Synchronous motors are generally used in applications requiring infrequent starting. Let's understand why this is the correct answer.

Synchronous Motors
A synchronous motor is a type of AC motor that rotates at a constant speed in synchronization with the frequency of the power supply. It operates based on the principle of magnetic fields generated by the stator winding interacting with the rotor. Synchronous motors are known for their precise speed control and high efficiency.

Infrequent Starting
When it comes to starting a motor, there are generally two types of starting methods: direct-on-line (DOL) starting and star-delta starting. Both of these starting methods are commonly used for asynchronous motors (such as induction motors), but not for synchronous motors.

Direct-On-Line (DOL) Starting
In DOL starting, the motor is directly connected to the power supply, causing a sudden surge of current during starting. This sudden surge can be problematic for synchronous motors as it can lead to excessive mechanical stress on the rotor and stator, potentially damaging the motor.

Star-Delta Starting
Star-delta starting is another common method used for starting asynchronous motors. It involves initially connecting the motor windings in a star configuration, which reduces the starting current. After a certain period, the windings are switched to a delta configuration, allowing the motor to reach its full speed. However, this method is not suitable for synchronous motors due to their inherent design and operating characteristics.

Variable Speed
While synchronous motors are not typically used for applications requiring variable speed, it's worth noting that they can be used with additional equipment such as variable frequency drives (VFDs) to achieve variable speed operation. However, this is not a common application for synchronous motors.

Sudden Applications of Heavy Loads
Synchronous motors are not well-suited for sudden applications of heavy loads. These types of applications require high starting torque, which synchronous motors do not possess. Induction motors are generally preferred in such cases as they have higher starting torque capabilities.

Frequent Stopping
Synchronous motors are not commonly used in applications requiring frequent stopping. This is because the starting and stopping processes can be complex and time-consuming for synchronous motors, and their design is better suited for continuous operation.

Conclusion
In conclusion, synchronous motors are generally used in applications requiring infrequent starting. They are not suitable for frequent stopping, sudden applications of heavy loads, or variable speed operation. It's important to consider the specific requirements of an application when selecting a motor type, and synchronous motors are best suited for applications where starting is infrequent.