Established solutions to the overvoltage produced by the reflections 

In order to protect drives, cables and motors from the overvoltage caused by reflections the obvious solution is to reduce the bus voltage [1] with respect to the cable insulation rating. Even with 100% overshoot in voltage, an IGBT drive with 300Vdc bus can use a standard 1000Vpk cable since the overshoot of 600V will be far from the cable insulation limits [2]. When this is not possible, most solutions are based on inserting especial electronic devices to reduce the standing wave via impedance matching, filtering or active modification of the PWM pulses [3,4].

Simpler solutions consist of an output reactor at the inverter end. In Ref. [1] it is proved how the insertion of a reactor at the drive end modifies the reflected waveform by changing it into a less destructive wave, with slower rise-time and possibly a reduced amplitude. The effects of adding a reactor at the motor end are similar to those of adding a reactor at the drive end. A very interesting solution mentioned in Ref. [1,7] is the potential installation of dv/dt filters which basically transform the PWM driver output into a smooth sinewave. These filters combine appropriate R-L-C components to form a damped low pass filter. The maximum cable length with which a sinewave would create reflections is several orders of magnitude greater than a PWM signal.

A low-loss LC filter clamped by diode bridges has been proposed in Ref.[7]. For this filter,the LC resonating voltage is clamped to the dc-bus voltage by virtue of the fast recovery diodes, and the rising time of the inverter output voltage can be controlled by modifying the values of L and C of the filter. Compared with the traditional LC filters the physical dimensions of diode clamping filters are smaller. Moreover, the power losses in the clamping circuit are decreased in comparison to the losses in the damping resistors of RLC filters [7]. However, the use of these devices require special features in the power drive to allow access to the DC bus which are rarely available.

A cost effective method for controlling the voltage reflection is a termination network or a terminator device that removes impedance discontinuities to maintain the reflected waveform below potentially destructive levels. These devices are installed in parallel with the motor where they load the receiving end of the cable with a Zload ohmic resistor value equal to the cable Z0, which makes Γ= 0 in Eq. (2) and eliminates the reflected wave.

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Posted in Control for teleoperation, Robotics

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