Source: Ali Bazzi, Department of Electrical Engineering, University of Connecticut, Storrs, CT.
Variable frequency drives (VFDs) are a type of adjustable speed drive, which are becoming standard equipment to power most AC induction motors. VFDs are common in industrial and automation applications and typically provide robust control of the motor in speed, torque, or position modes. The VFDs tested and simulated in this experiment focus on speed and open-loop control with constant voltage to frequency ratio (V/f) control. The induction motor typically operates at a rated stator flux, and this flux is approximately proportional to the V/f ratio. To maintain constant stator flux, the voltage and frequency applied to the stator are maintained at a constant ratio, which is the V/f ratio. The VFD used in this experiment is a 1 hp Yaskawa V1000 drive, but the procedure applies to most commercially available general purpose drives.
VFDs typically include a rectifier stage for AC/DC conversion, followed by an inverter stage for DC/AC inversion. The inverter and rectifier may be single-phase to supply single-phase motors or three-phase to supply three-phase motors. Rectifiers may also have a power factor correction stage, so the VFD and motor are seen at a high power factor from the grid side supplying the rectifier, to reduce current drawn from the grid into the VFD and motor. Inverters are usually switched with pulse width modulation (PWM), which is a switching pattern very close to a sinusoid. Having PWM voltages fed from the inverter into the motor makes the motor see voltages close enough to sinusoids, since most motors are designed to be line-fed (i.e., directly fed by the grid). In PWM switching, the VFD can adjust based on user input or by automatically controlling the frequency of the sinusoid into the motor and the voltage magnitude. Most commercial VFDs use open-loop control, where the V/f ratio is maintained as constant, when operating the motor at or below rated voltage; this maintains motor flux at a rated value. Other more advanced VFDs use "vector control," which is a closed-loop control scheme that provides tight speed or torque regulation.
1. Make sure the three-phase disconnect switch is off.
2. Check that the VARIAC is at 0%.
3. Perform the following connections at the machine and VARIAC terminals:
- Connect the induction machine stator terminals to the drive output (right-side connectors, when looking at the front of the drive).
- Connect the drive input (left-hand set of connectors, when looking at the front of the drive) to the VARIAC output.
- Connect the VARIAC input to the three-phase receptacle on the bench.
- Before power is applied, turn the dial on the VARIAC up to 75%. This starts the drive at about 210 V line-to-line later in the experiment.
- Turn on the three-phase disconnect switch. The main screen on the VFD should turn on and display "F000".
4. Press the "Lo/Re" button once to put the drive in local mode - the redlight on that button should turn on.
- The "Lo/Re" button allows the user to make a selection between the local (Lo) frequency setting and the remote (Re) frequency setting.
- The user is able to change the frequency (f), and the drive automatically sets the corresponding voltage (V) to maintain a constant V/f ratio.
5. Check that the drive parameters are the same as those shown in Table 1.
6. To perform basic voltage, current, and frequency measurements:
- Cycle through the menu and find the display with a "0.0u" after it - this displays the measurement of the voltage fed to the motor.
- From there, scroll up once to a screen that reads "0.00A" - this displays the current measurement when the drive is running.
- The next screen up from that reads "0.00" - this is the frequency measurement.
7. To set a different output frequency, and thus set a different motor speed since speed and electrical frequency are proportional:
- Go back to the main screen and search for the letter F.
- Change the frequency by pressing enter and then manipulating the value using the up and down arrows.
- Use the > (right arrow/reset) button to change between the column of values.
- Enact any changes by pressing enter.
- Cancel changes and continue to run at the present frequency by pressing "Esc."
8. Set the frequency to 10 Hz.
- Press the green "Run" button.
- Scroll to the voltage, current, and frequency readings and record their values.
- Repeat for the following frequencies: 25, 45, 60, and 70 Hz.
- Note that a maximum frequency limit might have been set to prevent the user from exceeding 60 Hz, so adjust the maximum frequency limit from the "E2" menu.
9. Note that if the drive overloads or faults: Press the red "Stop" button, and then press the > (right arrow/reset) button.
Table 1: Main VFD Settings
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!
VFDs typically provide a constant voltage-to-frequency ratio to maintain stator flux in an induction machine close to a constant. If a machine is rated at 60 Hz and 208 V (line-to-line, RMS), then the V/f ratio is 208/60 = 3.467 V/Hz. Therefore, when the machine is run at a lower frequency to reduce its speed, the voltage is weakened to maintain a V/f ratio at a constant. For example, if the machine is run at 30 Hz, voltage should be reduced to 104 V. Or, if the machine is run at a frequency of 15 Hz, then the voltage should be reduced to 52 V. Under no load conditions, current typically drops as voltage drops, since the machine's reactance drop with lower frequencies.
At higher than rated frequencies, VFDs are usually programmed to maintain rated voltage; therefore, a constant V/f does not apply. This is mainly due to the voltage ratings of the machine, where higher voltages than rated are kept away from to avoid breaking the machine insulation or causing more current to flow into the machine. For example, if the frequency for a 60 Hz machine is set at 70 Hz using a VFD, the voltage is maintained at 208 V instead of 242.67 V.
Applications and Summary
VFDs have a wide use in commercial, industrial, and automation systems, and they can save significant amounts of energy, as they adjust the operating point of a motor to draw as much energy as needed under variable speed operation. Inverters used in VFDs are also common in many motor control applications including transportation systems with more electric vehicles, in heating, ventilation, and air conditioning applications, and many others.