Three-Phase AC Generators and Induction Motors
(05) Engines: 3f-2 Phase Systems
General Concepts
A three-phase AC generator consists of three alternating electromotive forces (EMFs) with a 120-degree electrical phase delay between them. These are called phases and designated as R, S, and T.
A system is balanced when frequencies and RMS values are equal, and phases are symmetrically out of phase.
If impedances differ, causing different phase currents, it’s an unbalanced system, even with balanced voltages.
Advantages include simpler energy transport and high receiver performance.
Loads can be connected in star or triangle configurations.
- Phase Voltage (v): Measured between one phase and neutral.
- Line Voltage (U): Measured between any two phases.
- Phase Current (j): Current flowing through the load.
- Line Current: Current through conductors connecting generator and load.
Star Connection
- Phase current equals line current.
- Load voltage equals phase voltage (e.g., 220 volts).
- Line voltage is √3 times the phase voltage (vector sum).
- In a balanced system, neutral current is zero (phasor sum of line currents).
Triangle Connection
Neutral Failure
In systems with distributed loads and star connections, single-phase neutral failure can cause serious problems.
Starting 3f Motors
Induction motors‘ starting torque must exceed load torque. Special starting systems limit high starting current.
Connection Box – Three-Phase Motors
For a 3 x 380 Volt motor, connections can be star, triangle, or star-triangle.
Star-Triangle
For “star-triangle” starting, connect six cables from the switch in the same orientation (R to R, S to S, T to T).
Electrical Automation
Circuits feed actuators for work. Separate into power and control circuits for better maneuverability.
- Power Circuit: Transmits power to the driven element (1f or 3f).
- Control Circuit: Handles timing, self-hold, interlock, etc. (1f or DC).
Benefits include simplified schemes, cabling and element savings, and protection.
Protection Elements
Devices detect abnormal conditions and prevent harmful consequences, usually by interrupting power.
- Thermal Relays/Breakers and Fuses: Detect (relays) or detect and clear (fuses) overloads and short circuits.
- Differential Relay: Detects current leakage for personnel protection.
Electrical Installation Elements
Pushbutton
- Stable positions: normally closed (stop) or normally open (start).
Relay
- Used for protection and maneuver functions.
- Thermal Relays: Protect against overload; opening time depends on overcurrent.
- Magnetothermal Relays: Combine thermal and instantaneous electromagnetic action for short circuits.
- Time Relays: Delay connection or disconnection.
- Switching Relays: Open or close contacts without delay.
- Solid State Relays (SSR): Semiconductor-based, for resistive or inductive AC loads; advantages include high-frequency switching, silent operation, and no mechanical parts.
Switch
- Two stable positions; interrupts or sets rated current.
Automatic Switch
Establishes, supports, and interrupts currents under normal and abnormal conditions (e.g., short circuit).
Designation of Appliances and Connection Points
Devices are designated by three characters: AFN or ANF.
Direct Starting of an Induction Motor
Starting with Inversion of Rotation
Star-Triangle Starting
Starting Three-Phase Motors on Single-Phase Networks
A three-phase motor can operate on single-phase networks if stator windings are accessible.
Starting torque is reduced (40-50% of rated torque). Power loss is about 25%.
For a 220V network, use approximately 70 microfarads per kW.
Power Factor Correction
Inductive loads are offset with capacitors to correct the power factor.