Introduction to Motors
Electric motors continue to play a dominant role in many diverse applications. Most electric machines operate on the basis of interaction between current-carrying conductors and electromagnetic fields. In particular, electric motors rotate due to a principal based on Faraday's law of electromagnetic induction, e = Blv, where e is the voltage induced in a conductor of length (l), and whose velocity component (v) is perpendicular to a uniform magnetic field B. Maintaining the familiar properties of motor design, industry trends are adopting higher levels of integration and more sophisticated control techniques to strengthen other areas of machine operation.

The design of a modern motor control system is complex and requires a designer with a working knowledge of control system algorithms, microcontrollers, digital signal processors (DSPs), sensor signal measurement, A/D converters, high voltage interface, gate drivers, and output inverter power stages. Direct interfacing between intelligent control and power elements, whether actuator or indicator, is a challenge for semiconductor manufacturers striving to provide the highest level of integration for emerging systems.

Motor drives are used in a very wide power range, from a few watts to many thousands of kilowatts, and in applications ranging from very precise, high performance position-controlled drives in robotics to variable-speed drives for adjusting flow rates in pumps. In all drives with controlled speed and position, a power electronic converter interface is needed between the input power and the motor. A general block diagram depicting the control of motor drives is shown in Figure 1, where the process determines the requirements of the motor drive, i.e. servo-motor drive vs. adjustable speed drive.

This article is intended to provide a brief overview of electric motor controllers and new technologies that are being developed to maximize the performance of electric motors.

Alternating Current (AC) vs. Direct Current (DC)
DC brushed motors have dominated the velocity control market for many decades, even though recent competition from the inverter driven AC motors and brushless motors has increased. However, for constant torque and intermittent low speed applications, the inverter drives are still no match for the DC systems. DC motors can be connected directly to a DC supply, but then the torque and velocity control is lost. To combat this problem, there is a wide range of amplifiers that can be used to obtain precise velocity or torque control, such as pulse width modulation (PWM), linear, or SCR amplifiers. Torque mode is usually available in all amplifiers, and velocity mode can be configured if the motor has feedback, such as a tachometer.

DC brush motors are great for precise positioning control when coupled with a motion controller. Today, because of cost effectiveness and reliability, the Permanent Magnet DC motor (PMDC) is the motor of choice for applications involving fractional hp DC motors and most applications up to about three hp. At five hp and greater, various forms of the shunt wound DC motor are most commonly used because the more cost effective electromagnetic windings are permanent. In most DC motors, several sets of windings or permanent magnets are present to smooth out the motion. Controlling the speed of a brushed DC motor is not very difficult and can be accomplished through increased or decreased armature voltage to linearly increased or decreased rotation, respectively. Motors are available in certain standard voltages, which roughly increase in conjunction with higher horsepower ratings. The smallest industrial motors are rated at 90 VDC and 180 VDC, while larger units are rated at 250 VDC and higher. Specialty motors for uses in mobile applications are rated at 12, 24, or 48 VDC.

AC Motors
The induction motor is probably the most common of all motors. Torque is developed by the induction of a magnetic field induced by the AC current applied to the motor windings. For the motor to continue operation there must be relative motion between the rotor and stator field. The rotor, therefore, must run at a slower speed than the stator to create a small difference in speed (referred to as slip). Gradually the rotor picks up speed, rotor currents decrease, and the rotor reaches a point where the torque produced equals the torque demanded by the load. A Squirrel Cage motor design exemplifies this induction style of motor operation. However, to eliminate the slip and increase the efficiency of the motor, synchronous motors were developed to become one of the most efficient electric motors in the industry. They function at constant speeds and frequencies under steady state conditions, and operate from the relative motion of the conductors to the magnetic flux lines.

Synchronous motors tend to be used in very high power applications, with power levels in the kilowatt range and are often operated from a three-phase AC power source. Like the induction motor, the synchronous motor can operate as either a motor or a generator.

Comparisons
The AC motor has the advantage of being the lowest cost motor for applications requiring more than about 1/2 hp (325 watts) of power. This is due to the simple design of the motor. For this reason, AC motors are overwhelmingly preferred for fixed speed applications in industrial, commercial and domestic applications where AC line power can be easily attached. Over 90 percent of all motors are AC induction motors. They are found in air conditioners, washers, dryers, industrial machinery, fans, blowers, vacuum cleaners, and many other applications. The simple design of the AC motor results in extremely reliable, low maintenance operation. Unlike the DC motor, there are no brushes to replace. If it is run in the appropriate environment for its enclosure, the AC motor can expect bearing replacement after several years of operation, and up to ten years for a well designed application.

Motor Control Circuitry for Variable Speed - PWM
To achieve adequate levels of power handling capability, particularly in motor control applications that require multiple drive elements, power integrated circuits have been developed using hybrid constructions of discrete transistors. Hybrid techniques have been necessary and useful due to the power handling limitations of monolithic power integrated circuit technology. Power integrated circuit design has often been limited by the absence of power packages that provide the low thermal impedance and high performance switching necessary for reliable operation. The switching elements of these modules, which may be Insulated Gate Bipolar Transistors (IGBTs) or various forms of thyristors, "chop" low-frequency (e.g., 60 Hz or dc) voltages /currents at the input / output port into high-frequency square wave pulses of variable width (20 to 200 ms). These are then applied to the filter elements, which attenuate high-frequency components of the broad-spectrum pulses far more than low or mid-frequency components when passing the voltage (current) from their input to their output ports. By controlling the width of each pulse through precise timing of the switches, a process referred to as PWM, it is possible to manipulate the magnitude and phase of the power converter output voltage (or input current) for low- and mid-frequency components. Figure 3 shows the topology of a PWM circuit designed by ACI for an AC/AC inverter application integrated with the alternator.

A power stage employing PWM control techniques enables the generation of multiple, isolated, low-voltage outputs appropriate for power management applications. The majority of PWM controllers regulate a single power supply output and are unlikely to house all the decoding logic required to implement the various gate drive signals for multiple output applications. However, PWM waveform generation, dead time and fault diagnostic functions can be integrated into a single monolithic chip to allow multiple chip implementation for distributed power applications involving synchronous rectification for multiple, low-power, motor drive outputs.

Through previous accomplishments of the Power Electronics Teaching Factory, ACI has the knowledge and capability to maximize the operation of power switches (modules) through new packaging and interconnection techniques. Topology designs for motor controls will utilize power packages with improved thermal management and reduced operating deficiencies. Three-dimensional packaging with wireless bonds will prove to decrease power losses and increase operating efficiency, therefore maximizing the power switches at the device level for improved PWM system applications.

Gate Drive Considerations for Motor Drive Circuits
The core function of power modules is to perform motor power switching and amplification in electric variable speed and servo drives. Some modules integrate capabilities and intelligence beyond this basic requirement and integrate chipset architecture containing control algorithms for gate drive operation and power management. However, too much intelligence can increase product cost and reduce module flexibility, rendering the device application specific. For this reason, and in an attempt to achieve package standardization, gate drive logic is usually not integrated into the power module during the development stages. Drive manufacturers are then responsible to implement application specific gate drives to control the switching operation of the power transistor circuits contained within the power module package.

Considering the Insulated Gate Bipolar Transistor (IGBT) based modules, since they dominate industrial motor drive applications, the required gate drivers must perform fast switching (10kHz to 20kHz) of the voltage controlled MOS gates while delivering high peak output currents of 15A or more. These high peak currents are needed to rapidly charge the large input capacitance of the high current modules for turn-on during a typical PWM operation. The fast switching and higher operating voltages of IGBT based circuits produces high dv/dt that can couple noise into the gate drive circuit, therefore, selection of gate drive output devices, power supply decoupling, and circuit design are critical for efficient gate drive operation.

Use of IGBTs, Power MOSFETs for Low Loss Operation
High voltage and high current power switches are packaged according to many power module designs, but they all serve purpose for industrial motor drives and pulsed-power applications. Traditional high power applications employ the operation of gate turn-off thyristor (GTO) and IGCTs, but with the advancements of IGBT technology, evolving motor drive systems will implement the power switching capabilities of IGBT based power modules. The low drive power requirement of an IGBT's voltage controlled Metal Oxide Semiconductor (MOS) gate is one of the key advantages partly responsible for its rapid acceptance into industrial applications. Compared with GTOs and IGCTs, IGBT modules provide an advantage in motor driving, system control and package insulation due to MOS-gate control and isolated thermal management. IGBT modules in the medium voltage (MV) range of 6.5kV can be integrated into converter designs for industrial drive applications, and provide a safety margin for inductive overvoltage spikes and short circuit turn-off (at 4500Vdc).

Achieving these MV levels with success is due largely to advancements in semiconductor technology and innovative packaging for improved thermal management and low loss operation. The continuing process of altering the silicon characteristics of the IGBT and incorporating the fast recovery influences of new freewheeling diodes, leads to a reliable isolated package ideal for future industrial drives and pulse power applications.

ACI's Design
ACI is demonstrating a single-phase to three-phase power system that drives and controls a 5 to 10 horsepower motor. This inverter utilizes state-of-the-art IGBT technology as the power devices to drive the windings of a three-phase motor, to a speed that is adjustable and controllable. A ten HP synchronous motor is used to simulate a high power cooling fan application. The gate drive electronics are designed to accommodate expansion to higher power components, for higher powerloads. The rotational speed of the motor is tachometer-controlled and can be adjusted for a specific application. ACI is using a single supply, full H-bridge style power driver. Internal logic enables the gate drive circuitry to be used in a number of control schemes with only a few logic level inputs.

• "Chipset Streamlines Motor Drive Power Management," Takahashi, International Rectifier, PCIM, April 2001, pg. 18.
• "Power Management Solution Delivers Efficient Multiple Outputs," Andreycak, Texas Instr., PCIM, Aug 2001, pg. 29.
• "Gate Drive Techniques for Large IGBT Modules," Motto, Powerex Inc., PCIM, 1997, pg. 1.
• "6.5kV IGBT Module Delivers Reliable Medium-Voltage Performance," Schuetze, Berg, Schilling, PCIM, August 2001, pg. 14.