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| Variable Frequency Drives |
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| AC 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. AC motor drives are used with many types of AC motors. Induction motors induce current into the rotor windings without any physical connection to the stator windings. They are suitable for many different environments and are capable of providing considerable power as well as variable speed control. Synchronous motors are no-slip devices that operate at constant speed up to full load. Subcategories include reluctance motors and permanent magnet devices such as AC servomotors. Many AC servomotors use brushless commutation with feedback provided by Hall effect sensors. Pole changing motors use pole number control, a method for changing the number of poles on the primary winding. Vector drive motors provide independent control of both the voltage and frequency, resulting in low-speed torque outputs that approach those of DC motors. In some designs, encoders or resolvers are used to provide feedback about position and speed. Linear motors generate force in only the direction of travel. Common technologies include moving coil, moving magnet, and switched reluctance designs.
Selecting AC motor drives requires an analysis of application categories. Multi-axis controllers are used to control and monitor multiple, independent axes of motion. Motor speed controllers are application-specific and used to control machines such as conveyors. Robotic motion controllers use digital motion control hardware and software for the coordinated multi-axis control of industrial robots and robotic systems. Servo amplifiers are electronic modules that convert low level analog command signals to high power voltages and currents. Inverter drives convert AC power inputs to DC outputs. High frequency drives supply power to AC motors at frequencies that are considerably higher than those used in standard-power applications. Regenerative drives support motor braking. Variable speed drives support speed control and adjustment. AC motor drives that use microcontrollers, silicon controlled rectifiers (SCR), digital signal processors (DSP), and pulse width modulation (PWM) are also available.
AC motor drives differ in terms of electrical ratings, operating parameters, configurations, and features. Electrical ratings include maximum output voltage, rated power, continuous output current, peak output current, AC supply voltage, and DC supply voltage. AC motor drives use either single-phase or three-phase inputs at 50, 60, or 400 Hz. Operating parameters include specifications for setup and control. Some AC motor drives have manual controls such as knobs, DIP switches, jumpers or potentiometers. Others include a joystick, digital control panel, computer interface, or slots for PCMCIA cards. Control programs can be stored on removeable, nonvolatile storage media. Hand held devices are designed to be programmed remotely. Wireless and web-enabled controls are also available. Configurations for AC motor drives include several mounting styles. Most devices mount on a chassis, DIN rail, panel, rack, wall, or printed circuit board (PCB). Standalone devices and integrated circuit (IC) chips that mount on PCBs are also available. Features for AC motor drives include soft starting; dynamic, injection, or regenerative braking; brake outputs or auxiliary inputs/outputs (I/O); auto-tuning, self-diagnostics, and status monitoring; and alarms for conditions such as overvoltage.
Computer-based AC motor drives use many different types of buses and communication standards. Bus types include advanced technology attachment (ATA), peripheral component interconnect (PCI), integrated drive electronics (IDE), industry standard architecture (ISA), general-purpose interface bus (GPIB), universal serial bus (USB), and VersaModule Eurocard bus (VMEbus). Communications standards include ARCNET, AS-i, Beckhoff I/O, CANbus, CANopen, DeviceNet, Ethernet, small computer systems interface (SCSI), and smart distributed system (SDS). Many serial and parallel interfaces are also available. |
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| DC DRIVES |
DC motor drives act as an interface between a controller and the motor. The drive must match the control signals (voltage and power levels) and signal type (analog or digital). The drive produces power conversion, amplification, and sequencing of waveform signals. Types of DC motor drives include adjustable or variable speed control, servo control, and integral motion control. Some drives are capable of delivering adjustable or variable speed control. Servomotors are generally small, powerful for their size, and easy to control. Integral motion control includes feedback and a controller. Important specifications to consider when searching for DC motor drives include considering whether or not a brushless DC motor drive or amplifier is needed. Some DC motor drives operate in conjunction with a brushless DC motor. Types of DC brushless motor drives include sine wave, trapezoidal, hall sensor commutation, and no sensor.
Important operating specifications to consider when searching for DC motor drives include open loop control, positive feedback, tach or velocity module, BEMF or voltage control, torque or current control mode, switching frequency, bandwidth, maximum speed, continuous power, continuous output voltage, maximum continuous output current, and peak output current. Open loop control uses a tachometer or other device to monitor motor speed, rather than using position feedback to control motor. Position feedback uses position, force, or other feedback to control motor, in addition to speed sensors. Use of feedback is referred to as "closed-loop control." A tach or velocity mode gives direct feedback of motor speed. A drive with BEMF or voltage control can run without a tach using the motor's Back EMF voltage. A torque or current control module has constant current control, uses current feedback. Switching frequency is specified for drives with digital output waves, i.e., PWM. The bandwidth is the frequency range over which the device meets its accuracy specifications. Accuracy is degraded at lower and lower frequencies unless the device is capable of dc response, and at higher frequencies near resonance and beyond, where its output response rolls off. Frequencies in the database are usually the 3dB rolloff frequencies. The maximum device speed is the speed when the drive is burdened with no external forces, such as weight. The continuous power is the maximum amount of power the device can output continuously. The continuous output voltage is the maximum voltage the device can output continuously. The maximum continuous output current is the maximum current the device can output continuously. Maximum current value may require additional cooling. The peak output current is the peak current the device can output.
Interface options to consider when searching for DC motor drives include analog or digital drives. Manual adjustments are available for some drives. These include potentiometers, dip switches etc. for adjustment/set-up, not for daily use. An interchangeable module or EPROM is a 'module' that holds motor parameters to match a drive to a motor. Interchangeable modules allow the device to drive different motors. Computer interfaces and self-test diagnostics are also common features. Other important features to consider when specifying DC motor drives include PWM switching, dynamic braking, and regenerative braking. PWM or switching drives have an output that is pulse width modulated or switching. Dynamic braking uses a shunt resistor or direct short across the armature to reduce mechanical energy. Energy is dissipated to the environment as heat. Regenerative braking converts mechanical energy back to electrical energy and transfers it back to the energy source. In this form of braking, mechanical energy is fed back through the power transmission system and the motor acts as a generator to convert mechanical energy into electrical energy. Newly converted energy is stored in a battery. Regenerative braking is often used in conjunction with dynamic and other braking methods. An important environmental parameter to consider for DC motor drives is the operating temperature. |
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