Understanding Variable Frequency Drives
The Need for Variable Frequency Drives (VFDs)
While the development of the cage-rotor style induction motor was a major advancement in harnessing the power of electricity to do work, it soon became evident that the operation of the motor was limited. There became a need to better control the motor. That is, to be able to run the motor at different speeds and to control its acceleration, deceleration, and braking. As technology and time advanced, several solutions were developed to address these needs.
The Early Solutions
Perhaps one of the earliest and easiest solutions to vary the speed of the motor was to use different diameter pulleys with belts to mechanically connect the motor to the load as seen in Figure 16-1. This would result in a load speed that would be different from the rated speed of the motor. The same was true when using sprockets and chains. A drawback to this method was that the motor still operated at only one speed. For example, if the motor shaft speed was 1750 rpm and the desired speed of the load was 500 rpm, a set of pulleys or sprockets of a certain ratio would be used to mechanically convert the speed of the motor to the desired load speed. If it was determined that the speed of the load now needed to be 750 RPM, a different pulley ratio would be needed to produce the desired load speed. To change the speed, the diameters of the pulleys or sprockets had to be changed, but the final result was that the motor still ran at one particular speed. You could, however, change the direction of rotation by crossing the belt between pulleys as seen in Figure 16-2.
Different diameter pulleys provide different speeds as seen in this drill press.
Reversing direction by crossing the belt.
Another early solution to obtain a variable speed from the motor was the use of gearboxes, shown in Figure 16-3. Different gear ratios could be selected to produce different load speeds. However, just like the belts and pulleys or sprockets and chains, you could only select between different set speeds. You could not vary the speed of the motor between the fixed speeds. The configuration of the gears within the gearbox could also provide the ability to change the direction of rotation.
The ability to vary the speed of a motor between two set speeds is called continuously variable speed control. For example, a continuously variable speed control for a motor with a maximum shaft speed of 1750 rpm would allow the motor speed to be varied from 0 rpm to 1750 rpm and any speed in between.
Deceleration and braking were provided by the use of mechanical brakes as seen in Figure 16-4. These brakes used brake shoes that applied a friction material to a drum mounted on the motor’s shaft. By applying pressure, the friction caused the motor speed to slow and even come to a complete stop. Tension springs were used to lift the brake shoes from the brake drum, which released the braking action and allowed the motor to rotate again.
Electromechanical friction brake.
None of these methods, however, satisfied all the needs in one solution. In order to provide continuously variable speed control with acceleration, deceleration, and braking, a different approach to control the motor was needed.
The Next Stage
Fast Trax® Magenta Students will recall from Chapter 13, along with the three-phase caged-rotor induction motor, Mikhail Osipovich Dolivo-Dobrovolsky is also credited with inventing the three-phase wound-rotor induction motor. As the name implies, the rotor was constructed with windings, which were connected to slip rings that were mounted on the rotor shaft. Riding on the slip rings were carbon brushes. In this manner, an electrical connection could be made, through the brushes and slip rings, to the rotor windings, even while the rotor was rotating. A wound-rotor induction motor is pictured in Figure 16-5. Note the brushes and slip rings.
Wound-rotor induction motor. (Note windings on the rotor.)
A three-phase current is applied to the three-phase stator windings. This induced a three-phase current in the rotor windings, creating a magnetic field. The rotor would rotate, and a torque (dependent upon the amount of slip) would be produced. If a three-phase variable resistor were connected to the rotor windings (through the brushes and slip rings), as seen in Figure 16-6, the amount of magnetizing current flowing in the rotor windings could be varied. Doing so would vary the strength of the magnetic field. This would have the effect of slowing down or speeding up the speed of rotation of the rotor. This would produce a continuously variable speed motor. While varying the speed of rotation of the motor, the torque would also be varied. As the rotor speed decreases, slip increases and so does torque.
While being able to vary the speed and torque of a wound-rotor induction motor is a benefit, there is also a disadvantage—expense. The wound-rotor induction motor is more expensive to build due to the rotor windings, slip rings, and brushes. In addition, the wound-rotor induction motor requires more maintenance in the form of brush replacement. However, the wound-rotor induction motor was still a popular choice when variable speed and/or variable torque were required.
There is yet another somewhat different approach to speed control that had been popular until recently. Refer to Figure 16-7. This approach uses a DC motor as the prime mover of a three-phase alternator. In this manner, the speed of the DC motor could be varied by adjusting a field rheostat. By varying the speed of the DC motor, the speed of rotation of the three-phase alternator would vary accordingly. Since the frequency of the generated AC voltage at the output of the AC alternator is determined by the speed of rotation of the alternator, varying the speed of the DC motor (which drives the alternator) will vary the frequency of the generated AC voltage. Any three-phase motor connected to the output of the alternator will have the frequency of its applied AC voltage varied. Recall the equation for synchronous speed:
DC motor driving a three-phase alternator.
Remember that there are two variables that affect the synchronous speed. One is the number of poles (P) and the other is the frequency (f) of the applied voltage. We have just seen how the frequency of the applied voltage can be varied by connecting a motor to the output of an alternator, which is driven by a variable speed DC motor used as the alternator’s prime mover. In this manner, we have created a continuously variable speed control of the three-phase induction motor connected to the alternator.
As you may have already figured out, this is an expensive method. In order to obtain continuously variable speed control, you would need a DC motor and a three-phase alternator. The maintenance costs would also be high as there are brushes that will need replacement on the DC motor as well as on the alternator.
Enter the variable frequency drive or VFD. The VFD was developed around the late 1970s and early 1980s. It was during this time that many types of solid-state switching devices were being developed and improved, in particular transistors, MOSFETs (metal oxide semiconductor field effect transistors), and IGBTs (insulated gate bipolar transistors). Understanding the operation of transistors, MOSFETs, and IGBTs is beyond the scope of this text. Simply think of these devices as high-speed switches that are used to vary the frequency of the applied AC voltage. This in turn varies the speed of the motor.
The above article is an extract from our Fast Trax® Magenta Course.