Balancing Rotor on Small Motor 1

[JWT]

I have an old manufacturer's procedure for balancing the rotor on a small motor used in a gyroscope application. The motor is 400Hz, 3 phase and normally runs at 24,000 rpm. However in the balancing procedures the rotor is balanced at 10,800 rpm by setting the motor input frequency to 180Hz.. I'm looking for background info on why the rotor should be balanced at this low speed vs. the normal operational speed.

 

[nbucska]

Probably large initial unbalance would damage something <nbucska@pcperipherals.com>

 

[electricpete]

In general:

balancing at operating speed is required for rotors which are considered &quot;flexible&quot;, which means they operate near or above their first critical speed.

balancing at a lower (more convenient) speed is satisfactory for rotors which are considered &quot;rigid&quot;, which means they operate far below their first critical speed.

 

[JWT]

How is the &quot;First Critical Speed&quot; determined?

 

[UKpete]

JWT - the company I work for produce permanent magnet motors/alternators that work up to circa 100krpm. These are super-critical i.e. operate above 1st critical speed.

What electricpete says is correct though it is rare to test high speed rotors at operating speed, even super-critical rotors, because it is technically very difficult to make a balancing machine that will do it.

Our rotors are given a low speed balance to 1gm-mm (at about 3000rpm - about 3% of operating speed), we call this a rigid body balance. This gives satisfactory vibration performance at both operating speed and (most important) as it passes through critical speed. To achieve the latter the bearing system must have some damping e.g. squeeze-film. Non-contact bearing systems e.g. magnetic bearings, are more difficult although it is possible to balance these in-situ close to critical speed to get them through.

I would be suprised if your rotor is super-critical. It would have to be long relative to it's diameter. You can do a simple beam calculation to determine it's resonant frequency, but ideally you model it with finite element analysis.

 

[UKpete]

I should have added that although we balance at low speed, it is not really the same procedure as on a standard rotor - to achieve balance, material is removed (by grinding) at positions along the rotor between the bearings, with only a final trim at the outer ends of the rotor.

 

[JWT]

The entire motor-rotor assembly we are balancing is only 3.5 cm in length, so I suspect we are not super critical. FYI the &quot;rotor&quot; part of this assembly is a split cylinder with each half mounted to the end of the motor shaft.

I found a good thread in the Mechanical Engineers Forum that gives info on this subject: tread384-37343, Dynamic balancing at low speed.

Thx for all the info.