'Low-surge' inverters

Oct. 7, 2004
New circuit topology lets ac servodrives keep a lid on surge voltages that can otherwise shorten motor life.

Rafi Wilkinson
Yaskawa Electric America Inc.
Waukegan, Ill.

The G7 inverter provides both open-loop and closed-loop vector control. Open-loop mode uses a flux observer algorithm to extend the speed range and provide true torque control. Closed-loop mode can provide full torque at zero speed and a 1,000:1 control range. A zero-servo capability provides position control.

The three-level inverter is a neutral-point clamped circuit type. Twelve IGBTs and six clamp diodes form a connection to the midpoint between the plus and minus bus voltage to synthesize 0, 325, and 650-Vdc outputs.

PWM pulses produced by the G7 inverter have a voltage step height that is half that of ordinary two-level inverters. The lower ringing voltages in turn reduce surge voltage appearing at the motor terminals and common mode voltage that can produce pitting on motor shafts.

A comparison of motor bearing life for three-level and two-level inverters illustrates the improvements that arise from the use of lower surge voltages.

No question that developments in power semiconductors have made ac drives practical for numerous industrial applications. Specifically, the invention of high-power IGBTs (insulatedgate-bipolar transistor) in the 1990s represented a huge improvement in drive technology. IGBTs directly generate the power waveform that drives the motor. They have the ability to switch at frequencies up to 20 kHz. This is high enough to be out of the motor/drive bandwidth. For the same reason, they generate little noise in the audio range.

Recently, it has become apparent that these improvements have a downside. IGBT technology can cause bearing problems from electrical discharge. The problems arise because PWM inverters equipped with IGBT switches generate pulsed voltage waveforms that are associated with high rates of change of voltage (high dv/dt). The pulsed voltage waveform with high dv/dt causes voltage ringing with high amplitudes at the motor terminals. When the cable between the inverter and motor is long, voltages at the motor terminals are higher than those at the inverter terminals due to the high dv/dt and distributed inductance-capacitance combination of the cable. High voltage appearing across the motor terminals may damage the insulation material of the windings. High rate of voltage change also creates nonuniform voltage distribution among winding turns, affecting the life of insulation material.

Inverter switching also creates common-mode voltages. Common-mode voltage gives rise to common-mode current that flows through the parasitic capacitance in the cables between inverter and motor as well as through the parasitic capacitance formed between the stator winding and stator core. These currents make their way into the system ground and can interfere with the proper operation of other equipment sharing the same ground connection.

The common mode voltage also causes a current to flow from the stator windings to the rotor structure, which makes its way to the grounded stator core via the shaft bearing causing what is known as bearing currents. Undue heating of the lubrication material in the bearing due to current flow contributes to bearing problems.

At low speed, in case of ball bearings, the inner race and outer race are in contact with each other via the surface of the balls. As the speed increases, the balls in the bearing start to float in the lubricating medium. Under this condition, a high dv/dt across the inner and outer race can cause the lubricating medium to electrically breakdown thereby allowing a current pulse to travel from the inner race (shaft) to the grounded outer race via the lubricating media causing pitting and premature bearing failure.

Manufacturers have offered numerous options for overcoming bearing damage caused by electrical discharge. One widely used method is ceramic coating, which can be expensive. A shaft-grounding brush is a less-expensive option that short circuits the path between rotor and stator. However, this measure cannot be applied at all locations due to maintenance problems and an unfavorable or hostile environment at the motor shaft.

Recently a novel approach to IGBT switching has helped minimize surge voltage and bearing problems in an innovative way. G7 adjustable-frequency drives employ the world's first commercial three-level power architecture at 480 V. The three-level inverter uses a circuit configuration consisting of 12 IGBTs and six additional clamp diodes instead of the usual six IGBTs with six diodes found in two-level inverters. The 12 IGBTs synthesize three dc bus levels (0, 325, and 650 Vdc) rather then the two (0 and 650 Vdc) that conventional two-level PWM converters employ. The resulting waveform more closely resembles a sinusoid than does the two-level pulse-width-modulated waveform.

Switching takes place in 325-Vdc steps compared to 650-Vdc steps in case of twolevel PWM drives. The reduction in the step size is instrumental in reducing common mode voltage level and hence the corresponding common mode current.

A smaller step size also reduces the probability of insulation breakdown. The amplitude of the surge voltage at the motor produced by an inverter depends on the length and impedance of the power cables as well as the rise time of the power switches. Specifically, high dv/dt can damage the first coil of the motor and cause the motor insulation to deteriorate. Measurements of the G7 inverter confirm that it produces peak voltages in motor cables that are lower than what has been typical in two-level inverters. Actual long period tests were conducted to verify the superiority of the three-level inverter from bearing life point of view. Tests prove that the use of three-level topology can result in a significantly longer bearing life.

Similarly, the three-level technique tends to produce less noise than alternatives. The reason is that unlike the two-level case, the harmonic spectrum of the staircase waveform does not contain the switching frequency but contains components corresponding to twice the switching frequency. Tests show that peak noise in the audible range is typically 5 to 10 dB lower than that generated by typical two-level inverters, which run at up to twice the switching frequency of the G7.

Yaskawa Electric America Inc.,

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