Variable Frequency Drive Working
A variable frequency drive (VFDs) uses solid state electronic circuits to provide a signal to a motor to control its acceleration, deceleration, direction of rotation, torque, and horsepower. Drives do this by converting a fixed frequency (either 50 or 60 Hz) AC power source, to a variable frequency power supply to the motor. Drives have proven to reduce peak energy demand, decrease maintenance costs, and improve operating reliability.
A drive can be required to speed up or slow down a motor and change the rate at which the motor is adjusted. A drive can also be required to change the direction of rotation of the motor. Restated, a drive can be required to operate the motor in a counterclockwise (CCW) direction or a clockwise (CW) direction. That is, a drive can be capable of operating in one or more of four quadrants: CW, accelerating or decelerating, or CCW, accelerating or decelerating.
Motors must develop torque to move the connected load from a dead stop (breakaway torque) and accelerate the load to the desired rotating speed (acceleration/pull-up torque). Once the driven load has reached normal operating speed, the motor will be required to produce a lesser amount of torque (rated torque) than was needed to accelerate the load.
Some loads will demand that the motor produce a short-term rapid increase in torque to overcome a higher demand by the load. This occurs with machines such as hammer mills. Machines are grouped into two general classes, constant and variable torque loads. Centrifugal pumps and blowers are examples of variable torque loads. Cranes and coil winders are examples of constant torque loads.
Three-phase induction motors are capable of developing an amount of torque greater than the rated load torque. Breakaway torque is greater than rated torque; rated torque is less than breakdown torque.
When selecting a replacement drive it should be confirmed that the drive is capable of supplying the torque demanded by the driven load across its entire operating range and not a single point.
When a transient load is applied, a motor will slow down between 3 and 5 percent. Modern drives can hold the speed of a motor to within 1 percent of the set point. Many vector control drives can hold the speed of the motor to within 1/10 of 1 percent of the set point.
When a motor starts up it must develop enough torque to get the load moving. This is called breakaway torque. For the motor to produce this high level of torque, it must increase the force of the magnetic field. To do this, it increases the current drawn by the motor.
When a motor starts at full rated voltage, it draws between five and eight times as much power as it does when it is running. The Start-up current is called inrush current. All of the electrical system components in the circuit must be sized to handle this increased amount of power.
A VFD can reduce the initial start-up inrush current. A drive does this by reducing the voltage to the motor when it first begins to accelerate the load. This lower voltage reduces the motor’s current draw.
As the motor starts to rotate the load, the drive senses this movement and increases the voltage, allowing the amperage to rise, increasing the strength of the magnetic fields. The drive continues to sense the rotor’s increase in speed and in turn, increases the voltage supplied to the motor’s windings.
The drive also changes the frequency of the applied voltage, allowing the amperage to rise, increasing the strength of the magnetic fields. The drive continues to sense the rotor’s increase in speed and it increases the voltage supplied to the motor’s windings.
The drive also changes the frequency of the applied voltage as the motor increases its speed. The drive continues to increase the voltage and the frequency until the motor is operating at the preset speed. The drive can also raise the voltage applied to the motor above its rated voltage to allow additional torque to be developed during start-up and under heavy loads.
The relationship between the voltage supplied to the motor and the frequency of the electrical cycle is known as the voltage to Hertz ratio (V/Hz). That is, the voltage supplies the torque needed and the frequency controls the speed at which the motor is operated.
Control of Torque and Speed
When the load’s torque demand decreases, the motor will want to increase its speed. To maintain a constant speed, the drive will decrease the frequency of the power applied to the motor. Should the load increase, the motor will want to slow down. The drive can increase the frequency to speed up the motor.
To keep torque developed by the motor at a constant, after the driven load’s torque has increased, the drive can decrease the frequency—this will keep slip constant thus maintaining the motor’s torque output. Should the load torque demand decrease, the drive can increase the frequency of the electrical pulse supplied to the motor. Thus it can be said that modern VFDs respond very well to the requirements of the load.
Parts of Variable Frequency Drive Drive
Enclosure: The external housing is the part that protects people from accidental contact with energized parts. It also protects the drive’s electronic components from environmental contamination.
Terminal board: Two are provided, one for line and one for load conducts. Additional terminal boards are provided for the connection of the various input and output signals.
Keypad: Push buttons or keypads are used for entering and obtaining data from the drive.
LED: Drives will normally have LEDs providing visual communication with the operator.
COM: The drive may have the ability to communicate with a remote process control network, or an intranet and Internet.
External inputs: The drive will have terminals for connection to external devices such as remote start-stop stations, set point, reverse, jog, and alarms.
Inverter-converter: This section changes the AC input power to DC and then back to a variable AC signal. This is many times accomplished with rectifiers or diodes. Rectifiers and diodes allow power to pass in one direction only. This results in an output signal that looks more like a ripple than that of a pure AC sine wave.
DC Bus Section
The DC bus in a VFD smoothes out the ripples in the pulsed power produced by the converter section. Capacitors and other electronic devices are combined to smooth out the ripples in the pulse power produced by the converter section. Some drives require a variable voltage to the DC bus.
In these drives, the rectifier bridge may chop the pulses into smaller pieces using a chopper circuit. The larger the pulse produced by the circuit, the higher the voltage. The smaller the chop, the lower the voltage supplied to the DC bus.
The inverter section switches the DC signal on and off. This on and off switching is used to piece together what looks like to the motor an AC signal. This is accomplished by using either silicone-controlled rectifiers (SCR) or power transistors. Insulated gate bipolar transistors (IGBTs) that are used in many drives today can switch at a rate of about five million times per second. This allows the drive to produce a waveform that is much closer to a standard AC signal.
Pulse Width Modulation
The majority of drives manufactured today use a method called pulse width modulation (PWM) to invert the DC pulse back into an AC sine wave. On three-phase drives, six IGBTs are used—three for the positive bus and three for the negative DC bus. With PWM the voltage is pulsed on and off in sequence to produce a three-phase signal. The pulses are wider as the voltage is increased and narrower as the voltage decreases. This allows the drive to control both voltage and frequency signals.
Six-Step Method of Drive Control
The six-step method uses SCRs or IGBTs in the inverter section to switch the voltage from positive to negative in an offset pattern of pulses in each of the three phases. When this method is applied to the motor, the pulses create a pattern that simulates the rising and falling of the current of a phase of an AC sine wave. The drive changes the length of time the SCRs are ON. This changes the frequency of the signal to the motor.
By shortening the ON time, the frequency to the motor is increased. When the drive lengthens the ON time, it decreases the frequency of the signal to the motor. The voltage to the motor is changed in the converter. This is accomplished with either a rectifier or chopper circuit.
Soft Starters as Torque Controllers
When a motor starts it draws locked rotor amps (LRA). This can be between five and eight times the motor’s running full load amps. This large inrush current draw is necessary for the motor to produce sufficient breakaway torque to overcome the inertia of the rotor and the driven load. Motors that are started with full voltage reach rated speed quickly. This quick acceleration jolts both the rotor and the driven load.
The serving electric utility must provide enough power to supply these peak power demands. Power companies bill commercial customers a demand charge for these peak power demands. Electromechanical methods of starting motors use several different methods of reducing the inrush current draw of the motor. Some use resistors, others transformers.
Solid state drives are more reliable than older types of reduced voltage starters. Single-speed motors can be used in place of more expensive multispeed starters. Electromechanical starters still give a jolt to the drive train. The opening of contactors during starting can create voltage spikes in the electrical distribution system within a facility and will likely have a negative impact on sensitive electrical equipment.
Drives can be programmed by the operator. This allows the operator to preset how the drive operates. Many drives allowed this to be done through a menu-driven keypad operator interface program. These programs differ from one drive manufacturer to another and between the same manufacturer and individual models. These keypads are sometimes numbers only and sometimes alpha-numeric. On some brands, there is a local keypad and display, while on others a desktop or handheld PC can be used to program the drive. Still, others use an EPROM or EEPROM.
Many drives today have features that allow them to be preprogrammed to operate. The menu commonly offers several options from which to select. Some allow a value within a given range, while others allow only one of a few values to be selected. The following is a list of the features that can be set by the operator on several brands of drives available today.
Frequency: Different frequencies can be selected so that the motor can be operated at different speeds during each step of the process cycle. There may be both high and low-frequency limit set points. A word of caution: many motors will not operate at extremely slow speeds without overheating, so be careful that the low-speed setting is high enough to allow the motor to operate within its designed limits. Commonly motors will run too hot below 10 Hz, while some will operate as low as 5 Hz.
Frequency Avoidance: Machines that rotate have what is called a critical speed. Most everything mechanical has a natural frequency. When rotated at its lowest critical speed, the machine will resonate, which results in a significant increase in vibration. A drive should not be allowed to operate for prolonged periods at the machine’s resonate frequency. For this reason, some drives allow the operator to select a specific frequency at which the drive cannot operate.
Most squirrel case blower wheels have two critical speeds, the first can be passed through as the load accelerates and decelerates. The second critical speed will result in the blower wheel flying apart. The drive should not be allowed to operate the blower wheel at either of these speeds.
Jogging Frequency: When a motor must be operated in a jogging mode, the drive may have a selectable frequency for this task. Typically, this is between 5 and 10 Hz.
Acceleration-Deceleration Times: Sometimes this is called ramp-up and ramp-down rates. These are the rate of speed changes the motor will follow. This is a key element in reducing the amount of sudden jolt delivered to a drive train by the motor. Many drives offer options of more than one set of acceleration-deceleration ramp of rates.
Additional Features: The features available from drive manufacturers vary. The following is a short list of some common ones.
- Jog frequency
- Acceleration-deceleration rates
- Braking currents
- Volts to Hertz (V/Hz) ratio
- Frequency set points