VSD: Power supply connections, grounding requirements

Cable connection requirements

In accordance with accepted practice, power is normally provided to a VSD from a DB or a MCC. Adequate arrangements should be made to provide safety isolation switches and short-circuit protection at the power supply connection point. The short-circuit protection is required to protect the power cable to the AC converter and the input rectifier bridge at the converter. The converter provides the down-stream protection for the motor cable and the motor itself.

Adequate safety grounding should also be provided in accordance with the local 'Wiring Rules and Codes of Practice'. The metal frames of the AC converter and the AC motor should be grounded to keep touch potentials within safe limits. The chassis of the AC converter is equipped with one or more protective (ground) earth (PE) terminals, which should be connected to the common safety ground bar.

++++37 Power supply, motor and grounding connections --- Process controller-- Preferred ground; 4-20mA; PE; Power cable ground; Shielded motor cable; M; VSD

Power supply cables

The VSD should be connected to the power supply by means of a cable that is adequate for the current rating of the VSD. Reference can be made to Australian Standard AS 3008 when selecting cables. The AC converter requires a three-phase supply cable (red/white/blue) and a PE conductor (green/yellow), which means a four-core cable with copper or aluminum conductors. A neutral conductor is unnecessary and is usually not connected to the frequency converter. The AC converter is a source of harmonic currents that flow back into the low impedance of the power supply system. This conducted harmonic current is carried into other electrical equipments, where it causes additional heat losses and interference. Sensitive electronic instrumentation, such as magnetic flow-meters, thermocouples, and other microprocessor-based equipments, ideally should not be connected to the same power source, unless via a filtered power supply. In addition, interference can be radiated from the power supply cable and coupled into other circuits. These cables should, therefore, be routed well away from sensitive control circuits. The power supply cable should preferably be laid in a metal duct or a cable ladder and shielded in some way to reduce the radiation of emf due to the harmonic currents. For this purpose, steel wire armored (SWA) cables are particularly suitable. If the power cable is unshielded, the control and communications cables should not be located within 300 mm of the power cable. The conductor sizes should be selected in accordance with the normal economic cable selection criteria. These usually take into account the maximum continuous current rating of the VSD, the short-circuit rating, the length of the cable, and the voltage of the power supply system. The relevant local safety regulations should be strictly observed. However, when selecting the cable cross-sectional area for the power supply cables and upstream transformers, a de-rating factor of at least 10% should be included to accommodate the additional heating due to the conducted harmonic currents.

If a supply side harmonic filter is fitted at the converter, this may not be necessary.

Three-phase systems composed of three single-conductor cables should be avoided if possible. Power cables with a trefoil configuration produce a lower radiated emf.

Cables between converter and motor

The cable from the AC converter to the motor carries a switched PWM voltage, which is modulated at a high frequency by the inverter. This results in a higher level of harmonics than the power supply cable. Harmonic frequencies are in the frequency spectrum of 100 kHz-1 MHz. The motor cable should preferably be screened or located inside a metal duct. The control and communications cables should not be located close to this cable.

The level of radiated emf is higher for cables, with three separate single cores, laid horizontally on a cable ladder, than a trefoil cable with a concentric shield. The recommended size for the cable between the AC converter and the motor should preferably be the same as the power supply cable. The reasons are:

• It will be easier to add a bypass device in parallel with the frequency converter later, using the same cable, cable lugs, and connections.

• The load-carrying capacity of the motor cable is also reduced by harmonic currents and additionally by the capacitive leakage currents.

It should be borne in mind that the AC converter VSD provides short-circuit and overload protection, for the cable and the motor. A separate ground conductor between the converter and the motor is recommended for both safety and noise attenuation. The ground conductor from the motor should be reconnected to the PE terminal of the converter and not back to the DB. This will avoid any circulating high-frequency currents in the ground system.

When armored or shielded cables are used between the converter and the motor, it may be necessary to fit a barrier termination gland, at the motor end, when the cable is longer than 50 m. This is necessitated, because the high-frequency leakage currents flow, from the cable, through the shunt capacitance and into the shield. If these currents return via the motor or other parts of the grounding system, the interference is spread over a larger area. It’s preferable, for the leakage currents to return to the source, via the shortest route, which is via the shield itself. The shield or SWA should be grounded at both the converter end and also at the frame of the motor.

Control cables:

The control cables should be in accordance with the 'normal local practice'. These should have a cross-sectional area of at least 0.5 mm^2 for reasonable volt drop performance. The control and the communications cables, connected to the converter, should be shielded, to provide protection from the EMI. The shields should be grounded at one end only, at a point remote from the converter. Grounding the shield to the PE terminal of the drive should be avoided because the converter is a large source of interference. The shield should preferably be grounded at the equipment end.

Screened cables provide the best protection from coupled interference. The control cables should preferably be installed, on separate cable ladders or ducts and as far away from the power cables, as possible. If control cables are installed on the same cable ladder as the power cables, the separation should be as far as possible, with the minimum distance being about 300 mm. Long parallel runs on the same cable ladder should be avoided.

Grounding requirements

As mentioned earlier, both the AC converter and the motor must be provided with a safety ground, according to the requirements of local standards. The main purpose of this grounding is to avoid dangerous voltages, on the exposed metal parts, under fault conditions.

When designing and installing these ground connections, the requirements for the reduction of EMI should also be achieved, with these same ground connections. The main grounding connections of an AC converter are usually arranged.

The PE terminal, on the converter, should be connected back to the system ground bar, usually located in the DB. This connection should provide a low impedance path back to ground.

Common cabling errors

The following are some of the common cabling errors made when installing VSDs:

• The ground conductor, from the AC converter, is run in the same duct or cable ladder, as other cables, such as the control and power cables for other equipments. Harmonic currents can be coupled into sensitive circuits. Ideally, instrument cables should be run in separate metal ducts or steel conduits.

• Running unshielded motor cables, next to the supply cable, to the AC converter or the power cables for other equipments. High-frequency harmonic currents can be coupled into the power cable, which can then be conducted to other sensitive electronic equipment. Other cables should be separated from the motor cable or converter power cable by a minimum of 300 mm.

• Cables between the AC converter and the motor should be no longer than 100 m.

In case longer cables are used, motor filters are necessary to reduce the leakage current. Alternatively, the switching frequency may be reduced.