24 VDC PWM Causes Objectionable Motor Noises in Housing

I haven't come across this problem before. Are you running the DC variable speed controller at a very low speed? What's the motor speed and controller output (as say a percentage of range). If it is a resonant frequency thing it should only be happening in a very narrow band of motor operating speed.

Additionally have you checked the battery or input voltage to ensure that it is not sagging during the output pulse (making the problem worse).

The only time I have seen this happen is when the DC variable speed controller 'hunts' at low speed causing noticeable large pulses of speed at between 15 to 60 pulses per minute, but again this is more due to a faulty DC variable speed controller.

Have you checked the motor brushes for uneven wear or damage? What about the fan blade and bearings? Could be something mechanical rather than electrical.

The gearbox has a label that specifies the no-load output speed as 125rpm. Since the worm gear gives a 20:1 reduction, this suggests that the armature is operating at a no load speed of 2500 rpm. The same label specifies a coil voltage as 24 VDC.

I have driven it with a straight 24 VDC from a 50 amp power supply and without the PWM. It runs silently.

When installed there is an additional reduction via belt and pulleys of 10:1. With a no-load output speed at the gearbox output shaft of 125, that means that an unregulated rpm, at the turntable is 12.5.

The power supply that normally powers it is a regulated laboratory unit with five times the amperage capacity. It never occurred to me to suspect that the power supply may be causing the noise, not the PWM.

I'll hook up the motor with the PWM to a pair of 12 volt batteries in series and see if the humming still occurs.

There are two potentiometers on the circuit board. While I have the schematic diagram (see embedded diagram below) I can only find one symbol for a variable resistor in the schematic.

One pot controls speed so I suspect that It's varying the frequency. I suspect that the second one may regulate amplitude for torque compensation at low speeds but I don't know that.

The unit is brand new and has not seen more than 20 minutes of run time. It is one of two units and both have been tested and run fine of two different power supplies.

Thanks

L.J.

Generally speaking, one pot will control the switching frequency and the other will change the mark/space ratio.

Its the mark space ratio which will control the average voltage and therefore the speed of the motor.

The speed of switching (Frequency) is what you need to adjust to see if you can get to a frequency that will allow the motor to run quietly....

If you look here, you will probably understand the principles better, its well and simply explained:-

http://en.wikipedia.org/wiki/Pulse-width_modulation

One great sentence from here helps in your quest further I feel, especially the sentence I have underlined!! Using a frequency meter or oscilloscope may be needed to identify the frequency you are actually using....yours is probably too low.:-

The PWM switching frequency has to be much faster than what would affect the load (in your case motor), which is to say the device that uses the power. Typically switchings have to be done several times a minute in an electric stove, 120 Hz in a lamp dimmer, from few kilohertz (kHz) to tens of kHz for a motor drive and well into the tens or hundreds of kHz in audio amplifiers and computer power supplies.

Your PWM circuit may simply not be suitable (too low a frequency for example) for good quiet motor control...

Andy wrote: " Your PWM circuit may simply not be suitable (too low a frequency for example) for good quiet motor control..."

Andy, it may also be sufficient to damage the insulalion on the coils which could cause premature failure. There is a mechanical component to that noise and the energy has to go somewhere.

The main unit is apart so the mechanical parts can be donated to a second prototype. I will cobble together something and run some tests. I think in the final analysis, I'm better off shelving this PWM and buying one that has a higher frequency.

Thank you!

Best of luck.

Please DO keep us upto date with any changes you make, we are all REALLY interested......(or just plain Nosey?)

Will do Andy

Thanks for the generous support

L.J.

A quick view on the circuit diagram shows a triangle generator made with the first amplifier IC1/1 and the capacitor (C5?). The potentiometer (R4 30K?) is to change the frequency of this triangle signal.

The second amplifier is used as a threshold (adjustable with VR 30K).

If the instantaneous voltage the triangular signal exceeds the set value of the threshold, then conducts IC1 / 2 and that will drive the power stage of the control

With this combination you can adjust the switching frequency with R4 and the output voltage with VR.

That's the first time I've been able to comprehend how the system works. It would be embarrassing if it turns out that the noise was from driving the motor improperly. Thanks also for identifying the second variable device. I could not find it.

Thanks for the support.

L.J.

1. Do you get the vibration when running on batteries at similar speeds?

2. Have you looked at the PVM output at the problem speeds, to determine the pulse frequency?

3. Note that a square wave is built from multiple harmonics of the fundamental frequency, so the issue could well be with a one of the higher frequency components (use a microphone and your sound card to get a rough idea of the fundamental frequency of the vibrations).

4. If it turns out is is not a mechanical issue and is in fact an electrical issue at low drive frequencies, you might want to consider driving the motor at higher speeds and using a gear or belt speed reduction setup to get your desired lower speed.

Is it possible to place a low pass filter between motor and drive. (just a big/ heavy iron transformer and taking the secundairy winding as low pass filter. Do not short circuit the primairy)

I've not run the unit on batteries but have noticed that when at higher speeds the noise stops. This suggests that an additional, second stage reduction should be added so as to put the motor into a higher operating range. Similar to your suggestion.

Thanks

Its been a long time since I worked on inverters (PWM), but as I recall they all Sing at the base frequency. The last one I did was running at 10 KHz but wasn't particularly audible. It drove a 250 amp field on a starter generator then switched to 1 KHz for the regulation portion of the operation. the higher frequencies don't resonate the magnetics as badly.

Have you tried using a higher and or lower (if its already high) frequency of switching.

That will usually fix the problem if due to the frequency (or other multiples of) of the PW. But if its partly due to mechanical failings. this may only be a partial fix....

Best of luck.