Synchronous motor

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A synchronous electric motor is an AC motor distinguished by a rotor spinning with coils passing magnets at the same rate as the alternating current and resulting magnetic field which drives it. Another way of saying this is that it has zero slip under usual operating conditions. Contrast this with an induction motor, which must slip in order to produce torque. A synchronous motor is like an induction motor except the rotor is excited by a DC field. Slip rings and brushes are used to conduct current to the rotor. The rotor poles connect to each other and move at the same speed - hence the name synchronous motor. The speed at which synchronous motors rotate is dependent on the frequency of the AC power line, commonly in the United States, at 60 cycles per second (60Hz). They are used in analog electric clocks, timers and other devices where correct time is required.

Synchronous motors fall under the category of synchronous machines. There are two such machines : the alternator ( synchronous generator ) and the synchronous motor. These machines run under a speed, which is dependent only on the input AC power's frequency. These machines find numerous applications where constant speed is necessary. The speed-torque curve of these machines resemble a rectangular bar.

The speed of a synchronous motor is given by the expression below:

Speed ( in rpm ) = 120 * Supply frequency in Hz / Number of poles.

Contents 1 Parts 2 Operation 3 Starting methods 4 Special Properties 5 Uses 6 Advantages 7 Examples 8 See also

[edit] Parts

A synchronous motor is composed of the following parts:

• The stator is the outer shell of the motor, which carries the armature winding. This winding is spatially distributed for poly-phase AC current. This armature creates a rotating magnetic field inside the motor.
• The rotor is the rotating portion of the motor. it carries field winding, which is supplied by a DC source. On excitation, this field winding behaves as a permanent magnet.
• The slip rings in the rotor, to supply the DC to the field winding.

[edit] Operation

The operation of a synchronous motor is simple to imagine. The armature winding, when excited by a poly-phase ( usually 3-phase ) winding, creates a rotating magnetic field inside the motor. The field winding, which acts as a permanent magnet, simply locks in with the rotating magnetic field and rotates along with it. During operation, as the field locks in with the rotating magnetic field, the motor is said to be in synchronization.

Once the motor is in operation, the speed of the motor is dependent only on the supply frequency. When the motor load is increased beyond the break down load, the motor falls out of synchronization i.e., the applied load is large enough to pull out the field winding from following the rotating magnetic field. The motor immediately stalls after it falls out of synchronization.

[edit] Starting methods

Synchronous motors are not self-starting motors. This property is due to the inertia of the rotor. When the power supply is switched on, the armature winding and field windings are excited. Instantaneously, the armature winding creates a rotating magnetic field, which revolves at the designated motor speed. The rotor, due to inertia, will not follow the revolving magnetic field. In practice, the rotor should be rotated by some other means near to the motor's synchronous speed to overcome the inertia. Once the rotor nears the synchronous speed, the field winding is excited, and the motor pulls into synchronization.

The following techniques are employed to start a synchronous motor:

• A separate motor ( called pony motor ) is used to drive the rotor before it locks in into synchronization.
• The field winding is shunted or induction motor like arrangements are made so that the synchronous motor starts as an induction motor and locks in to synchronization once it reaches speeds near its synchronous speed.

[edit] Special Properties

Synchronous motors show some interesting properties, which finds applications in power factor correction. The synchronous motor can be run at lagging, unity or leading power factor. The control is with the field excitation, as described below:

• When the field excitation voltage is decreased, the motor runs in lagging power factor. The power factor by which the motor lags varies directly with the drop in excitation voltage. This condition is called under-excitation
• When the field excitation voltage is made equal to the rated voltage, the motor runs at unity power factor.
• When the field excitation voltage is increased above the rated voltage, the motor runs at leading power factor. And the power factor by which the motor leads varies directly with the increase in field excitation voltage. This condition is called over-excitation.

The leading power factor operation of synchronous motor finds application in power factor correction. Normally, all the loads connected to the power supply grid run in lagging power factor, which increases reactive power consumption in the grid, thus contributing to additional losses. In such cases, a synchronous motor with no load is connected to the grid and is run over-excited, so that the leading power factor created by synchronous motor compensates the existing lagging power factor in the grid and the overall power factor is brought close to 1 ( unity power factor ). If unity power factor is maintained in a grid, reactive power losses diminish to zero, increasing the efficiency of the grid. This operation of synchronous motor in over-excited mode to correct the power factor is sometimes called as Synchronous condenser.

[edit] Uses

Synchronous motors find applications in all industrial applications where constant speed is necessary

Improving the power factor as Synchronous condensers.

Electrical power plants almost always use synchronous generators because it is important to keep the frequency constant at which the generator is connected.

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

Mains synchronous motors are used for electric clocks.

[edit] 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, eg. 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).

[edit] Examples

• brushless permanent magnet DC motor.
• stepper motor.
• Slow speed AC synchronous motor.
• Switched reluctance motor.

[edit] See also

v • d • e Electric motors Broad Motor Categories Synchronous motor • AC motor • DC motor Conventional Electric Motors Induction • Brushed DC • Brushless DC • Stepper • Linear • Unipolar • Reluctance Novel Electric Motors Ball bearing • Homopolar • Piezoelectric • Ultrasonic • Electrostatic • Switched Reluctance Motor Controllers Adjustable-speed drive • Amplidyne • Direct torque control • Direct on line starter • Electronic speed control • Metadyne • Motor controller • Variable-frequency drive • Vector control • Ward Leonard control • Thyristor drive See also Barlow's Wheel • Nanomotor • Traction motor • Lynch motor • Mendocino motor • Repulsion motor • Inchworm motor • Booster (electric power) • Brush (electric) • Electrical generator • Alternator