Characteristics of DC Motors

Characteristics of DC Motors

• Three characteristic curves are important for DC motors:
  • Torque vs. armature current
  • Speed vs. armature current
  • Speed vs. torque
• These characteristics are determined by the relations: Ta ∝ ɸ.Ia and N ∝ Eb/ɸ
• Back emf (Eb) is given by the emf equation of a dc generator: Eb = PɸNZ / 60A

Characteristics of DC Series Motors

• Torque vs. armature current (Ta-Ia):
  • Before magnetic saturation, Ta α Ia2
  • After magnetic saturation, Ta ∝ Ia
  • The curve becomes a straight line after magnetic saturation
  • Shaft torque (Tsh) is less than armature torque (Ta) due to stray losses
• Speed vs. armature current (N-Ia):
  • Speed is inversely proportional to ɸ
  • Speed is inversely proportional to Ia for small currents
  • Speed becomes dangerously high when armature current is very small
• Speed vs. torque (N-Ta):
  • When speed is high, torque is low, and vice versa

Characteristics of DC Shunt Motors

• Torque vs. armature current (Ta-Ia):
  • Torque is proportional to armature current
  • The Ta-Ia characteristic is a straight line through the origin
• Speed vs. armature current (N-Ia):
  • Speed remains almost constant
  • The speed decreases only by 5 to 15% of full load speed

Characteristics of DC Compound Motors

• Cumulative compound motor:
  • Series winding takes care of heavy load
  • Shunt winding prevents the motor from running at high speed when the load is removed
  • Used in applications where high starting torque is required
• Differential compound motor:
  • Total flux decreases with increase in load
  • Speed remains almost constant or increases with increase in load
  • Used in limited applications in experimental and research work

Speed-Torque Characteristics of DC Shunt Motor

• The speed-torque characteristics are obtained from torque-current and speed-current characteristics
• The expression for back e.m.f is given by Eb = kaϕN
• The expression for torque is given by T = kaϕIa
• The speed-torque characteristics are affected by the armature reaction

Types of Electric Breaking

• Rheostat Breaking or Dynamic Breaking:
  • Armature is disconnected from the supply and connected across a breaking resistance Rb
  • The motor works as a generator, producing a breaking torque
  • The kinetic energy of moving parts is converted into electrical energy and dissipated in the form of heat
• Regenerative Breaking:
  • The motor is operated as a generator, converting kinetic energy into electrical energy
  • The electrical energy is returned to the supply source
  • The motor slows down, but does not come to a complete stop
• Plugging or Reverse Current Breaking:
  • The connections of the armature are reversed, providing a breaking effect
  • The motor tends to rotate in the opposite direction
  • The supply voltage and back emf act in the same direction, producing a high breaking torque

Comparison between Electrical Braking and Mechanical Braking

• Cost: Electrical braking has a high initial cost, while mechanical braking has a low initial cost

• Maintenance: Electrical braking requires very little maintenance, while mechanical braking requires frequent maintenance

• Replacement of brake shoes: Electrical braking does not require replacement of brake shoes, while mechanical braking requires frequent replacement

• Maintenance cost: Electrical braking has a low maintenance cost, while mechanical braking has a high maintenance cost

• Production of metal dust: Mechanical braking produces metal dust, which causes wearing of bearings, while electrical braking does not produce metal dust

• Braking action: Electrical braking provides a gradual and smooth braking action, while mechanical braking provides a sudden braking action

• Heat production: Electrical braking produces heat at a convenient place, while mechanical braking produces heat in brake shoes, which may damage the brake lining

• Regenerative braking: Electrical braking allows for regenerative braking, while mechanical braking does not

• Used alone or supplemented: Electrical braking is alone not sufficient, and it is supplemented by mechanical braking, while mechanical braking can be used alone### Motors

• Convert electrical energy to mechanical motion in the form of a rotor rotating around a stationary axis

• Used in various applications, including security cameras, smart locks, and 3D printers

Stepper Motors

• Convert electrical pulses into precise mechanical motion
• Operate in discrete steps, with each step being a precise angle of rotation (typically 1.8°)
• Main components:
  • Rotor: rotating component with teeth or magnetic poles
  • Stator: stationary segment with coils of wire that produce magnetic fields
  • Winding phases: either bipolar or unipolar, with different numbers of phases and windings
• Pulses and control: sequence of electrical pulses determines direction and distance of each step
• Highly precise and controllable, ideal for position-holding tasks, such as robotics and camera gimbals

Brushless Motor (BLDC)

• Electronically commutated DC motor without brushes
• Controller provides pulses of current to motor windings, controlling speed and torque
• Highly efficient, producing a large amount of torque over a wide speed range
• Smooth operation and holding torque when stationary
• Two main parts: rotor and stator
• Rotor: rotating part with permanent magnets
• Stator: stationary part with stator windings

Working of Brushless DC Motor

• Similar to brushed motor, but with electronic commutation instead of brushes
• Rotor magnets move the electromagnets to the stator
• High power transistors activate electromagnets for shaft turns
• Controller performs power distribution using a solid-state circuit

Types of Brushless DC Motors

• Outer rotor motor and inner rotor motor
• Main difference: design and heat dissipation
• Inner rotor design: rotor in the center, stator winding surrounds it, producing more torque and heat dissipation
• Outer rotor design: rotor surrounds the winding, trapping heat inside and operating at lower rated current

Advantages of Brushless DC Motor

• More efficient, with velocity determined by frequency of current supply
• No mechanical energy loss due to friction
• Can operate at high-speed under any condition
• No sparking, less noise, and more precise control
• Low rotor inertia, high performance, and large torque per cubic inch
• Reliable, maintenance-free, and less electromagnetic interference
• No air flow required for inside cooling

Disadvantages of Brushless DC Motor

• Cost more than brushed DC motor
• Limited high power supply, otherwise heat can damage magnets and insulation
• May weaken magnets and insulation if too much heat is generated