Read the text. In industry AC motors are more often used

Ac motors

In industry AC motors are more often used. AC motors operate from alternating power sources. The magnetic fields typically are generated using coils on the rotor and stator, and field movement occurs naturally in the stator due to the alternating nature of the input power. These motors are inexpensive to build and operate, reliable and usually run from standard line power. The power supply frequency determines the speed of an AC motor, so if operated from line power, the speed of rotation is always the same. Variable frequency power drives control the speed of AC motors, but such drives are expensive. Different industries use lots of electrical motors in their applications.

It is necessary to design the right motor for the right application. You need a motor that can handle the work you need (gives the speed and power you need), can be powered from the power source you want it to be powered with, can work in the conditions you have and is not too expensive for the application (to buy and maintain). Typically you will need to know the operating power (voltage, frequency, single or three phase), needed motor type and needed power.

AC induction motor is the most common motor used in industry and home appliances. Induction motors are also called squirrel cage motors. This type of motors are the most common type of industrial AC electric motors being rugged and requiring neither separate DC power source nor slip-rings. AC induction motors offer users simple rugged stator and is analogous to an AC transformer with a rotating secondary. The motors name comes from the alternating current induced in the rotor by the rotating magnetic flux produced in the stator. Motor torque is developed from interaction of currents flowing in the rotor bars and the stator’s rotating magnetic field.

The magnetic field rotates at synchronous speed, the motors theoretical top speed would result in no torque output. In actual operation, rotor speed always lags the magnetic fields speed. Slip increases with load. The actual rotation speed of AC induction motor depends on the motor design. The rotation direction is controlled on the order of the phases applied to the motor. If you change the order of two phase wires, you can change the rotation direction.

Induction motors are available with an extremely wide size range from very small units to hundreds of KW. AC induction motors are designed to run from three phase power.

Small AC induction motors are also sometimes used in single-phase power applications, and in those applications special arrangements need to be done to make the motor to start properly and have the right kind of fields in the running time.

Induction motors are practically fixed speed devices. There are practically only two methods to change the rotation speed of AC induction motor: use frequency converter or use motor with separate winding for different speeds. The applications where accurate speed control is needed, you need a frequency converter. A frequency converter can run a three phase AC motor at very wide speed range quite well. There are variable frequency drives that allow induction motors to run on different speeds. But on those applications mechanical load and the speed range must be considered, because on those applications motors can get very hot very fast. The problem is that a 60Hz motor does not have enough iron in it to allow efficient 25 Hz operation. A motor designed for variable frequency drive has more iron. Also, it might use different iron/steel alloy to allow different operation at higher frequencies. With a light mechanical load and good motor combined with a good variable frequency drive controller, it’s sometimes possible to get a reasonable speed range using a variable frequency inverter. A good variable frequency drive device controls both frequency and voltage. The better ones even take into account that at very low speeds the resistance of the coils cannot be neglected. Frequency converter does not work with AC induction motors that are run from single phase power source, because the operation of the needed motor phase conversion capacitor is very frequency sensitive. There are variable frequency drive devices that take in single power but can output three-phase power to the motor.

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