High Pitch Noise in DC Motor

Sorry I can't answer the question, but I lodge an interest in the replies, because a few years ago I investigated the possibility of using a DC electric motor to drive the propeller of my narrowboat, but a couple of demo boats I looked at had motors that gave off a shrill whine that was extremely uncomfortable on the ears.

"...they do that ! ...you'll get used to it !....." was the response.

They couldn't explain it. No thanks! I thought. Although oddly enough, in the confined space of the engine compartment , it was difficult to pinpoint the the exact spot where the noise was coming from.

I appreciate I am only talking of a couple of horsepower compared to your motor but it put me off the idea.

This motor has a VFD power drive unit? What's its PWM frequency? What frequency is the noise? Magnetostriction can be a pretty effective acoustic noise generator, and there may be a mechanical resonance involved as well. You said, E-M noise (observed how?), but then mention vibration in the same breath.

Dear WH,

Yes the Motor is being driven by 6 Pulse DC Drive. We have not measured the Noise Frequency, as do not have any equipment to measure the same.

We feel that this is EM Noise as the Noise practically reduces to ZERO when the Electrical power to the motor is Switched OFF, while Motor Armature is still in motion.

From the added information we now have, I agree that this is likely a magnetostriction noise problem. I discounted the idea originally because as you point out this happens with an older VFD design that switches in the audible frequency range. An older design and installation would presumably be operational with or without this noise already deemed acceptable. Something changed, I guessed a mechanical or other failure.

Hah, hah, ha!! Sorry, AC or DC, who's paying attention, not me! That magnetostriction whine completely blocked it out.

on a piece of equipment we had designed, there where (2) 40 HP Baldor Super 'E' AC motors with a frequency drive.

And the 'whine' that came from them was very uncomfortable. When I asked the elec. engineer about this, I believe he mentioned it had something to do with 'carrier frequency', I believe he mentioned something about it hitting a natural residence frequency. This was (10) years ago.

I never really experience quite as pronounced with over 12 years of designing I thought it had to do with using the efficiency motors.

I was aware of SAF playing with this a few years ago, and they were attaempting to use their AC drive to give a DC output on 2 phases, but the new DC drives in the power rating of 450kw DC from Rockwell (Powerflex DC) and ABB (DCS800) I have only seen the traditional rectified 6 or 12 pulse (creating 360HZ or 720Hz "noise").

I have always heard the "carrier" from theses drives, and often have complaints from the vibration folk they can see the carrier on their instruments, even though it is sometimes recorded even with the probe in mid air just beside the motor .

A 'scope picture of the DC supply voltage to the motor would tell a story, possibly identify the driving source and the frequency.

Is this a new condition that followed a drive or motor repair or replacement?

If this is a new condition following a drive failure and/or any work performed on the drive I would analyze all components of the drive for compliance with OEM design.

You did not state if the drive is an IGBT or GTO design or SCR or (?) Knowing this is also helpful when giving advice for troubleshooting.

Have you examined the reconstitued waveform with an oscilliscope to identify any/all harmonics present?

We had a similar issue with a drive that we traced to a failed capacitor in the harmonic filter section on the DC bus side of the drive.

It is very important that all OEM specifications are met when replacing any component in a DC or AC drive otherwise transient noise as well as drive and/or motor failure can occur.

My initial guess is a bearing in the motor has failed. That is if this is a mechanical and not an electromagnetic noise. Staying in the mechanical noise realm, my next two guesses are a loose rotor lamination or possibly excessive pitting of the commutation contact area. The only way to identify any of these failures are to disassemble the motor. Most people will just replace the motor. The failing motor can then be repaired by a firm with all of the tools and expertise to measure all of the critical dimensions.

A common mechanical problem that can damage the motor bearings is the coupling of motor to mechanical load may not be properly aligned. Again this can only be examined by a qualified person on site examining the machine. If you are very lucky, a misalignment can be corrected before motor damage happens.

I almost forgot, there could also be other mechanical damage that is not in the motor itself.

If the noise is an electromagnetic radiation that interferes with another piece of electronics then either a shielding drain wire or capacitor on the drive circuitry has failed.

Dear Redfred,

We have done the Vibration Spectrum Analysis of this motor under free suspended condition as per IEC guidelines. The Vibration spectrum does not show any significant peaks.

The problem with Bearing and Loose Lamination should have got recorded in the Vibration Spectrum. We have checked the commutator as well, but it is very clean ( No Pitting)

Thanks..

You have the equipment for vibration spectrum analysis, but you don't have a microphone to plug into the spectrum analyzer?

Hunh?

How about a frequency spectrum for the noise; what is its shape, are there any notable peaks, how does it roll-off, does voltage, current, or load, affect the pitch and/or shape? Next we need some design details of the motor; number of poles, slots, winding pitch for both stator and rotor, number of commutator faces, how connected, number of brush groups, angles, etc. Another thing, what is your DC source; DC generator, AC line rectification, active electronics AC/DC converter, etc.? What type of bearings, floating, fixed, roller, ball, tapered, sleeve, etc?

The fact that the noise diminishes when unpowered does not rule out bearing noise, it's possible that the bearings are being affected when the magnetic forces affect the centerline of the shaft. Before you tear the machine down for inspection I would run it at various combinations of load and speed (if it is a variable speed motor) and see how/if the noise spectrum changes, same thing for powered, unpowered. You've got to rule out as many variables as possible, and don't be surprised if the noise magically disappears after you inspect and reassemble the motor.

Dear RaMConsult,

We have arranged for recording the Noise Spectrum, shall come back with the details.

Today we have taken one more trial, we were feeling that the Noise is because of the Compensating winding design, so we have disconnected the Interpole and Compensating winding of this motor and run the motor at No Load 1500 RPM (Rated PRM), surprisingly the Noise did not disappear..

Does this gives any clue..

Thanks..

As it happens, I deal with the noise in a small Maxon motor, noise generated by some unbalance. I intended to perform a Fourier analysis and, because some 15 years ago I used a TI eval board for one of their DSP chips, performing Fourier, I was ready to buy one. Well, you mentioned FREE spectrum analyzers over the internet, and that's exactly what I am going to do. Thank you for opening my eyes.

I suppose this is a new installation? I.e. it hasn't operated OK in the past but the noise has just started?

If you have a smart phone, there is an free app called "Audalyzer" that will let you get a good estimate of the frequency. There's also an Iphone/Ipad app called SpectrumView (also free) that might help.

Just another thought: Is it possible it is a bearing noise and the bearing is being loaded when power is applied to the motor?

Have you tried using a stethescope (or a screwdriver held up to the ear) to try to localize the sound?

As a PS to my first post 1, the power supply was 48 volts from 4 x 12 vdc batteries connected in series. I do not know what form of speed control was used.

Reading through these posts, the OP still hasn't answered the question, that being 'is this 450kw motor being driven by pure dc or dc fed to a variable frequency drive feeding the motor.' Which is it, as this would help explain the source of the noise.