What is electromagnetic torque?

Electromagnetic torque is the torque required to start the rotation of a motor, and it is generally equal to the load torque. The output electromagnetic torque (torque produced by the armature on the air gap) is the load torque at no load, and the load torque is equal to the electromagnetic torque. Due to electromagnetic force, electrical torque is a circular motion. Torque is the radial force from an axis of rotation. It is the vector cross product of the radius and force vectors.

What is electromagnetic power?

The power carrier by the electromagnetic wave is defined as the electromagnetic power. Since, the magnetism is a form of energy reflected in or emitted from objects that can travel through space in electric waves and magnets. The electromagnetic power is created by the energy that flows in the wave and the energy so carried is proportional to the square of the amplitude of the wave. Examples for such wave carrying power includes radio waves, microwaves, infrared light, visible light (all colors of the spectrum we see), ultraviolet light, X-rays, and gamma rays.

Types of electromagnetic torque

The four types of torque in a motor, they are locked rotor torque (LRT), pull-up torque (PUT), breakdown torque (BDT), and full-load torque (FLT).

A motor connected to a load produces four types of torque: locked rotor torque (LRT), pull-up torque (PUT), breakdown torque (BDT), and full-load torque (FLT).
Full load Vs. Motor speed

Locked Rotor Torque

Locked rotor torque (LRT) is a torque produced by an engine whose rotor is stationary and full power is applied to the engine. The first torque, also known as a locked rotor torque, is the engine’s torque each time it is started with a limited voltage and frequency. It is the torque produced when power is applied to the engine at rest, i.e., when the synchronous motor is powered at full voltage, and the shaft is closed in place. This is the torque used to start accelerating the load. The locked rotor torque of the car is an important consideration. Locked rotor torque is an electrical torque motor that increases when it rests or at zero synchronous speed. The size of the current rotor lock is determined by the synchronous motor power and design features. Some important points for rotor torque are:

  • Reduced voltage soft starters: These are used to limit current start to reduce locked rotor torque or starting torque and are common in systems that are difficult to start or have to be carefully monitored, such as good transport pumps, crates, elevators, etc.
  • Squirrel-cage rotor: The squirrel-cage rotor consists of a metal-clad in the center with equally spaced copper bars or aluminum placed axially in the surrounding area, permanently shortened at the ends by rings.
  • Wound rotor: The rotor is a steel cylinder with a 3-degree curve that is evenly spaced 120 degrees apart and connected to a 'Y' suspension.
  • Non-salient rotor: The cylinder-shaped rotor is made of sturdy steel with spaces running along the outer length of the cylinder to hold the rotor field windings with taped copper bars inserted in the spaces and protected by wedges.

Pull-up torque (PUT)

Pull-up torque is a small torque generated by an electric AC motor when it goes from zero to full load speed (before it reaches the torque breakpoint).

Breakdown torque (BDT)

Breakdown torque is the highest torque available before the torque drops as the electric machine accelerates to operating conditions.

Full-load torque (FLT)

Full load torque requires the limited power of electric vehicles to be produced at full load speed.

Measurement of electromagnetic torque

Torque is a measure of the force which could reason an item to rotate around an axis. Power is what causes an object to accelerate in linear kinematics. In addition, torque is the cause of angular acceleration. Therefore, torque can be described because of the rotational equity of line power. The point at which an item rotates is the axis of rotation. In physics, torque is simply the tendency to turn or twist.

The relationship between force F, torque τ, linear force p, and angular force L in a rotating system is limited to a single plane (the intensity and frequency caused by gravity and sliding are not considered).
Electromagnetic torque

An easy way to calculate the torque is to determine the lever arm first and then multiply it by instantaeneous execution of force. The torque depends on the value of the force and vertical distance between the point and the factor of the applied force. Therefore, the mathematical torque is expressed as follows:

τ=F r sin θ

The particle is located in place r relative to its rotating axis. When force F is applied to particles, only the perpendicular portion of F⊥ produces torque.
Direction of torque

Energy density of electromagnetic wave

Consider a magnetic field that travels in a relaxed atmosphere in a positive x-axis direction. The electric field is associated with the wave changes in the y-direction, and the magnetic field alternates in the z-direction. The electric and magnetic fields are mathematically represented in the cosine form:

Eyx, t=E0 cos kx-ωtBzx, t=B0 cos kx-ωt

The energy stored in any part of an electric wave is the sum of the electrical energy density and magnetic field. The amount of energy stored per volume is the amount of electrical energy (U), which is the total amount of the electrical energy (UE) and the energy density of the magnetic field (UB).

The generalization of phenomenological equations for electromagnetic fields in superconductors is based on algebraic space-time.

The definition of a magnetic field density, in which 𝜇0 is permeability free space and 𝜺0 is permittivity free space, the equation for the power unit is Jm3.


Differences between electromagnetic power and electromagnetic torque

  • Power is the amount of energy consumed in terms of a unit of time, and torque is the level of electricity whose effect can revolve around an object.
  • The power unit is a second volume, and the torque unit is equal to a volume. The SI unit of power is watt, while the SI unit of torque is newton meter.
  • Electrical power is the result of a generator or battery, and the power of the electric machine is enhanced by doing the work.
  • Torque is caused when energy is applied to an object or body.

Common Mistakes

Remember that electrical motor torque is proportional to the output power of the magnetic flux and the armature current. Mechanical torque or load torque is proportional to the product of force and distance. The synchronous motor current and voltage vary according to the load torque used. When the synchronous motor runs stably, the armature current and voltage do not change, and the electrical torque is equal and opposite to the mechanical torque. So, here the mechanical torque is equal to the rotation of the line force. Depending on the study, it is also called a moment, a moment of strength, a rotating force, or a turning point. The mechanical torque idea came from Archimedes' studies on the use of levers.

Context and Applications

In each of the expert exams for undergraduate and graduate publications, this topic is huge and is mainly taught in

  • Bachelor of Technology in Electrical and Electronics Engineering
  • Bachelor of Science in Physics
  • Master of Science in Physics
  • Electromagnetic radiation 
  • Mechanical torque

Practice Problems

Q1. For the torque T=BIAcosθ find the angle at which the torque is minimum.

  1. 30
  2. 45
  3. 60
  4. 90

Correct option: (d)

Explanation: The torque of a conductor loop is given by T=BIA cos θ. The torque is minimum refers to zero torque. This is possible only when the angle is 90 or perpendicular as it makes the term cos90=0.


Q2. The value of power generated  by the wave when the H component of it is 0.75 units.

  1. 141.375
  2. 444.832
  3. 233.975
  4. 567.223

Correct option: (a)

Explanation: Consider the power equation is given by P=ηH22.

In air medium η=377, and given that H=0.75. A

Substitute the values and solve as,

 P=377×0.7522P=141.375 units


Q3. Determine the power of the wave with the intensity of field as 8 in the air.

  1. 0.05
  2. 0.02
  3. 0.08
  4. 0.09

Correct option: (c)

Explanation: Consider the formula for the Poynting  vector power P=E22η.

Substitute the values E=8 and η=377 air.

Solve for the power

 P=822×377P=0.08 units


Q4. Consider the electric field has intensity of 5 units and the density of filed is 7 units. Find the power per unit velocity produced by the wave.

  1. 90
  2. 66
  3. 35
  4. 19

Correct option: (c)

Explanation: The power per unit velocity formula is the product of electric field intensity and the density of the wave. Solve as,

Pv=E.dPv=5×7Pv=35 units


Q5. Consider the average power of the wave is 10 units and the wave has a surface density of 0.2 units. Determine the total power of the wave.

  1. 2.2
  2. 2.0
  3. 2.4
  4. 2.5

Correct option: (b)

Explanation: The total power formula isPavgds. Substitute the values Pavg=10 and ∫ds=0.2, the total power is 2 units.

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