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Do I need to monitor phase asymmetry when using VSD for motor+pump?

Published date: 06 February 2019

It is not mandatory by IEC standard in general to monitor phase asymmetry. 
However for some installations or application it may be necessary to monitor phase asymmetry as part of protection function. 

Regarding application (motor+pump), it is abvious that every induction motor must be protected against overload and short-circuit. In case of phase asymmetry, the consequence is that one or two motor phases will be overloaded. To monitor phase asymmetry is therefore optional, because the motor remain protected by thermal overload. 

If ATV is used to drive the motor, the situation is much easier because ATV provides protection for the motor even in cases of: 
- one motor phase missing 
- motor short-circuit (phase to ground or phase-to-phase) 
- motor overloaded (in any phase) 
Additionally if there is asymmetry in ATV supply, then no problem for the motor: 
- If asymmetry is small *, DC bus voltage in ATV will have some ripples, but motor will always have 3-phase voltage, balanced. 
- If asymmetry is significant (or in case of missing input phase), ATV will signalize fault and stop the motor. 

* Not applicable for ATV21, ATV212 because they do not have DC bus capacitors and so even small asymmetry can result in fault signalization. 

With ATV630, that is dedicated for pump applications, you will not make mistake. Phase asymmetry monitoring is not needed, although all ATV630 drives up to 160kW provides input voltage monitoring between each 2phase (L1-L2, L2-L3, L3-L1). Values can be displayed in Menu display - drive parameters

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