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The AC induction motor drive is the fastest growing segment of the motor control market in the world. There are various reasons for this fast growth of AC induction motor drive. These reasons are presented blow for your reference:

1. Ease of programming
2. Low investment cost for the development
3. Flexibility to add additional features with minimal increase in hardware cost
4. Faster time to market

The main disadvantage of this method is the need of the rotor position information, using the shaft mounted encoder. This means additional wiring and component cost. It increases the size of the motor. When the ac drive (VSD) and the motor are far apart, the additional wiring poses a challenge.

To overcome the sensor/encoder problem, today's main research focus is in the area of a sensorless approach. The advantages of the vector control are to better the torque response, compared to the scalar control (V/F control), full-load torque close to zero speed, accurate speed control and performance approaching direct current (DC) drive, among others.

Sinusoidal Pulse Width Modulation (PWM)

In this method of Sinusoidal Pulse Width Modulation (PWM), the sinusoidal weighted values are stored in the PICmicro microcontroller and are made available at the output port at user defined intervals.

The advantage of Sinusoidal Pulse Width Modulation (PWM) is that very little calculation is required. Only one look-up table of the sine wave is required, as all the motor phases are 120 electrical degrees displaced.

The disadvantage of Sinusoidal Pulse Width Modulation (PWM) is that the magnitude of the fundamental voltage is less than 90%. And the harmonics at Pulse Width Modulation (PWM) switching frequency have significant magnitude.

Six-Step Pulse Width Modulation (PWM)

The inverter of the frequency converter (ac drive) has six distinct switching states. When it is switched in a specific order, the three phase AC induction motor can be rotated.

In this type of control (scalar control, v/f Control), the motor is fed with variable frequency signals generated by the Pulse Width Modulation (PWM) control from an inverter, using the feature rich PICmicro microcontroller. Here, the V/f ratio is maintained constant in order to get constant torque over the entire operating range. Since only magnitudes of the input variables - frequency and voltage -  are controlled, this is known as "scalar control". Generally, the drives with such a control are without any feedback devices (open-loop control). Hence, a control of this type offers low cost and is an easy-to-implement solution.

In such controls, very little knowledge of the motor is required for frequency control. So, scalar control (v/f Control) is widely used. A disadvantage of scalar control (v/f control) is that the torque developed is load dependent, as it is not controlled directly. Also, the transient response of such a control is not fast due to the predefined switching pattern of the inverter.

Savings from variable speed drives (VSDs, variable frequency drives, VFDs, frequency converters, ac drives) come from reduced load of the fan, pump, or driven device. With fans and pumps, energy consumed is proportional to the cube root of shaft speed. If shaft speed is reduced by 10%, flow is reduced by 10%, while power consumption is reduced by 27%. And if shaft speed is reduced by 20%, power is reduced by 49%.

Compared to throttling as a means of flow control, speed reduction provides dramatic energy savings. Throttling to reduce flow in a fan or pump backs the device up on its operating curve, increasing pressure and often increasing power consumption.

Applications of energy efficiency variable speed drive (vsd, frequency converter):

The most common applications of ariable speed drive (vsd, frequency converter, adjustable frequency drive) are for pumps and fans to balance flows, and meet changing system needs. For example, Variable speed drives (frequency inverters) can be very cost-effective in retrofit or new construction of heating, ventilating, and air conditionin (HVAC) systems. Many heating, ventilating, and air conditionin (HVAC) systems were designed with constant flow pumps and fans that are throttled to meet changing operating conditions.

Variable speed drive (vsd, frequency converter, adjustable frequency drive) is also useful for loads, such as elevators, water and wastewater pumps, boiler fans, cooling towers, cranes and conveyors.

2. Inverter stage:

Electronic switches - power transistors or thyristors - switch the rectified DC on and off, and produce a current or voltage waveform at the desired new frequency. The amount of distortion depends on the design of the inverter and filter.

3. Control system:

An electronic circuit receives feedback information from the driven motor and adjusts the output voltage or frequency to the selected values. Usually the output voltage is regulated to produce a constant ratio of voltage to frequency (V/Hz). Controllers may incorporate many complex control functions.

Converting direct current (DC) to variable frequency alternating current (AC) is accomplished using an inverter.

Since an induction motor rotates near synchronous speed, the most effective and energy-efficient method to change the motor speed is to change the frequency of the applied voltage. Variable frequency drives (frequency changers) convert the fixed-frequency supply voltage to a continuously variable frequency, thereby allowing adjustable motor speed.

A VSD (frequency converter, variable speed drive, variable frequency drive, VFD) converts 60 Hz power, for example, to a new frequency in two stages: the rectifier stage and the inverter stage. The conversion process of frequency changers incorporates three functions:

1. Rectifier stage:

A full-wave, solid-state rectifier converts three-phase 60 Hz power from a standard 208, 460, 575 or higher utility supply to either fixed or adjustable DC voltage. The system may include transformers if higher supply voltages are used. (to be continued)

Variabl frequency drives (VFDs, variable speed drives, VSDs) are an excellent choice for adjustable-speed drive users. They allow drive users to fine-tune processes while reducing costs for energy and equipment maintenance.

At water and waste-water facilities, the applications of variable frequency drives (VFDs) are so popular.

Figure below illustrates the reduced energy consumption of Variabl frequency drives (VFDs, variable speed drives, VSDs) over valve control systems at water and waste-water facilities.

For applications where flow requirements vary, mechanical devices such as flow-restricting valves or moveable air vanes are often used to control flow, which is akin to riving a car at full throttle while using the brake to control speed.

Further, the rapid rise time of the pulses may cause trouble with the motor bearings. The stray capacitance of the windings provide paths for high frequency currents that close through the bearings. If the voltage between the shaft and the shield of the motor exceeds few volts the stored charge is discharged as a small spark. Repeated sparking causes erosion in the bearing surface that can be seen as fluting pattern. In order to prevent sparking the motor cable should provide a low impedance return path from the motor frame back to the inverter. Thus it is essential to use a cable designed to be used with variable frequency drives (VFDs, variable speed drives, VSDs).

In big motors, a slip ring with brush can be used to provide a bypass path for the bearing currents. Alternatively isolated bearings can be used.