Three-phase AC

Contents

Annotation.

AC motors are also fairly simple to understand.  They are a little trickier to make but will need single-phase or three-phase AC power to make them work.   In the little diagrams above, we have a squirrel cage ac induction motor, a permanent magnet synchronous machine, and a synchronous motor.  The inventor of the three-phase AC motor was Nikola Tesla, a pioneer in electromagnetism.

Introduction.

An AC motor is an electric motor that is driven by an alternating current. It consists of two basic parts, an outside stationary stator having coils supplied with alternating current to produce a rotating magnetic field, and an inside rotor attached to the output shaft that is given a torque by the rotating field.

There are two types of AC motors, depending on the type of rotor used. The first 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, which turns slightly slower than the supply frequency. The magnetic field on the rotor of this motor is created by an induced current.

1.Three-phase AC induction motors

An induction or asynchronous motor is a type of AC motor where power is supplied to the rotor by means of electromagnetic induction, rather than a commutator or slip rings as in other types of motor. These motors are widely used in industrial drives, particularly polyphase induction motors, because they are rugged and have no brushes. Single-phase versions are used in small appliances. Their speed is determined by the frequency of the supply current, so they are most widely used in constant-speed applications, although variable speed versions, using variable frequency drives are becoming more common. The most common type is the squirrel cage motor, and this term is sometimes used for induction motors generally.

2.Three-phase AC synchronous motors

A synchronous electric motor is an AC motor in which the rotation rate of the shaft is synchronized with the frequency of the AC supply current; the rotation period is exactly equal to an integral number of AC cycles. Synchronous motors contain electromagnets on the stator of the motor that create a magnetic field which rotates in time with the oscillations of the line current. The rotor turns in step with this field, at the same rate.

Another way of saying this is that the motor does not rely on "slip" under usual operating conditions, and as a result produces torque at synchronous speed. Synchronous motors can be contrasted with induction motors, which must slip in order to produce torque. The speed of the synchronous motor is determined by the number of magnetic poles and the line frequency.

Synchronous motors are available in sub-fractional self-excited sizes to high-horsepower direct-current excited industrial sizes. In the fractional horsepower range, most synchronous motors are used where precise constant speed is required. In high-horsepower industrial sizes, the synchronous motor provides two important functions. First, it is a highly efficient means of converting ac energy to work. Second, it can operate at leading or unity power factor and thereby provide power-factor correction.

3.Advantages

Synchronous motors have the following advantages over non-synchronous motors:

Speed is independent of the load, provided an adequate field current is applied.

Accurate control in speed and position using open loop controls, e.g. stepper motors.

They will hold their position when a DC current is applied to both the stator and the rotor windings.

Their power factor can be adjusted to unity by using a proper field current relative to the load. Also, a "capacitive" power factor, (current phase leads voltage phase), can be obtained by increasing this current slightly, which can help achieve a better power factor correction for the whole installation.

Their construction allows for increased electrical efficiency when a low speed is required (as in ball mills and similar apparatus).

They run either at the synchronous speed or they do not run at all.

4.Uses

        Three-phase AC synchronous motors and three-phase AC asynchronous motors find applications in all industrial applications where constant speed is necessary.

        Improving the power factor as synchronous condensers.

        Low power applications include positioning machines, where high precision is required, and robot  actuators.

        Mains synchronous motors are used for electric clocks.

5.Conclusion

An AC motor is an electric motor driven by an alternating current.

There are two main types of AC motors, depending on the type of rotor used. The first type is the induction motor, which runs slightly slower than the supply frequency. The magnetic field on the rotor of this motor is created by an induced current. The second type is the synchronous motor, which does not rely on induction and as a result, can rotate exactly at the supply frequency or a sub-multiple of the supply frequency. The magnetic field on the rotor is either generated by current delivered through slip rings or by a permanent magnet. Other types of motors include eddy current motors, and also AC/DC mechanically commutated machines in which speed is dependent on voltage and winding connection.

However, three-phase AC motor and – in particular – asynchronous motors are very robust so the problem of faulty magnetisation leading to load damage will only occur in continuous operation.

6. References

1.Available at : http://en.wikipedia.org/wiki/Three-phase_AC (accesed April 23 2012)

2.Available at : "http://en.wikipedia.org/wiki/AC_motor"Categories: AC motors

(accesed April 23 2012)

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