Glossary

Electric motors and generators harness to convert energy. In motors, opposes applied voltage, regulating speed and current. As motors spin up, back EMF increases, reducing current flow and torque until reaching .

Back EMF plays a crucial role in power calculations and energy conversion. It affects current draw, power dissipation, and efficiency. In generators, back EMF is the source of output voltage, converting mechanical energy into electrical power.

Back EMF in Motors and Generators

Back emf effects on motor operation

Top images from around the web for Back emf effects on motor operation

  • Back EMF of DC motor - protection - Electrical Engineering Stack Exchange View original

    Is this image relevant?

  • Back Emf | Physics View original

    Is this image relevant?

  • Back EMF of DC motor - protection - Electrical Engineering Stack Exchange View original

    Is this image relevant?

1 of 3

Top images from around the web for Back emf effects on motor operation

  • Back EMF of DC motor - protection - Electrical Engineering Stack Exchange View original

    Is this image relevant?

  • Back Emf | Physics View original

    Is this image relevant?

  • Back EMF of DC motor - protection - Electrical Engineering Stack Exchange View original

    Is this image relevant?

1 of 3
  • When a motor starts up, the back emf is initially zero
    • Allows a large current to flow through the motor windings ()
    • Large current produces a strong torque, causing the motor to start rotating ( begins spinning)
  • As the motor speeds up, the back emf increases
    • Increasing back emf opposes the applied voltage, reducing the current flow
    • Reduced current results in a decrease in torque as the motor reaches its normal operating speed (steady state)
  • At normal operating speed, the back emf is nearly equal to the applied voltage
    • Results in a small current flow, just enough to maintain the motor's rotation against friction and load (overcome resistance)
    • Back emf acts as a natural speed regulator, preventing the motor from spinning too fast ()

Power calculations using back emf

  • The power dissipated by a motor can be calculated using the formula:
    • PP is the power dissipated in watts (W)
    • II is the current flowing through the motor in amperes (A)
    • RR is the resistance of the motor windings in ohms (Ω)
  • At startup, the back emf is zero, so the current is determined by the applied voltage and the motor's resistance:
    • VV is the applied voltage (V)
    • Example: A 12 V motor with 2 Ω resistance will draw 6 A at startup (I=12V/2Ω=6AI = 12 V / 2 Ω = 6 A)
  • During normal operation, the current is determined by the difference between the applied voltage and the back emf, divided by the resistance:
    • ε\varepsilon is the back emf (V)
    • Example: If the back emf is 10 V, the current will be 1 A (I=(12V−10V)/2Ω=1AI = (12 V - 10 V) / 2 Ω = 1 A)
  • The power dissipated at startup is typically much higher than during normal operation due to the larger current flow
    • Startup power: P=(6A)2×2Ω=72WP = (6 A)^2 \times 2 Ω = 72 W
    • Normal operation power: P=(1A)2×2Ω=2WP = (1 A)^2 \times 2 Ω = 2 W

Back emf in energy conversion

  • In motors, the back emf is a result of the conversion of electrical energy into mechanical energy
    • As the motor rotates, the motion of the conductors through the magnetic field induces a voltage (back emf) that opposes the applied voltage
    • Work done by the motor in overcoming this opposition is the mechanical energy output (rotational kinetic energy)
    • Back emf is a consequence of , which states that induced emf opposes the change causing it
  • In generators, the back emf is a result of the conversion of mechanical energy into electrical energy
    • As an external force (turbine, engine) rotates the generator, the motion of the conductors through the magnetic field induces a voltage (back emf)
    • Induced voltage drives current through an external load, converting mechanical energy into electrical energy (power generation)
    • Back emf in generators is the source of the output voltage
  • The magnitude of the back emf depends on the speed of rotation and the strength of the magnetic field
    • Higher speed or stronger magnetic field will result in a larger back emf (directly proportional)
    • Formula: ε=kϕω\varepsilon = k\phi\omega, where kk is a constant, ϕ\phi is the , and ω\omega is the angular velocity

Electromagnetic Induction in Motors and Generators

  • Electromagnetic induction is the fundamental principle behind the operation of motors and generators
  • The , typically a coil of wire, rotates in a magnetic field
  • As the armature rotates, it experiences a changing , which induces a current in the coil
  • In motors, this interacts with the external magnetic field to produce torque
  • In generators, the induced current is the output power
  • The in DC motors and generators acts as a rotary switch, reversing the current direction in the armature at appropriate times to maintain rotation or power generation

Key Terms to Review (19)

$ ext{ω}$: $ ext{ω}$ is a Greek letter that represents angular velocity, a measure of how quickly an object rotates around a fixed axis. It is a fundamental concept in the study of rotational motion and is closely related to other important physical quantities such as angular acceleration and rotational kinetic energy.
$ i$: $ i$ is a symbol commonly used in physics to represent the magnetic flux, which is a measure of the total magnetic field passing through a given surface. It is a fundamental concept in electromagnetism and is closely related to the phenomenon of electromagnetic induction.
$arepsilon = karephiareomega$: $arepsilon = karephiareomega$ is a fundamental equation that describes the relationship between the electromotive force (emf) induced in a coil, the magnetic flux, and the angular velocity of the coil. This equation is particularly relevant in the context of back emf, which is the voltage generated by a motor when it is spinning.
$arepsilon$: $arepsilon$ is a Greek letter that represents a small quantity or change in a physical quantity. In the context of physics, it is often used to denote an induced electromotive force (emf) or voltage that opposes the change in the magnetic field, as described by Faraday's law of induction and Lenz's law. It is also associated with the concept of back emf in electric motors and the inductance of a circuit.
$I = (V - \varepsilon)/R$: $I = (V - \varepsilon)/R$ is an equation that describes the relationship between the current (I) flowing through a circuit, the voltage (V) applied to the circuit, the back electromotive force (ε or $\varepsilon$) generated by a motor or generator, and the resistance (R) of the circuit. This equation is particularly relevant in the context of understanding back EMF, which is a key concept in the study of electric motors and generators.
$I = V/R$: The equation $I = V/R$ is a fundamental relationship in electrical circuits, where $I$ represents the electric current, $V$ represents the voltage, and $R$ represents the resistance. This equation, known as Ohm's law, describes the direct proportionality between current, voltage, and resistance in a circuit.
$P = I^2R$: $P = I^2R$ is a fundamental equation in electrical engineering that describes the relationship between power (P), current (I), and resistance (R). It states that the power dissipated in a resistor is proportional to the square of the current flowing through it and the resistance of the circuit.
Armature: The armature is a crucial component in the operation of electric motors, generators, and other electromechanical devices. It refers to the rotating part of the device that carries the current-carrying windings, which interact with the magnetic field to produce torque or voltage.
Back EMF: Back EMF, or back electromotive force, is an induced voltage that opposes the change in current flowing through an inductor. It is a fundamental concept in understanding the behavior of electrical circuits involving inductors, such as in the context of 23.6 Back EMF, 23.9 Inductance, and 23.10 RL Circuits.
Commutator: A commutator is a device found in electric motors and generators that helps to convert the alternating current (AC) generated by the rotating armature into a direct current (DC) output. It plays a crucial role in the operation and efficiency of these electromechanical devices.
Electromagnetic Induction: Electromagnetic induction is the process by which a changing magnetic field induces an electromotive force (EMF) in a conductor, causing an electric current to flow. This phenomenon is the fundamental principle behind the operation of many electrical devices and systems, including transformers, generators, and motors.
Induced Current: Induced current refers to the flow of electric current that is generated in a conductor when it experiences a changing magnetic field. This phenomenon is a fundamental principle in electromagnetism and is the basis for the operation of many electrical devices and machines.
Lenz's Law: Lenz's law is a fundamental principle in electromagnetism that describes the direction of the induced current or electromotive force (emf) generated by electromagnetic induction. It states that the direction of the induced current is always such that it opposes the change in the magnetic field that caused it, in accordance with Faraday's law of induction.
Magnetic flux: Magnetic flux is the measure of the quantity of magnetism, taking into account the strength and extent of a magnetic field. It is calculated as the product of the magnetic field and the area through which it passes, perpendicular to the field.
Magnetic Flux: Magnetic flux is a measure of the total amount of magnetic field passing through a given surface or area. It represents the strength and distribution of a magnetic field and is a fundamental concept in the study of electromagnetism and its applications.
Rotor: The rotor is the rotating part of an electric generator or electric motor. It is the component that spins within the stationary stator, generating or converting electrical energy through electromagnetic induction.
Self-Limiting: Self-limiting refers to a process or system that has an inherent mechanism to regulate or restrict its own growth or activity, preventing it from continuing indefinitely or reaching an undesirable state. This concept is often observed in various scientific and engineering contexts.
Stator Coils: Stator coils are the stationary windings or electromagnets in an electric motor or generator that create a rotating magnetic field. They are an essential component that enables the conversion of electrical energy into mechanical energy, or vice versa, in these electromechanical devices.
Steady State: Steady state refers to a condition in which a system or process has reached a stable, unchanging state, where the input and output values remain constant over time. This concept is particularly relevant in the context of electrical circuits, where it describes the point at which the circuit has reached a stable, predictable behavior.
© 2024 Fiveable Inc. All rights reserved.
AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.
Back
Practice QuizGlossary
Practice QuizGlossary
Next guide