On a variable frequency ac drive, can you still use a amp-clamp to check the motor current? Are amp clamps only for 60 cycle power?
It is good practice to make a record of the current for all motors, but especially ones on a VFD, where your measuring procedure should agree with the display by the drive. Work out how to get proper readings and verify that all is within ratings beforehand. This is much more difficult to work out if there is a problem later. It may make sense to measure the DC bus of the VFD too. If you have a low pass filter for the meter, this shows proper voltages, and the frequency (PWM modulating frequency related to speed) can be measured on a suitable meter. You can try your clamp meter to determine whether it is meaningful or not.
Ordinary analog clamp-on ammeters have generally not been very good for checking VFD current. There are a lot of digital clamp-on ammeters that are designed to read true RMS current. Some may also read true fundamental current. Read the meter specifications and instructions carefully to determine if a particular meter is suitable. There is also a clamp-on ammeter with an iron vane movement that reads true RMS current from DC to above 60 Hz. It is the Columbia Tong Test meter. They were once considered the best meter to use, but now digital meters are preferred.
