WHAT IS MEANT BY VFD?
A variable frequency drive is an electronic controller that adjusts the speed of an electric motor by regulating the power being delivered. Variable-frequency drives provide continuous control, matching motor speed to the specific demands of the work being performed. Variable-frequency drives are an excellent choice for adjustable-speed drive users because they allow operators to fine-tune processes while reducing costs for energy and equipment maintenance.
NEED FOR A VARIABLE FREQUENCY DRIVE
Variable speed, depending upon the load requirement, provides significant energy saving. A reduction of 20% in the operating speed of the motor from its rated speed will result in an almost 50% reduction in the input power to the motor.This is not possible in a system where the motor is directly connected to the supply line.
If a variable voltage variable frequency drive is employed for speed control of a motor, the required level of pressure- head or Flowrate can be maintained at the drum which is different for different load conditions. VFDs allow a motor’s speed to be varied electrically instead of by mechanical means. This permits much greater efficiency and flexibility of operation. They can control both the speed of the motor and the torque. Without a VFD, industrial induction motors run at full speed continuously; valves, or other mechanical methods, are employed to control the machine output. Unfortunately, running a motor at maximum speed regardless of the varying demands of production means a great deal of electric power is wasted.
VFD OPERATION
When an induction motor is first connected to a full voltage supply, it draws several times (up to about 6 times) its rated current. As the load accelerates, the available torque usually drops a little and then rises to a peak while the current remains very high until the motor approaches full speed.
By contrast, when a VFD starts a motor, it initially applies a low frequency and voltage to the motor. The starting frequency is typically 2 Hz or less. Thus starting at such a low frequency avoids the high inrush current that occurs when a motor is started by simply applying the utility (mains) voltage by turning on a switch. After the start of the VFD, the applied frequency and voltage are increased at a controlled rate or ramped up to accelerate the load without drawing excessive current. This starting method typically allows a motor to develop 150% of its rated torque while the VFD is drawing less than 50% of its rated current from the mains in the low speed range.
A VFD can be adjusted to produce a steady 150% starting torque from standstill right up to full speed. Note, however, that cooling of the motor is usually not good in the low speed range. Thus running at low speeds even with rated torque for long periods is not possible due to overheating of the motor. If continuous operation with high torque is required in low speeds an external fan is usually needed. The manufacturer of the motor and/or the VFD should specify the cooling requirements for this mode of operation.
COMPONENTS OF A VFD
An ac drive converts the frequency of the network to anything between 0 to 300Hz or even higher, and thus controls the speed of motor proportionally to the frequency.
The technology consists of the following:
Rectifier unit: The ac drive is supplied by the electrical network via a rectifier. The rectifier unit can be uni- or bidirectional. When unidirectional, the ac drive can accelerate and run the motor by taking energy from the network. If bidirectional, the ac drive can also take the mechanical rotation energy from the motor and process and feed it back to the electrical network.
Dc circuit: The dc circuit will store the electrical energy from the rectifier for the inverter to use. In most cases, the energy is stored in high-power capacitors.
Inverter unit: The inverter unit takes the electrical energy from the dc circuit and supplies it to the motor. The inverter uses modulation techniques to create the needed three-phase ac voltage output for the motor. The frequency can be adjusted to match the need of the process.The higher the frequency of the of the output voltage is, the higher the speed of the motor, and thus, the output of the process.
COMPONENTS OF VFD IN DETAIL
RECTIFIER
A three-phase bridge rectifier is commonly used in high power applications. Herewe use a full wave bridge rectifier which can give six pulse ripples on the output voltage. The diodes are numbered in order of conduction sequences and each oneconduct for 120о. The conduction sequence for the diodes is D1-D2, D3-D2, D3-D4,D5-D4,D5-D6 and D1-D6.The pair of diodes which are connected between thatpair of supply lines having the highest amount of instantaneous line-to-line voltagewill conduct. The line-to-line voltage is √3 times the phase voltage of a three-phaseY-connected source.
The positive group of diodes (D1, D3, D5) conduct when these have the most positive anode. Similarly, negative group of diodes (D2, D4, D6) would conduct if these have the most negative anode. It is seen from the source voltage waveform Vs that from time period wt=300 to 1500, Voltage Va is more positive than the voltages vb,vc.Therefore,diode D1 connected to line a conducts during this interval.Likewise,from wt=150° to 270° voltage vb is more positive as compared to va,vc. therefore diode D3 connected to line b conducts during this interval.Similarly,diode D5 from the positive group conducts from wt=270° to 360° and so on.
Average value of the output voltage, V0=3Vm/∏
where Vm is the maximum value of line voltage
INVERTER
A device that converts DC power into AC power at desired output voltage and frequency is called an Inverter. Phase controlled converters when operated in the inverter mode are called line commutated inverters. But line commutated inverters require at the output terminals an existing AC supply which is used for their commutation. This means that line commutated inverters can’t function as isolated AC voltage sources or as variable frequency generators with DC power at the input.Therefore, voltage level, frequency and waveform on the AC side of the line commutated inverters can’t be changed. On the other hand, force commutated inverters provide an independent AC output voltage of adjustable voltage and adjustable frequency and have therefore much wider application.
A three-phase inverter is a six-step bridge inverter. It uses a minimum of six thyristors.In inverter terminology, a step is defined as a change in firing from one thyristor to the next thyristor in the proper sequence. For one cycle of 360° ,each step would be of 60° interval for a six-step inverter. This means that thyristors would be gated at regular intervals of 60° in proper sequence so that 3-phase ac voltage is synthesized at the output terminals of a six-step inverter. In the figure the thyristors are numbered in the order in which they are fired.
There are two possible patterns of gating the thyristors.In one pattern, each thyristor conducts for 180° and in other, each thyristor conducts for 120°.But in both these patterns, gating signals are applied and removed at 60° intervals of the output voltage waveform.
Comments
Post a Comment