Background :

A modern method to control the speed of A.C. motors is by using variable speed drives (VSD’s). These drives are also known as adjustable speed, pulse width modulated, and variable frequency drives. Advantages of using these modern drive systems include very precise control of speed, high efficiency and that the motor retains it’s torque when the VSD is adjusted to run it at very low speed.

Basically a VSD converts or rectifies the incoming 60 Hz power to D.C. Then it re-converts or inverts this D.C. back into an A.C. output but with a variable or adjustable fundamental frequency. This A.C. output (to the motor) is through pulse width modulation (PWM) in the VSD’s inverter section. This is accomplished by using solid state switching devices such as insulated gate bipolar transistors (IGBT’s), gate turn off (GTO’s), or bipolar junction transistors (BJT’s). These devices enable the VSD to produce an output with PWM. The PWM output is simply a series of D.C. voltage pulses of constant amplitude but with varying pulse width. This happens for both the positive and negative half cycles of the original incoming 60 Hz power supply. The newer VSD’s with IGBT’s produce D.C. pulses with very fast voltage rise times. This is good since it provides better more precise speed control but it can also cause problems in motors or in a cable system between a drive and motor.

Why are voltage spikes a problem ?

One problem for motors is caused by voltage spikes produced by the rapid rise time of pulses out of VSD’s. These voltage spikes are on top of the D.C. pulses travelling out to the motor. Since the motor looks like a high impedance to these high frequency (rapid rise time) pulses, the motor’s insulation can fail. Especially vulnerable are the first few turns of the stator windings. This problem has been identified for several years and has been largely eliminated through the use of VSD rated motors and with output reactors, output filters and line terminations. Poor quality or improperly insulated cables are also vulnerable to these rapid rise time pulses and their voltage spikes.

Why is reflected wave a problem ?

Cables connecting VSD’s to motors also have impedance and if this impedance does not match the motor impedance then the some of the energy in the high frequency pulses can actually be reflected back off the motor’s impedance. This is known as (voltage) reflection. If conditions are right a doubling of voltage can occur when an incoming pulse adds to the reflected pulse producing a doubling effect. A reflected voltage wave travels back along the cable toward the VSD. Here it could cause damage to the electronic components inside the VSD. In addition reflected voltage can also be harmful to motors and cables. Again the techniques mentioned above will eliminate voltage reflections. Cables are also subjected to these over voltages but no cable failures have been reported. This is due to the fact cables have an insulation thickness that is based, in part, on mechanical requirements. Electrically cables have more insulation thickness than they need, especially in the case of cross-linked polyethylene insulation (XLPE) which has very high dielectric strength. Cables using XLPE insulation and proper metal sheathing (not interlocked armouring) or shielding have lower capacitance. Therefore they can be used in longer runs before impedance mis-match and voltage reflections are likely to happen. One drive manufacturer recommends that PVC insulated cables not be used with their drives since PVC is more susceptible to failure from reflected wave and voltage spikes.

Several years ago a drive manufacturer conducted a study on which cable constructions were the best overall to use with VSD’s. They considered various technical concerns associated with the use of VSD systems as well as economic issues such as cable and connector cost, installation factors, and cable availability. Their study included cables that were commonly used for these applications as well as similar constructions with modifications to minimize or eliminate some of the unwanted side effects associated with the use of these modern VSD’s with their fast rise time outputs. Results of the study were presented in an I.E.E.E. paper at the 1996 Annual Pulp and Paper Industry Technical Conference.

Eight different cable constructions were set up to run the same motor from the same VSD. Six of the eight were U.L. style and two were I.E.C. style cables. This set up was constructed to simulate a real industrial environment, but also had test equipment installed to measure the various electrical data to be used for the comparison. There were five major electrical objectives of tests in this study:

a) to select a cable to minimize net induced ground currents into the drive system ground.

b) to select a cable to minimize common mode current.

c) to select a cable to minimize motor frame standing voltage and therefore bearing current.

d) to select a cable to minimize cross talk between adjacent cables.

e) to select the best electrical connection of sheath/shield and ground for optimum performance.

The results of tests for these five as well as the economic considerations mentioned previously were given different "weighting factors", based on their importance, to come-up with an overall rating. This overall rating finally compared one construction with the others. No one cable rated best in all categories and the best overall cable was not the best in more categories than the other seven, but where it counted it was the best.

Of the eight cables studied the best overall construction was one with three XLPE insulated conductors, three bare grounding (bonding) conductors, a continuous aluminum sheath and an overall jacket.

Resistance to Voltage Spikes and Reflected Wave :

Insulated conductors should be at least 600 and in some cases, according to the I.E.E.E.paper, 2 kV rated. This will enable the insulation to withstand the high voltage spikes and reflected voltage if it exsists. Since the study was done on six U.L. styles of cable they recommended a 2kV rated insulation for cables used on a 575 volt system where reflections could happen. U.L. has a 600 volt insulation level, then they jump to a 2 kV rating. C.S.A. has 600 volt then 1000 volt and then a 5 kV insulation level but no 2 kV level. Nexans DriveRx cables are C.S.A. approved and 1000 volt rated. Nexans MC Corflex VFD cables are U.L. approved and 600 volt rated.

All Nexans drive cables are manufactured with crosslinked polyethylene ( XLPE ) insulated phase conductors. XLPE is known to be an excellent insulation material with very high A.C. and impulse voltage breakdown levels. Because of this Nexans drive cables are the recommended cable construction for use with PWM variable speed drives.

In order to protect all of the equipment on the output side of the drive, the use of load reactors, etc is still recommended by Nexans and most drive manufacturers.

Reducing Common Mode Current and Bearing Current :

Three bare bonding (grounding) conductors ensure a balanced low resistance path to ground for minimizing or eliminating common mode currents and motor frame standing voltage. Reducing or eliminating motor frame standing voltage will reduce or eliminate bearing current. In addition three grounds in combination with the aluminum sheath ensures that this grounding path retains it’s low resistance as the cable ages. The total cross-sectional area of the three grounds must meet the cross sectional area requirements of the Canadian Electrical Code and the U.S. National Electrical Code. All three bonding conductors as well as a lead off a ground bushing on each cable connector are grounded to one common point in both the VSD and motor.

Reducing Crosstalk to Adjacent Cables :

A cable with a continuous aluminum sheath provides an excellent shield for the high frequency pulses being transmitted to the motor. This will reduce cross talk to and from other cables running adjacent to it. In addition this sheath retains it’s good shielding capability over it’s life. Copper tape shielded and interlocked armoured cables loose some of their shielding quality as they age. As mentioned previously the sheath in combination with the bonding conductors provides a long term low resistance path to ground for this high frequency electrical "noise" that could cause interference elsewhere.

Using Proper Connectors :

Testing conducted on the cables also included connectors. The resulting I.E.E.E. paper outlined the requirements for connectors that were found to provide the best combination of properties to meet objective e) above. These requirements include 360 degree contact to the sheath, the ability to accept a ground bushing and to have good high frequency techniques (ie: shielding). It was recommended that in order to meet these requirements a connector rated for Class II, Groups E,F, and G hazardous locations be used. This type of connector would most likely have the desired properties.

Why Have a Jacketed Cable?

A jacketed cable helps to prevent it’s sheath or armour from coming in contact with ground along it’s route to the motor. This helps to ensure that it’s grounded at the ends only and in this way stray injected ground currents being picked up by contact with building steel or the cable support structure are avoided.

Summary :

Using the recommended cable construction and connectors does not eliminate the need for output filters, line traps and other equipment suggested by drive manufacturers. However, using a proper cable will help reduce or eliminate some  undesired side effects associated with these modern drives. This includes crosstalk, common mode current, bearing currents, motor frame standing voltage and the possibility of cable failure due to voltage spikes and reflected wave.

In Canada the recommended cable construction is DriveRx,  C.S.A. type RA90. DriveRx is approved to C.S.A. Standard C22.2 No: 123-96 (R2005).