Abstract
In addition to controlling the voltage and current input signals, modern adjustable speed drives (ASD) or variable frequency drives (VFDs) also provide some motor protection functions. However, they fail to provide a biased thermal overload protection (BTP) that must reflect the motor heating due to overload, change in the ambient temperature, loss of cooling etc. A separate motor protection (SMP) relay is normally used to provide a complete protection and monitoring solution. However, various challenges are associated with the SMP when applied to protect a motor driven by ASD. These challenges are: significant distortion in the motor input signals, motor starting/running with continuous varying frequency resulting in varying fan cooling, limitations to motor differential and short circuit protection, allocation of instrument transformers etc. This paper discusses these practical challenges and proposes various solutions in order to achieve a more accurate, biased thermal protection solution. In addition, various general applications of motor protection relay with ASDs are also discussed.
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