Sunday, December 9, 2012

Permanent Magnet AC Motors

       Permanent magnet AC motors are nothing but polyphase synchronous motors consisting of permanent magnet rotors. Unlike synchronous machines, these machines do not have the field windings. In these machines slip rings and brushes are not required.

1.1 Construction 
       The constructional features of permanent magnet AC motors are as shown in Fig. 1 These motors are having cage rotor with rate earth permanent magnets.
Fig. 1  Cross section of Permanent Magnet a.c. motor

       When fixed frequency supply is given to such motors, then it is started as an induction motor initially and then runs synchronously. No d.c. excitation is required for the rotor makes the motor reliable and robust. These motors are similar to permanent magnet stepping motors having nonsalient stator. These motors have ratings upto 100 kW with maximum synchronous torque designed to about 150% of full load torque.
       The speed and positions sensors are mounted on the rotor shaft for controlling the motor. For shaft positioning number of techniques such as Hall effect devices, high emitting diodes and phototransistors in combination with pulsed wheel and inductance pick ups can be employed.
1.2 Operation 
       The operation of permanent magnet a.c. motor is similar to that of permanent magnet stepping motor. The shaft position sensor gives the information of the motor position and accordingly the phase windings are excited sequentially in such a way that the required speed and torque is produced. The motor speed is decided by the frequency of the supply which excites the phase windings. The angular position between the rotor magnetic axis and that of a phase winding produces the necessary torque for operation.
       As these motors are capable of measuring rotor speed and position and their phase windings may be excited with variable frequency and amplitude, these motors are highly controllable and exhibit wide range of operating characteristing whenever coupled with appropriate electronics and drive system. Hence in recent times, these motors are widely used in many applications.
       As these motors are capable of measuring rotor speed and position and their phase windings may be excited with variable frequency and amplitude, these motors are highly controllable and exhibit wide of range of operating characteristing whenever coupled with appropriate electronics and drive system. Hence in recent times, these motors are widely used in many applications.
       These motors are also referred as brushless motors or brushless d.c. motors as these motors have the characteristics similar to that of d.c. motors and no brushes are required since the armature current is electronically commutated by shaft position sensor and by switches like transistors and thyristors connected to armature windings.
1.3 Advantages 
1. As the slip rings or brushes are absent, there is no sparking.
2. These motor are flexible as they are highly controllable.
3. No d.c. field excitation is required so field losses are reduced.
4. It requires less space as field windings are absent.
5. They have low inertia and friction.
6. These motors are reliable and having long life.

1.4 Disadvantages 
1. The electronic circuity driving the motor is complex and complicated.
2. There is possibility of demagnetization due to excessive currents.

1.5 Applications
       These motors can be used in the applications where constant speed is required such as synthetic fibre drawing. They are also used in applications like turn table drives in record players, spindle drives in hard disk drives in computers and also in computer peripheral equipments.
       These motors are also used for tape drive for video recorders. They have applications in the field of aerospace, gyroscope motors and biomedical instruments like artificial heat pumps.

Conclusion 
       Permanent magnet synchronous motors are Ac motors the eliminate the slip characteristic of induction motors. They operate at exactly the synchronous speed of the three-phase power speed, usually 1800 or 3600 rpm. High-density magnets are imbedded in the rotor. Permanent magnet synchronous AC motors are ideally suited for processes requiring accurate speed control or synchronizing of several interconnected operations.

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