Induction machines – historical touch

Faraday discovered the electromagnetic induction law around 1831 and Maxwell formulated the laws of electricity (or Maxwell’s equations) around 1860. The knowledge was ripe for the invention of the induction machine which has two fathers: Galileo Ferraris (1885) and Nicola Tesla (1886).

Their induction machines are shown in Figure 1.1 and Figure 1.2.

Both motors have been supplied from a two-phase a.c. power source and thus contained two phase concentrated coil windings 1-1’ and 2-2’ on the ferromagnetic stator core. In Ferrari’s patent the rotor was made of a copper cylinder, while in the Tesla’s patent the rotor was made of a ferromagnetic cylinder provided with a short-circuited winding.

Though the contemporary induction motors have more elaborated topologies (Figure 1.3) and their performance is much better, the principle has remained basically the same.

That is, a multiphase a.c. stator winding produces a traveling field which induces voltages that produce currents in the short-circuited (or closed) windings of the rotor. The interaction between the stator produced field and the rotor induced currents produces torque and thus operates the induction motor. As the torque at zero rotor speed is nonzero, the induction motor is self-starting. The three-phase a.c. power grid capable of delivering energy at a distance to induction motors and other consumers has been put forward by Dolivo-Dobrovolsky around 1880.

In 1889, Dolivo-Dobrovolsky invented the induction motor with the wound rotor and subsequently the cage rotor in a topology very similar to that used today. He also invented the double-cage rotor. Thus, around 1900 the induction motor was ready for wide industrial use.

No wonder that before 1910, in Europe, locomotives provided with induction motor propulsion, were capable of delivering 200 km/h. However, at least for transportation, the d.c. motor took over all markets until around 1985 when the IGBT PWM inverter was provided for efficient frequency changers. This promoted the induction motor spectacular comeback in variable speed drives with applications in all industries.

The racehorse of high-tech

Mainly due to power electronics and digital control, the induction motor may add to its old nickname of “the workhorse of industry” the label of “the
racehorse of high-tech”. A more complete list of events that marked the induction motor history follows.

• Better and better analytical models for steady state and design purposes
• The orthogonal (circuit) and space phasor models for transients
• Better and better magnetic and insulation materials and cooling systems
• Design optimization deterministic and stochastic methods
• IGBT PWM frequency changers with low losses and high power density (kW/m3) for moderate costs
• Finite element methods (FEMs) for field distribution analysis and coupled circuit-FEM models for comprehensive exploration of IMs with critical (high) magnetic and electric loading
• Developments of induction motors for super-high speeds and high powers
• A parallel history of linear induction motors with applications in linear motion control has unfolded
• New and better methods of manufacturing and testing for induction machines
• Integral induction motors: induction motors with the PWM converter integrated into one piece