Electric motor

By Dmitry Levkin
An electric motor - is an electrical machine by which electrical energy is converted into mechanical energy, for driving various mechanisms. The electric motor is the main element of the electric drive.

In some operating modes of the electric drive, the electric motor performs the reverse energy conversion, that is, it operates in the mode of an electric generator.

By the type of created mechanical motion, electric motors are divided into rotating, linear, etc. By an electric motor, a rotating motor is most often meant, since it has received the greatest use.

The field of science and technology studying electrical machines is electromechanics. It is considered that its history begins in 1821 when the first electric motor was created by M. Faraday.

Motor construction

The main components of a rotating electric motor are the stator and the rotor. The stator is the fixed part. The rotor is the rotating part.

Construction of the rotating motor
Standard construction of the rotating motor

For most electric motors the rotor is located inside the stator. Electric motors in which the rotor is located outside the stator are called inside out electric motors.

The principle of operation of the motor

Principle of motor operation: Ampere's law
1. According to Ampere's law, a force F will act on a conductor with a current I in a magnetic field.
2. If a conductor with current I is bent into a frame and placed in a magnetic field, then the two sides of the frame, which are at right angles to the magnetic field, will experience oppositely directed forces F.
Principle of motor operation: forces acting on the frame
Principle of motor operation
3. The forces acting on the frame, create a torque or moment of force, rotating it.
4. The armature of the produced electric motors has several turns to provide greater constant torque.
Principle of motor operation
5. The magnetic field can be created both by magnets and electromagnets. An electromagnet is usually a wire wound on a core. Thus, according to the Faraday's law of induction, the current flowing into the frames will induce a current in the windings of an electromagnet, which in turn will create a magnetic field.

Motor classification

Rotating electric motor
Self-Commutated Externally Commutated
Mechanical-Commutator Motors Electronic-Commutator Motors1 Induction motors Synchronous motors
  • Universal motor
  • Repulsion motor
  • Electrically excited DC motor (wound stator)
      Winding connection
    • Separately excited
    • Series
    • Shunt
    • Compound
  • PM DC motor
  • BLDC motor
    (Brushless motor + EPC |+ RPS)
  • SRM
    (SyRM with salient pole rotor and with the concentrated stator winding + EPC |+ RPS)
  • Three phase
  • Two phase
    (condenser motor)
  • Single phase
    • with auxiliary winding
    • with shaded-pole
    • with asymmetrical stator
  • WRSM
  • PMSM
    • IPMSM
    • SPMSM
    • Hybrid
  • SyRM
  • Hysteresis
  • Hybrid SyRM-PM
  • Hysteresis-reluctance
  • Stepper3
Simple electronics Rectifier, transistors More elaborate
Most elaborate electronics (VFD)
when provided
  1. This category does not represent a separate class of electric motors, since the devices in the category under consideration (BLDC, SRM) are a combination of a brushless motor, an electric converter (inverter) and, in some cases, a rotor position sensor (e.g. Hall sensors). In these devices, the electric converter, in view of its low complexity and small dimensions, is usually integrated into an electric motor.
  2. The electric motors used in BLDC and SRM are AC motors. However, due to the presence of an electric converter in these devices, they are connected to the DC power grid.
  3. A stepper motor is not a separate motor class. Structurally, it is the PMSM, SyRM or the hybrid SyRM-PM.

Types of electric motors

Commutator motors

Сommutator machine is a rotating electrical machine, which has at least one of the windings involved in the main process of energy conversion, is connected to a collector [1]. In a collector motor, the brush-collector assembly performs the function of a rotor position sensor and a current switch in the windings.

Universal motor

Universal motor

Can work on AC and DC power supply. It is widely used in hand-held power tools and in some household appliances (in vacuum cleaners, washing machines, etc.). In the USA and Europe, it was used as a traction motor. Received a wide distribution due to its small size, relatively low price and simple control.
Brushed DC electric motor

Brushed DC electric motor

An electric machine that converts direct current electrical energy into mechanical energy. The advantages of the DC motor are high starting torque, speed, the ability to smoothly control the rotational speed, the simplicity of the device and control. The disadvantage of the motor is the need for maintenance of collector-brush assembly and limited lifetime due to wear of the collector.
  • With permanent magnet
  • With wound stator

Commutatorless motors

Commutatorless electric motors can have contact rings with brushes, so do not confuse commutatorless electric motors and brushless electric motors.

A brushless machine is a rotating electrical machine in which all electrical connections of the windings involved in the basic process of energy conversion are carried out without sliding electrical contacts [1].

Induction motor

Induction motor

The most common electric motor in the industry. The advantages of the electric motor are the simplicity of design, reliability, low cost, high service life, high starting torque, and overload capacity. The disadvantage of an induction motor is the complexity of the rotational speed control.
  • Single phase
  • Two phase
Synchronous motor

Synchronous motor

Synchronous motors are commonly used in tasks where precise control of rotational speed is required, or where the maximum value of parameters such as power/volume, efficiency, etc. is required.

Special electric motors



Servo motors are not a separate class of motors. As a servomotor can be used DC and AC electric motors with a rotor position sensor. The servo motor is used in the servo-mechanism for precise control of the angular position, speed, and acceleration of the actuator. To operate, a servomotor requires a relatively complex control system, which is usually developed specifically for a servo drive.

The main parameters of the motor

Motor torque

Torque (synonyms: a moment of force) is a vector physical quantity equal to the product of the radius-vector, drawn from the axis of rotation to the point of application of force, by the vector of this force.


  • where M – torque, Nm,
  • F – force, N,
  • r – radius-vector, m
Tip: Rated torque Mr, Nm, determined by the formula


  • where Pr – rated motor power, W,
  • nr - rated speed, min-1 [4]

The initial starting torque is the motor torque at start-up.

Tip: In the English system of measurement, force is measured in ounce-force (oz, ozf) or pound-force (lb, lbf)

1 oz = 1/16 lb = 0,2780139 N
1 lb = 4,448222 N

torque is measured in ounce-force inch (oz∙in) or pound-force inch (lb∙in)

1 oz∙in = 0,007062 Nm
1 lb∙in = 0,112985 Nm

Motor power

Motor power is the useful mechanical power at the motor shaft.

Mechanical power

Power is a physical quantity that shows what kind of work the mechanism performs per unit of time.


  • where P – power, W,
  • A – work, J,
  • t - time, s

Work is a scalar physical quantity equal to the product of the projection of the force on the direction F and the path s traversed by the point of application of force [2].


  • where s – displacement, m

For rotational motion


  • where – angle, rad,


  • where – angular velocity, rad/s,

Thus it is possible to calculate the value of mechanical power on the shaft of a rotating electric motor.

Tip: Rated value is the value of the electrical device parameter specified by the manufacturer, at which it should work, which is the source for counting deviations.

The energy conversion efficiency of the electric motor

Energy conversion efficiency of the electric motor is a characteristic of the machine effectiveness in relation to the conversion of electrical energy into mechanical energy.


  • where – efficiency of the electric motor,
  • P1 - input power (electrical), Вт,
  • P2 - useful output power (mechanical), W
    In this case, losses in electric motors are due to:
  • electric losses - in the form of heat as a result of heating of conductors with the current;
  • magnetic losses - core remagnetization losses: eddy current losses, hysteresis, and magnetic aftereffects;
  • mechanical losses - friction losses in bearings, on ventilation, on brushes (if any);
  • additional losses - losses caused by higher harmonics of magnetic fields arising due to the tooth structure of the stator, rotor and the presence of higher harmonics of the magnetomotive force of the windings.

The efficiency of the electric motor can vary from 10 to 99% depending on the type and design.

The International Electrotechnical Commission determines the requirements for the efficiency of electric motors. According to the standard IEC 60034-31: 2010, four efficiency classes are defined for synchronous and induction electric motors: IE1, IE2, IE3, and IE4.

IEC 60034-31

Rated speed

  • where n - is the rotation speed (frequency) of the electric motor, rev/min

Moment of inertia of the rotor

The moment of inertia - a scalar physical quantity, which is a measure of the inertia of a body in a rotational motion around an axis, is equal to the sum of the products of the masses of material points and the squares of their distances from the axis.


  • where J – the moment of inertia, kg∙m2,
  • m - mass, kg
Tip: In the English system of measures, the moment of inertia is measured in ounce-force-inch (oz∙in∙s2)

1 oz∙in∙s2 = 0,007062 kg∙m2

The moment of inertia is associated with the moment of force as follows


  • где – angular acceleration, s-2 [2]


Rated voltage

Rated voltage - is the voltage to which the power grid or equipment is designed and to which their characteristics is referred [3].

Electrical time constant

The electrical time constant is the time counted from the moment a DC voltage is applied to the electric motor, during which the current reaches a level of 63.21% (1-1/e) of its final value.


  • где – time constant, s

Torque-speed curve

The torque-speed curve (mechanical characteristic) of the motor is a graphically expressed dependence of the shaft speed from the electromagnetic torque at a DC supply voltage.

Comparison of characteristics of externally commutated motors

Below are the comparative characteristics of externally commutated electric motors, in terms of use as traction motors in vehicles.

  • Comparison of torque-speed curves of different types of electric motors with a limited stator current
  • The dependence of power from the shaft rotation speed for motors of different types with a limited stator current




Constant power over speed range
Torque per stator current
Efficiency over complete operating range
Orange color is low index; yellow is medium; light yellow is high.
  • SyRM-PM - Permanent Magnet Assisted Synchronous Reluctance Motor (Hybrid Synchronous Motor)
  • WRSM - Wound Rotor Synchronous Motor

In accordance with the above indicators, a hybrid synchronous electric motor, namely a synchronous reluctance electric motor with incorporated permanent magnets, is most suitable for use as a traction electric motor in the automotive industry (the choice was made for the BMW i3 & BMW i8 concept). The use of reactive torque provides high power in the upper speed range. Moreover, such a motor provides very high efficiency in a wide operating range [7].

Electric motors applications

Electric motors are the largest consumers of electricity in the world, they account for about 45% of the total electricity consumed [6].

    Electric motors are used everywhere, the main applications:
  • industry: pumps, fans, compressors, conveyors, driving force for other machines, etc.
  • construction: pumps, fans, conveyors, elevators, heating, ventilation and air conditioning systems, etc.
  • consumer devices: refrigerators, air conditioners, personal computers and laptops (hard drives, fans), vacuum cleaners, washing machines, mixers, etc.
Motor Functions Application
Rotating electric motors Pumps Water supply and drainage systems
Heating, cooling and chilling systems, HVAC1, irrigation systems
Sewage system
Oil pipeline
Fans Room air supply and exhaust, blowers, HVAC1
Compressors Cooling machines for air conditioning and commercial freezers, refrigerators and freezers, HVAC1
Compressed-air storage and distribution system, pneumatic systems
Liquification systems
Rotating, mix, stir Roller, rotors: metal, stone, plastics processing
Extruder: aluminium, plastics processing
Textile handling: weaving, washing, drying
Mixers, stirring: food, colour, plastics
Transport Passenger elevator, escalator, conveyor
Goods elevator, cranes, hoists, conveyor
Vehicles: train, tram, trolley, cars, buses, electric cars, bikes and bicycles, cog wheel train, cable car, ropeway
Angular position
(stepper motors, servomotors)
Valve (open/close)
Servo (setting position)
Linear motors Open/close Valve
Sort Production
Grab and place Robots
  1. HVAC - Heating, Ventilation, & Air Conditioning

Electric motor manufacturers

A country Manufacturer Induction motor Synchronous motor Universal Brushed DC motor
SCIM WRIM WRSM PMSM, servo SyRM, Hysteresis Stepper Brushed DC motor (wound stator) Brushed DC motor (permanent magnet stator)
Switzerland ABB Limited
USA Allied Motion Technologies Inc.
USA Ametek Inc.
USA Anaheim automation
USA Arc System Inc.
Germany Baumueller
Slovenia Domel
USA Emerson Electric Corporation
USA General Electric
USA Johnson Electric Holdings Limited
Germany Liebherr
Switzerland Maxon motor
Japan Nidec Corporation
Germany Nord
USA Regal Beloit Corporation
Germany Rexroth Bosch Group
Germany Siemens AG
Brazil WEG