UNIT - 3

Induction Generators: Principles of Operation, Steady-State Representation, Power and

Losses

1. Principle of Operation: An induction generator works by converting mechanical energy into

electrical energy. It operates based on Faraday's law of electromagnetic induction.

• Unlike synchronous generators, it does not need external DC excitation. It takes

reactive power (magnetizing current) either from the power grid or from capacitors.
• It only starts generating when the rotor speed is more than the synchronous speed.
This is called negative slip operation.

Key Components

The key components of an induction generator include:

1. Stator: The stationary part of the generator that carries the armature winding.

2. Rotor: The rotating part of the generator that is driven by a prime mover.

Working Mechanism
A rotating magnetic field is created in the stator. - The rotor is spun faster than the synchronous
speed. - This induces a current in the rotor bars in the opposite direction to that in motor
operation. - The induced rotor current interacts with the stator magnetic field to produce
electricity.

Advantages:

- Simple and robust construction: Induction generators have a simple and rugged design,
making them suitable for harsh environments.
- Low maintenance: Induction generators require minimal maintenance, as they have fewer
moving parts compared to other types of generators.
- Cost-effective: Induction generators are often less expensive than other types of generators,
making them a cost-effective option.

Applications:
- Wind turbines: Induction generators are widely used in wind turbines to convert wind energy
into electrical energy.
- Small hydroelectric power plants: Induction generators are used in small hydroelectric power
plants to generate electricity from water energy.

- Industrial applications: Induction generators are used in various industrial applications, such as
conveyor systems and pumps.

2. Steady-State Representation: In steady-state conditions, the behavior of an induction

generator can be modeled using an equivalent circuit similar to an induction motor. The main
difference is the negative slip value (since speed > synchronous speed). The rotor circuit has a
frequency dependent on slip, and voltage is induced in the rotor bars as it cuts the stator
magnetic field.

• In this condition, the rotor’s induced EMF produces a current that feeds back to the
stator, and thus electrical energy is delivered to the load.

Components of Equivalent Circuit:

• Stator resistance and reactance (R1,X1R_1, X_1R1,X1)

• Magnetizing branch (Xm,RcX_m, R_cXm,Rc)
• Rotor resistance and reactance referred to stator (R2′,X2′R'_2, X'_2R2′,X2′)

Applications: - Renewable energy sources like wind and micro-hydro. - Regenerative braking
systems. - Backup power generation.

3. Power and Losses:

In an induction generator, power flow refers to the conversion of mechanical energy into
electrical energy. The power flow can be analyzed by considering the various losses that occur
in the generator.
Losses in Induction Generators:

The losses in an induction generator can be categorized into several types:

1. Electrical Losses: These losses occur in the stator and rotor windings due to the resistance of
the conductors. Electrical losses can be further divided into:

- Copper losses: These losses occur in the stator and rotor windings due to the resistance of
the copper conductors.

- Stray losses: These losses occur due to the non-uniform distribution of current in the
conductors.

2. Magnetic Losses: These losses occur in the magnetic core of the generator due to hysteresis
and eddy currents.

- Hysteresis losses: These losses occur due to the magnetic properties of the core material.

- Eddy current losses: These losses occur due to the induction of currents in the core material.

3. Mechanical Losses: These losses occur due to the mechanical components of the generator,
such as friction and windage.

- Friction losses: These losses occur due to the friction between moving parts.

- Windage losses: These losses occur due to the resistance of the air to the rotating parts.

Power Output

The power output of an induction generator can be calculated by subtracting the losses from the
mechanical power input. The power output is typically measured in terms of active power (P)
and reactive power (Q).

Efficiency

The efficiency of an induction generator is defined as the ratio of the electrical power output to
the mechanical power input. It is an important parameter that determines the performance of the
generator.
Important Note: - Rotor copper losses are directly proportional to slip. - Since slip is negative in
generator mode, power is fed into the stator.

Self-Excited Induction Generator (SEIG): Magnetizing Curves and Mathematical Description

A Self-Excited Induction Generator (SEIG) works without a grid. It uses capacitors connected
across the stator terminals to provide the necessary reactive power (magnetizing current).

The process starts because of residual magnetism in the rotor. When the rotor is driven above
synchronous speed, the residual flux induces a small voltage. This voltage allows current to flow
through the capacitors, producing more flux, and the process grows until a stable voltage is
established.

Conditions for Self-Excitation:

• Enough residual magnetism in the rotor.

• Proper capacitor value for supplying sufficient magnetizing current.
• Speed of rotation must be higher than synchronous speed.
• Load must be within the capacity of the generator.

2. Magnetizing Curves: The magnetizing curve is a plot of magnetizing current vs terminal

voltage. It shows how much magnetizing current is needed to produce a given voltage in the
machine.

A capacitor curve is also plotted on the same graph. This shows the current supplied by a given
capacitor at different voltages.

The point where the magnetizing curve and the capacitor line intersect is the stable operating
point. If the curves do not intersect, voltage build-up will not occur.

3. Mathematical Model: The self-excitation process is governed by a nonlinear set of equations

involving the machine parameters and magnetizing characteristics.
The voltage build-up is an iterative process:

1. Residual magnetism → small voltage.

2. Capacitor supplies magnetizing current.
3. Voltage increases, which increases current, which increases flux.
4. A balance is reached when the magnetizing and capacitor curves intersect.

Advantages of SEIG:

• No need for a power grid.

• Useful in remote and rural areas.
• Simple and rugged.
• Suitable for renewable sources like wind and micro-hydro.

Limitations:

• Poor voltage and frequency regulation.

• Sensitive to load changes.
• Difficult to start under heavy load.
• Requires accurate capacitor sizing.

Interconnected and Stand-Alone Operation

1. Interconnected Mode (Grid-Connected Mode): In this mode, the induction generator is

connected to a large electrical grid that maintains constant voltage and frequency.

Key Features:

• The generator takes reactive power (magnetizing current) from the grid.
• Voltage and frequency are maintained by the grid.
• The generator only needs to supply real power.
• Rotor must be driven slightly above synchronous speed to generate power.

Benefits:

• No need for external capacitors.

 • Very stable operation.
• Easy integration with wind turbines or micro-hydro systems.
• Grid absorbs excess power and provides backup if load changes suddenly.

Operation Process:

1. Prime mover drives rotor faster than synchronous speed.

2. Generator starts producing real power.
3. Magnetizing current is drawn from the grid automatically.
4. Power is delivered directly to the grid or local loads.

2. Stand-Alone Mode (Isolated Mode):

This mode is used where no grid connection is available. The generator must be self-excited
using capacitor banks.

Key Features:

• Capacitors provide the magnetizing current.

• Generator supplies both real and reactive power.
• Voltage and frequency depend on load and speed.

Challenges:

• Voltage can drop if load increases.

• Frequency drops if speed decreases.
• Requires proper capacitor design and stable loads.

How It Works:

1. Rotor is spun using a prime mover (e.g., wind or hydro).

2. Residual magnetism starts voltage build-up.
3. Capacitor supplies required magnetizing current.
4. Generator starts supplying power to the connected load.
Speed and Voltage Control

Speed control is essential because:

• In grid-connected mode, the frequency is fixed by the grid, but the amount of power
depends on how much faster the rotor spins above synchronous speed.
• In stand-alone mode, both frequency and voltage depend directly on rotor speed.

Why Speed Affects Frequency:

Methods of Speed Control:

1. Mechanical Governors:
o Used in diesel engines or hydro turbines.
o Automatically adjust fuel or flow based on load.
2. Electronic Controllers:
o Use power electronics to adjust input torque.
o More precise and responsive.
3. Variable Frequency Drives (VFDs):
o Convert frequency to match load and generator speed.
o Expensive but efficient.
4. Load Controllers:
o Keep speed constant by adjusting or shedding load.

2. Voltage Control

Voltage depends on how much reactive power (magnetizing current) the generator receives.

In grid-connected mode:

• Grid provides stable voltage and frequency.

• Generator only handles real power.
• No control required.

In stand-alone (SEIG) mode:

• Voltage depends on:

o Capacitor size
o Load
 o Speed

If load increases or speed drops, voltage falls.

Methods of Voltage Control:

1. Capacitor Switching:
o Increase capacitance → Increase voltage
o Decrease capacitance → Decrease voltage
2. Automatic Voltage Regulators (AVRs):
o Adjust excitation (in advanced SEIGs or hybrid systems).
o Maintain constant voltage across varying loads.
3. Electronic Voltage Controllers:
o Sense output voltage and regulate by switching capacitors or using thyristors.

Key Observations:

• In SEIGs, load and speed changes directly impact performance.

• Too large a load can collapse voltage.
• Fast switching or regulation is required for sensitive equipment.

Economical Aspects

1. Cost of Installation

Induction generators are often chosen in renewable systems because they are simple and cost-
effective.

Why they’re cheaper:

• No DC excitation system needed.

• No brushes or slip rings (in squirrel cage types).
• Lower initial investment than synchronous generators.
• Easy to maintain—fewer moving parts.

They are ideal for small to medium power applications, especially where cost is a major factor.

2. Maintenance and Operational Costs

• Low maintenance: No commutator, no brush wear.

• Long lifespan: Rugged construction.
• Fewer breakdowns: Less chance of control failures.
• Lower operating cost: Especially when coupled with wind, hydro, or other renewables.
Induction generators are particularly useful in rural, off-grid, or backup power systems due to
their reliability.

3. Energy Efficiency

• Efficient energy conversion when properly loaded.

• Losses are limited to copper losses, core losses, and mechanical losses.
• No field winding = no excitation losses.
• High efficiency at rated load and speed.

However, voltage and frequency regulation is poor in stand-alone mode, which can reduce
performance if not controlled.

4. Application Suitability

• Wind Energy Systems: Especially grid-connected wind turbines.

• Micro-Hydro Plants: Simple and easy to manage in remote areas.
• Backup Power Units: Reliable for limited-use systems.
• Rural Electrification: Affordable and durable.

5. Economic Limitations

• Poor voltage/frequency control in stand-alone mode may require extra electronics,

increasing cost.
• Capacitor banks needed for SEIG operation are extra expense.
• Efficiency can drop sharply with poor loading or oversizing.