Tutorial: Mechanik - elektrotechnik Topic: Basics of electrical engineering the 2nd. year, Three-phase current

Prepared by: Škarka Miloš

Projekt Anglicky v odborných předmětech, CZ.1.07/1.3.09/04.0002

je spolufinancován Evropským sociálním fondem a státním rozpočtem

České republiky.

An inductive (asynchronous) machine is a rotary electric machine where its magnetic circuit is divided by the small gap into two parts: the stator and the rotor. Both parts have a winding. One winding (usually on the stator) is connected to the source of the alternating current and the other (usually on the rotor) is connected on short distance short circuited and current in it comes from the electromagnetic induction, which is similar to the transformer. That's why we call it an inductive machine.

The three-phase inductive motor is simple, reliable in operation and it doesn‘t require any special service or maintenance. It is being manufactured in wide range of performance: from a few watts up to 20MW with a very wide rotation range from tens of rotations per minute up to 100 000 rot/min. According to the design of a motor we distinguish:a) motors with the „squirrel cage“ armature on the short b) motors with the binder wound armature

Function principle

terminalventilator

terminal casing

stator winding

metal bundle of the stator

short circuit circle

shaft

metal bundle of the rotor

bearing

The motor with the „squirrel cage“ armature on the short has the rotor winding made of bars, which are short-circuited at the end connected on the short by conductive circles. The magnetic circuit is composed of laminated metals to minimize losses caused by turbulent currents.

The wound binder armature – Except of the bundle of rotor metals there are even collecting circlets on the shaft. In rebates of metals there is a three-phase rotor winding made of insulated conductors connected to the star, rarely to the triangle. The winding of the rotor is connected to three collecting circlets, which can be also connected to active resistors used for the engine start.

Armature with circlets Brushes on the collector

Stator windingConsist of three coils rotated by 120° powered by the three-phase current. This creates rotary magnetic field.

There are two ways how to connect the asynchronous motor

In case of the connection to the star the voltage on coils is 230V

In case of the connection to the triangle the voltage on coils is 400V

Terminal of the asynchronnous motor connected to the star and to the triangle.

Main parts of the asynchronous motor The difference between the squirrel cage

armature on the short and the wound binder armature

Describe the picture What is the voltage on coils?

Picture 1 Picture 2

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