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Boost industrial motor control with digital isolator

18 Dec 2014  | Dara O'Sullivan, Maurice Moroney

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The effect of the dead time is to distort the average voltage applied to the motor, especially at low speed. The dead time in effect injects an approximately constant magnitude error voltage ‎ [2] equal to:


 Equation


Where Verror is the error voltage, tdead is the dead time, ton and toff are the transistor turn-on and turn-off delay times and T is the PWM switching period; Vdc is the dc bus voltage, Vsat is the on-state voltage drop of the power transistor and Vd is the conduction voltage of the diode.

The error voltage changes sign when the current in a phase leg changes direction, and so step changes occur in the motor line-to-line voltages at the points where line currents cross zero. This creates harmonics of the sinusoidal fundamental voltage which in turn generate harmonic currents in the motor. This is a particular problem for larger low impedance motors used in open loop drives, where the harmonic currents can be significant, resulting in low speed vibration, torque ripple, and harmonic heating.

The impact of the dead time on distortion of the motor output voltage is at its worst for the following conditions:

 • High dc bus voltage
 • Long dead time
 • High switching frequency
 • Low speed operation, especially in open loop drives where no compensation is added by a control algorithm

Low speed operation is important because it is in this mode that the applied motor voltages are very low in any case, and the error voltage from dead time can be a significant fraction of the applied motor voltage. Moreover at low speed, any induced torque ripple has a more detrimental impact since the filtering effect of system inertia at higher speeds is not available.

Of all of these parameters, the length of the dead time is the only one that can be impacted by the isolated gate driver technology. Some of the dead time length is determined by the switching delay times of the power transistors, but the remainder is a function of the propagation delay mismatch. In this context, optical isolators are at a significant disadvantage to magnetic isolation technology.


Application example
In order to illustrate the impact of dead time on motor current distortion, results from a 3-phase inverter based open loop motor drive are illustrated. Magnetic isolators from Analog Devices (ADuM4223) are utilised for the inverter gate drivers, directly driving IRG7PH46UDPBF 1200V IGBTs from IR. The dc bus voltage is 700V. The inverter is driving a 3-phase induction motor in open loop V/f control mode. The line-to-line voltage and the phase currents are measured using resistive dividers and shunt resistors respectively, in conjunction with isolated sigma-delta modulators, again from Analog Devices (AD7403). The single bit data stream from each modulator is fed to the SINC filters of the control processor (ADSP-CM408 from Analog Devices) where the data is filtered and decimated to produce a precision representation of the voltage and current signals.

Measured line-to-line voltage from the SINC digital filter output is shown in figure 5. The actual line to line-to-line voltage is a high frequency switching waveform at 10kHz, but this is removed by the digital filter, allowing the lower frequency components of interest to be visualized.


 Line to Line Motor Voltage

Figure 5: Measured Line to Line Motor Voltage at (l) 500ns dead time (r) 1µs dead-time.


The consequent motor phase current is shown in figure 6.


 Motor Current

Figure 6: Measured Motor Current at (l) 500ns dead time (r) 1µs dead-time.



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