I have a question concerning an electrical test reading taken at the VFD bypass contactors. This VFD provides power to a 15 HP motor for an air handler unit. To check resistance across my contacts on my contactors I take voltage readings under load for each phase (from L1 to T1, L2 to T2 and L3 to T3). Readings are checked at the 3 contactors (bypass, VFD input & VFD output). I also do the same check on my overload which is in parallel to the load side of both the bypass and VFD output contactors.
In the VFD bypass mode, I get zero volts from line to load for each phase of the bypass contactor. The readings for each phase at the overload is also zero volts. In the VFD mode, I get zero volts from line to load for each phase of the VFD input contactor. At this point all readings taken are normal, but when I check the VFD output contactor I am puzzled by my readings. All readings at the VFD output contactor are 2.1 volts for all phases. The readings on the overload are also 2.1 volts. All six reading are exactly the same down to a tenth of a volt. When I raise the VFD speed to 60 Hz, all readings rise to 3.2 volts.
I assume there is a logical theory for these readings and would appreciate it if someone could shed some light on this.
In the VFD bypass mode, I get zero volts from line to load for each phase of the bypass contactor. The readings for each phase at the overload is also zero volts. In the VFD mode, I get zero volts from line to load for each phase of the VFD input contactor. At this point all readings taken are normal, but when I check the VFD output contactor I am puzzled by my readings. All readings at the VFD output contactor are 2.1 volts for all phases. The readings on the overload are also 2.1 volts. All six reading are exactly the same down to a tenth of a volt. When I raise the VFD speed to 60 Hz, all readings rise to 3.2 volts.
I assume there is a logical theory for these readings and would appreciate it if someone could shed some light on this.
