Electricity + Control May 2016

DRIVES, MOTORS + SWITCHGEAR

• The rotor of a motor is a critical component. • The rotor has a major influence on the motor performance. • Test and inspections on the rotor bars can and should be carried out when a motor is opened.

take note

and contraction of the bar, which leads to further stressing of the bar and joints • During stalled conditions the majority of the input power goes to the rotor resulting in over- heating of the rotor possible resulting in a meltdown as shown in Images 6, 7, 12 and 13 • Unbalance voltage supply system and harmonic can cause excessive heating of the rotor due to negative phase sequence and high frequency currents in the rotor

Testing of rotor bars There are a number of ways to test for broken rotor bars some in operational tests others static:

In operation: Current signature Vibration

With the rotor in a lathe or balancing machine: Induced voltage/current (the use of a search coil near to the surface of the rotor spinning in a magnetic field and the resultant wave form in the search coil displayed on an oscilloscope) Static with rotor on an inspection trestle: o Visual (Copper bar rotors) and server cases of cast aluminium rotors o Growler and noting the current as the rotor is turned or as the growler is moved from bar to bar around the rotor. o Note: A 50 Hz growler with a search coil will indicate an induced voltage or the current method is an indication as to the condition of the bars but may not penetrate to the bottom bars in a double cage arrangement o Passing a current through the bars by connecting a power source between the end-rings and checking the magnetic flux pattern Conclusion Rotor bar failures are a major concern as the effect the performance of the motor and any defect in the rotor bars whether Cast Aluminium, Copper Bar, Double Cage type or any other will get steadily worse and if not detected and rectified could result in a very serious motor failure. (Bar could lift damaging rotor core, stator core and windings). Test and inspections on the rotor bars can and should be carried out when a motor is opened for any reason. Vibration and current signatures will assist in indicating and potential rotor problems, and these can be dealt with at a convenient time as soon as possible. It is not possible to repair a cast aluminium rotor bar failure and a replacement is recommended.

Henry du Preez has a BSc degree from the University of the Witwatersrand, an MBL from UNISA, GED electrical engineering (Wits) and an Electrical and Mechanical government certificate of competency. He is a Fellow of the SAIEE and a registered Professional En- gineer. He has fifty years’ experience in the heavy engineering field, industry and mines and specialises in electrical machines and transformers. He currently works as a consult- ant, predominantly for repair and maintenance in mining and industry. He offers training courses in the field of machine and transformers aimed at users, engineers, maintenance

staff and the repair industry. Enquiries: Email henry@hdupreez.co.za

May ‘16 Electricity+Control

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