Month: January 2017

DC and AC Electric Induction Motors

The electric motors belong to two main groups: the direct current and the alternating current. Electric induction motors fit into asynchronous AC motors precisely because they can not operate at synchronous speed. About 80% of the energy consumed by the industries is used to convert electrical energy into mechanics through electric motors.

DC electric motors have this name because they are constant in time, having their value well defined, circulating in the same direction in a conductor. Already the AC motors are variants in the time, alternating the direction that they circulate in a conductor.

Alternating Current is used in numerous applications, especially in high power systems, industries and electrical machines, usually in the electric motors that equip appliances such as electric kettles, refrigerators and washing machines.

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All electronic circuits are powered by a source of electric power. Generally this source is a DC voltage source. For portable systems, batteries may be used, however, circuits need to be powered from the available local AC power grid. For these cases, there should be a part of the equipment that converts the alternating waveform of the network to DC power.

A little more history of electric motors

In 1879, Siemens & Halske presented the first electric locomotive with a power of 2 kW at the Berlin trade fair.

Only in 1886, Werner Siemens built a generator without the use of permanent magnet, proving that the machine could self-excite. The first dynamo of Siemens had a power of approximately 30 watts and a rotation of 1200rpm. The machine did not only function as an electricity generator, but could also operate as a motor, provided a direct current was used.

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In 1887, Yugoslavian Nikola Tesla presented a prototype electric motor with a short-circuit rotor, but due to poor performance Westinghouse, the project’s owner, abandoned development research.

In 1889, the German engineer Dobrowolsky of the AEG firm in Berlin filed a patent application for the design of a three-phase cage rotor motor. This one presented 80W of power with an efficiency of 80%. The advantages of the AC electric motor for the DC motor were striking: simpler construction, quiet, less maintenance and high safety in operation. In 1891 Dobrowolsky developed the first series line of asynchronous motors in varying powers of 0.4 to 7.5kW.

Drive Circuit

In electric motors the objectives of the elements in an electrical panel are to protect the operator and to provide a command logic.

As for operator protection, a sequence of elements required for starting and maneuvering motors are the isolating elements, protection against short-circuit currents, protection against overload currents and switching devices. To understand better, we will describe each item below:

Sectioning: it can only be operated without load and is used during the maintenance and verification of the circuit;

Protection against short-circuit currents: it is the protection of the conductors of the terminal circuit;

Protection against overload currents: protection of motor winding coils;

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Maneuvering Devices: refers to turning the engine on and off safely, without any contact of the operator in the power circuit.

The conventional motors are divided in two types, the type 1 coordination, which does not pose a risk to people and installations, with a safe disconnection of the short-circuit current, causing damage to the contactor and the overload relay, and Type 2 coordination which also does not pose a risk to people and installations, but there can be no damage to the overload relay or elsewhere, with the exception of a slight fusion of the contacts.