DC Motor Principles and Calculations

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Questions and Answers

What is a key characteristic of a DC shunt motor?

It is classified as a high-torque motor.

It shows good speed regulation. (correct)

It operates at varying speeds under different loads.

It has poor speed regulation.

What can be assumed about the flux in a shunt motor under normal operating conditions?

Flux remains constant, even under heavy loads.

Flux can be assumed practically constant, with minor decreases under heavy loads. (correct)

Flux decreases significantly at all loads.

Flux increases with armature current.

How does load affect the speed of a DC shunt motor?

Speed slightly decreases as load is increased. (correct)

Speed remains constant regardless of load.

Speed fluctuates wildly with changes in load.

Speed increases substantially with increased load.

How does the armature torque relate to the armature current in a DC shunt motor?

Armature torque is proportional to the armature current. Signup and view all the answers

What is the armature current at no load for the 230-V d.c. shunt motor?

2.5 A Signup and view all the answers

Which statement about DC compound motors is true regarding motor speed?

Motor speed may increase rapidly depending on the ampere-turns ratio. Signup and view all the answers

What is the shape of the electrical characteristic curve for a DC shunt motor's torque versus armature current?

A straight line through the origin. Signup and view all the answers

What is the effect on the speed of the d.c. shunt motor when rated load is applied?

Speed drops to 1,120 r.p.m. Signup and view all the answers

In what application would a DC shunt motor be most effective?

In situations needing constant speed with good load response. Signup and view all the answers

What happens to flux in a shunt motor at heavy loads due to armature reaction?

Flux decreases somewhat. Signup and view all the answers

What is the combined resistance of the armature and field in the motor?

115.5 Ω Signup and view all the answers

What distinguishes a DC shunt motor from other types of DC motors?

It has an independent field winding. Signup and view all the answers

Which statement correctly describes the behavior of a DC shunt motor under varying loads?

It exhibits a linear increase in torque with increased armature current. Signup and view all the answers

What is the armature resistance of the 230-V d.c. shunt motor?

0.5 Ω Signup and view all the answers

At what speed does the d.c. shunt motor operate at no load?

1,200 r.p.m. Signup and view all the answers

Study Notes

DC Motor

Principle of Operation: A current-carrying conductor in a magnetic field experiences a mechanical force (F = BIL). Fleming's left-hand rule determines force direction. DC motors and generators have the same construction.
Torque and Back EMF: Armature conductors experience forces due to current and stator field, creating torque. Back EMF (Eb) is generated in the armature due to rotation, opposing the applied voltage (V). Eb calculation: Eb = (POZN/60A). Note that Eb is always less than V.
Torque Equation: Torque (T) is the product of force and radius (T = F × r). Power developed (P) in the armature is T x ω (radians per second), where ω = 2πN/60. Important relationship is Pa = Ta x ω= Eb x Ia.
Significance of Back EMF: Back EMF regulates armature current (Ia). When load increases, armature speed decreases, thus Eb decreases. Allowing more current to flow, so driving torque increase.
Speed of a DC Motor: Speed (N) is related to back EMF (Eb), flux (Φ), and torque (T) N = (Eb / φ) x constant. Speed is inversely proportional to flux and directly related to V and inversely proportional to Ia. Constant speed motors are typically shunt motors
Classification of DC Motors:
- Separately excited
- Self excited (Shunt, Series, Compound)
- Short shunt
- Long shunt
Characteristics:
- Shunt Motor: Constant speed, relatively high starting torque, good speed regulation.
- Series Motor: High starting torque, variable speed, very poor speed regulation.
- Compound Motor: Variable speed, high starting torque (cumulative), improved speed regulation.
Starting of DC Motors: High starting current is typically required to overcome the inertial load, necessitating a starter. A three-point starter limits initial current, and a four-point starter allows for field rheostat control without impacting starting currents.
Losses and Efficiency: DC machines have constant losses (iron, friction) and variable losses (copper). Efficiency (η) is output/input.
Speed Control: Speed regulation can be achieved through field control or armature control methods. Field control adjusts magnetic flux, and armature control adjusts armature voltage, used mainly in series motors.

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Description

Explore the fundamental principles and calculations related to DC motors, including the operation of current-carrying conductors in magnetic fields, torque, back EMF, and motor speed. This quiz covers essential equations and concepts critical for understanding motor functionality and performance. Test your knowledge and understanding of these key concepts in electrical engineering.