Scope Of Motor Testing
It should be noted that the scope of motor testing depends upon the motor type and size, this being indicated on the inspection forms.
Motor vibration shall be measured in a tri-axial direction, i.e.:
- Point x axis – side of bearing housing at shaft height
- Point y axis – top of bearing housing
- Point z axis – axial of bearing housing at shaft height
The measurements shall be carried out with an instrument conforming to ISO 2954 (10-1000 Hz frequency range). With the motor at normal operating temperature, the vibration velocity shall not exceed 2.8 mm/s RMS, or 4 mm/s PEAK, in any direction.
For bearings fitted with proximity probes, the unfiltered peak-to-peak value of vibration (including shaft ‘run-out‘) at any load between no load and full load, shall not exceed the following values:
- 50 µm for two-pole motors
- 60 µm for four-pole motors
- 75 µm for six-pole or higher motors
Bearing temperature rise limits following a ‘heat run’ of 3.5 – 4 hours are as follows:
- Outer ring measurement max. 90 °C
- Temperature rise from ambient max. 50 °C
- Oil temperature max. 90 °C
- Bearing temperature rise by RTD max. 50 °C
- Lub. oil temperature rise from ambient max. 30 °C (for forced lub. oil systems).
This may be measured and rectified during installation or detected during running by the loosening of each holding-down bolt in turn while measuring motor vibration.
Motor ‘Soft Foot’ Condition
‘Soft feet’ are those which do not have solid flat contact with the base prior to the tightening of the holding-down bolts; one or more feet may be ‘soft’ as shown in Figures 1 to 3.
The profile of the foot contact area may be as shown in Figures 4 to 6.
- Figure 1 – Machine resting on 3 feet, foot 4 is raised or ‘soft’
- Figure 2 – Machine resting on diagonal formed by feet 3 and 4, feet 1 and 4 are ‘soft’
- Figure 3 – Bottoms of all 4 feet are not parallel with base, feet 3 and 4 are ‘soft’
NOTE: Re-machining of rotor feet is required in Figures 4 and 5; temporary use of wedge-shaped shims may be acceptable (maintenance).
Form 14 – Inspection of electric motor – Cage-induction type (incl. control unit)
Form 4 – Inspection of Switching Units – HV Switchgear
Form 11 – Inspection Of Outgoing Unit – LV Switchboard
Reference: Field Commissioning and Maintenance Of Electrical Installations and Equipment Manual