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Motor Guarding Transient Filters

The Problem

LineFilters catThe steep voltage wave fronts of the Pulse Width Modulated (PWM) output of Adjustable Frequency Drives (AFD’s) produce high frequency effects which may damage the insulation of motors operated by the equipment. The problems result from two distinct effects.

  1. High DV/DT Effects

    The rapid rate of voltage rise (dv/dt) at the leading edges of each output pulse of the PWM inverter, produces an uneven distribution of voltage within the motor windings. The result is a concentration of the voltage at particular points of the winding causing abnormal stress leading to breakdown of the insulation. This phenomenon has been described as “first coil breakdown” and is well documented.

  2. Reflections in Long Lines and Cables

    A long cable, in addition to resistance, has distributed inductance and capacitance, producing effects similar to a transmission line as shown below.Long transmission line
    Figure 1: Electrical representation of a long transmission line

The high frequencies present in the output of PWM waveforms cause reflections in long conductors connecting the motors to the drives (see Fig. 2). Harmful effects with conductors as short as 10 meters have been observed. However, the effects are most severe with cables of lengths greater than 50 meters leading to the doubling of the applied voltage. This translates to voltage peaks approaching 1600 Volts in 575 Volt systems

output unfiltered
Figure 2: Output of a PWM inverter reaching 200% due to reflections in long cables

The combination of these two effects stresses the winding insulation considerably beyond design limits and has been known to shorten the insulation life and in some instances leads to early catastrophic failure of motors.

The Solution

It has been demonstrated that these transient effects can be reduced by using filters placed at the output of the AFD’s thereby allowing safe operation of the motors and an expectation of full insulation life (see fig. 3). After years of experience and research, Rex Power Magnetics has developed a comprehensive line of state of the art output filters suitable for a wide range of prospective applications. The filters are constructed using optimized combinations of inductors and resistors.

output filtered
Figure 3: Voltage peak reduced to less than 125% appearing at the motor, due to the addition of a Rex Motor Guarding Filter.

Features of Rex Output Filters for Adjustable Frequency Drives

  1. The prospective voltage spike of 200% at the motor due to reflections in the cable is typically reduced to less than 125% as shown in Figure 6b. This is a significant improvement over using Line Reactors alone.
  2. The dv/dt is reduced promoting more uniform voltage distribution among the motor windings.
  3. Standard filter designs are suitable for a wide range of applications including line lengths of 100 meters and switching frequencies up to 5 kHz. A wider range and mix of applications and conditions are possible by consulting the office.
  4. Filter designs are capable of reducing reflection transients even in the most severe cases involving cables with grounded metal casings.
  5. The losses introduced with the use of these filters are small, approaching the losses experienced in using cables without filters. The use of Rex output filters generally introduces no additional losses but merely transfers the losses associated with charging and discharging the line capacitance from the cable to the filter. Thus the use of Rex filters has little effect on the efficiency of the system.
  6. Components used in the filters are specified for specialized qualities and are operated at a fraction of their design limitations to reduce stress and provide reliability. These steps ensure that the filters will operate without overheating even at the limits of their specifications at the extremes of the longest metal-sheathed cables.

Dimensions and Specifications

Motor Guarding Transient Filter

Filter Reactors
Rating [A]
Part No. Dimensions
Open Core & Coil
Enclosure # Weight (Lbs)
Open Enclosed
2 2C5LF 8.25/6.50/7.00/7.25/2.00 0 5 14
4 4C5LF 8.25/6.50/7.00/7.25/2.00 0 6 15
8 8C5LF 8.25/6.50/7.00/7.25/2.00 0 9 19
11 11C5LF 8.25/6.50/7.00/7.25/2.00 0 10 20
17 17C5LF 8.25/6.50/7.00/7.25/2.00 1 11 23
27 27C5LF 8.25/7.20/7.00/7.25/2.00 1 19 32
34 34C5LF 8.25/7.20/7.00/7.25/2.00 1 25 37
45 45C5LF 11.00/9.00/8.10/10.00/4.00 3 29 48
60 60C5LF 11.00/9.00/8.10/10.00/4.00 3 30 50
80 80C5LF 11.00/12.00/6.80/10.00/4.00 4 42 68
100 100C5LF 11.00/12.00/6.80/10.00/4.00 4 47 78
130 130C5LF 11.00/12.00/6.80/10.00/4.00 4 48 79
160 160C5LF 11.00/13.00/8.20/10.00/4.00 4 49 88
200 200C5LF 11.00/13.00/8.20/10.00/4.00 4 68 99
255 255C5LF 11.00/13.00/8.20/10.00/4.00 4 84 115
320 320C5LF 11.00/13.00/8.20/10.00/4.00 6 90 150
410 410C5LF 14.00/14.50/14.00/13.00/6.00 6 95 158
500 500C5LF 20.00/16.00/18.00/19.00/8.00 6 140 200
600 600C5LF 20.00/17.50/18.00/19.00/8.00 6 148 210
  • Rex filters are current rated, therefore easy to specify. Simply use the total motor load current to select the appropriate filter rating.
  • Filters are available enclosed or open style. Add the suffix /E at the end of the part number to denote enclosed style.

Enclosed Dimensions

Size No. Length Depth Height
 0 9.50 7.00  8.00
12.00 9.00 9.50 
11.00 11.00 14.00 
15.50 11.00 14.00 
15.75 16.00 21.00 
20.50 16.00 21.00 
20.50 20.75 26.50 
24.50 21.75 31.50 
30.75 29.75 31.75 
40.00 38.00 45.75 
10  46.00 50.00 64.50 

Note: All Rex enclosures larger than #3 are standard NEMA 3R sprinkler proof. Boxes #7 and smaller can be floor or wall mounted. Consult the website or our engineering department for the most up-to-date information on enclosures.