Understanding the Initial Stages of a Synchronous Motor and Its Behavior as an Induction Motor

Many engineers and enthusiasts often wonder whether a synchronous motor behaves like an induction motor during its initial transition before receiving power supply to its field coils. This article aims to demystify this concept by breaking down the mechanics, criteria, and implications of such a transition.

Criteria for Initiating a Synchronous Motor

The inherent challenge of starting a synchronous motor lies in the fact that it must align with the stator's magnetic field before commencing operation. Unlike an induction motor that can start on its own, a synchronous motor requires auxiliary machinery to get it to near-synchronous speed. This auxiliary machinery, often referred to as a starting motor, can either share the stator or be built integrally with the synchronous motor to share a common shaft.

Behavior Before Field Supply

The question of whether shorting the field coils can make the synchronous motor behave like an induction motor during the initial stages is a valid one. Shorting the field may indeed allow the motor to start as an induction machine, which could significantly reduce the complexity and energy consumption of the start-up phase. However, the critical point is whether this configuration can achieve a speed close enough to the synchronous speed to avoid serious torque pulsations when switching to synchronous operation.

Implications of Additional Torque and Coil Dedication

If additional torque is necessary to achieve the start-up speed, the rotor may need specific squirrel cage induction coils to assist in the induction start-up process. Dedicated squirrel cage coils can significantly affect the synchronous machine's capacity, reducing it somewhat. Therefore, this configuration must be carefully balanced to ensure optimal performance during both the start-up and operational stages.

The Role of Variable-Frequency Drives

A more modern approach to starting synchronous motors involves the use of variable-frequency drives. These devices can provide a controlled start-up phase by allowing the motor to reach the synchronous operating frequency gradually. Once the desired frequency is achieved, the motor can be smoothly integrated into the main power supply system. This method ensures that the torque changes during the transition are minimized, providing a smoother operation.

Low-Torque Applications

In applications where the inertial loading of the rotor is minimal and the motor can reach synchronous speed within a single cycle of the applied frequency, shade-pole synchronous motors can be exceptionally useful. These motors are particularly common in low-torque applications such as mains-driven clockworks. According to reliable sources, such as Wikipedia, shade-pole synchronous motors can function efficiently without a squirrel cage induction winding due to their low inertia load.

Conclusion

The transition behavior of a synchronous motor in its initial stages can significantly impact its operational efficiency and the complexity of the start-up process. Whether it behaves as an induction motor before receiving field supply and how this influences the design and operation are essential factors that must be considered. Whether through ancillary machinery, squirrel cage coils, or modern variable-frequency drives, the ultimate goal is to ensure a smooth and effective transition into synchronous operation.