Hi,

I have concern about electric motor allowed level of vibration in rms mm/s. After motor has been serviced I got report it has balancing tolerance for 20 grams. I know what this mean and I agree it should be them much bay ISO standard.

But I want to test this by measuring level of vibration when I get it in my shop.

How to know was it good balance by measuring level of vibration, what is electric motor allowed level of vibration?

A basic question, what does 20 g imbalance mean? This is not a proper unit of imbalance.

According to ISO standards, the allowable vibration for a motor depends on the size of the motor.

Enrique,
You, as the customer should supply the motor repair shop with your own standard for balancing, for example the shop told you they balanced it to 20 grams. That probably represents 20 grams at the balance ring radius on both ends. So, if the radius is 5 inches, then the residual unbalance is 100 gram inches.
Ask the shop to further explain what they mean by 20 grams. One end or both? At the balance radius?
The attached nomogram represents the tolerance for precision balance results. Notice that for a given rotor, you need to look up the rotor mass in lbs. and then cross reference the speed the rotor when in operation.
So, for a 40 lb rotor operating at 3600 rpm, the recommended tolerance in gram-inches would be 2.5 gram-inches total for the whole rotor, or 1.25 gram-inches per end.
This nomogram is copied from the manual by Schenck Trebel, a manufacturer of hard bearing balancing machines.
Now, you get the motor to your shop and you run it to see what vibration levels you get.
First, if there is no coupling on the motor shaft, remember to fill the keyway with a half key (make the keyway flush with the shaft surface). It is also a good idea to make sure the shop that balanced your motor did this as well. If they balanced the motor without a half key, you will not have a balanced motor when you cut the key to lock the coupling or sheave.
Do not put the motor on concrete. If you have a steel test bed, shim the motor, bolt it down, correct for softfoot and then test.
If you do not have a steel test bed, then put the motor on isolation pads, making sure they are not too hard or too soft. I believe the proper isolation pad must not deflect more than 10% of it's unloaded thickness, but I could be wrong on that.
Now, the motor is yours, it's in your shop and you want to know if it is with a specification. That specification should be yours.
NEMA (a US based association supported by motor manufacturers) has a spec that calls for unfiltered velocity levels to be no more than 0.10 inch/second at 1800 rpm or less and no more than 0.15 inch/second at 3600 rpm. In my opinion, these levels are slightly high. Most of my customers will not accept anything over 0.10 inch/second and some customers require less than 0.075 at 1x (most affected by unbalance).
Hope this helps.
Ron Brook

The vibration levels in an electric motor after repairs should not focus on just the unbalance in the motor but the overall vibration. I was associated with an electric motor shop for many years and our spec. was less than .1 In/Sec overall peak vibration at 1000 Hz Fmax.

Thanks to all.
Unit of unbalance per ISO 1940 is gram. I have rotor mass of 2150 kg, radius 400 mm and distance between bearing 1200 mm, 1485 rpm.
When I calculate allowed level of unbalance and refers to ISO 1940 I have G6.3 it is very good job done by service shop.

Ron i am not sure is it one ore both. By the geometry of this rotor it should be dynamically balanced?

quote:

Unit of unbalance per ISO 1940 is gram.

Not so.

Read the definition in 3.5 of ISO 1940-1, and you will see units of g-mm in Note 2 to this definition; moment imbalances are g-mm^2.

Later on 1940 goes on to say the SI unit would be kg-m, but most of the time for balancing this is ridiculous (maybe not for wind turbines?).

Aplication of ISO1940 to this rotor suggests that a balance grade of G6.3 requires a specific unbalance 40.5 gram-mm/kg.

For the dimensions given, assuming that the radius is 400mm at both ends, and that the allowable unbalance is split equally between both ends then G6.3 for this rotor corresponds to an unbalance of 108grams at each end.

Whilst an actual unbalance of 20grams looks good, I would suggest the classes of machine defined in ISO1940 are a 'little slack' and that you should be aiming for for G1.0 (17.2 grams at each end) or better for any machine in a workshop balancing environment (or 4W/N): the cost (in $ and time) to achieve this is negligible and provides the best possible opportunity to have a machine that exhibits low levels of vibration when installed.

I tried to attach the balance tolerance for both a G1.0 and G0.4. I agree that a G1.0 is easily attained and should be what you ask for.

Always remember that a balance specification must stipulate mass AND radius.

G1.0_G0.4_Nomogram.doc (78 Kb, 22 downloads)

Yes unit for unbalance is grmm/kg but what I thought when said 20 gram is apsolute level od unbalance which I get by formula:

Ma=(grmm/kg)*mass of rotor/radius and for G6.3 I get it is alowed 64 gram.

Thanks a lot for advice to all concerning application of standard.

And I think so it is on booth end...