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

• 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 the particular points of the winding causing abnormal stress leading to breakdown of the insulation. This phenomena has been described as "first coil breakdown" and is well documented.

   

• Reflections in Long Lines & Cables
  A long cable, in addition to resistance, has distributed inductance and capacitance, producing effects similar to a transmission line as shown below.

   

  Delay Line Equivalent Circuit of Long Cable

  The high frequencies present in the output of PWM wave forms cause reflections in long conductors connecting the motors to the drives (see image below). 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.

  
  Cable Input & Output Voltage - Using No Filter

  On the output of PWM inverters the voltage peak due to reflections in long cable lines can be 200% (as shown in the image above).

  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 AFD's thereby allowing safe operation of the motors and an expectation of full insulation life (see figure below). After years of experience and research,  a line has been 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, capacitors and resistors.

Cable Input & Output Voltage - Using Filter

By adding a motor guarding filter to the output of PWM inverters the voltage peak is reduced to less than 125% appearing at the motor as shown above.

Features of Rex AFD Output Filters

1. The dv/dt is reduced to less than 200 volts per micro second thereby insuring uniform voltage distribution along them the motor winding.
2. The prospective spike of 200% of applied voltage due to reflections in long cables is reduced to less than 125% as shown above. This and the combination of low dv/dt translates to safe operation even at 575 volts input.
3. Filter designs are suitable for a wide range of applications including drive frequencies of 5 kilohertz and lines up to 200 meters in length.
4. Filter designs are capable of suppressing reflection transients even in the most severe cases involving cables with grounding metal casings.
5. The losses introduced in the use of these filters are small and approach the losses experienced in using cables without filters. The use of Output Filters generally introduce no additional losses but merely transfer losses associated with charging and discharging the line capacitance, from the cable to the filter. Thus the use of filters practically has no 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 insure that the filters will operate without overheating while exhibiting full performance even at the limits of their specifications at the extremes of the longest metal sheathed cables.