03-01-2013, 04:26 PM
Synchronous Machines
1Synchronous.ppt (Size: 6.42 MB / Downloads: 292)
INTRODUCTION
Synchronous generators or alternators are used to convert mechanical power derived from steam, gas, or hydraulic-turbine to ac electric power
Synchronous generators are the primary source of electrical energy we consume today
Large ac power networks rely almost exclusively on synchronous generators
Synchronous motors are built in large units compare to induction motors (Induction motors are cheaper for smaller ratings) and used for constant speed industrial drives
Operation Principle
The rotor of the generator is driven by a prime-mover
A dc current is flowing in the rotor winding which produces a rotating magnetic field within the machine
The rotating magnetic field induces a three-phase voltage in the stator winding of the generator
Voltage Regulation
A convenient way to compare the voltage behaviour of two generators is by their voltage regulation (VR). The VR of a synchronous generator at a given load, power factor, and at rated speed is defined as
Where Vfl is the full-load terminal voltage, and Enl (equal to Ef) is the no-load terminal voltage (internal voltage) at rated speed when the load is removed without changing the field current. For lagging power factor (PF), VR is fairly positive, for unity PF, VR is small positive and for leading PF, VR is negative.
Open-circuit test
The generator is turned at the rated speed
The terminals are disconnected from all loads, and the field current is set to zero.
Then the field current is gradually increased in steps, and the terminal voltage is measured at each step along the way.
It is thus possible to obtain an open-circuit characteristic of a generator (Ef or Vt versus If) from this information
Short-circuit Ratio
Another parameter used to describe synchronous generators is the short-circuit ratio (SCR). The SCR of a generator defined as the ratio of the field current required for the rated voltage at open circuit to the field current required for the rated armature current at short circuit. SCR is just the reciprocal of the per unit value of the saturated synchronous reactance calculated by
Active and reactive power-angle characteristics
The real and reactive power delivered by a synchronous generator or consumed by a synchronous motor can be expressed in terms of the terminal voltage Vt, generated voltage Ef, synchronous impedance Zs, and the power angle or torque angle d.
Referring to Fig. 8, it is convenient to adopt a convention that makes positive real power P and positive reactive power Q delivered by an overexcited generator.
The generator action corresponds to positive value of d, while the motor action corresponds to negative value of d.
1Synchronous.ppt (Size: 6.42 MB / Downloads: 292)
INTRODUCTION
Synchronous generators or alternators are used to convert mechanical power derived from steam, gas, or hydraulic-turbine to ac electric power
Synchronous generators are the primary source of electrical energy we consume today
Large ac power networks rely almost exclusively on synchronous generators
Synchronous motors are built in large units compare to induction motors (Induction motors are cheaper for smaller ratings) and used for constant speed industrial drives
Operation Principle
The rotor of the generator is driven by a prime-mover
A dc current is flowing in the rotor winding which produces a rotating magnetic field within the machine
The rotating magnetic field induces a three-phase voltage in the stator winding of the generator
Voltage Regulation
A convenient way to compare the voltage behaviour of two generators is by their voltage regulation (VR). The VR of a synchronous generator at a given load, power factor, and at rated speed is defined as
Where Vfl is the full-load terminal voltage, and Enl (equal to Ef) is the no-load terminal voltage (internal voltage) at rated speed when the load is removed without changing the field current. For lagging power factor (PF), VR is fairly positive, for unity PF, VR is small positive and for leading PF, VR is negative.
Open-circuit test
The generator is turned at the rated speed
The terminals are disconnected from all loads, and the field current is set to zero.
Then the field current is gradually increased in steps, and the terminal voltage is measured at each step along the way.
It is thus possible to obtain an open-circuit characteristic of a generator (Ef or Vt versus If) from this information
Short-circuit Ratio
Another parameter used to describe synchronous generators is the short-circuit ratio (SCR). The SCR of a generator defined as the ratio of the field current required for the rated voltage at open circuit to the field current required for the rated armature current at short circuit. SCR is just the reciprocal of the per unit value of the saturated synchronous reactance calculated by
Active and reactive power-angle characteristics
The real and reactive power delivered by a synchronous generator or consumed by a synchronous motor can be expressed in terms of the terminal voltage Vt, generated voltage Ef, synchronous impedance Zs, and the power angle or torque angle d.
Referring to Fig. 8, it is convenient to adopt a convention that makes positive real power P and positive reactive power Q delivered by an overexcited generator.
The generator action corresponds to positive value of d, while the motor action corresponds to negative value of d.