Electric motors are by far the biggest consumers of energy produced in the world. According to the Department of Energy, of the total energy produced in the U.S., about 60 percent to 65 percent is consumed by electric motors. Among the largest components of this are home and commercial refrigeration appliances and HVAC systems. It is estimated that by building more efficient drives, the U.S. can save about 25 percent in home refrigeration costs, 80 percent in HVAC costs and about 60 percent in pump and fan control costs.

Governments all over the world are taking decisive steps by mandating the reduction of energy consumption, forcing manufacturers of new appliances to develop motors and drives that are more energy efficient.

Historically, the most efficient systems have been three-phase motors driven by variable-speed drives (VSDs). The role of variable-speed drives is to transform electrical energy into mechanical energy in the most efficient way possible. Unfortunately, many applications are still being driven at fixed speeds, regardless of their loads, and often the motors and power-conversion circuits are oversized to be able to cope with the extremely large currents needed at startup.

VSDs can provide a soft startup and control speed as required by the load. Open-loop or closed-loop inverter systems are controlled by microcontrollers with pulse-width modulated (PWM) signals. Speed, torque, and position are regulated based on preset values and feedback signals from the motor.

The most common types of motors driven by power inverters are brushless DC (BLDC), permanent magnet AC (PMAC) and AC induction motors. BLDC motors, which are driven by trapezoidal voltages with amplitudes controlled by the duty cycles of the PWM signals, are considered lower in performance than PMAC or AC induction motors, which are driven by sinusoidal voltages.

Enlarge this picture Fig. 2. V850E/IG3 block diagram.

To generate sinusoidal voltages at the motor terminals, the microcontroller generally uses up/down counters for the high-frequency PWM carrier and lookup tables or sine functions to generate a low-frequency, modulating sine wave.

The simpler sinusoidal drive is a constant voltage/frequency (V/F) type that is typically used for open-loop AC induction motors. V/F drives are also known as scalar drives because they only adjust the magnitudes of the control variables. Although good for many common applications, such as fans and pumps, their limited performance due to the coupling effects between the stator currents and the induced rotor flux make them unusable for higher-performance drives.

For applications that demand faster dynamic response, such as clothes washers and servos, vector-controlled sinusoidal drives are used. Vector drives not only adjust the magnitudes of the control variables, they also deal with the phase relationship between them, eliminating the previously mentioned coupling effects. The sinusoidal vector drives used in both PMAC and AC induction motors deliver superior performance, making them the most desirable of the drives today, but they are more difficult to implement and require powerful computational resources.

To implement vector control, motor speed and phase currents are typically measured using a shaft encoder for speed and Hall sensors for current. The costs of these sensors are quite high, which is why sensor-less methods were developed using only motor terminal currents and voltages. In the sensor-less method, the speed sensor is totally eliminated and inexpensive low-side sensing resistors are used to measure the phase currents. (See Fig. 1.)

The three phase currents — Iu, Iv, Iw — are converted to Id, Iq orthogonal currents and then referenced, first to the stator frame, and then to the rotor frame, by two operations known as Clarke and Park transforms. The net result of these transformations is the simplification of the three-phase motor model, making it look like a separately excited DC motor with similar torque expressions.

Enlarge this picture Fig. 3. Embedded operational amplifiers and comparators.

Proportional-integral (PI)-type algorithms are performed on Id and Iq in an inner torque loop, and another PI loop is used for the outer speed loop. The adjusted Id and Iq are transformed back to the three-phase coordinate system though inverse Park and Clarke transforms. The result is the adjustment of PWM duty cycles of the three-phase voltages.

Sensor-less vector control algorithms require fast computations within one PWM cycle and need devices capable of doing that and other tasks related to system control.

From the beginning, digital signal processors (DSPs) were considered to be the most appropriate processors for vector drives, but soon a variety of microcontroller vendors developed similar products running at high speeds with special features implemented in hardware to ease CPU load and eliminate the need for external peripherals. This trend has resulted in more compact and less expensive drives, as well as ease of use for firmware developers because most of them are more familiar with microcontrollers.

In addition, the existence of common microcontroller peripherals, such as I/O ports, communication ports, on-chip flash memory, and SRAM, has helped to combine the motor drive requirements with other tasks, resulting in what is known as single-chip system solutions at substantially lower costs.

NEC Electronics has a line of RISC architecture-based 32-bit microcontrollers that can operate at speeds up to 200 MHz and execute up to 400 million instructions per second (MIPS; Dhrystone 1.1). Over the years, the company has developed several application-specific standard products (ASSPs) for the high-end, motor-control market. The V850™ line, for example, was designed with real-time process control in mind. Equipped with a 5-stage pipeline, these microcontrollers can execute one instruction per clock cycle, including 32-bit by 32-bit multiplication instructions and 32-bit shift instructions supported in hardware. Newer V850-based motor control ASSPs have features such as embedded analog circuits, noise filters and safety circuits that are even more unique and make the devices ideal for high-performance vector drives.

Enlarge this picture Fig. 4. High-impedance output control.

One of NEC Electronics’ most recent developments is the V850E/Ix3™ line of six microcontrollers, which operate at 64 MHz, execute 86 Dhrystone 1.1 MIPS and have two identical inverter control functions able to drive two independent sinusoidal vector drives. The V850E/IG3™ microcontroller configuration is illustrated in Fig. 2.

Special peripherals in the V850E/IG3 microcontroller include the 16-bit TABn inverter timer, the TAAn analog-to-digital (A/D) trigger generation timer, the dedicated TMT encoder timer, the 12-bit high-speed A/D converter with analog operational amplifiers (op amps) and comparators on the input side, and safety blocks such as the power-on-clear (POC) circuit, low-voltage indicator (LVI) and watchdog timer.

The 16-bit inverter timer is an up/down counter with four double-buffered compare registers, one for the PWM carrier cycle and the rest for three waveforms with independent duty cycles. A series of option registers is used to generate three complementary signals to make up the six PWM signals needed to drive the power stage. A variable dead-time interval can be inserted to prevent accidental shoot-through of the power transistors and the interrupts generated at the half-way point in the PWM cycle, and the timer overflows can be “culled” (counted before they are acknowledged).

The PWM duty cycle can be updated any time due to the double buffering of the three compare registers, but the new values are transferred to the internal registers all at once — either on the inverter timer overflow interrupt or the half-cycle interrupt. This allows a duty-cycle change without stopping the timer and without CPU intervention.

In vector control, precise current measurements must be performed within every PWM cycle. The V850E/IG3 microcontroller’s two high-speed 12-bit A/D circuits with independent sample and hold (S&H) circuits have multiple sources for triggering. One of those sources is timer TAA when used in synchronous mode with the inverter timer. Two double-buffered compare registers can be employed to produce A/D triggering at any two points within the PWM cycle. The A/D circuit is equipped with five double-buffered conversion result registers. In scan mode, five measurements can be performed within one PWM cycle and all measurements stored in separate buffers. The CPU’s assistance is not required during this process and the data is saved for later processing.

Enlarge this picture Fig. 5. Two-motor drive with one microcontroller.

One of the more unique features of this microcontroller is the analog front end of embedded op amps and analog comparators shown in Fig. 3.

More often, motor currents need some amplification before they can be properly measured with the A/D converter under very noisy conditions. Some of the most sensitive components to noise are the small signal op amps that must be used. Having the op amps embedded in the microcontroller is an effective way to increase noise immunity and also reduce the total component count and system cost. The gain of the embedded op amps can be adjusted through software settings in 13 steps from 2.5x to 10x.

For motor safety, two sets of analog comparators are used to monitor A/D input voltage against two variable thresholds that can be set on the microcontroller pins and also to produce a total hardware shutdown if non-safe operating conditions are detected.

Safe operation of the motor and power conversion circuits is one the most important tasks of the drive control system. Multiple hardware and software components have to be employed to avoid unsafe operation that may have catastrophic outcomes. An internal watchdog timer is used to issue either an interrupt or a total system reset if an accidental software runaway is detected. The V850/Ix3 microcontroller also has multiple embedded peripherals to address safety, since the fastest response is achieved with hardware. Fig. 4 shows a combination of sources to shut down the six PWM outputs in case there is a problem in the system. Two pins on the microcontroller can be triggered either by external hardware when an over-current situation is detected, or by an external watchdog circuit.

Other sources for high-impedance control are the embedded comparators mentioned earlier. The main clock oscillator is constantly watched by a clock monitor circuit to ensure proper operation.

Enlarge this picture Fig. 5b. Two-motor drive with one microcontroller.

Other safety features implemented in hardware are the POC and LVI circuits. The microcontroller will be kept in reset by the POC circuit until the power supply voltage reaches a minimum threshold at which the CPU and its peripherals can operate properly. The power-supply voltage is further monitored by the LVI circuit. If the power-supply voltage falls bellow a threshold voltage, an interrupt or an internal reset is generated.

In addition to motor control-specific peripherals, the V850E/Ix3 microcontroller has an large set of general-purpose hardware to support system tasks. A total of 13 available 16-bit timers, including the motor control timer, can be configured in up to nine different modes of operation to control the PWM output, interval timer, input capture, one-shot pulse generation, pulse-width measurement, external event count, external trigger-pulse output, six-phase inverter PWM output and encoder counter. An additional 10-bit/8-channel A/D converter is provided for other analog device interfacing such as potentiometers and temperature sensors.

A 16-bit address bus and 16-bit data bus are available for external memory access on the V850E/IG3, and most of the pins can be configured general-purpose I/O pins. Internal pull-up resistors under software control can be connected to any I/O pin, excluding the A/D inputs but including the inverter timer’s six PWM outputs. An on-chip direct-memory access controller (DMAC) can be used for high-speed data transfer between various memory devices, I/O registers or serial ports without CPU intervention. Other peripherals include four high-speed serial interfaces with multiple functions such as UART, UART with FIFO, clocked serial interface (CSI) and IIC.

The variety and number of specialized and general-purpose peripherals make the V850E/IX3 microcontroller an ideal single-chip solution for motor and system control implementation. The 32-bit RISC architecture-based 86 MIPS performance and 64 MHz operating speed exceeds the requirements of a vector-controlled sinusoidal motor drive. In fact, the microcontroller can control two high-performance vector drives. (See Fig. 5). The on-chip op amps, comparators, pull-up resistors, internal voltage regulator and POC and LVI circuits allow a drastic reduction in external component count, resulting in a more compact and less expensive motor control system design.

For more information, email:  j.pocs@am.necel.com