VIBRATION

The International Standardisation Organitation, ISO, has issued international standard covering balancing and vibration characteristics. ISO 2373 is of particular interest for electric motors. It governs permitted vibration level on delivery and applies to motors with shaft heights in the range 80 t0 400 mm. The vibration level is expressed in mm/s rms ( milimeteres per second root mean squared ) and must be measured at no load with the motor on elastic mounting. ISO 2373 requires the shaft extension to be fited with a full – size key during vibraton measurement. The requirements apply in the measurement range 10 to 1000 Hz.

Grade of	Speed	Maximum Vibration Velocity in mm/s rms
quality	r / min	at shaft height, mm
		8 - 132	160 - 225	250 - 400
N	600   ≤   3 600	1.8	2.8	4.5
Normal
R	600   ≤   1800	0.71	1.12	1.8
Reduced	>  1 800  ≤  3 600	1.12	1.8	2.8
S	600   ≤   1800	0.45	0.71	1.12
Special	>  1 800  ≤  3 600	1.71	1.12	1.8

The corresponding standard for large machines has not yet been issued, but a figure of 2.8 mm/s can be taken as guide, at least for squirrel– cage motors. Measurement with the motor bolted fast to a solid base may occur, as may measurement with a half key fitted to the shaft extension, the smooth shaft method.

Rotor balancing is a relatively simple opration and the balance is easy to check. However, the final vibration resistance is also iinfluenced by other factors, mainly the nature of base on wich he motor is mounted, although the method of clamping, the aligment and the electromagnetic forces also play part.