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Tutorial: Mechanik - elektrotechnik
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
The rotary field created by the stator has a higher speed
than rotor revs. The basis of asynchronous motor
function is a creation of the rotary magnetic field, which
comes from the alternating current passing through the
winding of the stator. This magnetic field induces the
voltage in the rotor and a formed current causes the
power for rotating the rotor. Rotary field revs are
determined by the frequency of the powering voltage
taken from the network and by the number of poles in
the three-phase motor.
Where f is the frequency of the current and p is the
number of pole pairs in the stator
terminal casing
metal bundle of the stator
stator winding
metal bundle
of the rotor
short circuit circle
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
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 threephase 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 winding
Consist 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
Terminal of the asynchronnous motor connected to the
star and to the triangle.
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
Main parts of the asynchronous motor
The difference between the squirrel cage
armature on the short and the wound binder
Describe the picture
What is the voltage on coils?
Picture 1
Picture 2