25-09-2010, 09:13 PM
Abstract—
In this paper, several aspects of high frequency related
issues of modern AC motor drive systems, such as common mode
voltage, shaft voltage and resultant bearing current and leakage
currents, have been discussed. Conducted emission is a major prob-
lem in modern motor drives that produce undesirable effects on
electronic devices. In modern power electronic systems, increasing
power density and decreasing cost and size of system are market
requirements. Switching losses, harmonics and EMI are the key
factors which should be considered at the beginning stage of a
design to optimise a drive system.
Introduction:
Nowadays, more than 60 percent of the world’s energy is used
to drive electric motors. Due to growing requirements of speed
control, pulse width modulated inverters are used in adjustable
speed drives. Rapid developments in semiconductor technolo-
gy have increased the switching speed and frequency of power
switches dramatically. In a motor drive system, a voltage source
converter with hard switches generates high dv/dt, which
causes leakage currents due to stray capacitances in an electric
motor.A modern power electronic drive
consists of a filter, a rectifier, a DC link capacitor, an inverter
and an AC motor. Many small capacitive couplings exist in the
motor drive systems which may be neglected at low frequency
analysis but the conditions are completely different at high
frequencies.
Electromagnetic Interference (EMI) is a major problem in
recent motor drives that produces undesirable effects on elec-
tronic devices. In modern power electronic systems, increasing
power density and decreasing cost and size of a system are mar-
ket requirements. Switching losses, harmonics and EMI are the
key factors which should be considered at the beginning stage
of a design to optimise a drive system.
Common mode voltage creates shaft voltage through electro-
static couplings between a rotor and stator windings and the ro-
tor and a frame which can cause bearing currents when the shaft
voltage exceeds a breakdown voltage level of the bearing grease.
An increase in the carrier frequency of voltage-source Pulse
Width Modulated (PWM) inverters based on high-speed
switching devices has improved operating characteristics of the
inverters.