Synchronous Machines Overview

The difference between mechanical power in a synchronous generator and electrical power out is due to losses like Copper losses, Core losses, Friction losses, and Windage losses.

Yes, a 60 Hz synchronous generator can be operated at 50 Hz by derating the operating voltage to 83.3% of its original value.

Synchronous reactance (XS) can be approximated by dividing the internal generated voltage (EA) by the short-circuit current (IASc).

Approximate winding resistance of a synchronous generator can be determined by applying a DC voltage to the windings while stationary and measuring the current.

Two ways to increase the voltage generated by a synchronous generator are 1) Increasing the speed of rotation and 2) Increasing the field current.

The terminal voltage (Vφ) of a synchronous generator is less than the internal generated voltage (Ea) due to the voltage drop across the armature resistance and reactance.

The synchronous generator terminal voltage vs. field current curve flattens out as field current increases due to rotor saturation. Additional field current results in little increase in the magnetic field.

A synchronous motor can be started by 1) Reducing the stator field speed to a low value for rotor acceleration or 2) Using an external prime mover to accelerate the motor to synchronous speed. It can also be started by bringing it online as a generator.

A synchronous machine acts as a motor when its prime mover is turned off or disconnected. This causes the rotor to slow down and eventually sync with the rotating magnetic field, resulting in torque production.

Damper windings are used to start synchronous machines as induction motors. They allow the machine to accelerate to synchronous speed, after which the current in the windings decreases to zero.

A synchronous motor operated without any load (no mechanical output). It is used to improve power factor by supplying reactive power to the grid.

Changing the field current (IF) in a synchronous motor while keeping the load constant affects the power factor. The output power remains constant.

No current flows through the rotor of an induction motor when it operates at synchronous speed. This is because the rotor is moving at the same speed as the rotating magnetic field, so no relative motion exists.

The frequency of the rotor current (fr) in an induction motor is directly proportional to the slip (s) and the stator frequency (fe): fr = sfe.

Pullout torque refers to the maximum torque an induction motor can produce before it loses synchronism and stalls.

When an induction motor is forced to rotate faster than its synchronous speed, it becomes a generator. It produces electrical power instead of consuming it.

The resistance experienced by the rotor currents in an induction motor model. It can be found by subtracting the stator winding resistance (R1) from the resistance measured during a locked rotor test (RLR).

The inductance that represents the magnetic field created by the stator winding in an induction motor. It's determined by subtracting the stator winding leakage inductance (X1) from the total inductance measured when the motor is running close to synchronous speed.

The energy lost during the conversion of electrical energy into mechanical energy in an induction motor. These include stator copper losses, core losses, rotor copper losses, friction losses, and windage losses.

The magnetic field created by the current flowing in the armature of a DC machine. This field interacts with the main field, affecting the overall magnetic field strength.

Two methods can reduce armature reaction in DC machines: using interpoles (small windings placed between the main poles) or compensating windings (placed in slots on the pole faces).

The induced voltage in a machine depends on the strength of the magnetic flux (φ), the rotational speed of the rotor (ω), and a constant that depends on the machine's construction.

Windings used in DC machines that produce low voltage and high current. The coils are connected in parallel.

Windings used in DC machines that produce high voltage and low current. The coils are connected in series.

The small amount of magnetism remaining in the poles of a DC generator even after it's de-energized. It's crucial to start the generator.

When a shunt DC generator fails to produce voltage during starting, it usually means there's a problem with the field circuit.

A technique used to introduce residual flux in a DC generator when it's missing. It involves connecting the field winding to an external DC source briefly.

Two main ways to control the output voltage of a shunt DC generator: changing the speed of rotation or adjusting the field resistance.

The relationship between terminal voltage and load current is worse in series generators compared to shunt generators because they saturate easily.

A cumulatively compounded DC generator where the terminal voltage at full load is lower than the no-load voltage.

A cumulatively compounded DC generator where the terminal voltage at full load is higher than the no-load voltage.

A DC generator where the series field winding aids the shunt field winding, increasing the total magnetic field and terminal voltage as load increases.

A DC generator where the series field winding opposes the shunt field winding, decreasing the total magnetic field and terminal voltage as load increases.

The terminal voltage drops significantly as the load increases, leading to poor performance and potential instability.

The motor will accelerate rapidly due to the absence of field flux, potentially reaching dangerously high speeds and risking damage.

The motor will operate at its no-load speed, which is typically higher than its loaded speed.

Armature reaction weakens the main field flux, leading to increased motor speed because DC shunt motor speed is inversely proportional to flux.

1. Field Resistance Control: by adjusting the field resistance, you control the field flux and therefore the motor speed. 2. Armature Voltage Control: by adjusting the voltage supplied to the armature, you directly control the motor speed.

Field resistance control should be used to control motor speed above the rated speed (base speed). This prevents excessive current in the field winding.

There are two main types of compounded DC motors:

1. Cumulatively compounded DC motor - where the series field winding assists the shunt field winding.
2. Differentially compounded DC motor - where the series field winding opposes the shunt field winding.

A cumulatively compounded DC motor combines the best features of both shunt and series motors. It provides high starting torque like a series motor and avoids overspeeding at no load like a shunt motor.

A differentially compounded DC motor becomes unstable as its load increases. This is because the series field opposes the shunt field, causing the motor to speed up uncontrollably.

A compounded DC machine can be used as both a motor and generator under specific conditions. However, it cannot operate as a differentially compounded machine when power flow direction is reversed.

Permanent magnet DC motors have some drawbacks compared to conventional DC motors:

1. Limited field strength: They cannot produce as strong a field as conventional motors, leading to lower torque capabilities.
2. Demagnetization: Their magnet poles can demagnetize over time, making them unusable.

Ferromagnetic materials with high residual flux are best suited for permanent magnet DC motor poles.

A brushless DC motor is a synchronous motor controlled by a power electronic interface. It uses electronic commutation instead of brushes, resulting in higher efficiency and longer lifespan.

Brushless DC motors are more efficient than traditional DC motors because electronic commutation reduces energy losses associated with mechanical friction and wear from brushes.