Electrical Machines: DC and AC Synchronous Machines

• Electrical machines convert electrical energy into mechanical energy (motors) or vice versa (generators).
• They are essential components in many industrial applications.

Types of Electrical Machines

• Rotating electrical machines are classified into DC machines, AC synchronous machines, and AC induction machines.
• Linear electrical machines are available but less common.

DC Machines

• DC machines feature a rotating armature and a stationary field winding.
• Commutator and brushes convert AC voltage generated in the armature into DC voltage at the terminals.
• Types:
  • Separately excited DC machines
  • Shunt DC machines
  • Series DC machines
  • Compound DC machines

AC Synchronous Machines

• AC synchronous machines have a rotating field winding and a stationary armature winding.
• The rotor operates at synchronous speed, determined by the AC supply's frequency and the number of poles.
• Types:
  • Salient pole synchronous machines
  • Cylindrical rotor synchronous machines

AC Induction Machines

• AC induction machines include a stationary stator winding and a rotating rotor winding.
• The rotor winding is short-circuited; current is induced via the stator winding's rotating magnetic field.
• Types:
  • Squirrel cage induction machines
  • Wound rotor induction machines

Operating Principles

DC Machines

• Operation relies on the principle that a current-carrying conductor in a magnetic field experiences a force.
• This force generates torque, causing the armature to rotate.
• In a generator, armature rotation in a magnetic field induces an electromotive force (EMF) in the armature winding.

AC Synchronous Machines

• Operation is based on magnetic locking between the stator's rotating magnetic field and the rotor's magnetic field.
• In generators, an external prime mover drives the rotor at synchronous speed; in motors, the AC supply does.

AC Induction Machines

• Operation relies on electromagnetic induction.
• The stator winding's rotating magnetic field induces a current in the rotor winding.
• Interaction between the rotor current and stator magnetic field produces torque, leading to rotor rotation.
• Rotor speed is slightly less than synchronous speed; the difference is termed slip.

Performance Characteristics

DC Machines

• Performance characteristics:
  • Torque-speed characteristic
  • Output power characteristic
  • Efficiency characteristic
  • Voltage regulation (for generators)
• Characteristics vary depending on the specific type.

AC Synchronous Machines

• Performance characteristics:
  • V-curve (relationship between armature current and field current)
  • Capability curve (limits of active and reactive power)
  • Torque-angle characteristic
  • Voltage regulation (for generators)

AC Induction Machines

• Performance characteristics:
  • Torque-speed characteristic
  • Current-speed characteristic
  • Power factor characteristic
  • Efficiency characteristic

Applications in Industry

DC Machines

• Applications:
  • Variable speed drives
  • Traction (trains, trolleys)
  • Battery-powered equipment
  • DC generators for supplying DC power

AC Synchronous Machines

• Applications:
  • Power generation (in power plants)
  • Synchronous motors for constant speed drives
  • Power factor correction (synchronous condensers)

AC Induction Machines

• Applications:
  • Pumps
  • Fans
  • Compressors
  • Conveyor belts
  • Machine tools
  • General-purpose industrial drives

Maintenance and Troubleshooting

General Maintenance

• Regular machine inspection is essential.
• Check for loose connections, worn insulation, and signs of overheating.
• Maintain cleanliness, removing dust and dirt.
• Lubricate bearings as needed.
• Conduct periodic insulation resistance tests.

Troubleshooting

• Common problems:
  • Open circuits
  • Short circuits
  • Ground faults
  • Overheating
  • Vibration
  • Noise
• Diagnosis requires a multimeter, megohmmeter, etc.
• Adhere to safety procedures when working on electrical machines.

DC Machine Specific Issues

• Commutator and brush wear are common.
• Arcing at the commutator can damage the brushes and surface.
• Check brush pressure and replace worn brushes.
• Clean the commutator surface.

AC Machine Specific Issues

• Stator winding insulation failure is common.
• Rotor winding faults can occur.
• Check winding resistance and insulation resistance.
• Inspect for overheating or winding damage.

Motor Starting Issues

• Starting problems include failure to start or tripping during startup.
• Possible causes: Overload, low voltage, starter issues.

Vibration and Noise

• Excessive vibration/noise can indicate mechanical problems.
• Check for loose parts, imbalances, and bearing problems.
• Use vibration analysis to identify the source.