DC Machines: Characteristics and Applications

Why it matters

DC machines are crucial in various applications due to their ability to provide precise speed control and high starting torque. Conventional brushed DC machines illustrate traction, hoist and variable-speed drive principles; the selected motor technology depends on the application.

Key ideas

• Types of DC Machines: DC machines are classified into DC motors and DC generators. Each type can be further divided based on their field winding connections: shunt, series, and compound.
• Characteristics of DC Motors:
  • Shunt Motor: Offers approximately constant speed at fixed supply voltage and field flux, suitable for applications requiring stable operation.
  • Series Motor: Provides high starting torque, ideal for traction applications.
  • Compound Motor: Combines features of both shunt and series motors, used in applications needing variable speed and torque.
• Characteristics of DC Generators:
  • Shunt Generator: Used for battery charging and as exciters for AC generators.
  • Series Generator: Rarely used alone, but can be part of compound generators.
  • Compound Generator: Can be designed for approximately constant terminal voltage with cumulative compounding, used in lighting and power supply systems.
• Applications: Traditional examples include DC traction drives, cranes and elevators, and DC generators for excitation or charging. These are application examples, not a claim that modern systems always use brushed DC machines. A series motor must not be operated unloaded because its speed can rise dangerously.

Formulas

• E = V - I·R (motor operation, neglecting brush voltage drop)
  • E: Back EMF (V)
  • V: Supply Voltage (V)
  • I: Armature Current (A)
  • R: Armature Resistance (Ω)
• T = k·Φ·I
  • T: Torque (Nm)
  • k: Machine Constant
  • Φ: Flux per Pole (Wb)
  • I: Armature Current (A)

Worked example

Given: A DC shunt motor has a supply voltage of 220 V, armature resistance of 0.5 Ω, and draws an armature current of 10 A.

1. Calculate the back EMF (E):

   • Formula: E = V - I·R
   • Calculation: E = 220 V - 10 A · 0.5 Ω = 220 V - 5 V = 215 V
   • Back EMF (E) = 215 V

2. Calculate the torque (T) if the flux per pole is 0.02 Wb and machine constant is 150:

   • Formula: T = k·Φ·I
   • Calculation: T = 150 · 0.02 Wb · 10 A = 30 Nm
   • Torque (T) = 30 Nm

Common mistakes

• Confusing the types of DC machines and their applications.
• Incorrectly calculating back EMF by not considering armature resistance.
• Misapplying formulas for torque and speed characteristics.

For GATE EE

Questions often involve calculating back EMF, torque, and efficiency of DC machines. Practice problems on speed-torque characteristics and applications of different types of DC motors and generators.

Quick check

1. What is the main advantage of a DC shunt motor?
2. How does a series motor differ from a shunt motor in terms of torque?
3. What type of DC generator is used for stable voltage output?

Answers: 1. Approximately constant speed at fixed voltage and flux, 2. High starting torque, 3. An appropriately designed cumulatively compounded generator.