EMC and VFDs

 

Why do we recommend screened cable on the output of drives?

Is a screened cable really necessary?


Most manufacturers recommend that screened cables are used to connect the output of VFDs to motors, and in a lot of the documentation, the reason given is to minimize the radiated high frequency content of the output of the VFD, but this is not strictly correct.

The real issue is the conducted noise caused by the capacitively coupled currents that flow between the motor windings and the motor frame. These currents need to get back to the DC bus within the drive. (source of the energy)

The output voltage of the drive is made up of a PWM high speed voltage waveform that is synthesized to cause a sinusoidal current to flow in the motor windings. The voltage is effectively switched between the positive and negative DC bus rails with the transition happening at a very high speed (perhaps 50ns). With this very rapid transition applied to the capacitance between the motor windings and the motor frame, there is a very high impulse of charging or discharging current through the capacitance to the motor frame.

This causes high, fast voltage transients to appear on the frame of the motor which in turn cause high fast impulse currents to flow through all possible paths from the motor frame, back to the DC Bus of the VFD. In many drives, particularly CE compliant drives, there are high frequency capacitors connected between the DC bus and the chassis of the VFD and this provide part of the return path for these switching currents. Otherwise, the return path must be via the three phase input so all connected components that have internal EMC filters between phase and earth help to provide that path. -- The net result is that the charging and discharging currents flow through numerous paths and causing interference with everything else on the local network.

The best solution, is to provide a very low impedance return path between the motor frame and the DC bus of the VFD. The currents flowing are primarily in the order of hundreds of KHz, so the return path is very much on the surface of the conductors (outer 0.2mm). For this reason, it is imperative that the return path has a large surface area over the whole path from the motor frame to the VFD.

The surface area of the screen of an EMC cable, can exhibit a surface area in the order of 50 times the surface area of the phase conductors. This reduces the voltage on the frame of the motor by a factor of 50 and so the "stray" currents will reduce by a very large proportion and the interference will be greatly reduced.

The use of the screen as a return path is no imperative, it is a low cost way of getting a low impedance high frequency return path between the motor frame and the VFD. An alternative path can perform equally well provided there are no restrictions in the surface area on the return path .