Variable speed drives how does it work

This is definition used in all topical discussion on this paper. The AC-induction motor can have its speed changed by changing the frequency of the voltage used to power it. This means that if the voltage applied to an AC motor is 50 Hz used in countries like China , the motor works at its rated speed. If the frequency is increased above 50 Hz, the motor will run faster than its rated speed, and if the frequency of the supply voltage is less than 50 Hz, the motor will run slower than its rated speed.

According to the variable frequency drive working principle, it's the electronic controller specifically designed to change the frequency of voltage supplied to the induction motor.

In the s, VFDs had rather small solid-state components that limited the amount of current the VFD could supply to the motor. This usually limited the size of the motor that could be controlled by a frequency and they were not commonly used.

When larger transistors became available in the s, VFDs allowed the largest motors to have their speed controlled. Jumpers and dip switches were used provide ramp-up acceleration and ramp-down deceleration features by switching larger or smaller resistors into circuits with capacitors to create different slopes.

The arrival of advanced microprocessors has allowed the VFD works as an extremely versatile device that not only controls the speed of the motor, but protects against overcurrent during ramp-up and ramp-down conditions.

Newer VFDs also provide methods of braking, power boost during ramp-up, and a variety of controls during ramp-down. The biggest savings that the VFD provides is that it can ensure that the motor doesn't pull excessive current when it starts, so the overall demand factor for the entire factory can be controlled to keep the utility bill as low as possible.

This feature alone can provide payback in excess of the price of the VFD in less than one year after purchase. It is important to remember that with a traditional motor starter, they will draw locked-rotor amperage LRA when they are starting. When the locked-rotor amperage occurs across many motors in a manufacturing plant, it pushes the electrical demand too high which often results in the plant paying a penalty for all of the electricity consumed during the billing period.

Today the VFD is perhaps the most common type of output or load for a control system. As applications become more complex the VFD has the ability to control the speed of the motor, the direction the motor shaft is turning, the torque the motor provides to a load and any other motor parameter that can be sensed. Newer VFDs have a variety of parameters that can be controlled by numbers programmed into it or downloaded from another microprocessor-controlled system such as a programmable controller PLC.

These VFDs are also available in smaller sizes that are cost-efficient and take up less space. The block diagram of a typical VFD can be divided into three major sections: the power-conversion section the microprocessor control section CPU and the control section that includes the external switches an signals to control the VFD operations the power section where AC voltage is converted to DC and then DC is inverted back to 3-phase AC voltage The block diagram below contains three separate sections to indicate the basic working principle of a VFD: the rectifier the filter the switching section that uses regular transistors, darlington pair transistors, or insulated gate bipolar transistors IGBT to invert the DC voltage back to AC voltage with the proper frequency.

VFD connect to standard AC induction motors, and have capabilities of adjustable speed, torque, and horsepower control similar to the principles of DC drives. These devices can control de speed of serial direct current motors, permanent magnet motors, shunt motors and compound motors.

It works through the regulated activation of a field coil , which generates a magnetic field that is transmitted to the input rotor, which in turn transmits it to the output rotor.

The higher the intensity of the magnetic field, the higher the speed and torque that are transmitted. An AC tachometer is used to control the output speed.

The input drive needs to be on a constant speed shaft, while the output rotor will work on an adjustable speed shaft. It has a predetermined ratio between each motor or pump revolution and the fluid flow in use at any given moment. In this way, the resulting torque that is transmitted is known and foreseeable, since it proportionally responds to the pressure exerted by the hydraulic cylinder on the discs.

The use of gear motors in various types of machines is justified by the wide range of advantages that they offer:. This is called the synchronous speed. The initial downward curve is labelled as "Pull-up Torque" and the drop off after the peak is labelled as "Breakdown Torque".

The rotor windings in a squirrel cage motor are aluminum or copper alloy bars that are positioned along the direction of the shaft and short circuited by end rings as shown in the following diagram. The shape of the bars and the resistance of the alloy used in their construction influences the torque-speed characteristics of the motor.

When operated from a constant frequency power source typically 60Hz , AC induction motors are fixed speed devices. A variable frequency drive controls the speed of an AC motor by varying the frequency supplied to the motor.

The first step in this process is to convert the AC supply voltage into DC by the use of a rectifier. DC power contains voltage ripples which are smoothed using filter capacitors. This DC voltage is then converted back into AC. The output voltage is turned on and off at a high frequency, with the duration of on-time, or width of the pulse, controlled to approximate a sinusoidal waveform.