The hazard from VSDs

AC pulse-width-modulated (PWM) variable speed drive systems are a special case of power switching converter. They represent a particularly serious source of radio frequency emissions, because the high voltage and current, switched at the full dv/dt and di/dt of the converter, is deliberately routed out of the drive module and to the motor load. In industrial installations this can be several tens of metres from the drive cabinet. This is in direct contrast to other types of switching converter, where power at the switching frequency is carefully kept inside the module. Thyristors as used in high-power DC 6-pulse drives are relatively slow-switching devices, which limits their emission spectrum to about 1MHz. IGBTs on the other hand may extend to about 30MHz. With a poorly installed system, interference is likely to occur in the 100kHz-10MHz range where emission is strongest. As SiC transistors become the preferred choice for such applications, one can only anticipate that EMC challenges will become even more pronounced.

The drive unit itself is not an important source of direct emission; there may be strong electric and magnetic fields close to the drive housing, but they diminish rapidly, by an inverse cube law, with increased distance within the enclosure. However the wiring connected to the drive can extend some distance and is likely to be long enough to form an effective coupling path. The main mechanism by which RF energy leaves the system is by conduction through the input and output power cables. In order of importance, the routes are:

the motor cable: carries the highest dv/dt and di/dt since the PWM switching power is taken outside the control cabinet to the motor, by design

the supply cable: although this does not intentionally carry switching voltages, improperly terminated return currents can leak onto this cable; this is more damaging because it is connected to the system's supply network and can therefore affect other equipment by conduction or radiative coupling. A longer motor cable increases the amplitude of the leaked parasitic currents

the earthing or grounding system: the drive ground wire carries RF current returning from the motor; the capacitance of the power conductors in the cable, and the motor windings, present a lowering impedance as the harmonic frequency increases, and this current has to return via the grounding system. Without a well defined low impedance grounding path, the current will take whatever path it can, which is likely to disturb other components of the installation.

The most important aspect of drive installation is to ensure a direct, low impedance path for the return of the parasitic RF current.

The nature of drive emissions

The main circuit elements of an AC inverter VSD are shown in the diagram. The output 3-phase PWM waveform at U,V,W has typical rise-times of the order of 50-100ns, containing significant harmonic energy up to about 30MHz. This voltage is present both between output phases and also as a common-mode voltage between phases and ground. The common-mode voltage is primarily responsible for emission effects, because it results in high-frequency current flowing to ground through the stray capacitances of the motor windings to the motor frame, and of the motor cable power conductors to the ground conductor and/or screen. The capacitance of a motor winding to its frame may be in the range 1nF to 100nF, depending on its rating, and the capacitance from the cable power cores to ground is generally between 100pF and 500pF per metre. These values cause significant current pulses at the edges of the PWM voltage waveform. The amplitude can be calculated to a first order from

I          =          C · dv/dt

So, for instance, a 20m cable with a capacitance of 250pF/m, when subjected to an IGBT-driven PWM output with a peak voltage swing of 600V and a rise time of 50ns, will pass a parasitic screen current of 60A, not counting that which flows through the motor frame.

The current returns through a variety of paths which are difficult to control. Current may find its way from the motor frame back to the supply through any part of the installation's metalwork, and if it passes through ground wires in sensitive measuring circuits it may disturb them. Also, a major return route to the drive is through the supply wiring, so any equipment sharing the supply may be affected.

As well as the threat of ground return currents, the motor cable will be an effective radiating antenna at frequencies where its length is an odd number of quarter wave-lengths. For example, a 30m cable (one wavelength long at 10MHz) will have greatest emission at about 2.5MHz and also at 7.5MHz and 12.5MHz. The exact frequencies will be detuned to some extent by the presence of the motor and by the distance of the cable from surrounding metalwork.

Precautions for installation

Segregation

At the very least, the drive supply and output cables must be segregated from cables carrying small signals. Recommended segregation distances vary with different installers, but the cable installation topic here provides some guidelines. The motor cable is always Class 4 (very noisy). It is also good practice to allow some separation distance within the control cabinet between the drive module(s) and other equipment; and the input connections to the mains filter must be segregated from the drive itself and from the motor cable.

Earthing/grounding of motor and drive

The essential objectives of the grounding structure are:

to define preferential paths for parasitic high frequency currents

to minimise the enclosed area for these currents

to make sure that the defined paths are not shared with sensitive circuits

Power cables should include their corresponding ground wire, which should be connected at both ends. These wires should not be shared with any signal-carrying functions. The drive enclosure should have a metal backplate which functions as a low impedance grounding structure; all modules to be grounded are connected immediately to the metal plate by short conductors with large cross-sectional area, preferably flat, or by direct metal-to-metal bonding. Screened cables have their screens clamped directly to the structure at both ends. Safety earth connections are still provided by green-and-yellow wires where required by safety regulations, but these are in addition to the EMC ground plane and are redundant in normal operation. The backplate should also be used as the sole reference point for shared signal circuits.

Screened cabling

Except for very short runs in well controlled installations, it will be standard practice to use screened cable from the drive to the motor. As discussed elsewhere, the coaxial nature of the screen, coupled with a low impedance ground connection at each end, encourages the HF common mode return current to flow in the screen rather than anywhere else. Copper braided screen is to be preferred, but steel wire armoured (SWA) cable is acceptable. The drive end of the screen or SWA should be directly connected to the drive earth terminal if possible or to the backplate near to the drive otherwise, using a metal clamp arrangement; the motor end should be directly connected to the motor frame ground terminal. There should be no contact, accidental or intentional, with the installation's metalwork at any point in between. If either SWA or screen have to be bonded at the enclosure entry, the internal run of wiring to the drive should also be screened to ensure continuity of coverage of the internal conductors.

Filtering

It is also standard practice to use a filter on the mains input to the drive module, in order to meet conducted emissions regulations; with a less-than-perfect installation it may be necessary for the proper functioning of the whole system as well. Many drives are available with a matching filter at extra cost, sometimes mounting directly underneath the drive unit (a "footprint" filter). The important requirement is that the installation of the filter should follow the recommendations outlined above with respect to grounding and segregation of wiring.

A halfway house, when the installation requirements are not onerous, is simply to add capacitors of 0.5-1µF between each phase and the grounding plate next to the drive module supply terminals. These will not be helpful if there is already a filter installed.

The need for filtering of any kind is directly related to the length of the motor cable, since the capacitance to ground added by this largely determines the amplitude of the parasitic ground currents. Cables with an insulating sheath between the inner conductors and the screen or SWA will have a lower capacitance per unit length. A series common mode choke on the motor output, between the terminals and the screen termination, will reduce the higher frequency currents; this would typically be achieved by winding two or three turns of all three motor phase conductors around a toroidal ferrite core. A manganese-zinc (lower frequency) ferrite has high loss in the 1-10MHz frequency range where motor cable resonance occurs, and this gives useful damping of the resonance and a substantial reduction in the peak current. You need to be careful that the common mode current peaks don't saturate the core.

For improved performance, you can use a sinusoidal filter on the output of the drive. This converts the PWM output to a near-sinusoid which is then passed down the motor cable, and with proper installation, can largely eliminate EMC issues without the need for a screened cable, as well as reducing the stress on the motor caused by high dv/dt. Such filters are large and expensive, though.