
- Electrical Machines - Home
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- Transformers
- Electrical Transformer
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- DC Machines
- Construction of DC Machines
- Types of DC Machines
- Working Principle of DC Generator
- EMF Equation of DC Generator
- Derivation of EMF Equation DC Generator
- Types of DC Generators
- Working Principle of DC Motor
- Back EMF in DC Motor
- Types of DC Motors
- Losses in DC Machines
- Applications of DC Machines
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- DC Generator
- DC Generator Armature Reaction
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- Stepper vs DC Motors
- DC Shunt Generators Critical Resistance
- DC Machines Commutation
- DC Motor Characteristics
- Synchronous Generator Working Principle
- DC Generator Characteristics
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- DC Motor Voltage & Power Equations
- DC Generator Efficiency
- Electric Breaking of DC Motors
- DC Motor Efficiency
- Four Quadrant Operation of DC Motors
- Open Circuit Characteristics of DC Generators
- Voltage Build-Up in Self-Excited DC Generators
- Types of Armature Winding in DC Machines
- Torque in DC Motors
- Swinburne’s Test of DC Machine
- Speed Control of DC Shunt Motor
- Speed Control of DC Series Motor
- DC Motor of Speed Regulation
- Hopkinson's Test
- Permanent Magnet DC Motor
- Permanent Magnet Stepper Motor
- DC Servo Motor Theory
- DC Series vs Shunt Motor
- BLDC Motor vs PMSM Motor
- 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
- Characteristics of 3-Phase Induction Motor
- Speed Regulation and Speed Control
- Methods of Starting 3-Phase Induction Motors
- More on Induction Motors
- 3-Phase Induction Motor Working Principle
- 3-Phase Induction Motor Rotor Parameters
- Double Cage Induction Motor Equivalent Circuit
- Induction Motor Equivalent Circuit Models
- Slip Ring vs Squirrel Cage Induction Motors
- Single-Cage vs Double-Cage Induction Motor
- Induction Motor Equivalent Circuits
- Induction Motor Crawling & Cogging
- Induction Motor Blocked Rotor Test
- Induction Motor Circle Diagram
- 3-Phase Induction Motors Applications
- 3-Phase Induction Motors Torque Ratios
- Induction Motors Power Flow Diagram & Losses
- Determining Induction Motor Efficiency
- Induction Motor Speed Control by Pole-Amplitude Modulation
- Induction Motor Inverted or Rotor Fed
- High Torque Cage Motors
- Double-Cage Induction Motor Torque-Slip Characteristics
- 3-Phase Induction Motors Starting Torque
- 3-phase Induction Motor - Rotor Resistance Starter
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- 3-Phase Induction Motor - Rotating Magnetic Field
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- Winding EMFs in 3-Phase Induction Motors
- Split-Phase Induction Motor
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- Repulsion Induction Motor
- PSC Induction Motor
- Single-Phase Induction Motor Performance Analysis
- Linear Induction Motor
- Single-Phase Induction Motor Testing
- 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
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- 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
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- Synchronous Motor Excitation Voltage Determination
- Hunting Synchronous Motor
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- 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
- Characteristics of Single-Phase Motor
- Types of AC Generators
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- Distribution Factor
- Electrical Machines Basic Terms
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- Metering CT vs Protection CT
- Stator and Rotor in Electrical Machines
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- Discussion
Transformers Parallel Operation: Reasons and Conditions
When the primary windings of two or more transformers are connected to a common voltage supply and their secondary windings are connected to a common load. Then, the transformers are said to be connected in parallel, i.e., parallel operation of the transformers. The figure shows two transformers T1 and T2 connected in parallel.

Reasons for Parallel Operation of Transformers
The primary reasons for operating transformers in parallel are as follows −
- For large loads, it may be impractical or uneconomical to have a single large transformer so that many small transformers are paralleled to meet the load demand.
- By operating the transformers of standard size in parallel at the substations, the spare capacity of the substation can be reduced.
- There is always a scope of future expansion of a substation to supply a load of capacity greater than that of the transformers already installed. Hence, in future by connecting a new transformer in parallel with existing transformers, the total capacity of the substation can be increased.
- If a transformer is damaged in a system of transformers connected in parallel and is removed for repair and maintenance, then there is no interruption of power supply for essential services.
Conditions for Parallel Operation of Single-Phase Transformers
Necessary Conditions
For satisfactory parallel operation of the transformers, two main conditions are necessary −
- The parallel connected transformers must be of the same polarities. Otherwise, huge circulating currents flow in the windings.
- The voltage ratios must be equal of the parallel connected transformers. If the voltage ratios are not same, then the secondaries will not show the same voltage even if the primaries are connected to the same voltage supply. As a result of this, a current circulates in the secondaries and hence there will also be circulating currents on the primary side. Due to this, a considerable amount of current is drawn by the transformers even without the load.
Desirable Conditions
For efficient parallel operation of the transformers, following conditions are desirable −
- The internal impedances of the parallel connected transformers should be equal.
- The ratios of the winding reactances to the resistances should be equal for all the parallel connected transformers. This ensures that the transformers operate on the same power factor, hence, sharing the active and reactive powers according to their ratings.
Conditions for Parallel Operation of Three-Phase Transformers
The conditions for satisfactory parallel operation of three-phase transformers are as follows −
- The parallel connected transformers should have same polarities.
- The parallel connected transformers must have identical primary and secondary voltage ratings.
- The winding reactances to the resistances ratios in the parallel connected transformers should be the same.
- The phase sequence of all the parallel connected transformers must be the same.
- The phase shift between the primary and secondary voltages must be the same for all the parallel connected transformers.
- All the parallel connected transformers should be in the same vector group.
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