DC Motor Characteristics: Quiz and Flashcards

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Questions and Answers

What happens to the speed of a series motor when the armature current is very small?

It remains constant

It becomes moderately high

It becomes very low

It becomes dangerously high (correct)

What is the characteristic of a DC series motor?

Speed is inversely proportional to armature current (correct)

Torque is directly proportional to armature current

Speed is directly proportional to armature current

Torque is inversely proportional to armature current

Why should a series motor never be started without a mechanical load?

To prevent electrical shock

To prevent overheating

To prevent the motor from running at a dangerously high speed (correct)

To prevent damage to the motor

What is the torque-armature current characteristic of a DC shunt motor?

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

What happens to the back emf (Eb) of a DC shunt motor with an increase in load?

It decreases slightly Signup and view all the answers

What is the purpose of a shunt winding in a compound motor?

To prevent the motor from running at a high speed Signup and view all the answers

What is the characteristic of a cumulative compound motor?

It is suitable for applications with sudden and temporary loads Signup and view all the answers

What happens to the speed of a differential compound motor with an increase in load?

It increases Signup and view all the answers

What is the expression for the back emf (Eb) of a DC motor?

Eb = kaϕN Signup and view all the answers

What is the effect of armature reaction on the air gap flux (ϕ) of a DC motor?

It decreases the air gap flux Signup and view all the answers

What is the main drawback of the plugging method of breaking?

It is highly inefficient due to energy waste in resistances Signup and view all the answers

What is a major advantage of electrical braking over mechanical braking?

It requires less maintenance Signup and view all the answers

What is a disadvantage of mechanical braking?

It requires frequent replacement of brake shoes Signup and view all the answers

What is a benefit of regenerative braking in electrical braking?

It can feed back 60% to 80% of kinetic energy to the supply line Signup and view all the answers

What is a characteristic of mechanical braking?

It requires frequent maintenance Signup and view all the answers

What is the main application of DC servo motors?

In numerically controlled machines and computers Signup and view all the answers

What is a component of a DC servo motor?

Field winding Signup and view all the answers

What is the function of a gearbox in a DC servo motor?

To change the motor's output Signup and view all the answers

What is a type of DC servo motor?

Armature-controlled servo motor Signup and view all the answers

What is connected to the motor's mechanical shaft in a DC servo motor?

A load Signup and view all the answers

What is the main function of a position sensor?

To provide a voltage proportional to the motor shaft's absolute angle Signup and view all the answers

What is the purpose of the comparator in a closed-loop system?

To compare the output of a position sensor and a reference point Signup and view all the answers

What is the role of the amplifier in a closed-loop system?

To amplify the error signal from the comparator Signup and view all the answers

What is the purpose of the pulse width modulator in a closed-loop system?

To generate the pulse width modulation Signup and view all the answers

What is the characteristic of a closed-loop system?

It provides zero steady-state error Signup and view all the answers

What is the transfer function of a DC servo motor?

The ratio of the Laplace transform of the output variable to the Laplace transform of the input variable Signup and view all the answers

What is the advantage of an armature-controlled DC servo motor?

Superior performance due to the closed-loop system Signup and view all the answers

What is the specification that defines the speed at which the shaft turns?

Shaft speed Signup and view all the answers

What are the three characteristic curves considered important for DC motors?

Torque vs. armature current, Speed vs. armature current, and Speed vs. torque Signup and view all the answers

What determines the required torque for a DC servo motor?

The speed-torque characteristics of the load Signup and view all the answers

What is the relationship between torque and armature current in DC series motors before magnetic saturation?

Ta ∝ Ia² Signup and view all the answers

What is the type of torque required while starting the servo motor?

Starting torque Signup and view all the answers

What is the relationship between torque and armature current in DC series motors after magnetic saturation?

Ta ∝ Ia Signup and view all the answers

Why is the shaft torque (Tsh) less than armature torque (Ta) in DC series motors?

Due to stray losses Signup and view all the answers

What is the relationship between speed and armature current in DC series motors for small load currents?

N ∝ 1/Ia Signup and view all the answers

What is the expression for back emf in a DC motor?

Eb = PɸNZ / 60A Signup and view all the answers

What type of DC motors are used where high starting torque is required?

DC series motors Signup and view all the answers

What is the relationship between torque and armature current in DC motors according to the equation Ta ∝ ɸ.Ia?

Torque is directly proportional to the product of armature current and field flux Signup and view all the answers

What is the main difference between bipolar and unipolar stepper motors?

Number of winding phases Signup and view all the answers

What is the purpose of electrical pulses in a stepper motor?

To determine the direction and distance of each step Signup and view all the answers

What is the characteristic of stepper motors that makes them ideal for position-holding tasks?

High holding torque Signup and view all the answers

What is the main advantage of brushless DC motors over brushed DC motors?

Less noise and no sparking Signup and view all the answers

What is the function of the controller in a brushless DC motor?

To provide pulses of current to the motor windings Signup and view all the answers

What is the basic difference between inner rotor and outer rotor brushless DC motors?

Design and construction Signup and view all the answers

What is the advantage of inner rotor design in brushless DC motors?

Easier heat dissipation Signup and view all the answers

What is the characteristic of brushless DC motors that allows them to operate at high speed?

Frequency of current supply Signup and view all the answers

What is the advantage of brushless DC motors in terms of noise and sparking?

Less noise and no sparking Signup and view all the answers

What is the characteristic of brushless DC motors that makes them high-performance motors?

High torque per cubic inch Signup and view all the answers

What happens to the speed of a DC shunt motor when the torque is increased?

It decreases Signup and view all the answers

What is the main purpose of regenerative breaking in DC motors?

To control the speed above the no-load speed Signup and view all the answers

What is the name of the method of electric breaking where the motor works as a generator?

Dynamic breaking Signup and view all the answers

What is the necessary condition for regenerative breaking?

The back EMF must be greater than the supply voltage Signup and view all the answers

What happens to the armature current during regenerative breaking?

It reverses Signup and view all the answers

What is the purpose of the current limiting resistor in plugging or reverse current breaking?

To limit the armature current to a safe value Signup and view all the answers

What happens to the motor when the connections of the armature are reversed in plugging or reverse current breaking?

It rotates in the opposite direction Signup and view all the answers

What is the approximate voltage across the armature during plugging or reverse current breaking?

Double of the supply voltage Signup and view all the answers

What is the difference between continuous torque and the torque mentioned in the beginning?

The initial torque is normally higher Signup and view all the answers

What is the main disadvantage of rheostat breaking or dynamic breaking?

It is inefficient Signup and view all the answers

What is the purpose of a 20 to 30% margin in motor ratings?

To ensure reliability Signup and view all the answers

What is the main advantage of regenerative breaking?

It is more efficient than other methods Signup and view all the answers

What is the characteristic of a DC servo motor that allows for quick torque response?

High armature resistance Signup and view all the answers

What is the main difference between AC servo motor and DC servo motor?

The type of electrical input Signup and view all the answers

What is the characteristic of an AC servo motor?

Delivers low output power Signup and view all the answers

What is the characteristic of a DC servo motor?

Delivers high output power Signup and view all the answers

What is the purpose of a stepper motor?

To convert electrical pulses into precise mechanical motion Signup and view all the answers

What is the typical angle of rotation of a stepper motor?

1.8° Signup and view all the answers

What is the rotating component in a stepper motor?

Rotor Signup and view all the answers

What is the stationary segment of a stepper motor?

Stator Signup and view all the answers

Study Notes

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

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Description

Learn about the characteristics of DC motors, including torque vs armature current, speed vs armature current, and speed vs torque curves. Understand the equations that determine these characteristics and how they relate to the back emf of a DC motor.