was no heat coming from the registers. However the boiler heated water. The pump's motor ran.

Then a serviceman disconnected the pump and found the plastic vanes had broken off inside the circulating part of the pump.

 

[meshgate]

BradtheRat,
thanks for your suggestions. I checked if the rotor is forcing, there are no problems and when working there is no intelligible noise, I have to put ears very close to the body of the motor in order to hear the noise of rotation . Also the water is at correct level and pressure as well. Very strange is that: when water circulation starts early in the morning ( cold liquid) the pump runs, during the day when controller stops circulation the pump sometimes does not start again (hot liquid).
I would exclude bodies in the liquid, since, switching off and on the supply, the motor stats without any noise at all. That is because I was thinking the capacitor woul be fault ( it should be an intermittent fault), but it is a bit difficult for me to take it
off in order to measure the value. I do not know if this kind of motors could sometimes start also without capacitor connected.