VFDs in Hazardous Zones.

VFDs Increase Risk

VFDs operate by converting the incoming AC into DC by the use of a rectifier circuit. This circuit is either a single phase of three phase full wave rectifier circuit and the DC is smoothed by a large capacitor bank.

This DC voltage is then converted into AC by a three phase chopper circuit comprising 6 x AC switches. The output waveform is a PWM waveform that causes a sinusoidal current to flow through the motor windings.

Modern VFDs use IGBTs as the switches to create the PWM waveform and as the IGBTs switch very fast, there are steep edges to the output pulses. These pulse edges cause capacitively coupled impulse currents to flow between the windings of the motor and the frame of the motor. The amplitude of the pulses are a function of the capacitance and the rate of rise of voltage (dv/dt). In addition, there are capacitively coupled currents into the ground circuits associated with the output cabling, ie.the screen of screened cable, and other cables and metal work in close proximity to the phase conductors between the VFD and the motor.

The net result is, that there is High Frequency current flowing from the frame of the motor to all conductive elements in contact with it. These currents are flowing through all conductive paths back to the frame of the VFD (provided that it includes DC Bus decoupling capacitors internally), or back to the three phase cirrcuits connected to the input of the VFD.

If the high frequency impedance between the frame of the motor and the DC bus is greater than 0.0 ohms, there willl be a High Frequency noise voltage on the frame of the motor. This voltage is carried by all conductors connected to the motor and can travel quite some distance. The voltage can be high enough to cause an arc to other grounded metalwork and as such, there is the potential for ignition in a combustible atmosphere.

Voltage higher than 160 Volts have been measured on the frame of the motor and associated metalwork.

Minimising the Risks

There are a number of issues about the operations of motors and VFDs in hazadous regions.
The advice on this page is purely related to the minimisation of stray currents and voltages that can incease the risk of ignition due to driving a motor with a VFD where the stray voltages on the motor can result in an increase in voltages in hazadous regions.

Where practical, mount the motor and the VFD on the same metalic frame with the motor and VFD well bonded to the frame and to each other. When the motor and the VFD must be mounted separately, use a sinusoidal filter on the output of the VFD where the filter provides phase to phase filtering and phase to earth filtering. The standard low cost filters provide phase to phase filtering only resulting in a high common mode voltage to earth. This type of filter does not eliminate common mode voltage from the frame of the motor.

Insulate the motor from any metalwork that encroaches into the hazadous region. For example, where a pump is used, connect the pump to the pipework via an insulated section so that any high frequency voltage on the pump frame is isolated from the pipework.

from : vfd-emc