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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 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, 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 them 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.
* Information
is subject to change without prior notice.
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