
- Electrical Machines - Home
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- DC Machines
- Construction of DC Machines
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- Induction Motors
- Introduction to Induction Motor
- Single-Phase Induction Motor
- 3-Phase Induction Motor
- Construction of 3-Phase Induction Motor
- 3-Phase Induction Motor on Load
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- 3-Phase Induction Motor Fault Types
- Synchronous Machines
- Introduction to 3-Phase Synchronous Machines
- Construction of Synchronous Machine
- Working of 3-Phase Alternator
- Armature Reaction in Synchronous Machines
- Output Power of 3-Phase Alternator
- Losses and Efficiency of an Alternator
- Losses and Efficiency of 3-Phase Alternator
- Working of 3-Phase Synchronous Motor
- Equivalent Circuit and Power Factor of Synchronous Motor
- Power Developed by Synchronous Motor
- More on Synchronous Machines
- AC Motor Types
- Induction Generator (Asynchronous Generator)
- Synchronous Speed Slip of 3-Phase Induction Motor
- Armature Reaction in Alternator at Leading Power Factor
- Armature Reaction in Alternator at Lagging Power Factor
- Stationary Armature vs Rotating Field Alternator Advantages
- Synchronous Impedance Method for Voltage Regulation
- Saturated & Unsaturated Synchronous Reactance
- Synchronous Reactance & Impedance
- Significance of Short Circuit Ratio in Alternator
- Hunting Effect Alternator
- Hydrogen Cooling in Synchronous Generators
- Excitation System of Synchronous Machine
- Equivalent Circuit Phasor Diagram of Synchronous Generator
- EMF Equation of Synchronous Generator
- Cooling Methods for Synchronous Generators
- Assumptions in Synchronous Impedance Method
- Armature Reaction at Unity Power Factor
- Voltage Regulation of Alternator
- Synchronous Generator with Infinite Bus Operation
- Zero Power Factor of Synchronous Generator
- Short Circuit Ratio Calculation of Synchronous Machines
- Speed-Frequency Relationship in Alternator
- Pitch Factor in Alternator
- Max Reactive Power in Synchronous Generators
- Power Flow Equations for Synchronous Generator
- Potier Triangle for Voltage Regulation in Alternators
- Parallel Operation of Alternators
- Load Sharing in Parallel Alternators
- Slip Test on Synchronous Machine
- Constant Flux Linkage Theorem
- Blondel's Two Reaction Theory
- Synchronous Machine Oscillations
- Ampere Turn Method for Voltage Regulation
- Salient Pole Synchronous Machine Theory
- Synchronization by Synchroscope
- Synchronization by Synchronizing Lamp Method
- Sudden Short Circuit in 3-Phase Alternator
- Short Circuit Transient in Synchronous Machines
- Power-Angle of Salient Pole Machines
- Prime-Mover Governor Characteristics
- Power Input of Synchronous Generator
- Power Output of Synchronous Generator
- Power Developed by Salient Pole Motor
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- Synchronous Motor Excitation Voltage Determination
- Hunting Synchronous Motor
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- Effect of Load Change on Synchronous Motor
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- Output Power of Synchronous Motor
- Input Power of Synchronous Motor
- V Curves & Inverted V Curves of Synchronous Motor
- Torque in Synchronous Motor
- Construction of 3-Phase Synchronous Motor
- Synchronous Motor
- Synchronous Condenser
- Power Flow in Synchronous Motor
- Types of Faults in Alternator
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- Solid State Motor Starters
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- Types of AC Generators
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- Electrical Machines Basic Terms
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- Stator and Rotor in Electrical Machines
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- Discussion
Significance of Short Circuit Ratio of Alternator (Synchronous Machine)
In a synchronous machine, the short-circuit ratio (SCR) is an important factor. The SCR affects the physical size, operating characteristics and cost of the synchronous machine.
Low Value of SCR
With a low value of SCR, the synchronous machine is very sensitive to the load variations, i.e., the alternator has a large variation in terminal voltage with a change in load. In order to keep the terminal voltage constant, the field current of the machine is to be varied over a wide range. Also, for a small value of SCR, the synchronising power of the machine is small. The synchronising power keeps the machine in synchronism, thus, the synchronous machine with a low value of SCR has a low stability limit. Therefore, an alternator with a low SCR is less stable when operating in parallel with other alternators. The armature current of the synchronous machine under short-circuit conditions is small for a low value of SCR
High Value of SCR
With a high value of SCR, a synchronous machine has a better voltage regulation and improved steady-state stability limit, but the armature current under short-circuit conditions is high.
Effect of SCR on the Size, Weight, and Cost of Machine
The physical size and cost of the synchronous machine are affected by the short-circuit ratio. The excitation voltage for a synchronous machine is given by,
$$\mathrm{\text{Excitation voltage, } \: E_{f} \:=\: 4.44 \: k_{w} \: f \: \phi \: T_{f} \:\: \dotso \:(1)}$$
Therefore,
$$\mathrm{E_{f} \: \propto \: \text{ field flux per pole}}$$
$$\mathrm{E_{f} \: \propto \: \frac{\text{Field MMF per pole}}{\text{Reluctance of air gap}} \:\: \dotso \:(2)}$$
Also,
$$\mathrm{\text{Synchronous inductance,} \:L_{s}\: \propto\: \frac{1}{\text{Reluctance of air gap}}\:\: \dotso \:(3)}$$
And
$$\mathrm{SCR \: \propto \: \frac{1}{L_{s}}\:\: \dotso \:(4)}$$
$$\mathrm{\therefore \: SCR \: \propto \: \text{ Reluctance of air gap or Length of air gap }\:\: \dotso \:(5)}$$
Hence, it is clear from eq. (5) that the SCR of the synchronous machine may be increased by increasing the length of air-gap. With the increased length of air-gap, the field MMF is to be increased for the same value of Ef . To increase the field MMF, either the field current (If) or the number of turns in field winding (Tf) is to be increased and it requires greater height of the field poles. As a result, the overall diameter of the machine increases. Therefore, a high value of SCR will increase the physical size, weight, and cost of the machine.
Typical values of short-circuit ratio (SCR) of various types of synchronous machine are as follows −
- Cylindrical rotor synchronous machine – 0.5 to 0.9
- Salient-pole synchronous machine – 1.0 to 1.5
- Synchronous compensators – 0.4