SPEED CONTROL OF MOTORS

There is a wide range of appliances that are part of our lives, from household electrical appliances used in the home like vacuum cleaners and hair dryers to pumps, conveyor belts, and machine tools in large industrial plants. Wherever any machines use electric motors some type of motor speed control is invariably part of that machine.

Speed Control of AC Motors

The AC induction motor is essentially a constant speed device. The speed of the rotating magnetic field is referred to as synchronous speed. The synchronous speed (S) of a motor is defined as S = 120(F) ÷ P, where (F) is the incoming line frequency, and (P) is the number of poles in the motor. 

Since in the United States, the AC line frequency is 60 Hertz at 120 Volts, therefore an AC induction motor with four poles would have a synchronous speed of 1,800 rpm. In the field, however, as the load is applied to the motor it will run at less than 1,800 rpm. This difference in speed is called the slip and is usually expressed as a percentage. Because the number of poles in a machine is fixed, the only variable that’s left to change is the incoming line frequency — this is the basis for the operation of a motor speed controller known as the Variable Frequency Drive (VFD).

The VFD performs two functions: First, it converts the incoming AC signal to a DC signal by rectification; secondly it inverts the rectified DC signal back to a variable frequency AC signal. 

A DC motor could have its speed changed by increasing or decreasing the applied voltage. This is not the case for an AC motor. An AC induction motor will be damaged if the incoming supply voltage varies significantly. 

AC Motor Speed Controller

An AC Controller controls the speed of the AC Motor is also be referred to as a Variable Frequency Drive (VFD), Adjustable Speed Drive (ASD), and Frequency Converter (FC). The AC Motor receives power with a varying frequency generated by the AC Controller. This adjustable output allows precise control of motor speed. 

Components of an AC Speed Controller

There are three main components of an AC Speed Controller:

  1. Rectifier
  2. Inverter
  3. DC Link

AC input is converted to DC by a rectifier. The inverter converts the DC voltage back AC as output at the desired frequency. 

Types of Variable Frequency Drives 

Variable Frequency Drives (VFDs) come in three basic types: 

  1. Current Source Inverter (CSI)
  2. Voltage Source Inverter (VSI)
  3. Pulse Width Modulation (PWM). 

The sections of a VFD include a Converter, DC Link, and Inverter.  

Current Source Inverter (CSI)

A Current Source Inverter (CSI) converts the incoming AC voltage as well as controlling the frequency and voltage that supplies the AC Induction Motor. The CSI converts incoming AC to a variable DC voltage using: 

  • Silicon Controlled Rectifiers (SCR) 
  • Gate-Commutated Thyristors (GCT)
  • Symmetrical Gate-Commutated Thyristors (SGCT)

Voltage Source Inverter (VSI)

Voltage Source Inverter (VSI) conversion functions similarly to that of the CSI – converting incoming AC to DC voltage. The difference is that the VSI uses a diode bridge rectifier to achieve the conversion. The bridge uses capacitors to maintain consistent DC voltage, as well as storing energy for the drive system. 

The inverter section uses many types of transistors and thyristors which act like switches to create a Pulse Width Modulation (PWM) output to control the frequency and voltage applied to the motor.

Pulse Width Modulation (PWM)

Pulse Width Modulation (PWM) employs a diode bridge rectifier, like a VSI, converting incoming AC voltage to DC. Ripples generated by the rectifier are smoothed out using the DC Link’s large capacitors. This ensures a stable DC bus voltage. 

High power rated Insulated-Gate Bipolar Transistors (IGBTs) are used in the multi-step inverter stage of the driver to switch on and off to control both the frequency and voltage given to the motor in a sine-like waveform output. Varying the voltage pulse width results in an average power voltage which is supplied to the motor. The number of waveform transitions per second determines the frequency the motor requires.

Speed Control of DC Motors

DC motors are used in precise speed control applications because of their ability to provide rotation from a stop position to full speed quite easily and efficiently. Control of the speed of a DC series motor when the field is in series with the armature is done by increasing or decreasing the applied voltage to the circuit. 

In a DC shunt motor where the field is in parallel with the armature, the speed is controlled by increasing or decreasing the applied voltage to the armature utilizing a rheostat. 

Instead of rheostats now Silicon Controlled Rectifiers (SCRs) are used as they can handle large amounts of power with no heat dissipation issues. Additionally, SCRs are much smaller in size and interface easily with Programmable Logic Controllers (PLCs).

Flux Control

The speed of a DC motor can be controlled by changing the flux applied to it as the speed of the motor is inversely proportional to the flux per pole. To control the flux, a variable resistor or a rheostat is added in series with the field winding. Increasing the resistance will increase the speed as it will decrease the flux. In shunt motors, the field current is quite very small, so this method works quite efficiently. Flux control is an easy and convenient method for speed control as the power loss is small due to small because of the shunt field current.

Armature Control 

The speed of a DC motor is directly proportional to the back EMF. That means, when supply voltage and the armature resistance are kept constant, then the speed is directly proportional to armature current. Thus, if we add resistance in series with the armature, the current decreases, and therefore the speed will also decrease. Greater is the resistance in series with the armature, the greater the decrease in speed. In Armature Control Method a large amount of power is wasted and it is useful for small motors.

Voltage Control

Multiple Voltage Control

The shunt field is given a fixed exciting voltage but varying voltages are applied to the armature. This voltage across the armature is controlled by suitable switchgear and the speed of the motor is generally proportional to the voltage across the armature.

Ward-Leonard System

Ward-Leonard system speed control of DC motors is used where very accurate speed control of the motor is required. In this method, the output from the generator is fed to the armature of the motor whose speed is to be controlled. The output voltage of the generator can be varied employing the field regulator from zero to its maximum value, thereby varying the armature voltage smoothly, which results in very smooth control of the speed of the DC motor.  

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