Servo drives, motors, and the devices that control them represent a sustainable growth opportunity, driven by innovations in automotive and industrial systems and advances in production technology. The automotive and transportation industries are expected to account for the largest share of servo motors and drive sales through 2022 to 2022. Demand for drives, controllers, and servo motors remains strong to help companies improve efficiency in industrial environments.
Guiding and speed/torque control requirements and technologies vary by motor type, ranging from simple control of voltage and current for DC motors and general-purpose motors to the use of inverters for AC motors, feedback switching of different phases in brushless motors, and complex digital circuitry stepper motor drives. Even for traditional analog motors such as induction motors and switched reluctance types, today's traditional analog technology is accompanied by increasingly sophisticated digital control methods that allow solutions to be implemented at low cost. The use of microelectronic devices enables better speed, position and torque control, as well as higher efficiency.
Figure 1: Motor control IC block diagram (Image: Maxim Integrated)
Motor control circuits must quickly switch current on and off in motor coils while minimizing switching or conduction losses. both MOSFETs and insulated gate bipolar transistors (IGBTs) meet the needs of motor control in a variety of applications. These electrical control devices have similar functions and attributes, and there is overlap in their internal design. In most applications, they are used in an H-bridge configuration to control the current path to two or more motor coils. This allows full control over the speed and direction of the motor (Figure 1).
Motor Overview
Any design project that includes the requirement of a motor or mechanical drive must evaluate whether to use a continuous current design or a stepper or servo motor. In a continuous motor, permanent magnets or windings are used to create a static magnetic field in the stator. The rotor consists of coils where current enters through graphite brushes pressed into a manifold on a rotating shaft. Current flows through successive windings to maintain rotation.
AC motors can be synchronous or asynchronous. In asynchronous motors (also known as induction motors), the stator windings are arranged to form a roughly sinusoidal distribution. Synchronous motors include brushless DC and AC motors as well as switched reluctance motors and motors powered by sinusoidal voltage sources.
In brushless motors, the rotor has permanent magnets and the windings located in the stator are driven by control electronics in the appropriate sequence. A brushless DC motor is driven by a continuous signal switching sequence on different stator windings. Brushless AC motors can be made as synchronous AC motors with permanent magnets; in this case they are driven by sinusoidal signals. The absence of brushes increases efficiency by eliminating sources of friction. The absence of mechanical parts on the switches allows higher rotational speeds to be achieved.
Stepper motors are brushless synchronous motors powered by DC. The rotor remains stationary at a specific position. Stepper motors can very accurately rotate the rotor shaft a few degrees without using sensors to detect the angular position.
Key Parameters
As with most electronic components, a number of key and specific performance parameters determine the initial correspondence between the device and the application. The key parameters for motor control devices are the current and voltage management values, as these determine whether a particular component can support the motor load requirements.
For MOSFETs, the next key parameters are the active resistance (RDS(on)) and gate capacitance. Lower resistance reduces resistance losses and voltage drop during the on state, which reduces dissipative load and improves efficiency. The gate capacity determines the frequency and current speed required to fully enable and disable the gate at the desired transition time (switching speed). For IGBTs, the next critical parameter is the voltage drop (Vdrop), which is the sum of the contributions from diodes and internal MOSFETs passing through the PN junction. Temperature and current levels affect the RDS(on) and Vdrop parameters.
In general, MOSFETs offer higher switching speeds (in MHz) and higher peak currents. IGBTs offer current values around 10 A and are robust, but have slower switching speeds. For motor control applications, the basic rule is that MOSFETs are a better choice for lower voltage and current and higher switching frequency, while IGBTs are a better choice for higher voltage/current and lower frequency.