EMC and VFDs



VFDs and EMC

What is EMC?

EMC is an acronym for ElectroMagnetic Compatibility and is a regime of standards designed to ensure that interference between items of equipment is minimized. It is some times referred to as EMI or RFI, but with most electronic equipment, there is a) noise generated by the equipment (emissions) and b) a maximum threshold of noise that the equipment can withstand without causing problems. (Immunity)
There are two types of emissions, radiated emissions and conducted emissions. Radiated emissions are generally high frequency emissions above several megahertz and the conducted emissions are the lower frequency emissions. (below ten megahertz)


What is EMI?

EMC is an acronym for ElectroMagnetic Interference and is the effect that a strong electromagnetic field can have on the correct operation of equipment.


What does EMI do?

Electromagnetic Interference causes other equipment to alter the way in which it performs and this can range from very minor interference to catastrophic interference.
An example of EMI, is the buzzing noise that can be heard on an AM radio station. If this is minor, it will be a background noise, but if it is major, it will totally block reception of the radio program. EMI can cause lines to appear on top of a TV program, it can cause measuring equipment to read incorrectly and it can even cause equipment to fail.
In cow shed (dairy parlor) installations, incorrectly installed or designed VFDs can cause the cows to be subjected to electric shocks and this in turn can increase the SCC and indirectly cause an increase in mastitis.


What causes EMI with VFDs?

The output stage of a VFD comprises six solid state switching devices which are switched in a manner to provide a PWM sinusoidal output current to the motor. Modern switching elements are usually MOS FETs or IGBTs. These devices switch from fully OFF to fully ON very quickly, typically 150 - 250 nS.
The very rapid transition of the output voltage from low to high results in switching noise being produced and this can be coupled to the frame of the motor and from there conducted by the ground circuits, eventually returning to the DC Bus in the VFD.


What are conducted emissions and conducted noise?

When an electrical or electronic switch operates, there is a noise impulse that can cause high frequency radiated emissions, and low frequency conducted emissions. The conducted emissions or noise, are carried by the phase and earth conductors to other equipment. noise generated by the equipment that is below 1 Mz is usually conducted between equipment rather than radiated.
The current carried in the earth conductor between the VFD and the Motor can be very high in amplitude although short in duration. The source voltage is high, (hundreds of volts) and a poor connection on the earth circuit can result in major arcing which is very dangerous in a hazadous environment.


How do you minimize EMI and conducted emissions from VFDs?

The best way to minimize the emissions from VFDs, is by firstly selecting a VFD that has been correctly designed to minimize the noise produced, and then to install the VFD correctly. The most important part of the installation, is to provide a very low impedance path from the frame of the motor to the chassis of the VFD.
This would usually entail the use of a screened cable with proper EMC glands or clamps at each end of the cable. The screen must be correctly terminated at the VFD end and the motor end. Pigtails severly reduce the effect of the low impedance screen path.


What is the impact of dv/dt on conducted emissions?

The rate of rise of voltage determines the bandwidth of the noise produced. Steeper wave fronts produce noise over a wider spectrum (to a higher frequency) than less steep wave fronts. Reducing the dv/dt (rate of rise of voltage) concentrates the noise produced to a lower frequency range. This can actually make it harder to eliminate interference in a practical installation.


What is the impact of pigtails on conducted emissions?

The use of pigtails to terminate the screen on a screened cable increases the return earth impedance for the noise voltage on the frame of the motor and increases the conducted emmissions considerably.


NO B.S. Guide to installing VFDs for minimum conducted noise problems.

A simple step by step guide for installing VFDs (VSDs) to minimise interference problems. This applies to all VFDs.