Synchronous Motor Operation for Power Factor Improvement Explained

A synchronous motor has the unique ability to operate at leading, lagging, or unity power factor depending on its excitation level. This characteristic makes it a valuable tool for improving the power factor of an electrical system, especially when operated in a specific mode.

How Synchronous Motors Improve Power Factor

The power factor of any AC electrical system is influenced by the type of load connected. Inductive loads (like induction motors, transformers, fluorescent lights) draw lagging reactive power, causing the overall power factor to be lagging. A synchronous motor, when over-excited, behaves like a source of reactive power, supplying leading reactive power to the system. This leading reactive power cancels out some of the lagging reactive power drawn by inductive loads, thereby improving the overall power factor of the system towards unity.

The relationship between excitation and power factor can be visualized using V-curves, which plot armature current versus field current (excitation) for constant real power output. When the field current is increased beyond the point corresponding to unity power factor, the motor is said to be over-excited, and it operates at a leading power factor.

Why Operation at No Load with Over Excitation is Key

When a synchronous motor is used specifically for power factor improvement without performing significant mechanical work, it is often referred to as a synchronous condenser or synchronous compensator. For this purpose, two conditions are important:

• Over Excitation: As discussed, over-exciting the field winding causes the motor to draw leading current from the supply. This leading current corresponds to the machine supplying reactive power to the grid, which is essential for counteracting the lagging reactive power from other loads.
• Operation at No Load: When the motor is operated at no load, it draws minimal real power from the supply (just enough to cover its internal losses like friction, windage, and core losses). The majority of the power it handles is reactive power. By operating at no load, the motor's full reactive power capability can be utilized for power factor correction without being limited by mechanical load requirements. If it were loaded, part of its capacity would be used for real power output, reducing the available reactive power for power factor correction.

Therefore, operating a synchronous motor at no load with over-excitation is the standard practice when its primary function is to improve the power factor of the connected electrical system.

Analyzing the Options

• Option 1: Operated with load - While a loaded synchronous motor can still operate at a leading power factor if over-excited, its primary function becomes providing mechanical power. Its reactive power contribution capacity for the grid might be limited compared to operating at no load.
• Option 2: Operated at no load with over excitation - This aligns perfectly with the principle of using a synchronous motor as a synchronous condenser to maximize leading reactive power delivery to the grid for power factor improvement.
• Option 3: Connected along with capacitor bank - A synchronous motor can be used *instead* of or *in addition* to a capacitor bank for power factor correction, but being connected *along with* one is not the defining condition for *its own* use for power factor improvement. It functions based on its excitation and load, not necessarily requiring a capacitor bank connection.
• Option 4: Operated at no load with under excitation - Under-excitation causes a synchronous motor to draw lagging current, meaning it consumes reactive power. This would worsen the system's power factor, not improve it.

Based on this analysis, the condition under which a synchronous motor should be used for power factor improvement is operating at no load with over excitation.

Revision Table: Synchronous Motor Power Factor Modes

Additional Information on Synchronous Motor Power Factor Correction

When a synchronous motor is used solely for power factor correction, it is often referred to as a synchronous condenser or synchronous compensator. These machines are essentially synchronous motors running without any mechanical load, purely to control the reactive power flow in the transmission or distribution system. They can supply or absorb reactive power by varying their field excitation:

• Over-excited: Acts like a capacitor, supplying leading reactive power (improves power factor).
• Under-excited: Acts like an inductor, absorbing lagging reactive power (worsens power factor).

Synchronous condensers offer dynamic power factor correction compared to static capacitor banks, as their reactive power output can be smoothly and rapidly adjusted by controlling the field excitation.