What is motor load?

An electric motor is a gadget that changes electrical energy into mechanical energy over to follow up on a mechanical burden. The weight set on the motor because of this mechanical movement is known as motor load. Appropriately matching the motor burden to the engine is vital to forestall harm to the engine or wasteful and unnecessary expensive activity. Engines perform such undertakings as moving a thing starting with one spot then onto the next, cutting it, changing its shape, etc. The motors might be evaluated as far as a few variables including power yield, voltage, current, and temperature. Harm to an engine might happen when it is worked under over the top engine load contrasted with its appraised limit.

What is torque?

Torque is a rotational equivalent of force exerted at a distance from an object’s axis of rotation. A torque is applied to an object will cause it to rotate around a pivot point.

This is the torque force equivalence at one-meter leverage.
CC0 1.0 Universal Public Domain Dedication | https://commons.wikimedia.org | Zoiros

Power quality requirement

Power quality can be defined as the sinusoidal voltage being supplied and a sinusoidal current is drawn. Consumers typically draw into the categories of lighting, heating, and electric motors. In general, they were not very sensitive to momentary variations in the supply voltage.

Two major changes in the characteristics of consumer loads are:-

  • Sensitivity of the load themselves.
  • The fact that these sensitive loads are interconnected in extensive networks and automated processes.

Effects of motor load

If the load increase on the motor then it will draw more power from the source, which would result in an increase in the form of higher current flowing to the motor as the voltage of the electrical source is constant. Since the majority of power supplies are voltage sources, motors will drain more current to compensate for the load increase.

  • Induction motors lowers their shaft speed, hence rotational current increases which will deplete magnetic field and increase stator current.
  • DC motor will lower shaft speed which causes decrease in back-emf and hence current will increase.
  • Synchronous machines will increase the lagging angle between rotor field and armature field which effects in increasing current.

Under any conditions, the electric motor can work with overload currents up to the rated value. Minor overloads are accepted under restricted conditions.

Types of motor load

There are three types of motor load:

Variable torque load

Variable torque loads require much lower torque at low speeds than at high speeds. The torque required varies as the square of the speed and the horsepower required varies as the cube of the speed. Variable torque loads include most centrifugal and axial pumps, fans and blowers, and many mixers and agitators.

As the speed is decreased, the torque will decrease by the square of the speed decrease and the horsepower required decreases by the cube of the speed decrease.

Speed vs. torque graph for variable torque load.
GNU Free Documentation License | https://en.wikipedia.org |Smilesgiles89

Constant torque load

Constant torque loads require similar measure of force at low paces as at high paces. Force stays consistent all through the speed range and the torque increments and diminishes in direct extent to the speed. Steady force loads incorporate best uprooting and responding siphons and blowers just as foothold drives and transports. With steady force stacks, the force isn't a component of speed. As speed is changed, the heap force will remain genuinely consistent and the strength will change directly with the speed.

Speed vs. torque graph for constant torque load.
Attribution-Share Alike 3.0 Unported | https://commons.wikimedia.org | Bgrayrob

Constant power load

Constant power load requires high force at low velocities and low force at high rates, which implies steady strength at any speed. Steady strength loads incorporate processors, winding machines, and machines. For steady strength stacks, the force stacking is an element of accelerating to 100 percent working pace. As the speed of the activity is diminished, the force increments with the goal that the pull required remaining parts basically steady.

Calculations of torque

The magnitude of the torque vector τ for a torque produced by a given force F is

τ=Fr sin θ

Here, r is the length of the moment arm, and θ is the angle between the force vector and moment arm.

The force is at right angles 90° to the moment arm, so the sine term becomes 1, and τ=Fr.

The direction of the torque vector is found by convention using the right-hand grip rule. If a hand is curled around the axis of rotation with the fingers pointing in the direction of the force, the torque vector points will be in the direction of the thumb.

Deviations in power quality requirements

There are many deviations in the quality of the power:


  • Variations in the peak or root mean square (RMS) voltage are important to different types of equipment.
  • When the RMS voltage exceeds the nominal voltage by 10 to 80% for 0.5 cycles to 1 minute, the event is called swell.
  • Over voltage occurs when the nominal voltage rises above 110% for more than 1 minute.


  • Nonzero low-recurrence impedance (when a heap draws more power, the voltage drops).
  • Nonzero high-recurrence impedance (when a heap requests a lot of currents, then, at that point, abruptly quits requesting it, there will be a dunk or spike in the voltage because of the inductances in the power supply line).
  • Varieties in the wave shape, normally portrayed as sounds at lower frequencies and depicted as common mode distortion or inter harmonics at higher frequencies.


  • The swaying of voltage and current preferably follows the type of a sine or cosine work.
  • Generators cause voltage twists, and loads cause current contortions.
  • Low symphonious substance in a waveform is ideal since music can cause vibrations, humming, hardware bends, and misfortunes and overheating in transformers.

Common Mistakes

  • During light load conditions, the power factor of the motor drops to a very low value. This is because, during the start, the motor draws a large magnetizing current to overcome the reluctance offered by the air gap between the Stator and the Rotor.
  • Torque is less fuel-efficient than a manual gearbox or a CVT.
  • Problems in power quality requirements are automatic resets, data errors, equipment failure, and circuit board failure.

Context and Applications

In each of the expert exams for undergraduate and graduate publications, this topic is mainly used for the following:

  • Bachelor of Electrical Engineering
  • Bachelor of Electronics Engineering
  • Masters of Electrical Engineering
  • Three phase induction motor
  • Stator
  • Rotational motion
  • Transient voltages and currents

Practice Problems

Q1 No-load speed of which of the following motor is the highest?

  1. Differentially compound motor
  2. Cumulative compound motor
  3. Shunt motor
  4. Series motor

Correct option-(d)

Explanation- At the time of starting, flux is very less in DC series motor. Therefore, the DC series motor is never started at no load.

Q2 Which part of the DC motor can sustain maximum temperature rise?

  1. Armature winding
  2. Field winding
  3. Slip ring
  4. Commutator

Correct option-(d)

Explanation- The commutator comprises copper portions. This copper portion is protected from one another with the assistance of mica. Mica has an extremely high dielectric stress of around 1180 kV/cm, and another best is diamond. It can endure high temperatures, up to 600°C.

Q3 If we increase the angular speed of the rotor, its power will __________.

  1. decrease
  2. increase
  3. first increase then decrease
  4. first decrease then increase

Correct option-(b)

Explanation: The following equation gives the power of a rotation rotor:


So, if we increase the angular speed, the power will increase.

Q4 What is the effect on torque when angular momentum increases?

  1. Increases.
  2. Decreases
  3. Constant
  4. None

Correct option-(a)

Explanation- Net torque is directly proportional to the angular moment, so when angular momentum increases, net torque also increases.

Q5 The number of the pole in a small DC motor up to 5 HP is ____________.

  1. 2 poles
  2. 4 poles
  3. 8 poles
  4. 10 poles

Correct option-(a)

Explanation- Small HP motors require only two poles because the number of poles is inversely proportional to the speed. Therefore, a two-pole motor runs at high speed than a four-pole motor.

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