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Motor failure

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2 Motors of our newly commissioned AHUs (AC SYS) failed on numerous occasions.


These motors are driven by VSDs.


Distance between the vsds and the motors are nearly 60 m.


What is the reason?




there are not enough info and data to give a useful reply.


Motor power and voltage? Type of motor (inverter duty or normal duty), type of load? Type of failure encountered? Tipe of cable between inverter and motor (screened, non screened)




Mario Maggi - Italy - http://www.evlist.it - https://www.axu.it

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Hi Suren, there could be a couple of different reasons for these failures, have you had the motors inspected by the supplier after failure? One cause could be overheating caused by the motor running at low speed for extended periods of time thus reducing the cooling efficiency of the motors fan. However you say that the motors are on AHUs so generally at low speed there is also a low torque requirement and therefore low current due to the centrifugal fan laws. Also any quality VSD should have internal thermal models that will take account of the speed and current of the motor and trip before damage is done. If this is the cause then you should ensure that the thermal model protection is turned on (this is not always on as a default setting) and also set a minimum speed setting on the VSD. Also if thermisters are available on the motor then these should be connected to the VSD as these will provide the best protection for the windings.

Another reason for the damage could be high peak voltage on the motor windings. Although 60 metres is not a long cable run you should check with the VSD manufacturer what the peak voltage would be for that model of VSD at that cable length and then check with the motor manufacturer what the motor can handle.

Good luck


An expert is one who knows more and more about less and less until he knows absolutely everything about nothing
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