- #1

niko_bellic

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In a 3-phase squirrel cage induction machine, why the rotor terminals are short circuited?

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In summary, the rotor terminals are short circuited on a 3-phase squirrel cage induction machine because we want a large induced current to flow in these windings. This current flow produces the magnetic field which makes the rotor turn. Unlike a dc brush motor, there are no windings on the rotor. The only way the stator magnetic field can produce a torque on the rotor is when there are induced currents in the squirrel cage. The are no induced currents in the rotor itself because it is laminated. The currents are induced in the squirrel cage only when the rotor RPM slips relative to the synchronous RPM.

- #1

niko_bellic

- 3

- 0

In a 3-phase squirrel cage induction machine, why the rotor terminals are short circuited?

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- #2

vk6kro

Science Advisor

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This current flow produces the magnetic field which makes the rotor turn.

- #3

Bob S

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Bob S

- #4

m.s.j

- 215

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Shorting of rotor bars in form of squirrel cage cause the suitable low impedance path for rotor induced currents. The impedance characteristics (R & X) of rotor winding infects the motor performance, starting torque, and total losses consequently motor efficiency.

When the rotor is stationary, the revolving magnetic field cuts the short-circuited secondary conductors at synchronous speed and induces in them line-frequency currents. To supply the secondary IR voltage drop, there must be a component of voltage in time phase with the secondary current, and the secondary current, therefore, must lag in space position behind the revolving air-gap field. A torque is then produced corresponding to the product of the air-gap field by the secondary current times the sine of the angle of their space-phase displacement.

At standstill, the secondary current is equal to the air-gap voltage divided by the secondary impedance at line frequency. As the rotor speeds up, with a given air-gap field, the secondary induced voltage and frequency both decrease in proportion to s. Thus, the secondary voltage becomes sE_{2}, and the secondary impedance R_{2}+jsX_{2} .

The maximum torque is approximately equal to E^{2}/2X. This gives the basic rule that the percent maximum torque of a low-slip polyphase motor at a constant impressed voltage is about half the percent starting current.

By choosing the value of R_{2}, the slip at which maximum torque occurs can be fixed at any desired value. The maximum-torque value itself is affected, not by changes in R_{2}, but only by changes in X and to a slight degree by changes in X_{M}.

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Creative thinking is breezy, Then think about your surrounding things and other thought products. http://electrical-riddles.com

When the rotor is stationary, the revolving magnetic field cuts the short-circuited secondary conductors at synchronous speed and induces in them line-frequency currents. To supply the secondary IR voltage drop, there must be a component of voltage in time phase with the secondary current, and the secondary current, therefore, must lag in space position behind the revolving air-gap field. A torque is then produced corresponding to the product of the air-gap field by the secondary current times the sine of the angle of their space-phase displacement.

At standstill, the secondary current is equal to the air-gap voltage divided by the secondary impedance at line frequency. As the rotor speeds up, with a given air-gap field, the secondary induced voltage and frequency both decrease in proportion to s. Thus, the secondary voltage becomes sE

The maximum torque is approximately equal to E

By choosing the value of R

--------------------------------

Creative thinking is breezy, Then think about your surrounding things and other thought products. http://electrical-riddles.com

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A squirrel cage rotor is short circuited in order to produce the rotating magnetic field necessary for the operation of an induction motor. Without the short circuit, the rotor would not be able to interact with the stator's magnetic field, and the motor would not function.

A short circuited squirrel cage rotor means that the rotor bars are connected together by end rings, creating a closed loop. This allows for the flow of current and the creation of the rotating magnetic field.

No, the material used for the rotor bars and end rings must have low resistance in order for the short circuit to be effective. Copper is the most commonly used material for its high conductivity.

The main advantage of a short circuited squirrel cage rotor is its simplicity. There are no additional moving parts, such as brushes or slip rings, which helps to reduce maintenance and increase reliability. It also allows for a more rugged design, making it suitable for harsh environments.

One disadvantage of a short circuited squirrel cage rotor is that the speed cannot be easily controlled. This can make it unsuitable for applications that require precise speed control. Additionally, the starting torque may be lower compared to other types of rotors, such as wound rotors with external resistors.

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