Protection of VFD-fed motors

How to properly protect an inverter-duty variable-speed electric motor? What protection is covered by the VFD? What shall be done externally? Which protection elements are most suitable? The article on protection of VFD-fed motors brings the answers.

General considerations

System and equipment protection is of crucial importance. It has direct impact on reliability, availability and especially safety. The protection system shall ensure following requirements:

Selectivity
Sensitivity
Speed
Security
Reliability

These are general items that fully apply for the protection of VFD-fed motors as well.

So the protection functions shall provide protection in the respective zone, it shall have the necessary sensitivity, react fast enough (to minimize risk and the magnitude of hardware damages), avoid major hazards and escalation and work in a reliable way (proven concepts, backup protection etc).

Differences between direct on-line and inverter duty motor

Inverter fed motor is operated differently than a direct on-line working machine. First obvious difference is the variable frequency. The sensors of electric signals must work reliably across the whole frequency range and must keep required accuracy and sensitivity (for example, “standard” 50 Hz voltage transformer might have guaranteed accuracy only in specified frequency range, e.g. 45-65 Hz).

Due to that, measurements with external devices may not perform very good. It is normally a better choice to rely on the sensors that the VFD inherently includes.

Moreover, the voltage waveform (in case of VSI drive) is not perfectly sinusoidal but contains voltage steps. The more levels the inverter has the closer the waveform to a sine wave. However, even the multi-level inverter topologies do not perfectly synthesize the sine wave.

An inverter duty motor is designed to cope with such voltage waveform. The same is expected from the protection elements.

Attention: There are protection elements commonly used in DOL machines which are not suitable for VFD supplied motors (at least you should carefully double-check their suitability). Using such devices may not contribute to a better protection but rather increase the probability of a fault.

Do you want to know more? Sign up for our premium subscription!

Protection elements for VFD-fed motors

Now how to protect VFD-fed electric machines? And is the VFD the only “protection element” that covers all protection functions?

1. VFD

The VFD is an intelligent device. In a way it is a large “smart sensor”. So far its potential has not yet been fully utilized. What machine protection functions does a typical VFD provide?

Overload protection
Overcurrent protection
Stall protection
…
…
…

Availability of certain protection functions also depends on the control algorithms being used. In general, closed loop schemes allow not just more precise control but also a better (faster and more accurate) motor protection.