A motor driver is a little current amplifier; the function of motor drivers is to take a low-current control signal and then turn it into a higher-current signal that can drive a motor.
A typical motor drive system is expected to have some of the system blocks indicated in Fig. 27.1. The load may be a conveyor system, a traction system, the rolls of a mill drive, the cutting tool of a numerically controlled machine tool, the compressor of an air conditioner, a ship propulsion system, a control valve for a boiler, a robotic arm, and so on.
The power electronic converter block may use diodes, MOSFETS, GTOs, IGBTs, or thyristors. The controllers may consist of several control loops, for regulating voltage, current, torque, flux, speed, position, tension, or other desirable conditions of the load. Each of these may have their limiting features purposely placed in order to protect the motor, the converter, or the load.
DC Motor Drives
Direct-current motors are extensively used in variable-speed drives and position-control systems where good dynamic response and steady-state performance are required. Examples are in robotic drives, printers, machine tools, process rolling mills, paper and textile industries, and many others. Control of a dc motor, especially of the separately excited type, is very straightforward, mainly because of the incorporation of the commutator within the motor. The commutator brush allows the motor-developed torque to be proportional to the armature current if the field current is held constant. Classical control theories are then easily applied to the design of the torque and other control loops of a drive system.
The mechanical commutator limits the maximum applicable voltage to about 1500 Vand the maximum power capacity to a few hundred kilowatts. Series or parallel combinations of more than one motor are used when dc motors are applied in applications that handle larger loads. The maximum armature current and its rate of change are also limited by the commutator.
Small servo-type dc motors normally have permanent magnet excitation for the field, whereas larger size motors tend to have separate field-supply Vf for excitation. The separately excited dc motors represented in Fig. 27.2a have fixed field excitation, and these motors are very easy to control via the armature current that is supplied from a power electronic converter.
Thyristor ac–dc converters with phase angle control are popular for the larger motors, whereas duty-cycle controlled pulse-width modulated switching dc–dc converters are popular for servo motor drives.
The series-excited dc motor has its field circuit in series with the armature circuit as shown in Fig. 27.2b. Such a connection gives high torque at low speed and low torque at high speed, a pseudo-constant-power-like characteristic that may match traction-type loads well.
We recall the block diagram for an armature-controlled DC motor:
Converters for dc Drives
Depending on application requirements, the power converter for a dc motor may be chosen from a number of topologies. For example, a half-controlled thyristor converter or a singlequadrant PWM switching converter may be adequate for a drive that does not require controlled deceleration with regenerative braking. On the other hand, a full four-quadrant thyristor or transistor converter for the armature circuit and a two-quadrant converter for the field circuit may be required for a high-performance drive with a wide speed range.
Thyristors are used to construct the first stage of an electric motor drive in order to vary the amplitude of the voltage waveform across the windings of the electrical motor as it is shown in Fig. 3.35.
An electronic controller controls the gate current of these thyristors. The rectifier and inverter sections can be thyristor circuits. A controlled rectifier is used in conjunction with a square wave or pulse-width modulated (PWM) voltage source inverter (VSI) to create the speed-torque controller system. Figure 3.36 shows a square-wave or PWM VSI with a controlled rectifier on the input side. The switch block inverter is made of thyristors (usually GTOs) for high power. Lowpower motor controllers often use IGBT inverters.
One of the basic functions in Power Electronic is Switching. Based on Figure 1.15, Switching Functions can be characterized completely with three parameters:
- The duty ratio D is the fraction of time during which the switch is on. For control purposes the pulse width can be adjusted to achieve a desired result. We can term this adjustment process as pulse-width modulation (PWM), perhaps the most important process for implementing control in power converters.
- The frequency fswitch =1/T (with radian frequency ω=2πfswitch) is most often constant, although not in all applications. For control purposes, frequency can be adjusted. This is unusual in power converters because the operating frequencies are often dictated by the application.
- The time delay t0 or phase Ø0=ωt0: Rectifiers often make use of phase control to provide a range of adjustment. A few specialized ac-ac converter applications use phase modulation
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