Variable Frequency Drive for Induction Motor

Introduction

Single-phase induction motors are extensively used in appliances and industrial controls. The Permanent Split Capacitor (PSC) single-phase induction motor is the simplest and most widely used motor of this type.

By design, PSC motors are unidirectional, which means they are designed to rotate in one direction. By adding either extra windings, and external relays and switches, or by using gear mechanisms, the direction of rotation can be changed. In this idea, we will discuss in detail, how to control the speed of a PSC motor in both directions using a PIC16F72 microcontroller and power electronics.

The PIC16F72 microcontroller was chosen because it is one of the simplest and low-cost general-purpose microcontrollers Microchip has in its portfolio.  Even though it does not have the PWMs in hardware to drive complementary PWM outputs with dead-band inserted, all PWMs are generated in firmware using timers and output to general-purpose output pins.

What is Variable Frequency Drive?

Variable Frequency Drive or VFD is the way that enables controlling of the speed of induction motor by applying varying frequency of AC supply voltage. By controlling the output AC frequency, it is possible to drive the motor at different speeds based on the requirements. These are adjustable speed drive largely used in industrial applications such as pumps, ventilation systems, elevators, machine tool drives, etc It is essentially an energy-saving system. Therefore the first requirement is to generate a sine wave with different frequencies for VFD.

What is the technology adopted in VFD?

It is the system that gives AC output with varying frequency to control the speed of the motor according to the needs. Single Phase Variable Frequency Inverters are more common since most devices are working in Single-phase AC supply. It consists of a full-wave bridge rectifier to convert 230 / 110 Volt AC to approximately 300 / 150 volt DC. The output DC from the bridge rectifier is smoothed by a high-value smoothing capacitor to remove the ripples of AC. This fixed voltage DC is then fed to the frequency generating circuit formed of MOSFET (Metal oxide field effect transistor) / IGBT (Isolated Gate Bipolar Transistor) transistors. This MOSFET/IGBT circuit receives the DC and converts it into AC with variable frequency to control the speed of the device.

The frequency change can be achieved using electronic circuits or Microcontroller. This circuit varies the frequency of voltage (PWM) applied to the gate drive of the MOSFET / IGBT circuit. Thus AC voltage of varying frequency appears at the output. The Microcontroller can be programmed to change the frequency of the output according to the needs.

The VFD System:

The Variable Frequency Device has three parts like an AC motor, a Controller, and an Operating interface.

The AC motor used in VFD is generally a three-phase induction motor even though the single-phase motor is used in some systems. Motors that are designed for fixed-speed operation is generally used, but some motor designs offer better performance in VFD than the standard design.

The Controller part is the solid electronic power converter circuit to convert AC to DC and then to quasi sine wave AC. The first part is the AC to DC converter section having a full-wave rectifier bridge usually a three phases / single phase full wave bridge. This DC intermediate is then converted into quasi sine wave AC using the inverter switching circuit. Here MOSFET / IGBT transistors are used for inverting DC to AC.

The inverter section converts DC to three channels of AC to drive the three-phase motor. The Controller section can also be designed to give improved power factor, less harmonic distortion and low sensitivity to input AC transients.

Volts / Hz controlling:

The controller circuit regulates the frequency of the supplied AC to the motor through the volts per hertz control method. AC motor requires variable applied voltage when the frequency changes to give the specified torque. For example, if the motor is designed to work in 440 volts at 50Hz, then the AC applied to the motor must be reduced to half ( 220 volts) when the frequency changes to half (25Hz). This regulation is based on the Volts/ Hz. In the above case, the ratio is 440/50 = 8.8 V/Hz.

Other voltage controlling methods:

Besides Volts/Hz controlling, more advanced methods like Direct Torque Control or DTC, Space Vector Pulse Width Modulation (SVPWM), etc are also used to control the speed of the motor. By controlling the voltage in the motor, the magnetic flux and torque can be controlled precisely. In the PWM method, the inverter switches produce a quasi sine wave through a series of narrow pulses with Pseudo sinusoidal varying pulse durations.

Operating Interface:

This section allows the user to start/stop the motor and to adjust the speed. Other facilities include motor reversing, switching between manual and automatic speed control, etc. The operating interface consists of a panel with display or indicators and meters for showing the speed of the motor, applied voltage, etc. A set of keypad switches are generally provided for controlling the system.

Inbuilt -Soft Start:

In an ordinary induction motor, switched on using an AC switch, the current drawn is much higher than the rated value and can increase with the increased acceleration of the load to attain the full speed of the motor.

On the other hand in a VFD controlled motor, initially low voltage at low frequency is applied. This frequency and voltage increase at a controlled rate to accelerate the load. This develops almost more torque than the rated value of the motor.

VFD Motor Commutation:

The frequency and applied voltage are first reduced to a controlled level and then kept on decreasing until it becomes zero and the motor shuts down.

Application Circuit to control the speed of Single Phase Induction Motor

The approach is relatively easy as far as the power circuit and control circuit are concerned. On the input side, voltage doublers are used and on the output side an H-bridge, or 2-phase inverter, is used as shown in Figure 2. One end of the main and start windings are connected to each half-bridge and the other ends are connected to the neutral point of the AC power supply.

The control circuit requires four PWMs with two complementary pairs with a sufficient dead band between the complementary outputs. The PWM dead bands are PWM0-PWM1 and PWM2-PWM3. The PIC16F72 does not have PWMs designed in the hardware to output the way we need. Concerning VF, the dc bus is synthesized by varying the frequency and amplitude. This will give two sine voltages out of phase.

If the voltage applied to the main winding lags the start winding by 90 degrees, the motor runs in one (i.e., forward) direction. If we want to change the direction of the rotation then the voltage applied to the main winding is to conduct the start winding.

I hope you have got an idea about the variable frequency drive for the induction motor from the above article. so if you have any queries on this concept or the electrical and electronic project please leave the comments section below.