High-pitched, shrill sound from MCB as the current increases. Is the MCB the culprit?

[Wrichik Basu]

We bought a 4HP semi-commercial treadmill that requires a 16A outlet. Our mains are 230V RMS @ 50Hz. Just got the electricians to install a new set of wires from the ground up. The new outlet box that I bought on Amazon came with a 25A SP MCB, one 16A outlet, and four 6A outlets. The circuit is like this at the moment:

Here's the issue: When I am switching the treadmill on and increasing the speed, I can hear a shrill sound. A pretty high-pitched shrill sound. As I increase the speed (max speed is 22km/h at 6HP), the noise becomes really loud. I mean, concerning loud. And I am 100% sure that it is coming from the MCB in this outlet box because yesterday I tested the treadmill using a separate 16A extension outlet that has a fuse, and there was no such sound. I tested this new outlet box at normal running speed, and it was fine. My bad, I should have tested this too at full power.

I can ask the electrician to bypass that MCB and connect to the 20A SP MCB. That's fine. But is there anything I can check to make sure that the sound is indeed coming from this MCB? I am really disappointed because now I can't return this thing as holes have already been drilled.

Also, how damaging can this be for the treadmill? I have ordered a servo stabilizer, but it will take time to reach.

 

[Baluncore]

The treadmill probably uses a DC servo motor. The AC supply is rectified, then PWM by an H-bridge, to control the motor. You are somehow hearing magnetostriction, as the PWM current passes through some iron core inductor, in the MCB, but more probably the motor.

 

[Wrichik Basu]

The treadmill probably uses a DC servo motor. The AC supply is rectified, then PWM by an H-bridge, to control the motor. You are somehow hearing magnetostriction, as the PWM current passes through some iron core inductor, in the MCB, but more probably the motor.

This treadmill came with an AC motor.

Also, if it is magnetostriction, I believe I should've heard the sound for the past three days too?

 

[Baluncore]

This treadmill came with an AC motor.

Then, how is the speed changed?

What mechanism is used by the MCB? Could the MCB be responding to the motor controller, or a power factor controller?

 

[Wrichik Basu]

Then, how is the speed changed?

What mechanism is used by the MCB? Could the MCB be responding to the motor controller, or a power factor controller?

Zero idea about how the internals work. But AC motor treadmills are pretty common these days, especially if you want a higher max weight and higher speed than DC motor treadmills.

 

[Baluncore]

Zero idea about how the internals work.

Then what is the make and model of the MCB?
Is it a thermal, or a magnetic overcurrent, detector?

 

[Wrichik Basu]

Then what is the make and model of the MCB?
Is it a thermal, or a magnetic overcurrent, detector?

It is an SP C-curve 25A MCB. To be exact, this one.

 

[Baluncore]

I expect your MCB has a bimetallic thermal trip for detecting overcurrent, and an electromagnetic trip for sudden short circuits. You may be hearing the electromagnet responding to the high frequency current drawn by the variable frequency motor controller. I would put a low-pass noise filter in the line from the MCB to the treadmill, to see if it changes the sound.

 

[Wrichik Basu]

Could it be an earthing issue? I just measured neutral vs PE, it's coming to be 4.87V, unlike last time, where it was 0.3V. Live vs neutral and live vs PE has a difference of around that same value.

Maybe the electricians messed it up somewhere?

 

[Baluncore]

Maybe the electricians messed it up somewhere?

Anything is possible in India.

The sound must come from a "transducer" somewhere. You need to identify that transducer as the source of the sound.

Use a half metre long, clear vinyl tube, about 8 or 10 mm internal diameter. Hold one end to an ear, search out the source of the noise with the other. If the noise is coming from the MCB, then it really must be the electromagnet coil, core, and armature. But the MCB is not the problem, the problem is the source of the high frequency current noise from the motor controller, reaching the external circuit.

Maybe the lack of a PE on the treadmill circuit is preventing the noise filter on the treadmill motor controller from functioning. Maybe a connector clamp has not been tightened, so there is a current sensitive spark gap in the AC supply circuit.

Chasing AC voltages with a high impedance multimeter will only lead you astray. Instead, try plugging in a 240 volt, 100 watt filament lamp, or an electric water heater on the same circuit. As that will change the RF impedance of the local wires, if the noise is travelling electrically over the wires, changing the impedance and load point, will change the character of the noise.

 

[Wrichik Basu]

Anything is possible in India.

Agreed, sadly.

If the noise is coming from the MCB, then it really must be the electromagnet coil, core, and armature. But the MCB is not the problem, the problem is the source of the high frequency current noise from the motor controller, reaching the external circuit.

Maybe the lack of a PE on the treadmill circuit is preventing the noise filter on the treadmill motor controller from functioning.

I believe you just located the source of the problem. Explains why the buzzing starts as soon as I start the motor, but not when the treadmill has power in general and just the software is running.

This electrical noise is detrimental to the AC motor of the treadmill too, right? I turned on the treadmill thrice at almost max speed while trying to reproduce the noise with different configurations and to take readings.

Chasing AC voltages with a high impedance multimeter will only lead you astray. Instead, try plugging in a 240 volt, 100 watt filament lamp, or an electric water heater on the same circuit. As that will change the RF impedance of the local wires, if the noise is travelling electrically over the wires, changing the impedance and load point, will change the character of the noise.

I am not turning on the machine again until that PD is gone/is negligible. We saved up a fortune for this; I am not letting it go to waste because of some idiots. Oh man, I am so done with chasing these issues.

 

[berkeman]

Use a half metre long, clear vinyl tube,

Can I use an optically translucent one instead?

 

[Baluncore]

Can I use an optically translucent one instead?

You are only limited by your imagination.
Clear Vinyl Tubing, CVT, is a widely available, low-cost product.

Avoid electrically conductive tubes, unless you want to be the first to hear the shocking news.

 

[Tom.G]

Then what is the make and model of the MCB?
Is it a thermal, or a magnetic overcurrent, detector?

The circuit breaker is listed as Thermal-Magnetic.
Which means that for mild overloads, the Thermal trip eventially opens the breaker. (delay trip)
For heavy overloads, the Magnetic trip acts as a fast trip, not waiting for the Thermal part to heat up.

See page 23 of:
UNO_Catalogue_20231226151543567399372675.pdf

Cheers,
Tom

 

[DaveE]

I just measured neutral vs PE, it's coming to be 4.87V

Too much IMO. But that assumes it's 50/60Hz. Some meters may also be measuring the higher frequency noise that you are hearing. Try it with an incandescent light bulb or heater element, they don't (shouldn't) make electrical noise.

One potential cause is bad connections in the neutral wiring (too much resistance). You could also try measuring the resistance WITH EVERYTHING TURNED OFF from the PE to another ground (a water pipe, another outlet). Also from an unloaded neutral to various PEs and grounds. Do it carefully. Measure voltage first and verify it's zero so you don't kill your meter or yourself. Even 5V could hurt your meter, or at least give you bad data. Swapping hot and neutral isn't an uncommon error in careless wiring.

You might also be able to trace the voltage drop upstream to see where it goes bad if you know the wiring well enough to find upstream outlets and such.

 

[Wrichik Basu]

Try it with an incandescent light bulb or heater element, they don't (shouldn't) make electrical noise.

"Try it" in the sense, switch on an incandescent bulb or heater element and then measure the PD?

It's so irritating, this morning, it's down to 0.5V. I basically have nothing to show to the electricians, except for my word that it was 4.87V last night.