The Problem 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, Rex
Manufacturing 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, capacitors and resistors.
Cable Input & Output Voltage - Using Rex Filter
By adding a Rex 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
- The dv/dt is reduced to less than 200 volts per micro second thereby insuring uniform
voltage distribution along the motor winding.
- 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.
- Filter designs are suitable for a wide range of applications including drive frequencies
of 5 kilohertz and lines up to 200 meters in length.
- Filter designs are capable of suppressing reflection transients even in the most severe
cases involving cables with grounding metal casings.
- The losses introduced in the use of these filters are small and approach the losses
experienced in using cables without filters. The use of Rex 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 Rex filters practically has no effect on the efficiency of the system.
- 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.
Rex dvdt filters supplied by: Electram Rotary Equipment Ltd.
