Speed Control of AC and DC Motors

Speed Control of DC Motors

• Armature Resistance Control:
  • Involves adding a variable resistor in series with the armature
  • Increases resistance drops more voltage across the resistor, reducing the voltage across the armature, and decreasing the speed
  • Simple but less efficient due to power loss in the resistor
• Field Flux Control:
  • Varies the strength of the magnetic field through which the motor operates
  • Reduces the field current to decrease the magnetic flux, increasing the speed of the motor
  • Provides a wide range of speed control but can affect the motor torque
• Armature Voltage Control:
  • Varies the voltage applied to the motor's armature
  • Done using a variable supply voltage or electronically through pulse-width modulation (PWM)
  • Efficient and provides good speed regulation without significant torque loss

Speed Control of AC Motors

• Pole Changing:
  • Certain AC motors designed with variable pole numbers that can be selected via wiring configurations
  • Changing the number of poles changes the motor's synchronous speed, offering discrete speed options
  • Efficient and simple to implement but doesn't allow for continuous speed control
• Supply Voltage Variation:
  • For some types of AC motors, varying the supply voltage can influence the speed to a limited extent
  • Less effective and rarely used for precise speed control since AC motor speed is more dependent on frequency than voltage
• Frequency Control (using VFDs):
  • Variable Frequency Drives (VFDs) adjust the frequency of the power supply to the motor
  • Allows for precise speed control and is highly efficient
  • Provides smooth control over a wide range of speeds without sacrificing torque
• Slip Control (for Slip Ring Induction Motors):
  • Involves controlling the slip of the motor by varying the resistance in the rotor circuit
  • Increases the rotor resistance to increase the slip and decrease the motor speed
  • Allows for good torque characteristics at lower speeds but is less efficient due to power loss in the external resistances

Thyristorized Stator Voltage Control of 3-Phase Induction Motor

• Principle of Operation:
  • Varies the amplitude of the voltage applied to the motor's stator windings by controlling the firing angle of the thyristors
  • Adjusts the effective RMS voltage applied to the motor windings, thereby controlling the motor speed
• Circuit Diagram:
  • Consists of three-phase AC power supply, three-phase thyristor bridge converter, triggering circuit, control unit, and three-phase induction motor
• Operation:
  • Triggering circuit controls the firing of the thyristors in the converter based on the firing angle set by the control unit
  • Adjusting the firing angle controls the amplitude of the output voltage applied to the motor
  • Smooth and continuous speed control over a wide range
• Advantages:
  • Smooth speed control
  • High efficiency
  • Low starting torque
  • Compact and lightweight
• Disadvantages:
  • Harmonics generation
  • Complex control
  • Cost

(V/F) Control Method

• Principle of Operation:
  • Maintains the ratio of voltage to frequency (V/F) constant to achieve stable and efficient motor operation
  • Allows for precise control of the motor speed without compromising torque characteristics
• Circuit Diagram:
  • Consists of AC power supply, rectifier stage, DC link capacitor, inverter stage, control unit, and three-phase AC induction motor
• Operation:
  • Rectification: AC power supply is rectified to convert AC to DC
  • DC Link: Rectified DC voltage is smoothed using a DC link capacitor
  • Inversion: Smoothed DC voltage is inverted back to variable-frequency AC using power electronic switches
  • Control: Control unit sets the frequency of the output voltage according to the desired motor speed
  • Voltage is adjusted proportionally to maintain the V/F ratio constant
• Advantages:
  • Wide speed range
  • High efficiency
  • Smooth starting and stopping
  • Torque control
• Disadvantages:
  • Limited overload capacity
  • Complex control
  • Harmonic generation
  • Cost

Slip Power Recovery Schemes

• Principle of Operation:
  • Recovers and converts the slip power (power dissipated in the rotor due to slip) into useful electrical energy
• Common Slip Power Recovery Schemes:
  • Rotor Resistance Control (Rotor External Resistance Control)
  • Static Scherbius Drive
  • Cycloconverter Drive
• Advantages:
  • Improved efficiency
  • Energy savings
  • Variable speed control
  • Regenerative braking
• Disadvantages:
  • Complexity
  • Harmonics and power quality issues
  • Maintenance
  • Cost