Motors
A synchronous electric motor is an AC motor distinguished by a rotor spinning with coils passing magnets at the same rate as the power supply frequency and resulting rotating magnetic field which drives it.
Another way of saying this is that it does not rely on slip under usual operating conditions and as a result, produces torque at synchronous speed. Synchronous motors can be contrasted with an induction motor, which must slip in order to produce torque. They operate synchronously with line frequency. As with squirrel-cage induction motors, speed is determined by the number of pairs of 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.
Type
There are two major types of synchronous motors: ‘non-excited’ and ‘direct-current excited’, which have no self-starting capability to reach synchronism without extra excitation means, such as electronic control or induction.
With recent advances in independent brushless excitation control of the rotor winding set that eliminates reliance on slip for operation, the ‘brushless wound-rotor doubly-fed electric machine’ is the third type of synchronous motor with all the theoretical qualities of the synchronous motor and the wound-rotor doubly-fed motor combined, such as power factor correction, highest power density, highest potential torque density, low cost electronic controller, highest efficiency, etc.
Non-excited motors
These are manufactured in permanent magnet, reluctance and hysteresis designs. Reluctance and hysterisis designs employ a self-starting circuit and require no external excitation supply. Permanent magnet designs require electronic control for practical operation (see Permanent magnet synchronous generator).
Reluctance motor designs have ratings that range from sub-fractional to about 30 hp. Sub-fractional horsepower motors have low torque, and are generally used for instrumentation applications. Moderate torque, integral horsepower motors use squirrel cage construction with toothed rotors. When used with an adjustable frequency power supply, all motors in the drive system can be controlled at exactly the same speed. The power supply frequency determines motor operating speed.
Hysteresis motors are manufactured in sub-fractional horsepower ratings, primarily as servomotors and timing motors. More expensive than the reluctance type, hysteresis motors are used where precise constant speed is required.
Synchronous speed
The “synchronous speed” of a synchronous motor is determined by the following formula:
where v is the speed of the rotor (in rpm), f is the frequency of the AC supply (in Hz) and n is the number of magnetic poles. Different from all other synchronous motors, the synchronous brushless wound-rotor doubly-fed electric machine operates from sub-synchronous to super-synchronous speeds or twice synchronous speed.
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 may be supplied by a DC source. On excitation, this field winding behaves as a permanent magnet.Some machines use permanent magnets in the rotor.
The slip rings on the rotor, to supply the DC to the field winding.
The stator frame contains and supports the other parts and may include bearing housings.
Large machines may include additional parts for cooling the machine, supporting the rotor, lubricating and cooling the bearings, and various protection and measurement devices
Operation
The operation of a synchronous motor is simple to imagine. The armature winding, when excited by a poly-phase (usually 3-phase) supply, 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.
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 most basic property of synchro motor is that it can be used both as a capacitor or inductor. Hence in turn it improves the power factor of system.
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.
Uses
Synchronous 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.
Record player turntables
Written by Deepak Kumar
Electrical & Electronic Engineer
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