VFD's & Vibration

Is there any thing in the programing or hardware that could cause a motors shaft to oscillate at a high rate. I monitor vibration on VFD operated crane gearboxes and have come across a strange situation. We blieeve it may be coming from the control system.
Thanks

What type of VFD?
What type of motor?

More info please!

Is there any thing in the programing or hardware that could cause a motors shaft to oscillate at a high rate.

Click to expand...

At what motor speed? Is the high rate of oscillation occurring when the motor is at rated speed or above, or is it when you really slow it down? We do not know your situation, so you have to provide way more info than if you were explaining your situation to a colleague at work.

There are many things a VFD can do to cause vibrations in the load.

It mainly comes down to the waveshape of the current applied to the motor windings. Purely sinusoidal currents will cause no more vibration than across the line operation, however, there are also other considerations.

For example, let us assume your mechanical load has a resonant vibration when it is run at the equivalent of 3Hz. When the VFD runs the load at this speed, the vibrations have the opportunity to "multiply" very much like Ella Fitzgerald's glass in the famous "Is it live or is it Memorex?" commercials. This assumes the VFD output is sinusoidal at 3Hz.

If your drive is exciting the motor at one of its resonant frequencies, then you need to implement a "band skip" function which bypasses the known bad frequencies. In the above example, I'd probably put in a band skip from about 1.5 to 5 Hz and see if it helped. The drive will typically still excite the motor in the "skipped band," but will either operate continuously above or below the skipped band, going through the skip as quickly as possible according to ramp parameters.

If the applied waveform is NOT sinusoidal, then it will excite the motor/load in such a way as to cause additional vibrations resonant with the harmonics causing the non-sinusoidal waveforms. Older VFD systems that had the notorious "six-step" outputs had harmonic noises at the 5th and 7th (and 11th, and 13th, and all odd harmonics not divisible by 3). These harmonics were present on the VFD output, and filters could only account for "so much" of this distortion.

The worst case is when you have an electrical harmonic (causing vibrations) that corresponds to a mechanical resonance (multiplying the vibrations). When this occurs, I've seen Bentley Nevada vibration monitors practically pick up a telephone to dial 911. You would swear the floor is about to collapse.

So, the answer to your question is "Yes."

The motor is a 6 pole induction motor operating at either 60 or 90 Hz.
Some other units are wound rotor motors utilizing eddy current control. This is according to the manufacturer.

The vibration is occuring at a frequency that is a non integer of running speed. Looks like a bad bearing. However, this frequency component is through out the vibration spectrum on all data points. Including the gearbox that the motor is driving. It is even non characteristic of a resonance.

Problem here is this is an unusual problem. What I am loking for is an if a VFD can affect the operational characteristics of the motor in this manner.

Many Thanks

In my experience, the output pulses from a PWM inverter can cause a large variety of vibration readings on an analyser. Some of these are proportional to the drive carrier frequency and some are not.

The important thing about this subject is that I have never seen any detremental effects from these vibrations.

One of the most sensitive applications for shaft vibrations is metal polishing. Years ago, the polishers were all driven by DC motors and the pattern of the commutator bars would be clearly visible in the polished surface. Later, we attempted to do this job with six-step voltage source drives but that wasn't smooth enough either.

When PWM came along, even with darlington transistor outputs and carriers as low as 600hz, the problem was solved. No more shaft pulsing patterns in the polished surface. We don't even think about it at today's carriers of 2khz and higher.

Thanks all,

The vibration levels that I am seeing are extreme. We have come up with a way to verify if it is electrical VS mechanically induced.

The motor is driving a parallel shaft 4 stage gear reducer. The gears are helical cut and generally do not react well to a cyclic motion like I am referring to.

What I am getting from this post is: The shape of the inverter output wave form can cause this however, normally not to a degree that would be damaging.

I'll post back the results when we determine the cause.

Hello!
I ones had a problem a little like yours, motor or gearbox made a lot of noises. We tought it was broken checked the motor and gearbox no problems ?
Then we started to check the cabels ! Ahaa one of the phases was missing! The motor was running with only two phases that was what caused the noise.
The motor will start running if it gets a "jump start" from outside. This motor was in a overhead crane traversing movement which had four motors. The other motors gave the faulty motor it´s jump start...

just remembered that the noise was at the worst when driving in slow speed