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Lecture-7.pdf - Rama University

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Memuat.... (Lihat teks lengkap sekarang)

Unduh sekarang ( 4 Halaman )

Teks penuh

(1)

FACULTY OF MEDICAL SCIENCES FACULTY OF MEDICAL SCIENCES

FACULTY OF ENGINEERING & TECHNOLOGY

Electrical Machine-ii

Amit Kumar Singh

(2)

ASSEMBLY OF 3-PH INDUCTION MOTER

INDUCTION MOTOR

(3)

Principle of Operation I.M

This rotating magnetic field cuts the rotor windings and produces an induced voltage in the rotor windings

Due to the fact that the rotor windings are short circuited, for both squirrel cage and wound-rotor, and induced

current flows in the rotor windings

The rotor current produces another magnetic field

A torque is produced as a result of the interaction of those two magnetic fields

Where

ind is the induced torque and BR and BS are the magnetic flux densities of the rotor and the stator respectively

i ndkB R B s

When a 3 phase stator winding is connected to a 3 phase voltage supply, 3 phase current will flow in the windings, which also will induced 3 phase flux in the stator. These flux will rotate at a speed called a Synchronous Speed, ns. The flux Is called as Rotating magnetic Field.

Synchronous speed is given by the expression

Where p = is the number of poles, and f = the frequency of supply

INDUCTION MOTOR

(4)

SLIP

• The force exerted by the rotor bars causes the rotor to turn in the direction of the rotating magnetic field.

• The difference between the rotor speed, nr, and the synchronous speed, ns, is called the Slip Speed.

Slip Speed = ns ― nr ( rpm or rev/s )

• The ratio (ns ― nr)/ ns is called the Fractional Slip or just the Slip, s, and is usually expressed as a percentage.

Thus

Slip , S = (ns― nr)/ ns

• Typical values of slip between no load and full load are about 4 to 5 per cent for small motors and 1.5 to2 per cent for large motors.

Rotor E.M.F. and Other Parameters

• Rotor e.m.f

When an induction motor is stationary, the stator and rotor windings form the equivalent of a transformer.

• when running, rotor e.m.f. per phase = Er = SE2

• rotor e.m.f. per phase

Referensi

Unduh sekarang ( PDF - 4 Halaman - 587.38 KB )

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