- #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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- Thread starter niko_bellic
- Start date

- #1

niko_bellic

- 3

- 0

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

- #2

vk6kro

Science Advisor

- 4,081

- 41

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

- #3

Bob S

- 4,662

- 7

Bob S

- #4

m.s.j

- 215

- 1

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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