VFD Testing with MTAP’s – Part 2

Connecting test equipment to energized motors through an easy access Motor Test Access Panel (MTAP) has greatly improved the safety and efficiency of electric motor testing. However, when it comes to medium voltage VFD’s some considerations are important. The last tip covered the Line Side of VFD’s. For this tip we will be focusing on the Load Side of the VFD.

Load Side Current of the VFD:
The Load Side current of the VFD can be monitored using CT’s. However, care must be taken as the low end frequency range of our standard CT’s is only 40Hz. Thus, the drive must be running at 40Hz or higher to obtain accurate results. Should the drive be expected to run at slower speeds than 40Hz, appropriately rated (Frequency Range and Voltage) CT’s will be required for that application. The standard CTs we provide are also rated for 600V class applications. For medium voltage such as 4160V applications, custom CT’s rated for 4160V are required.

Load Side Voltage of the VFD:
The Load Side Voltage of a low voltage VFD (<600v) can be connected directly to the MTAP and has a low end frequency range of 15Hz. Load Side Voltage on medium voltage VFD applications often cannot be monitored due to the lack of PT’s installed. PT’s are not installed due to the fast rise-times of the voltage waveforms from the output of the VFD. Additionally, standard off the shelf PT’s are typically only rated for 50/60Hz. Contact your drive vendor to see if they have a source of PT’s that will work with the output of their drives for your application.