DC Motor Drives Information
Image Credit: Dart Controls, Inc.; Electromate; Baldor Electric Company
DC motor drives are defined as amplifiers or power modules that interface between a controller and a DC motor. They convert step and direction input from the controller to currents and voltages compatible with the motor. These units are sometimes called variable speed drives, referring to a majority of DC motor drives which adjust shaft speed. In industry, a 'drive controller' is a motor drive which incorporates functions of a programmable logic controller (PLC) and drive interface to regulate the speed, torque, horsepower, and direction of a DC motor.
DC motors tend to be less complex than AC motors and are normally less expensive for most horsepower ratings. They are capable of providing large startup torques exceeding 400% of the rated continuous torque. They have a long history of use in variable speed applications with a wide range of options available for this purpose.
Types of DC Drives
DC motor drives are classified based on the type of DC motor being used. These types include brushed, brushless, servo, linear, and voice coil motors.
- Brushed motors commutate via physical contacts, often spring-loaded graphite brushes biased against the commutation bar.
- Brushless motors commutate electronically with no physical brush contact.One common technique for positional feedback to control commutation is the use of Hall effect sensors to detect rotor position. Commutation options include trapezoidal and sinusoidal drive signals to the motor.
- Servomotors can be of brush or brushless design, and include an internal sensor for position control and other industrial automation applications. If brushless, the commutation of the three phases will typically be either trapezoidal or sinusoidal; the term "DC brushless" often connotes trapezoidal commutation of a brushless motor with Hall Effect sensor feedback for commutation control.
- Linear motors generate force only in the direction of travel. The motor technology resembles rotary motor technologies simply oriented in a linear fashion. Linear motors are capable of extremely high speeds, quick acceleration, and accurate positioning.Linear motor technologies include moving coil, moving magnet, AC switched reluctance design, AC synchronous design, AC induction or traction design, linear stepping design, DC brushed design, and DC brushless design.
- Voice coil motors consist of a magnetic coil placed in a magnetic field. When current is applied to the coil, electromagnetic flux is generated that causes the coil to move. The motor's name is derived from its resemblance to audio speaker operation.
DC motor drives can also be classified based on types of control functions (e.g. integral motion controllers, variable speed drives, motor speed controllers, etc.).
There are a wide range of operating specifications to consider when searching for DC motor drives. The most important of these include:
- Continuous current - the current applied to the motor during continuous operation.
- Input frequency - the AC input frequency accepted by the device.
- Power - the rated power output of the drive motor system.
- Supply voltage - the voltage supplied to the drive.
- Operating temperature - the operating temperature of the power supply.
Other important characteristics include the mounting configuration and accompanying features.
Drive mounting configuration is important for compatibility with the motor system. Drives can be mounted in various ways based on the design, including onto a PCB (printed circuit board), PC board, panel, DIN rail, or rack. Other drives may be stand-alone devices or designed to be incorporated into specific products by original equipment manufacturers (OEMs).
DC motor drives may also include various features which may be important for certain applications. These include:
- Regeneration - Method of braking in which the motor is disconnected from the power supply and power generated from the rotating motor is sent back to the supply.
- Programmability and configuration - Device can be programmed with routine configurations and commands for greater functionality and process control.
- Auto restart - Drive is designed to automatically restart operation after a stall.
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AC Motor Drives
AC motor drives interface controllers to AC motors. They match the control signals (voltage and power levels) as well as the signal type (analog or digital). They also provide power conversion, amplification, and the sequencing of waveform signals.
Motion Control Systems
Motion control systems contain matched components such as controllers, motor drives, motors, encoders, user interfaces and software. Components in these systems are optimally matched by the manufacturer.
Motion controllers range from simple linear controllers to complex, user-programmable modules that act as controllers within complex integrated multi-axis motion systems.
Motor controllers receive supply voltages and provide signals to motor drives that are interfaced to motors. They include a power supply, amplifier, user interface, and position control circuitry.
Motor Speed Controllers
Motor speed controllers are electronic devices that control motor speed. They carry specifications for drive type, product classification, electrical ratings, and operating parameters.
Servo drives provide electrical drive outputs to servo motors in closed-loop motion control systems where position feedback and corrective signals optimize position and speed accuracy.
Stepper Motor Drives
Stepper motor drives power unipolar and bipolar stepper motors in full step, half step, and microstep motion control applications.