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VFD-fed AC Induction Machine

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Variable-frequency drives, also known as VFDs, are affordable, reliable controllers with the ability to adjust the speed of induction motors for optimal performance. VFDs are becoming standard equipment for powering small to large motors in fans, pumps, compressors, drills, and many other applications. Unlike fixed speed controllers, which instantly turn on a motor to full speed, VFDs can soft start a motor by gradually increasing speed to the desired level. Soft starts eliminate high starting torques and surge currents, reduce mechanical stresses, and increase equipment life and reliability. Furthermore, because loads torque and power vary with the square and cube of speed respectively, adjusting motor speed by even a small amount can save considerable energy. This video will demonstrate the configuration of a variable-frequency drive and its use in the control of a three-phase AC induction motor.

An AC induction motor has only two main parts, the stator and the rotor, and most commonly uses three-phase AC power. Three-phase current through the stator coils generates a stator magnetic field, which rotates with a angular velocity proportional to the AC frequency. This stator magnetic field spins the rotor. As a result, motor speed is proportional to the input power frequency. For more information on the induction motor operation, please watch the JoVE Science Education video: AC Induction Motors. If the motor is directly connected to three-phase mains power, it operates at a fixed speed which is determined by the constant 60 hertz line frequency. For adjustable speed, a variable frequency drive, or VFD, must provide the power. VFDs adjust motor speed by setting the output frequency and voltage. First, a rectifier converts the 60 hertz AC input to DC power. Then, a DC to AC inverter uses pulse width modulation to switch this DC power on and off in a particular pattern. Finally, a low pass filter transforms the pulse stream into a roughly sinusoidal wave form and generates AC output power at the chosen frequency, which governs motor speed. A sinusoidal wave form is necessary because most induction motors are designed to use power from AC mains. Single-phase motors use VFDs with single phase rectifiers and inverters, and three-phase motors use VFDs with three phase rectifiers and inverters. For more information on rectifiers and inverters, please watch the JoVE Science Education videos: Single Phase Rectifiers and Single Phase Inverters. Advanced VFDs used closed loop, or vector control, for good regulation of speed or torque. A microprocessor receives feedback about the motors magnetic field and torque, and continually adjusts the VFD power according to a control algorithm. When operating a motor at or below its rated voltage, most VFDs use open loop control to simply output constant drive power without feedback or adjustments. With open loop control, VFDs maintain a chosen voltage to frequency ratio, which is approximately proportional to the stator magnetic field, and therefore also proportional to motor speed. For example, if a motor is rated at 208 volts and 60 hertz, then the voltage to frequency ration is about 3.5 volts per hertz. To reduce motor speed, the VFD reduces the frequency, but must also reduce voltage to maintain a constant voltage to frequency ratio. Therefore, if the VFD drives the motor at 30 hertz instead of 60 hertz, it decreases the voltage proportionally to 104 volts from 208 volts, and the voltage to frequency ratio remains 3.5 volts per hertz. When operating a motor above its rated frequency, VFDs usually restrict output to the rated voltage. This precaution avoids exceeding voltage or current limits of the insulation and coils. For example, the motor rated at 208 volts and 60 hertz has a voltage to frequency ratio of 3.5 volts per hertz. A VFD that increases the speed of this motor by increasing frequency to 120 hertz, would not increase the output to 460 volts as required for a constant voltage to frequency ratio. Instead, the VFD would limit its output to the rated 208 volts to prevent damage to the motor. Now that the basics of VFDs have been explained, let's examine a VFD connected to a three-phase AC induction motor. In this experiment, the VFD operates with open loop control of motor speed and a constant voltage to frequency ratio.

With the three-phase power turned off and the Variac set to 0%, connect the induction motors stator terminals to the VFD drive output. When viewed from the front of the VFD, the drive output connectors are on the right side. Connect the Variac input to the three-phase receptacle on the bench. Adjust the control knob of the Variac to 75% and the turn on the three-phase power. With this Variac setting, the line to line voltage is about 210 volts. Now the VFDs main screen should light up and display F 000. The local remote button allows the user to select the method of frequency selection. Local control allows use of the keypad to operate the VFD. While remote control requires analog or digital communications, press the local remote button once to put the drive in local mode. Set the VFD perimeters to those shown in the table. To do so, set the motor speed by using the arrow keys to reach the frequency menu, letter F on the main screen. Then set the frequency to 10 hertz. To measure the voltage input to the motor, select the menu with the display of 0.0v. To measure the current driving the motor, scroll up to the screen that reads 0.00A. To measure the VFD frequency, scroll to the frequency measurement screen. Press the green run button to start the motor. The drive automatically outputs the necessary voltage to maintain a constant voltage to frequency ratio, which is preset to 3.47. Scroll to the displays of voltage, current, and frequency, and record their values. If the drive overloads or faults, press the red stop button and then press the reset button. Use a strobe light to measure the motors rotation speed. Adjust the course frequency knob until the shaft looks almost stationary, then adjust the fine frequency knob until the shaft looks motionless. Repeat this procedure for frequencies 25, 45, 60, and 70 hertz. Plot the motor speed versus frequency to obtain a graph of motor behavior under control of the variable frequency drive.

Variable frequency drives control the speed of AC induction motors, and can reduce mechanical stresses, increase reliability, and decrease maintenance costs. In addition, VFDs allow operation of motors at an optimal speed for improving energy efficiency. Because of these benefits, VFDs are useful in many applications, such as adjusting the speed of a fan. When incorporated in a ventilation system, fans like this can respond to manual or automatic controls that increase fan speed and air circulation when temperatures are high, or decrease fan speed when temperatures are low. Drill presses, laids, milling machines, and similar equipment use VFDs to control their motors. Plastics require low speed machining to prevent charring or melting, while hard metals like steel tolerate high speed machining for faster work. With VFDs, machining equipment is more versatile and better able to handle a wide range of situations.

You've just watched JoVE's Introduction to Variable Frequency Drives for AC Induction Motors. You should now understand how VFDs work, and how the input power frequency determines motor speed. Thanks for watching!

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