Difference Between a Synchronous Motor and an Induction Motor

Synchronous motor

Synchronous motor is an AC motor that generates a torque by a DC-powered excitation magnetic field that interacts with a rotating magnetic field of the armature to rotate at a synchronous speed.

The structure of the synchronous motor is basically the same as that of the synchronous generator, and the rotor is also divided into a salient pole and a hidden pole. But most synchronous motors are salient. The installation form is also divided into horizontal and vertical.

Induction motor

An induction motor, also known as an "asynchronous motor", is a device that places a rotor in a rotating magnetic field and obtains a rotational moment under the action of a rotating magnetic field, thereby rotating the rotor.

The stator is the part of the motor that does not rotate. The main task is to generate a rotating magnetic field. The rotating magnetic field is not achieved mechanically. Instead, it is connected to a pair of electromagnets by alternating current, so that its magnetic pole properties change cyclically, so it is equivalent to a rotating magnetic field.

Working principle

The rotating magnetic field generated by the stator (the rotational speed is the synchronous rotational speed n1) and the relative motion of the rotor winding, the rotor winding cutting magnetic induction line generates an induced electromotive force, thereby generating an induced current in the rotor winding. The induced current in the rotor winding interacts with the magnetic field to generate electromagnetic torque that causes the rotor to rotate. Since the induced current gradually decreases as the rotor speed gradually approaches the synchronous speed, the generated electromagnetic torque also decreases accordingly. When the asynchronous motor operates in the motor state, the rotor speed is less than the synchronous speed.

Difference between synchronous motor and induction motor

The synchronous motor and induction motor are the most widely used types of AC motor. The difference between the two types is that the synchronous motor rotates at a rate locked to the line frequency since it does not rely on current induction to produce the rotor's magnetic field. By contrast, the induction motor requires slip: the rotor must rotate slightly slower than the AC alternations in order to induce current in the rotor winding.

Small synchronous motors are used in timing applications such as in synchronous clocks, timers in appliances, tape recorders and precision servomechanisms in which the motor must operate at a precise speed; speed accuracy is that of the power line frequency, which is carefully controlled in large interconnected grid systems.

Synchronous motors are available in self-excited sub-fractional horsepower sizes to high power industrial sizes.

In the fractional horsepower range, most synchronous motors are used where precise constant speed is required. These machines are commonly used in analog electric clocks, timers and other devices where correct time is required. In higher power 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.

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