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AC Drives (variable speed drives) 

The closed loop system is also controlled directly by an input signal. It has two basic units: an amplifier and a motor too. It seems the same as an open loop control system. But the closed loop system has an automatic feedback mechanism which ensures that current does not flow in one direction.  

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 first type of AC motors is the synchronous motor, which rotates exactly at the supply frequency or a submultiple of the supply frequency. The magnetic field on the rotor is either generated by current delivered through slip rings or by a permanent magnet.

The second type is the induction motor. The induction motor runs slightly slower than the supply frequency. The magnetic field on the rotor of the induction motor is created by an induced current.

A typical name plate on an AC induction motor is shown below. Generally the name tag includes: type, horsepower (H.P.), amps, volts, hertz, rpm...
Typical name plate on AC motor:

Below I explain these parameters of the typical name tag on ac induction motor.

Volts: Rated terminal supply voltage.
Amps: Rated full-load supply current.
H.P.: Rated motor output.

We have introduced 5 design types of the National Electrical Manufacturers Association (NEMA) in our previous articles. Below we explain the motor standard: the International Electrotechnical Commission (IEC).

The International Electrotechnical Commission (IEC) Torque-Speed Design Ratings practically mirror those of NEMA. The IEC Design N motors are similar to the National Electrical Manufacturers Association (NEMA) Design B motors, the most common motors for industrial applications. The International Electrotechnical Commission (IEC) Design H motors are nearly identical to NEMA Design C motors.

The IEC Duty Cycle Ratings are different from those of the National Electrical Manufacturers Association (NEMA)'s. Where NEMA usually specifies continuous, intermittent or special duty (typically expressed in minutes), the International Electrotechnical Commission (IEC) uses nine different duty cycle designations (IEC 34-1).

The standards, shown in the table below, apart from specifying motor operating parameters and duty cycles, also specify temperature rise (insulation class), frame size (physical dimension of the motor), enclosure type, service factor and so on.
 

Below are the disadvantages of ac motors.

1) Expensive speed control

Speed control is expensive. The electronics required to handle an AC inverter drive are considerably more expensive than those required to handle a DC motor.
 

2) Inability to operate at low speeds

3) Poor positioning control

Below are advantages of ac induction motors used in the indstry.

4. Easily Found Replacements of AC motors

The wide use of AC motors has resulted in easily found replacements. And many manufacturers adhere to either European (metric) or American (NEMA) standards. (For Replacement Motors)

5. Variety of Mounting Styles

AC Motors are available in many different mounting styles

6. Many Different Environmental Enclosures

Below are advantages of ac induction motors used in the indstry.

1. Simple Design

The simple design of AC motor

Simply a series of three windings in the exterior (stator) section with a simple rotating section (rotor). The changing field caused by the 50 or 60 Hertz AC line voltage causes the rotor to rotate around the axis of the ac motor.

2. Low Cost

3. Reliable Operation