In 3 phase AC induction motor, what is slip?,Give its expression which relates speed & frequency?
and what is the application of this 3ph IM?
Answer:
The synchronous speed of an induction motor is given by speed(RPM) = 120 X frequency(Hz) / Number of motor poles. Motors are wound with 2, 4,6 or more poles (always in pairs). The motor will run at nearly the synchronous speed if there is no load coupled to the motor shaft. With a load applied, the motor runs at a slightly lower speed the difference between synchronous speed and the actual running speed is slip. For the most common design of 3 phase induction motor, the speed at full load is 2 or 3 percent less than the synchronous speed.
There are many types of starters for 3 phase induction motors. The most common type is the direct on line or across line starter. It is simply a contactor that is closed to connect the motor directly to the 3-ph supply. There are various types of electromechanical and electronic reduced voltage starters.
Three phase induction motors are used to drive most industrial machinery requiring more than 2 or 3 horsepower (1.5 or 2.25 kW). They are often called the workhorse of industry.
Edit 1
The most common type of 3-phase induction motor is the cage rotor or squirrel cage motor. Squirrel cage motors are made in a number of designs according to IEC standards and NEMA standards. The NEMA B design and its IEC equivalent is the most common. There are also designs with higher starting torque and higher slip. The torque vs. speed curves show the differences among the designs.
There are also wound rotor induction motors with slip rings for inserting resistance and or control circuitry in the rotor circuit.
The field windings in the stator of an induction motor set up a rotating magnetic field around the rotor. The relative motion between this field and the rotation of the rotor induces electrical current flow in the conductive bars. In turn these currents flowing lengthwise in the conductors react with the magnetic field of the motor to produce force acting at a tangent to the rotor, resulting in torque to turn the shaft. In effect the rotor is carried around with the magnetic field but at a slightly slower rate of rotation.
The difference in speed is called “slip” and increases with load.
http://en.wikipedia.org/wiki/squirrel-ca...
slip = [( synchronous speed ) - ( rotor speed ) / ( synchronous speed )] x 100
slip as expressed in %
speed in rpm
frequency in hz
http://www.ieee-kc.org/library/motors/mo...
as also,
http://www.engineeringtoolbox.com/electr...
the squirrel cage rotor induction motor is the most widely used motor because of it's sheer ruggedness, low cost ..
speed control has simplified with frequency controls becoming commonplace
dol (direct on line) & star-delta are the most common forms of starting, besides solid-state soft starters & frequency controllers
__ __ __ __ __
main types,
i. squirrel cage
ii. wound rotor
wound rotor has very limited applications & normally used for medium duty motors, using resistance cut-out for starting
more of a thing of the past, though not entirely
s=(n1-n)/n1
n1=syncronism speed
*The Slip can be expressed as:
S = (ωs - ωa) 100% / ωs (1)
where
S = slip
ωs = synchronous speed of magnetic field (rev/min, rpm)
ωa = shaft rotating speed (rev/min, rpm)
*An AC (Amplitude Current) induction motor consists of two assemblies - a stator and a rotor. The interaction of currents flowing in the rotor bars and the stators' rotating magnetic field generate a torque. In an actual operation, the rotor speed always lags the magnetic field's speed, allowing the rotor bars to cut magnetic lines of force and produce useful torque.
This speed difference is called the slip. The slip increase with load and is necessary for torque production. Slip speed is equal to the difference between rotor speed and synchronous speed. Percent slip is slip multiplied by 100. When the rotor is not turning, the percent slip is 100%.
*Slip and Voltage:
When the motor starts rotating, the slip is I00% and the voltage is at maximum. The slip and voltage are reduced when the rotor begins to turn.
*Slip and Frequency:
Frequency will decrease when the slip decrease.
{AC Drives
The speed of an AC motor is determined for the most part by two factors: The applied frequency and the number of poles.
N=120f/P
Where:
N = RPM[synchronous motor speed]
[% slip=synchronous speed-rotor speed * 100 / synchronous speed]
f = frequency
P = number of poles
Some motors such as in a typical paddle fan have the capability to switch poles in and out to control speed. In most cases however, the number of poles is constant and the only way to vary the speed is to change the applied frequency. Changing the frequency is the primary function of an AC drive. However, one must consider that the impedance of a motor in determined by the inductive reactance of the windings. Refer to the equation below.
XL= 2*Pi*f*L
Where:
XL = Inductive reactance in Ohms
f = Line frequency
L = inductance
This means that if the frequency applied to the motor is reduced, the reactance and therefore impedance of the motor is reduced. In order to keep current under control we must lower the applied voltage to the motor as the frequency is reduced. This is where we get the phrase “volts per hertz”.}
*The AC Induction Motor is the most commonly used AC motor in industrial applications because of its simplicity, rugged construction, and relatively low manufacturing costs. The reason that the AC induction motor has these characteristics is because the rotor is a self-contained unit, with no external connections. This type of motor derives its name from the fact that AC currents are induced into the rotor by a rotating magnetic field.
http://www.engineersedge.com/motors/indu...
*A.C. Induction motors are traditionally started and stopped by applying and removing the A.C. supply. In some cases, a full voltage start is acceptable, but in many situations, the start current must be reduced, and so a reduced voltage starter is employed.
1.Direct On Line:
The simplest form of motor starter for the induction motor is the Direct On Line starter. The DOL starter comprises a switch and an overload protection relay.
2.Primary Resistance.
The Primary Resistance starter will have one or more sets of resistors which, during start, are connected in series with the supply to the motor. The series resistors limit the starting current drawn by the motor, and thus reduce the starting torque of the motor.
3.Primary Reactance.
A Primary reactance starter is similar to a primary resistance starter except that the resistors are replaced by a three phase reactor to limit the starting current. The operation of the primary reactance starter is essentially the same as that of the primary resistance starter, but the use of a three phase reactor in place of the resistors offers the advantage of reduced heat loss and greater ease of start current setting due to the ability to change taps on the reactor.
4.Auto transformer :
An Auto transformer starter uses an auto transformer to reduce the voltage applied to a motor during start. The auto transformer may have a number of output taps and be set-up to provide a single stage starter, or a multistage starter. Typically, the auto transformer would have taps at 50%, 65% and 80% voltage, enabling the motor to be started at one or more of these settings.
5.A soft starter:
is another form of reduced voltage starter for A.C. induction motors. The soft starter is similar to a primary resistance or primary reactance starter in that it is in series with the supply to the motor. (Three wire or standard connection) The current into the starter equals the current out. The soft starter employs solid state devices to control the current flow and therefore the voltage applied to the motor. In theory, soft starters can be connected in series with the line voltage applied to the motor, or can be connected inside the delta loop of a delta connected motor, controlling the voltage applied to each winding. (Six wire or Inside Delta connection)
*3-Phase AC Induction Motor:
The AC induction motor is a rotating electric machine designed to operate from a three-phase source of alternating voltage. The stator is a classic three phase stator with the winding displaced by 120°. The most common type of induction motor has a squirrel cage rotor in which aluminum conductors or bars are shorted together at both ends of the rotor by cast aluminum end rings. When three currents flow through the three symmetrically placed windings, a sinusoidally distributed air gap flux generating the rotor current is produced. The interaction of the sinusoidally distributed air gap flux and induced rotor currents produces a torque on the rotor. The mechanical angular velocity of the rotor is lower then the angular velocity of the flux wave by so called slip velocity.
*Applications :
Washing machines
Compressors
Air conditioning units
Pumps
Simple industrial drives
Electric cars
Industrial machines (most popular in world).
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