2. Electric motors

An electric motor uses electrical energy to produce mechanical work, nearly always by the interaction of magnetic fields and current-carrying conductors. The reverse process, that of using mechanical energy to produce electrical energy, is accomplished by a generator or dynamo.

The World's first electric motor He was an inventor, engineer, physicist, Roman Catholic priest, He formulated the concept of the dynamo at least 6 years prior to Siemens and Wheatstone. He invented the carbonated water too Ányos Jedlik's electric motor (Hungary, 1828).

The Jedlik Dynamo

Jedlik's electric car model in 1828

Lorentz force The force on a point charge due to electromagnetic fields. F is the force (in newtons) E is the electric field (in volts per metre) B is the magnetic field (in teslas) q is the electric charge of the particle (in coulombs) v is the instantaneous velocity of the particle (in metres per second) × is the vector cross product qE: electric force, qv × B: magnetic force.

Right-hand rule At right angles

A positively charged particle will be accelerated in the same linear orientation as the E field. A positively charged particle will curve perpendicularly to both the velocity vector v and the B field according to the right-hand rule (thumb: v, index finger: B, middle finger: F).

Beam of electrons moving in a circle, due to the presence of a magnetic field. Purple light is emitted along the electron path, due to the electrons colliding with gas molecules in the bulb.

Force on a current-carrying wire When a wire carrying an electrical current is placed in a magnetic field, moving charges can create a force on the wire. In the case of a straight, stationary wire: I = current in wire, measured in amperes L = a vector, whose magnitude is the length of wire (measured in meters), and whose direction is along the wire, aligned with the direction of conventional current flow.

Categorization of electric motors Direct current Alternating current – Synchronous Uni-phase - Poly-phase – Induction - Uni-phase

DC motors A simple DC motor has a coil of wire that can rotate in a magnetic field. The current in the coil is supplied via two brushes. The coil lies in a steady magnetic field. The forces exerted on the current-carrying wires create a torque on the coil.

The effect of the brushes on the split ring: When the plane of the rotating coil reaches horizontal, the brushes will break contact, and the current then flows in the opposite direction, which reverses force pair. Torque:

In practice DC motors often have a high permeability core inside the coil, so that large magnetic fields are produced by modest currents. This is shown in the figure below in which the stators (the magnets which are stationary) are permanent magnets.

A DC motor is also a DC generator. The coil is being turned, which generates an electromagnetic field (emf).

The motors of trains become generators when the train is slowing down. Recently, the electric motors used to drive the car are also used to charge the batteries when the car is stopped. It is called regenerative braking.

AC motors We mostly use AC motors. With AC currents, we can reverse field directions without having to use brushes. We can avoid - ozone production - the ohmic loss of energy (brushes) - brushes wear out.

Rotating field Single phase AC Two currents are out of phase (capacitor) Two field components, the vector sum is rotating

If we put a permanent magnet in this rotating field, or if we put in a coil whose current always runs in the same direction, then this becomes a synchronous motor. The motor will turn at the speed of the magnetic field.

Single phase induction motors (asynchronous) We have a time varying magnetic field, we can use the induced emf in a coil to make the rotor a magnet. It does not have any direct supply onto the rotor; instead, a secondary current is induced in the rotor. There is a difference between the speed of the rotor and speed of the rotating magnetic field. Due to this an induction motor is sometimes referred to as an asynchronous machine.

This is the most commonplace motor. „Squirrel cage” rotor This is the most commonplace motor. The rotating stator field induces current in the cage creating a magnetic field which causes the rotor to follow the stator field. First Induction Motor, 1888 Inventor Nikola Tesla 1894 Induction Motor. World’s largest when new.

No modern home should be without one – or maybe a dozen No modern home should be without one – or maybe a dozen. You'll find an induction motor in the fan, fridge, vacuum cleaner, washing machine, dishwasher, clothes drier… Advantages: • Cheap • Quiet • Long lasting • Creates no interference Disadvantages: • Wants to turn at constant speed • Cannot turn faster than 1500rpm (4-pole motor) • Draws a massive starting current, or is inefficient, or both • Kind of big and bulky for the power it develops • Capacitors are expensive

Three phase AC induction motors Single phase is used in domestic applications for low power applications. Industrial high power applications use three phase extensively. The three wires carry three voltages which are out of phase with each other by 120° Three stators give a smoothly rotating field

Animation of a squirrel-cage AC motor

If one puts a permanent magnet in such a set of stators, it becomes a synchronous three phase motor. If any conductor is placed in this rotating field then this becomes a three phase induction motor . Three phase AC induction motors are capable of high efficiency, high power and high torques over a range of rotation rates.

Motor efficiency matters In U.S. Industry, electric motors consume: ~680 billion kWh/year ~63% of all industrial electricity consumption ~23% of all U.S. consumption These percentages are typically higher in developing countries, while the motors are typically less efficient

How is power lost in a motor? Mechanical (friction and windage) losses Magnetic losses (eddy current losses) Electrical (I2R) losses Miscellaneous losses associated mainly with electromagnetic radiation Eddy current is caused when a conductor is exposed to a changing magnetic field

How is efficiency determined? There are different standards in use around the world for the determination of motor efficiency. They yield slightly different results. IEEE 112-B (United States) IEC IEC60034.2 (International Electrotechnical Commission) JEC-37 (Japanese Electrotechnical Committee) C-390 (Canadian Standards Association)

1 HP = 745.7 W

Electrical motors constructed according NEMA (National Electrical Manufacturers Association) must meet the efficiencies below:

Basic calculations We will use SI units. It is officially accepted in electrical engineering in the USA. Ohm’s Law: where: I – current, measured in amperes (A); V – applied voltage, measured in volts (V); R – resistance, measured in ohms (Ω). The consumed electrical power of the motor: where: Pin – input power, measured in watts (W).

Motors supposed to do some work and two important values define how powerful the motor is. It is motor speed and torque – the turning force of the motor.  Output mechanical power of the motor: where: t – torque, measured in Newton meters (Nm); w – angular velocity, measured in radians per second (rad/s). where: n – rotational speed in rev/s; N – rotational speed in rev/min; rpm; where: h – efficiency