Testing Challenges in Motor Drive Systems

Figure 1: The power section of a motor drive system requires
measurements of line input, PWM output, and efficiencies

Motors are everywhere in our world, and nowhere more so than in  our vehicles. For example, when's the last time you had to crank a car window up and down to pay a highway toll? Or, for that matter, when did you last manually adjust the seat position or rear-view mirror angles? These aspects of vehicles are all typically motorized these days.

There are other motor applications in which we demand great precision in their speed, positioning, and start/stop times. Industrial equipment, UAVs and other military/aerospace systems, and even childrens' toys depend on precise motor operation for safety and proper function. And for all of these motors, there must be a motor drive system controlling the action.

Design of motor drive systems is a highly complex affair. A commonly-encountered motor drive is the variable-frequency drive (VFD), a circuit that converts AC line voltage to DC, and then converts the DC voltage to a pulse-width-modulated AC signal that is applied to the motor terminals. VFDs that operate from a battery input forego the AC rectification. Figure 1 shows a simplified, yet typical schematic of the complete power-conversion section of an AC-AC, three-phase variable-frequency drive (VFD).

Figure 2: Torque, speed,
position, and power must
be measured in the drive's
integration section

In the power section (Figure 1), the three-phase AC line input is rectified by a six-pulse (six-diode) rectifier and filtered to a low-ripple, stable DC bus voltage in the DC Bus/Link section. Also shown are signals present in the circuit at the AC line input, DC bus, motor drive output, and gate drive of the power semiconductor devices. This form of VFD is highly representative of 95% or more of VFD designs.

Input to the VFD is typically a 50/60-Hz, single- or three-phase signal (in the latter case, the phases are typically designated A, B, and C) at a voltage anywhere from a nominal 120 V to nominal 600 V. The three-phase PWM motor drive output, when applied to the motor's winding, induces a near-sinusoidal current to flow in the winding. The characteristics and quality of the output voltage signal is related to the PWM control methodology. Control of the width of the PWM signal results in more or less voltage applied to the winding at a given frequency, resulting in more or less current draw in the motor. The current draw provides the correct speed, torque, power, efficiency, etc. from the motor.

In the above-described power section of the VFD, there are measurements to be made on the line input, pulse-width modulation output, and efficiencies. These are primarily low-frequency measurements in the range of 1 to 5 MHz. Ideally, we'd want to correlate power-section behaviors to the high-frequency behaviors of the embedded control system.

Moving ahead to the motor-integration section of the drive system (Figure 2), there are more measurements related to the motor's torque, speed, position, and power (motor (mechanical) power = torque times speed). These are very low-frequency measurements in the kilohertz range. Here, it's desirable to have a simple means of integration to the measurement instrument(s).

Figure 3: Analog, digital, and PWM signals join with serial data
and control-loop signals to make debug of the motor drive's
control system a highly complex task

Debugging of the control section of the motor drive is perhaps the most complex task (Figure 3). The controller comprises analog and digital signals, serial data, the control loop itself, and PWM signals. Here, we encounter high frequencies of 100 MHz and higher. Mixed-signal debugging and validation of this nature calls for an oscilloscope with a good amount of analysis functionality.

A high-voltage isolated gate driver connects the control/logic system to the power electronics. The controls take feedback signals from the motor and other parts of the circuit and then applies algorithms to calculate how the gates of each power transistor should be switching on/off to create the appropriate PWM signal. Because the controls connect to the gates of the power semis, there is no ground reference for many of the signals on the controls. Moreover, the controls themselves are not at ground potential.

The motor shaft can be instrumented for position, speed, or torque measurements. The motor as a whole can be instrumented for current inputs, temperature, vibration, or other physical characteristics. Some of these instrumented signals provide feedback to the VFD control system while others serve only design validation and/or testing.

While all VFDs have much in common, there can be significant differences in components, control systems, feedback signals, and output PWM waveforms. We'll dig into those differences in an upcoming post.