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Squirrel Cage Motor

Squirrel Cage Motor Squirrel Cage Motor

 

The squirrel cage motor receives its name from the type of rotor utilized in the motor. A squirrel cage rotor is made by linking bars to two end rings. If the metal lamination’s were eliminated from the rotor, the result would look very similar to a squirrel cage (shown below). A squirrel cage is a cylindrical device constructed of heavy wire. A shaft placed within the center of the cage allows the cage to spin around the shaft. A squirrel cage is put inside the cage of small animals such as squirrels and hamsters to permit them to work out by running inside of the squirrel cage. A squirrel cage rotor is shown below.

 

 

Concept of Operation

The squirrel cage motor is an induction motor. That suggests that the current flow in the rotor is produced by induced voltage from the turning magnetic field of the stator.

In the figure below, a squirrel-cage rotor is shown inside the stator of a three-phase motor. It will be presumed that the motor given includes 4 poles per phase, which produces a turning magnetic field with a synchronous rate of 1800 revolutions per minute. The stator is linked to a 60-hertz line. When power is first connected to the stator, the rotor is not turning. The electromagnetic field of the stator cuts the rotor bars at a rate of 1800 revolutions per minute. This cutting activity causes a voltage into the rotor bars. This induced voltage will be the same regularity as the voltage put on the stator. The quantity of induced voltage is identified by three aspects:.

1. The strength of the electromagnetic field of the stator.
2. The number of turns of wire cut by the magnetic field (in the case of a squirrel-cage rotor, this will be the number of bars in the rotor).
3. The speed of the cutting action.

squirrel cage inside motor Squirrel Cage Motor

induced voltage inside squirrel cage motor

Since the rotor is stationary at this time, optimum voltage is induced into the rotor. The induced voltage triggers current to stream through the rotor bars. As current flows with the rotor, an electromagnetic field is produced around each bar.

The magnetic field of the rotor is brought into the electromagnetic field of the stator, and the rotor starts to turn in the exact same direction as the turning magnetic field. As the rate of the rotor boosts, the rotating electromagnetic field cuts the rotor bars at a slower rate. For example, presume the rotor has actually increased to a speed of 600 rpm. The synchronous speed of the rotating electromagnetic field is 1800 rpm. Therefore, the rotor bars are being cut at a rate of 1200 rpm (1800 rpm – 600 rpm = 1200 revolutions per minute). Considering that the rotor bars are being cut at a slower rate, less voltage is induced in the rotor, lowering rotor current. When the rotor existing decreases, the stator current reduces also.
As the rotor continues to increase, the turning magnetic field cuts the rotor bars at a lowering rate. This decreases the quantity of induced voltage and, for that reason, the quantity of rotor current. If the squirrel cage motor is running without a load, the rotor continues to speed up till it reaches a rate close to that of the rotating magnetic field.

Starting Features of Squirrel Cage Motors

When a squirrel cage motor is first turned on, it has a present draw several times greater than its regular running current. The actual quantity of starting current is identified by the kind of rotor bars, the horse power rating of the motor, and the applied voltage. The kind of rotor bars is indicated by the code letter found on the nameplate of a squirrel cage motor.

Torque

The quantity of torque produced by an AC induction motor is figured out by three factors:.
1. The strength of the electromagnetic field of the stator.
2. The strength of the electromagnetic field of the rotor.
3. The phase angle distinction in between rotor and stator fields.

Notice that one of the factors that determines the quantity of torque produced by an induction motor is the strength of the magnetic field of the rotor. An induction motor can never ever reach synchronous speed. Another element that determines the amount of torque developed by an induction motor is the phase angle distinction in between stator and rotor field flux. Maximum torque is developed when the stator and rotor flux are in phase with each other.

Motor Rotation

On many types of equipment, the direction of motor rotation is critical. The direction of rotation of any three-phase motor can be changed by reversing 2 of its stator leads. This causes the direction of the turning electromagnetic field to reverse. When a motor is linked to a device that will not be damaged when its motor rotation is reversed, power can be momentarily applied to the motor to observe its direction of rotation. If the rotation is incorrect, any two line leads can be interchanged to reverse the motor’s rotation.

When a motor is to be linked to a machine that can be damaged by incorrect motor rotation, the direction of rotation must be figured out before the motor is linked to its load. The direction of motor rotation can be figured out in two standard ways. One means is to make an electric connection to the motor before it is mechanically connected to the load. The direction of rotation can then be tested by briefly putting power to the motor prior to it’s connection to the load.

Phase Rotation Motor Rotation

There could be occasions when it is not useful or is very bothersome to apply power to the motor before it is linked to the load. In such a case, a phase rotation meter can be utilized. The phase rotation meter compares the phase rotation of two various three-phase connections. The meter consists of six terminal leads. 3 of the leads are connected to one side of the meter and are labeled MOTOR. These 3 motor leads are identified A, B, or C. The LINE leads are found on the other side of the meter and are labeled A, B, or C.

To determine the direction of motor rotation, first zero the meter by following the directions offered by the manufacturer. Then set the meter selector switch to MOTOR, and connect the three MOTOR leads of the meter to the “T” leads of the motor (Image above). The phase rotation meter contains a zero-center voltmeter. One side of the voltmeter is labeled INCORRECT, and the opposite is identified CORRECT. While observing the zero-center voltmeter, manually turn the motor shaft in the direction of preferred motor rotation. The zero-center voltmeter will immediately swing in the CORRECT or INCORRECT direction. When the motor shaft stops turning, the needle might swing in the opposite direction. It is the first indication of the voltmeter that is to be made use of.

If the voltmeter needle shows CORRECT, tag the motor T leads A, B, or C to correspond with the MOTOR leads from the stage rotation meter. If the voltmeter needle shows INCORRECT, alter any 2 of the MOTOR leads from the stage rotation meter and again turn the motor shaft. The voltmeter needle ought to now show CORRECT. The motor T leads can now be labeled to correspond with the MOTOR leads from the stage rotation meter.

 

motor rotation meter Motor Rotation

After the motor T leads have actually been identified A, B, or C to correspond with the leads of the phase rotation meter, the rotation of the line providing power to the motor must be figured out. Set the selector switch and turn on the phase rotation meter to the LINE position. After making sure the power has actually been shut off, link the three LINE leads of the stage rotation meter to the inbound cable (Picture above). Turn on the power and observe the zero-center voltmeter. If the meter is pointing in the CORRECT direction, switch off the power and label the line leads A, B, or C to correspond with the LINE leads of the phase rotation meter.

If the voltmeter is pointing in the INCORRECT direction, switch off the power and alter any both of the leads from the phase rotation meter. When the power is turned on, the voltmeter must point in the CORRECT direction. Turn off the power and tag the line leads A, B, or C to correspond with the leads from the stage rotation meter.
Now that the motor T leads and the incoming power leads have been labeled, connect the line lead labeled A to the T lead identified A, the line lead identified B to the T lead labeled B, and the line lead identified C to the T lead identified C. When power is linked to the motor, it will run in the appropriate motor rotation.

Rotating Magnetic Field

The operating principle for all three-phase motors is the rotating magnetic field. There are 3 elements that trigger the magnetic field to turn. These are:

1. the fact that the voltages in a three-phase system are 120° out of phase with each other.
2. the fact that the three voltages alter polarity at routine intervals.
3. the arrangement of the stator windings around the inside of the motor.

3 phase operating principles and rotating magnetic field

rotating magnetic field 2 Rotating Magnetic Field

The image above shows 3 AC sine waves 120° out of phase with each other, and the stator winding of a three-phase motor. The stator illustrates a two-pole three phase motor. 2 pole implies that there are 2 poles per phase. AC motors do not normally have actual pole pieces like this image, but they will be utilized below to aid in comprehending how the rotating magnetic field is developed in a three-phase motor.

Notice that pole pieces A1 and A2 are found opposite each other. The exact same is true for poles B1 and B2 and C1 and C2. Pole pieces A1 and A2 are wound in such a manner that when current flows with the winding they will develop opposite magnetic polarities. This is likewise true for poles B1 and B2 and C1 and C2. The windings of poles B1 and C1 are wound in the exact same direction in relation to each other, but in opposite instructions from the winding of pole A1. The beginning end of the winding for poles A1 and A2 is connected to Line 1, the beginning end of the winding for poles B1 and B2 is linked to Line 2, and the beginning end of the winding for poles C1 and C2 is connected to Line 3. The finish ends of all three windings are joined to form a wye connection for the stator.

rotating magnetic field 3 Rotating Magnetic Field

To understand exactly how the magnetic field turns around the inside of the stator (image above). A dashed line labeled A has been drawn with the 3 sine waves of the three-phase system. This line is utilized to show the condition of the 3 voltages at this point in time. The arrows drawn inside the motor indicate the greatest concentration of magnetic lines of flux; the arrows are pointing in the direction that shows magnetic lines of flux from north to south. Line 1 has actually reached its maximum peak voltage in the favorable direction and Lines 2 and 3 are less than maximum and in the adverse direction. The magnetic field is focused in between poles A1 and A2. Weaker lines of magnetic flux likewise exist in between poles B1 and B2 and C1 and C2. Also note that poles A1, B1, and C1 are all a south magnetic polarity. Poles A2, B2, and C2 form a north magnetic polarity.

In the image above, line B is drawn at a time when the voltage of Line 3 is zero and the voltages of Lines 1 and 2 are less than optimum but opposite in polarity. The magnetic field is now concentrated in between the pole pieces of phases A and B. Phase C has no current flow at this time and therefore no electromagnetic field.

Line D indicates when Line 1 is zero and Lines 2 and 3 are less than max and opposite in polarity (image above). The electromagnetic field is now concentrated between the poles of stages B and C. At the end of one total cycle the magnetic field finishes a complete 360° of rotation. The speed of the turning magnetic field is 3600 rpm in a two-pole motor connected to a 60-Hz line.