RACA Journal November 2020 November 2020 | Page 42

40
Support
This is a very important aspect that is often neglected . Failure to do a root cause analysis could result in a repeat failure . Why did the fault occur ? Why did the component fail ? Did the component fail prematurely ? Is the component failure the symptom or the cause ? What can be done to prevent future failures ?
Looking at electrical motors it is important to know that there are seven fault zones , namely : 1 . Power quality 2 . Power circuit 3 . Insulation 4 . Rotor ( with its bearings ) 5 . Stator 6 . Air Gap 7 . Cooling
By looking at the trouble-shooting charts , we can diagnose most faults with regards to an electric motor .
Let ’ s look at an example where a motor would not start . In gathering information , the client indicated that there was a burning smell .
On testing the windings , we found that the resistance readings on all windings were incorrect . In addition , the motor windings tested as down to earth . On opening the motor all was revealed .
In this case the motor supply was the cause . On testing , it was found that the supply voltage drop was excessive . This created a situation where the motor continued to run but in an overloaded state , drawing excessively high amperage . The overload on the power circuit was not operational and the result was a burnt-out motor . After all repairs were undertaken to the motor , we remembered to correct the root cause .
GENERAL TROUBLESHOOTING : If an electric motor is subject to improper operating conditions , electrical , mechanical or environmental , the winding life will be reduced significantly .
Insulation defects can be caused by contaminants , abrasion or voltage fluctuation and overheating .
Complete insulation burn out on all phases of a three-phase motor is often caused by motor overload or under or over voltages . A locked rotor can also cause the windings to burn out .
Rapid start / stop scenarios will cause heat build-up in a motor – if temperatures become excessive , burnt insulation will result .
We may find that we have phase defect . This is a consequence of a power supply interruption in one phase . A burnt fuse , an open contactor , one power supply interrupted , or a poor connection normally causes this defect ( also known as single phasing ).
The insulation burn-out in one phase of the stator winding can be a result of uneven voltage between phases . Uneven voltages are usually caused by unbalanced loads in the power supply originated by poor connections at motor terminals or by bad contact . 1 % of voltage unbalance can cause a current unbalance of 6 % to 10 %.
On a single-phase motor burn-out to the start winding is normally caused by the non-opening of the centrifugal or start relay where the coil remains switched on longer than the specified time .
Jaco , I hope that this assists you with your installations and motor issues . Remember to always determine and correct the root cause – this is often overlooked and will result in repeat failures . RACA
Thank you for all your questions . Send your problems ( and sometimes your creative solutions ) to acra @ netactive . co . za with ‘ Solutions Page ’ in the subject line . You may include pictures .
REFERENCES : 1 . ACRA

40

RACA Journal I November 2020 www . hvacronline . co . za