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Speed Control System DC Motors
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Outlines Introduction Speed Control of D.C. Shunt Motors Flux control method. Armature control method. Voltage control method Speed Control of D.C. Series Motors Flux control method. Armature-resistance control method. Series-parallel And Resistance Control
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Introduction 1.DC motors hold significant industrial importance despite the prevalence of AC motors. 2.The primary advantage of DC motors lies in their ability to undergo speed changes across a broad range through straightforward means. 3.AC motors typically lack the capability for fine speed control, unlike DC motors. 4.Fine speed control stands as a key factor contributing to the competitive edge of DC motors in modern industrial settings. 5.This chapter will delve into the diverse methods employed for controlling the speed of DC motors.
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Speed Control of D.C. Shunt Motors The speed of a shunt motor can be changed by: Flux control method. Armature control method. Voltage control method. The first method (i.e. flux control method) is frequently used because it is simple and inexpensive.
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Flux Control Method It is based on the fact that by varying the flux f, the motor speed (N α 1/f) can be changed and hence the name flux control method. In this method, a variable resistance (known as shunt field rheostat) is placed in series with shunt field winding as shown in Figure Beside.
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Flux Control Method The shunt field rheostat reduces the shunt field current Ish and hence the flux f. Therefore, we can only raise the speed of the motor above the normal speed (See Figure Beside). Generally, this method permits to increase the speed in the ratio 3:1. Wider speed ranges tend to produce instability and poor commutation.
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Armature Control Method This method is based on the fact that by varying the voltage available across the armature, the back e.m.f and hence the speed of the motor can be changed. This is done by inserting a variable resistance RC (known as controller resistance) in series with the armature as shown in Figure beside.
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Armature Control Method Due to voltage drop in the controller resistance, the back e.m.f. (Eb) is decreased. Since N µ Eb, the speed of the motor is reduced. The highest speed obtainable is lhat corresponding to RC = 0 i.e., normal speed. Hence, this method can only provide speeds below the normal speed (See Figure beside).
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Voltage Control Method In this method, the voltage source supplying the field current is different from that which supplies the armature. This method avoids the disadvantages of poor speed regulation and low efficiency as in armature control method. However, it is quite expensive. Therefore, this method of speed control is employed for large size motors where efficiency is of great importance.
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Voltage Control Method Multiple Voltage Control: This method involves connecting the shunt field of a motor to a fixed voltage source while allowing the armature to connect across different voltages via switchgear. Speed changes are achieved by adjusting the armature voltage, typically proportional to speed. Intermediate speeds can be attained using a shunt field regulator.
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Speed Control of D.C. Series Motors The speed control of d.c. series motors can be obtained by: 1.Flux control method. 2.Armature-resistance control method. The latter method is mostly used.
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Flux Control Method Field Diverters: 1.Method: A variable resistance, known as a field diverter, is connected in parallel with the series field winding of a motor. 2.Effect: The diverter shunts a portion of the line current from the series field winding, weakening the field and increasing motor speed (speed ∝ 1/field strength). 3.Lowest Speed: The lowest achievable speed is when the diverter carries zero current (i.e., open position), corresponding to the motor's normal speed. 4.Speed Range: This method can only provide speeds above the normal speed, making it suitable for applications requiring speeds higher than the motor's baseline speed. 5.Application: Commonly used in traction work for controlling the speed of motors.
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Flux Control Method Armature Diverter: 1.Method: A variable resistance, known as an armature diverter, is connected in parallel with the armature of the motor. 2.Effect: The diverter shunts some of the line current, reducing the armature current. 3.Flux and Speed Relationship: With a decrease in armature current (Ia), the flux (f) must increase (flux ∝ Ia) for a given load. Since speed (N) is inversely proportional to flux (N ∝ 1/f), decreasing armature current leads to a decrease in motor speed. 4.Speed Adjustment: By adjusting the armature diverter, speeds lower than the normal speed can be achieved. 5.Application: This method is utilized for obtaining speeds below the normal speed in motor operations.
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Flux Control Method Tapped Field Control: 1.Method: In this technique, the flux is decreased (resulting in increased speed) by reducing the number of turns of the series field winding. 2.Implementation: A switch (S) is used to short circuit different segments of the field winding, effectively reducing the flux and raising the motor speed. 3.Speed Adjustment: With the full turns of the field winding engaged, the motor operates at its normal speed. As the field turns are progressively cut out using the switch, speeds higher than the normal speed are attained. 4.Application: Tapped field control is employed to adjust motor speed, allowing for speeds above the normal operating range by reducing the field flux.
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Flux Control Method Tapped Field Control: 1.Method: In this technique, the flux is decreased (resulting in increased speed) by reducing the number of turns of the series field winding. 2.Implementation: A switch (S) is used to short circuit different segments of the field winding, effectively reducing the flux and raising the motor speed. 3.Speed Adjustment: With the full turns of the field winding engaged, the motor operates at its normal speed. As the field turns are progressively cut out using the switch, speeds higher than the normal speed are attained. 4.Application: Tapped field control is employed to adjust motor speed, allowing for speeds above the normal operating range by reducing the field flux.
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Flux Control Method Paralleling field coils: This method is usually employed in the case of fan motors. By regrouping the field coils as shown in Figure below, several fixed speeds can be obtained.
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Armature-Resistance Control Series Resistance Control: 1.Method: A variable resistance is directly connected in series with the motor's power supply, reducing the voltage available across the armature and consequently lowering the speed. 2.Speed Adjustment: By altering the resistance value, any speed below the normal speed can be achieved. 3.Common Usage: This method is widely employed for controlling the speed of DC series motors. 4.Speed Regulation: While this method offers poor speed regulation, it is not significant for series motors because they are typically used in applications requiring varying speeds. 5.Power Loss Consideration: Although the series resistance results in power loss, it is not a serious concern for many series motor applications. The control is often utilized to reduce speed under light-load conditions and intermittently when the motor is operating at full load.
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Speed Control DC Motor By Using PWM Pulse Width Modulation (PWM) is a widely-used technique for controlling the speed of DC motors. Essentially, it involves rapidly switching a digital signal on and off at a fixed frequency. By varying the ratio of the time the signal is on (known as the "duty cycle") to the time it's off, you can effectively control the average voltage supplied to the motor, thus regulating its speed.
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Speed Control DC Motor By Using PWM To implement PWM speed control for a DC motor, you'll need a few key components. Firstly, you'll require a microcontroller or PWM generator to generate the PWM signal. Additionally, you'll need a MOSFET or motor driver circuit to switch the power supplied to the motor based on the PWM signal. Of course, you'll also need the DC motor itself. Connect the microcontroller's PWM output pin to the input of the MOSFET or motor driver, and then connect the motor to the output of the MOSFET or motor driver. Ensure all power supply connections are properly made to avoid any issues.
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References 1."Electric Motors and Drives: Fundamentals, Types and Applications" by Austin Hughes and Bill Drury - This comprehensive book covers various aspects of electric motors, including DC motor control techniques. 2."Control of Electric Machine Drive Systems" by Seung-Ki Sul - This book focuses on the control strategies and techniques used in electric machine drive systems, including DC motor control. 3."Electric Motor Drives: Modeling, Analysis, and Control" by R. Krishnan - This book covers the modeling, analysis, and control of electric motor drives, including DC motors. 4."Power Electronics and Motor Drives: Advances and Trends" edited by Bimal K. Bose - This book discusses recent advances and trends in power electronics and motor drives, which includes topics related to DC motor control. 5."Practical Variable Speed Drives and Power Electronics" by Malcolm Barnes - This practical guide provides insights into the design and implementation of variable speed drives and power electronics, including DC motor control. 6.IEEE Transactions on Industry Applications - This journal regularly publishes research articles and papers related to industrial applications of electric motors, including DC motor control techniques.
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