In industry, induction motors are used mainly because of their low cost, robust construction, good starting torque and low maintenance. Synchronous motors are rarely used in industry for drive applications. Below ZCL explains some of the advantages and disadvantages associated with synchronous motors and their applications.

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Synchronous Motors

A synchronous electric motor is an AC electric motor in which, at a steady state, the rotation of the shaft is synchronized with the frequency of the supply current; the rotation period is exactly equal to an integer number of AC cycles. Synchronous motors use electromagnets as the stator of the motor which create a magnetic field that rotates in time with the oscillations of the current. The rotor with permanent magnets or electromagnets turns in step with the stator field at the same rate and as a result, provides the second synchronized rotating magnet field. Doubly-fed synchronous motors use independently-excited multiphase AC electromagnets for both rotor and stator.

Synchronous and induction motors are the most widely used AC motors. Synchronous motors rotate at a rate locked to the line frequency since they do not rely on induction to produce the rotor's magnetic field. Induction motors require slip: the rotor must rotate at a frequency slightly slower than the AC alternations to induce current in the rotor.

Advantages of Synchronous Motors

1 . Synchronous motors can operate over a wide range of power factors, including hysteresis and overrunning.

2 . They can be constructed with a wider air gap than induction motors, which makes these motors more mechanically stable.

3 . Synchronous motors are less costly in certain kW and speed ranges. They typically operate at higher efficiencies at low speeds with uniform power factors.

5 . It can be constructed with a wider air gap than an induction motor, which allows it to be mechanically stroked.

5. Over-excited synchronous motors can have an over-excited power factor and can be operated in parallel with induction motors and other lagging power factor loads, thus improving the system power factor.

6. In synchronous motors, the speed remains constant regardless of the load. This characteristic helps industrial drives.

7. Compared to induction motors, synchronous motors usually operate at higher efficiency.

Disadvantages of Synchronous Motors

1,. Synchronous motors require DC excitation, which must be provided by an external power source.

2. Synchronous motors are not inherently self-starting motors and require special methods to make them self-starting.

3. The cost per kW output is generally higher than induction motors.

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4. These motors cannot be used for variable speed applications because the speed cannot be adjusted unless the input power frequency is adjusted.

5. Synchronous motors cannot be started with a load. If the load changes suddenly, hunting will occur.

6. When a synchronous motor is loaded beyond its capacity, the rotor and stator rotating magnetic fields lose synchronization and the motor stops.

7. Synchronous motors require collector rings and brushes, resulting in increased maintenance.

8. Synchronous motors are not suitable for applications that require frequent starting or require high starting torque.

Synchronous Motors Applications

1 . An overexcited synchronous motor runs at the leading power factor and takes the leading current from the sink bar, so it can be used to improve the power factor of the overall installation.

2 . Synchronous motors are also used to regulate the voltage at the end of the transmission line.

3 . Typical applications for high-speed synchronous motors are fans, blowers, pumps and compressors.

4 . Since synchronous motors can have speeds as low as 120 RPM, they are ideal for direct connection to reciprocating compressors.

5 . Pulp mixers and mills, steel and metal mills, and rock and ore crushers are often connected to motors or geared.

Advantages & Disadvantages of Synchronous Motors

In industries induction motors are employed mostly because of less cost, rugged construction, good starting torques and very less maintenance. Synchronous motors are rarely used in industries for drive applications. They are generally used as power factor correction device. In industries they are employed to improve the power factor of the system. Some of the advantages and disadvantages related to synchronous motors are explained below:

Advantage or Merits:

• One of the major advantage of using synchronous motor is the ability to control the power factor. An over excited synchronous motor can have leading power factor and can be operated in parallel to induction motors and other lagging power factor loads thereby improving the system power factor.
• In synchronous motor the speed remains constant irrespective of the loads. This characteristics helps in industrial drives where constant speed is required irrespective of the load it is driving. It also useful when the motor is required to drive another alternator to supply at a different frequency as in frequency changes
• Synchronous motors can be constructed with wider air gaps than induction motors which makes these motors mechanically more stable
• In synchronous motors electro-magnetic power varies linearly with the voltage
• Synchronous motors usually operate with higher efficiencies ( more than 90%) especially in low speed and unity power factor applications compared to induction motors

  Disadvantages or Demerits:

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• Synchronous motors requires dc excitation which must be supplied from external sources
• Synchronous motors are inherently not self starting motors and needs some arrangement for its starting and synchronizing
• The cost per kW output is generally higher than that of induction motors
• These motors cannot be used for variable speed applications as there is no possibility of speed adjustment unless the incoming supply frequency is adjusted (Variable Frequency Drives)
• Synchronous motors cannot be started on load. Its starting torque is zero
• These motors have tendency to hunt
• When loading on the synchronous motor increases beyond its capability, the synchronism between rotor and stator rotating magnetic field is lost and motor comes to halt
• Collector rings and brushes are required resulting in increase in maintenance
• Synchronous motors cannot be useful for applications requiring frequent starting or high starting torques required

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