DC Motor Drives
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Introduction: DC Motor Drives
Direct current (dc) motors have variable characteristics and are used extensively in variable-speed drives. DC motors can provide a high starting torque and it is also possible to obtain speed control over a wide range. The methods of speed control are normally simpler and less expensive than those of AC drives. DC motors play a significant role in modern industrial drives. Both series and separately excited DC motors are normally used in variablespeed drives, but series motors are traditionally employed for traction applications. Due to commutators, DC motors are not suitable for very high speed applications and require more maintenance than do AC motors. With the recent advancements in power conversions, control techniques, and microcomputers, the ac motor drives are becoming increasingly competitive with DC motor drives. Although the future trend is toward AC drives, DC drives are currently used in many might be a few decades Copyright 2003 industries. It Chapter 2 Power Semiconductor before the DC drives are 2-2 by John Wiley & replaced by AC drives. Sons, Inc. Switches: An Overview completely
Introduction: DC Motor Drives
Controlled rectifiers provide a variable dc output voltage from a fixed ac voltage, whereas a dc-dc converter can provide a variable dc voltage from a fixed dc voltage. Due to their ability to supply a continuously variable dc voltage, controlled rectifiers and dc-dc converters made a revolution in modern industrial control equipment and variable-speed drives, with power levels ranging from fractional horsepower to several megawatts. Controlled rectifiers are generally used for the speed control of dc motors. The alternative form would be a diode rectifier followed by dc-dc converter. DC drives can be classified, in general, into three types:
1. Single-phase drives 2. Three-phase drives 3. DC-DC converter drives
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Controlled Rectifier- and DC-DC Converter-Fed Drives
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Operating Modes
In variable-speed applications, a dc motor may be operating in one or more modes:
motoring, regenerative braking, dynamic braking, plugging, and four quadrants.
Motoring: The arrangements for motoring are shown in Figure 15.7a. Back emf Eg is less than supply voltage Vy. Both armature and field currents are positive. The motor develops torque to meet the load demand.
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Armature Reversal
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Field Reversal
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Operating Modes
Regenerative braking: The arrangements for regenerative braking are shown in Figure 15.7b. The motor acts as a generator and develops an induced voltage Eg. Eg must be greater than supply voltage Va. The armature current is negative, but the field current is positive. The kinetic energy of the motor is returned to the supply. A series motor is usually connected as a self-excited generator. For self-excitation, it is necessary that the field current aids the residual flux. This is normally accomplished by reversing the armature terminals or the field terminals.
Copyright 2003 by John Wiley & Sons, Inc.
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Operating Modes
Dynamic braking: The arrangements shown in Figure 15.7c are similar to those of regenerative braking, except the supply voltage Va is replaced by a braking resistance Rb,. The kinetic energy of the motor is dissipated in Rb.
Copyright 2003 by John Wiley & Sons, Inc.
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Operating Modes
Plugging: Plugging is a type of braking. The connections for plugging are shown in Figure 15.7d. The armature terminals are reversed while running. The supply voltage Va and the induced voltage Eg act in the same direction. The armature current is reversed, thereby producing a braking torque. The field current is positive. For a series motor, either the armature terminals or field terminals should be reversed, but not both.
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Operating Modes
Four Quadrants: Figure 15.8 shows the polarities of the supply voltage Va, back emf Eg, and armature current Ia for a separately excited motor. In forward motoring (quadrant I), Va, Eg, and Ia are all positive. The torque and speed are also positive in this quadrant. During forward braking (quadrant II), the motor runs in the forward direction and the induced emf Eg continues to be positive. For the torque to be negative and the direction of energy flow to reverse, the armature current must be negative. The supply voltage Va should be kept less than Eg. In reverse motoring (quadrant III), Va, Eg, and Ia are all negative. The torque and speed are also negative in this quadrant. To keep the torque negative and the energy flow from the source to the motor, the back emf Eg must satisfy the condition | Va | > | Eg |. The polarity of Eg can be reversed by changing the direction of field current or by reversing the armature terminals. During reverse braking (quadrant IV), the motor runs in the reverse direction. Va, and Eg continue to be negative. For the torque to be positive and the energy to flow from the motor to the source, the armature current must be positive. Copyright 2003 Chapter 2 Power Semiconductor 2-11 The induced emf Eg must satisfy the condition | Va | < | Eg |. by John Wiley & Sons, Inc. Switches: An Overview
Operating Modes
Four Quadrants: Figure 15.8 shows the polarities of the supply voltage Va, back emf Eg, and armature current Ia for a separately excited motor.
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Single-Phase Drives
If the armature circuit of a dc motor is connected to the output of a single-phase controlled rectifier, the armature voltage can be varied by varying the delay angle of the converter a. The basic circuit agreement for a single-phase converter-fed separately excited motor is shown in Figure 15.9. At a low delay angle, the armature current may be discontinuous, and this would increase the losses in the motor. A smoothing inductor, Lm, is normally connected in series with the armature circuit to reduce the ripple current to an acceptable magnitude. A converter is also applied in the field circuit to control the field current by varying the delay angle f.
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Single-Phase Drives
Depending on the type of single-phase converters,
single-phase drives may be subdivided into: Single-phase half-wave-converter drives. Single-phase semi converter drives. Single-phase full-converter drives. Single-phase dual-converter drives.
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Single-Phase Half-Wave-Converter Drives
A single-phase half-wave converter feeds a dc motor, as shown below. The armature current is normally discontinuous unless a very large inductor is connected in the armature circuit. A freewheeling diode is always required for a dc motor load and it is a one-quadrant drive. The applications of this drive are limited to the 0.5 kW power level. Figure shows the waveforms for a highly inductive load. A half-wave converter in the field circuit would increase the magnetic losses of the motor due to a high ripple content on the field excitation current.
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Single-Phase Half-Wave-Converter Drives
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Single-Phase Full-Wave-Converter Drives
The armature voltage is varied by a single-phase full-wave converter, as shown in Figure 15.13a. It is a two-quadrant drive, as shown in Figure 15.13b, and is limited to applications up to 15 kW. The armature converter gives + Va or - Va, and allows operation in the first and fourth quadrants. During regeneration for reversing the direction of power flow, the back emf of the motor can be reversed by reversing the field excitation.
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Single-Phase Full-Wave-Converter Drives
The converter in the field circuit could be a full, or even a dual converter. The reversal of the armature or field allows operation in the second and third quadrants. The current waveforms for a highly inductive load are shown in Figure 15.13c for powering action.
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Single-Phase Full-Wave-Converter Drives
The converter in the field circuit could be a full, or even a dual converter. The reversal of the armature or field allows operation in the second and third quadrants. The current waveforms for a highly inductive load are shown in Figure 15.13c for powering action.
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Single-Phase Dual-Converter Drives
Two single-phase full-wave converters are connected. Either converter 1 operates to supply a positive armature voltage, Va, or converter 2 operates to supply a negative armature voltage, - Va. Converter 1 provides operation in the first and fourth quadrants, and converter 2, in the second and third quadrants. It is a four-quadrant drive and permits four modes of operation: forward powering, forward braking (regeneration), reverse powering, and reverse braking (regeneration). It is limited to applications up to 15 kW. The field converter could be a full-wave or a dual converter.
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Single-Phase Dual-Converter Drives
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