2l Single Phase Im.pdf
This document was uploaded by user and they confirmed that they have the permission to share it. If you are author or own the copyright of this book, please report to us by using this DMCA report form. Report DMCA
Overview
Download & View 2l Single Phase Im.pdf as PDF for free.
More details
- Words: 2,193
- Pages: 28
Lecture 2: Third Class Electrical Engineering Department College of Engineering University of Kerbala
2018/2019 By
Dr. Ali Altahir
Equivalent Circuit of 1-Phase Induction Motor ο It was stated earlier that when the stator of a single-phase I.M is connected to a single-phase supply, the stator current produces a ΙΈ pulsating flux that is equivalent to two-constant-amplitude fluxes π 2 revolving in opposite directions at the synchronous speed (double-field revolving theory). ο Each of these fluxes induces currents in the rotor circuit and produces induction motor action. Therefore, a single-phase I.M can imagine to be consisting of two motors, having a common stator winding, but with their respective rotors revolving in opposite directions. ο Each rotor has resistance and reactance half the actual rotor values. ο The two equivalent circuits are connected in series.
ο The current, power and torque can be calculated from the combined equivalent circuit using the Ohm Law. ο The equivalent circuit of a single-phase I.M in running condition is shown in the following Figure
Equivalent Circuit of a 1-Phase induction motor ο Input power: P in = Is * Vs
output power:
Equivalent Circuit of a Single-I.M In Running Condition.
Starting Torque for a Single - Phase I.M The single phase I.M are classified based on starting method: 1- Split-Phase Induction Motor ο The stator of a split-phase I.M is provided with an auxiliary or starting winding, S in addition to the main or running winding, M. ο The starting winding is located 90Β° electrical from the main winding and operates only during the brief period when the motor starts up. ο The two windings are so re-signed that the starting winding S has a high resistance and relatively small reactance while the main winding M has relatively low resistance and large reactance to be as inductance (the current delay with voltage) to make shifting current as shown in the schematic connections in Fig.
Single β Phase Induction Motor Applications
ο Consequently, the currents flowing in the two windings have reasonable phase difference (25Β° to 30Β°) as shown in the phasor diagram this shifting in current its necessary for starting torque.
Principle of Operation for a Split-Phase I.M (i) When the two stator windings are energized from a single-phase supply, the main winding carries current Im while the starting winding carries current Is. (ii) Since main winding is made highly inductive while the starting winding highly resistive, the currents Im and Is have a reasonable phase angle a (25Β° to 30Β°) between them. Consequently, The starting torque is given by; Ts = k * Im * Ia sin (Ο) where k is a constant whose magnitude depends upon the design of the motor. ο When the motor reaches about (70 β 80) % of the synchronous speed, the centrifugal switch opens the circuit of the starting winding. The motor then operates as a single-phase I.M and continues to accelerate till it reaches the normal speed. The normal speed of the motor is below the synchronous speed and depends upon the load on the motor.
Split β Phase I.M Characteristics
(i) The starting torque is 2 times the full-loud torque. (ii) Due to their low cost, split-phase I.Ms are most popular single phase motors in the commercial markets. (iii) Since the starting winding is made of fine wire, the current density is high and the winding heats up quickly. ο If the starting period exceeds 5 seconds, the winding may burn out unless the motor is protected by built-in-thermal relay. This motor is, therefore, suitable where starting periods are not large. ο An important characteristic of these motors is that they are essentially constant-speed motors. The speed variation is 2-5% from no-load to full load. (a) Fans (b) Washing machines (c) Oil burners (d) Small machine tools etc. The power rating of such motors generally lies between 60 W and 250 W.
2- Capacitor-Start Induction Motor ο The capacitor-start I.M is identical to a split-phase IM except that the starting winding has a capacitor. ο Moreover, a capacitor value bounded between C (3 - 20 ΞΌF) is connected in series with the starting winding. The value of capacitor is so chosen that Is leads Im by about 80Β° which is considerably greater than 25Β° found in split-phase motor. Consequently, the starting torque is, Ts = k * Im * Ia sin (Ο) is much more than that of a split-phase motor. ο Again, the starting winding is opened by the centrifugal switch when the motor attains about 80% of synchronous speed. The motor then operates as a single-phase I.M and continues to accelerate till it reaches the normal speed.
Capacitor βstart I.M Characteristics (i) The starting characteristics of a capacitor-start I.M are better than those of a split-phase I.M, both machines possess the same running characteristics because the main windings are identical. (ii) The phase angle between the two currents is about 80Β° compared to about 25Β° in a split-phase IM. Consequently, for the same starting torque, the current in the starting winding is only about half that in a split-phase motor. Therefore, the starting winding of a capacitor start I.M heats up less quickly and is well suited to applications involving either frequent or prolonged starting periods. (iii) Capacitor-start IM are used where high starting torque is required and where the starting period may be longe. Applications: (a) Compressors (b) Large fans (c) Water pumps (d) High inertia loads The power rating of such motors lies between 120 W and (7-5) kW.
Principle of Operating
3- Capacitor-Start Capacitor-Run Motor
οThis motor is identical to a capacitor-start IM except that starting winding is not opened after starting so that both the windings remain connected to the supply when running as well as at starting. Two designs are generally used. (i) In first design, a single capacitor C is used for starting and running. This design eliminates the need of a centrifugal switch and at the same time improves the power factor and efficiency of the motor. (ii) In second design, two capacitors C1 and C2 are used in the starting winding as shown in Figure below. οThe smaller capacitor C1 required for optimum running conditions is permanently connected in series with the starting winding. The much larger capacitor C2 is connected in parallel with C1 for optimum starting and remains in the circuit during starting. The starting capacitor C2 is disconnected when the motor tends 80 % of synchronous speed. The motor then runs as a single-phase IM.
Capacitor start β capacitor run induction motor
Characteristics (i) The starting winding and the capacitor can be designed for perfect 2-phase operation at any load. The motor then produces a constant torque and not a pulsating torque as in other single-phase motors. (ii) Because of constant torque, the motor is vibration free and can be used in: (a) hospitals (6) studios and (c) other places where silence factor is important index.
Academic Example A 250W, 230 V, 50 Hz capacitor start I.M has the following impedances at standstill. Main winding, Zm = 7 + j5 . Auxiliary winding, Za = 11.5 + j5 Ξ©. Find the value of capacitor to be connected in series with the auxiliary winding to give theoretical maximum starting torque between the currents in two windings. Draw the circuit and phasor diagram for I.M.
Solution: Let Xc be the capacitive reactance to be connected with auxiliary winding at start, Za = 11.5 + j (5-Xc ) Ξ© Zm = 7 + j5 Ξ© = 8.6023 Now Ia and Im must have a phase difference of 90o. Im will lag the voltage by 35.5376o hence Ia must lead the voltage by (90o- 35.5376o ) i.e. 53.4624o . ...
The phase angle of Za is, Ξ¦a = tan-1((5 - Xc )/11.5) = - 53.4624o As leads, the phase angle must be negative, hence taken as tan(-53.4624o ) = (5 - Xc )/11.5 i.e. -1.34956 = (5 - Xc )/11.5 ... Xc = 20.52 Ξ© = 1/(2ΟfC) ... C = 1/(2Ο x 50 x 20.52) = 155.1217 ΞΌF
Academic Example
Academic Example
Academic Example
Academic Example
4 - Shaded-Pole Induction Motor οThe shaded-pole I.M is very popular for ratings below 0.05 H.P. (~ 40 W) because of its extremely simple construction. οIt has salient poles on the stator excited by single-phase supply and a squirrel cage rotor as shown in Figure below. οA portion of each pole is surrounded by a short-circuited turn called shading coil ( or shading ring ).
Principle of Operation for shaded pole I. M ο ο ο ο ο
The main winding produces a pulsating flux that links with the squirrel cage rotor. This flux induces a voltage in the shorted winding The induced voltage produces a current in the shorted winding. This current generates a flux that opposes to the main flux in the shaded pole. The result is that the flux in the un-shaded and shaded parts of the pole will be unequal. ο These two fluxes generate an unbalanced rotating field. The field amplitude changes as it rotates. ο This rotating field produces a torque, which starts the motor in the direction of the shaded pole. It may be seen that the motor is self-starting unlike a singlewinding, single-phase motor.
Characteristics (i) The salient features of this motor are extremely simple construction and absence of centrifugal switch. (ii) The starting torque is small, but it is sufficient for fans and other household equipment requiring small starting torque. (iii)The motor efficiency is poor, but it is cheap cost. (iv) Since small starting torque, efficiency and power factor are very low, these motors are only suitable for low power applications e.g., to drive: (a) small fans (b) toys (c) hair driers (d) desk fans.
Shaded pole I.M for household fan.
Motors Efficiency Comparisons ο When comparing efficiency of an EC motor to an AC shaded pole motor or an AC permanent-split capacitor motor, shaded pole motors have an efficiency range of 15 to 25%, permanent-split capacitors (PSC) range from 30 to 50%, and EC motors have an efficiency of 60 to 75%. ο EC (Electronic Circuit) motors were found to be the most efficient upgrade option for current motor applications.
Blocked Rotor test for Induction Motor ο Blocked rotor test is conducted on an induction motor. It is also known as short circuit test or locked rotor test or stalled torque test. ο From this test, short circuit current, power factor on short circuit, total leakage reactance, starting torque of the motor can be found. ο The test is conducted at low voltage (Why?) Answer: because if the applied voltage was normal voltage then the current density flowing through the stator windings were high enough to over heat the winding and damage (or deterioration) them.
ο The blocked rotor test is not performed on a wound rotor motors because the starting torque can be varied as desired.
0 -10A
0-300V
Some one can press on this shaft to block the rotation
Procedures ο
A low voltage is applied on the stator terminals so that full load current flows in the stator winding.
ο The current, voltage and power input are measured at this point. ο When the rotor is stationary, the slip ,S=1. ο The test is conducted at 1/4th the rated frequency as recommended by IEEE and NEMA Recommendations (Why?). Answer: This is because the rotor's effective impedance at low frequency may differ at high frequency.
ο The test can be repeated for different values of voltage to ensure the values obtained are consistent. ο
As the current flowing through the stator may exceed the rated current, the test should be conducted quickly.
No Load Test of Induction Motor ο By the name no - load test, it means that there is no load- that is load is zero. But it is exact opposite. No - load means infinite load test. It is because in no load there is NO load , and no load means it is open circuit . Open circuit means infinite resistance. ο If slip=0, then load resistance will be infinite and you can make slip zero by making synchronous speed Ns equal to actual rotor speed Nr. ο So, slip will be zero. Load resistance will be infinite. 0-10A
0-300V
No load at output of motor
Procedures ο Connect the circuit.
ο Supply the rated voltage to induction motor, keep it running. ο The current drawn by motor is quit low
ο Take care of the voltmeter should be of voltage ratings of induction motor & the ratings of ammeter should be low because the current drawn by motor is very small. ο Take the readings of voltmeter, ammeter and wattmeter.
Universal Motor with AC Source ο The universal motor, like a series DC motor, has a very high no-load speed that drops rapidly as the load increases. ο Unlike the induction motor variations, the universal motor is not limited to operating below synchronous speed. ο The speed of a universal motor is typically controlled by means of electronic devices. ο Universal motors are used in portable drills, saws, routers, vacuum cleaners, and similar applications.
Torque / Speed C/Cs
2018/2019 By
Dr. Ali Altahir
Equivalent Circuit of 1-Phase Induction Motor ο It was stated earlier that when the stator of a single-phase I.M is connected to a single-phase supply, the stator current produces a ΙΈ pulsating flux that is equivalent to two-constant-amplitude fluxes π 2 revolving in opposite directions at the synchronous speed (double-field revolving theory). ο Each of these fluxes induces currents in the rotor circuit and produces induction motor action. Therefore, a single-phase I.M can imagine to be consisting of two motors, having a common stator winding, but with their respective rotors revolving in opposite directions. ο Each rotor has resistance and reactance half the actual rotor values. ο The two equivalent circuits are connected in series.
ο The current, power and torque can be calculated from the combined equivalent circuit using the Ohm Law. ο The equivalent circuit of a single-phase I.M in running condition is shown in the following Figure
Equivalent Circuit of a 1-Phase induction motor ο Input power: P in = Is * Vs
output power:
Equivalent Circuit of a Single-I.M In Running Condition.
Starting Torque for a Single - Phase I.M The single phase I.M are classified based on starting method: 1- Split-Phase Induction Motor ο The stator of a split-phase I.M is provided with an auxiliary or starting winding, S in addition to the main or running winding, M. ο The starting winding is located 90Β° electrical from the main winding and operates only during the brief period when the motor starts up. ο The two windings are so re-signed that the starting winding S has a high resistance and relatively small reactance while the main winding M has relatively low resistance and large reactance to be as inductance (the current delay with voltage) to make shifting current as shown in the schematic connections in Fig.
Single β Phase Induction Motor Applications
ο Consequently, the currents flowing in the two windings have reasonable phase difference (25Β° to 30Β°) as shown in the phasor diagram this shifting in current its necessary for starting torque.
Principle of Operation for a Split-Phase I.M (i) When the two stator windings are energized from a single-phase supply, the main winding carries current Im while the starting winding carries current Is. (ii) Since main winding is made highly inductive while the starting winding highly resistive, the currents Im and Is have a reasonable phase angle a (25Β° to 30Β°) between them. Consequently, The starting torque is given by; Ts = k * Im * Ia sin (Ο) where k is a constant whose magnitude depends upon the design of the motor. ο When the motor reaches about (70 β 80) % of the synchronous speed, the centrifugal switch opens the circuit of the starting winding. The motor then operates as a single-phase I.M and continues to accelerate till it reaches the normal speed. The normal speed of the motor is below the synchronous speed and depends upon the load on the motor.
Split β Phase I.M Characteristics
(i) The starting torque is 2 times the full-loud torque. (ii) Due to their low cost, split-phase I.Ms are most popular single phase motors in the commercial markets. (iii) Since the starting winding is made of fine wire, the current density is high and the winding heats up quickly. ο If the starting period exceeds 5 seconds, the winding may burn out unless the motor is protected by built-in-thermal relay. This motor is, therefore, suitable where starting periods are not large. ο An important characteristic of these motors is that they are essentially constant-speed motors. The speed variation is 2-5% from no-load to full load. (a) Fans (b) Washing machines (c) Oil burners (d) Small machine tools etc. The power rating of such motors generally lies between 60 W and 250 W.
2- Capacitor-Start Induction Motor ο The capacitor-start I.M is identical to a split-phase IM except that the starting winding has a capacitor. ο Moreover, a capacitor value bounded between C (3 - 20 ΞΌF) is connected in series with the starting winding. The value of capacitor is so chosen that Is leads Im by about 80Β° which is considerably greater than 25Β° found in split-phase motor. Consequently, the starting torque is, Ts = k * Im * Ia sin (Ο) is much more than that of a split-phase motor. ο Again, the starting winding is opened by the centrifugal switch when the motor attains about 80% of synchronous speed. The motor then operates as a single-phase I.M and continues to accelerate till it reaches the normal speed.
Capacitor βstart I.M Characteristics (i) The starting characteristics of a capacitor-start I.M are better than those of a split-phase I.M, both machines possess the same running characteristics because the main windings are identical. (ii) The phase angle between the two currents is about 80Β° compared to about 25Β° in a split-phase IM. Consequently, for the same starting torque, the current in the starting winding is only about half that in a split-phase motor. Therefore, the starting winding of a capacitor start I.M heats up less quickly and is well suited to applications involving either frequent or prolonged starting periods. (iii) Capacitor-start IM are used where high starting torque is required and where the starting period may be longe. Applications: (a) Compressors (b) Large fans (c) Water pumps (d) High inertia loads The power rating of such motors lies between 120 W and (7-5) kW.
Principle of Operating
3- Capacitor-Start Capacitor-Run Motor
οThis motor is identical to a capacitor-start IM except that starting winding is not opened after starting so that both the windings remain connected to the supply when running as well as at starting. Two designs are generally used. (i) In first design, a single capacitor C is used for starting and running. This design eliminates the need of a centrifugal switch and at the same time improves the power factor and efficiency of the motor. (ii) In second design, two capacitors C1 and C2 are used in the starting winding as shown in Figure below. οThe smaller capacitor C1 required for optimum running conditions is permanently connected in series with the starting winding. The much larger capacitor C2 is connected in parallel with C1 for optimum starting and remains in the circuit during starting. The starting capacitor C2 is disconnected when the motor tends 80 % of synchronous speed. The motor then runs as a single-phase IM.
Capacitor start β capacitor run induction motor
Characteristics (i) The starting winding and the capacitor can be designed for perfect 2-phase operation at any load. The motor then produces a constant torque and not a pulsating torque as in other single-phase motors. (ii) Because of constant torque, the motor is vibration free and can be used in: (a) hospitals (6) studios and (c) other places where silence factor is important index.
Academic Example A 250W, 230 V, 50 Hz capacitor start I.M has the following impedances at standstill. Main winding, Zm = 7 + j5 . Auxiliary winding, Za = 11.5 + j5 Ξ©. Find the value of capacitor to be connected in series with the auxiliary winding to give theoretical maximum starting torque between the currents in two windings. Draw the circuit and phasor diagram for I.M.
Solution: Let Xc be the capacitive reactance to be connected with auxiliary winding at start, Za = 11.5 + j (5-Xc ) Ξ© Zm = 7 + j5 Ξ© = 8.6023 Now Ia and Im must have a phase difference of 90o. Im will lag the voltage by 35.5376o hence Ia must lead the voltage by (90o- 35.5376o ) i.e. 53.4624o . ...
The phase angle of Za is, Ξ¦a = tan-1((5 - Xc )/11.5) = - 53.4624o As leads, the phase angle must be negative, hence taken as tan(-53.4624o ) = (5 - Xc )/11.5 i.e. -1.34956 = (5 - Xc )/11.5 ... Xc = 20.52 Ξ© = 1/(2ΟfC) ... C = 1/(2Ο x 50 x 20.52) = 155.1217 ΞΌF
Academic Example
Academic Example
Academic Example
Academic Example
4 - Shaded-Pole Induction Motor οThe shaded-pole I.M is very popular for ratings below 0.05 H.P. (~ 40 W) because of its extremely simple construction. οIt has salient poles on the stator excited by single-phase supply and a squirrel cage rotor as shown in Figure below. οA portion of each pole is surrounded by a short-circuited turn called shading coil ( or shading ring ).
Principle of Operation for shaded pole I. M ο ο ο ο ο
The main winding produces a pulsating flux that links with the squirrel cage rotor. This flux induces a voltage in the shorted winding The induced voltage produces a current in the shorted winding. This current generates a flux that opposes to the main flux in the shaded pole. The result is that the flux in the un-shaded and shaded parts of the pole will be unequal. ο These two fluxes generate an unbalanced rotating field. The field amplitude changes as it rotates. ο This rotating field produces a torque, which starts the motor in the direction of the shaded pole. It may be seen that the motor is self-starting unlike a singlewinding, single-phase motor.
Characteristics (i) The salient features of this motor are extremely simple construction and absence of centrifugal switch. (ii) The starting torque is small, but it is sufficient for fans and other household equipment requiring small starting torque. (iii)The motor efficiency is poor, but it is cheap cost. (iv) Since small starting torque, efficiency and power factor are very low, these motors are only suitable for low power applications e.g., to drive: (a) small fans (b) toys (c) hair driers (d) desk fans.
Shaded pole I.M for household fan.
Motors Efficiency Comparisons ο When comparing efficiency of an EC motor to an AC shaded pole motor or an AC permanent-split capacitor motor, shaded pole motors have an efficiency range of 15 to 25%, permanent-split capacitors (PSC) range from 30 to 50%, and EC motors have an efficiency of 60 to 75%. ο EC (Electronic Circuit) motors were found to be the most efficient upgrade option for current motor applications.
Blocked Rotor test for Induction Motor ο Blocked rotor test is conducted on an induction motor. It is also known as short circuit test or locked rotor test or stalled torque test. ο From this test, short circuit current, power factor on short circuit, total leakage reactance, starting torque of the motor can be found. ο The test is conducted at low voltage (Why?) Answer: because if the applied voltage was normal voltage then the current density flowing through the stator windings were high enough to over heat the winding and damage (or deterioration) them.
ο The blocked rotor test is not performed on a wound rotor motors because the starting torque can be varied as desired.
0 -10A
0-300V
Some one can press on this shaft to block the rotation
Procedures ο
A low voltage is applied on the stator terminals so that full load current flows in the stator winding.
ο The current, voltage and power input are measured at this point. ο When the rotor is stationary, the slip ,S=1. ο The test is conducted at 1/4th the rated frequency as recommended by IEEE and NEMA Recommendations (Why?). Answer: This is because the rotor's effective impedance at low frequency may differ at high frequency.
ο The test can be repeated for different values of voltage to ensure the values obtained are consistent. ο
As the current flowing through the stator may exceed the rated current, the test should be conducted quickly.
No Load Test of Induction Motor ο By the name no - load test, it means that there is no load- that is load is zero. But it is exact opposite. No - load means infinite load test. It is because in no load there is NO load , and no load means it is open circuit . Open circuit means infinite resistance. ο If slip=0, then load resistance will be infinite and you can make slip zero by making synchronous speed Ns equal to actual rotor speed Nr. ο So, slip will be zero. Load resistance will be infinite. 0-10A
0-300V
No load at output of motor
Procedures ο Connect the circuit.
ο Supply the rated voltage to induction motor, keep it running. ο The current drawn by motor is quit low
ο Take care of the voltmeter should be of voltage ratings of induction motor & the ratings of ammeter should be low because the current drawn by motor is very small. ο Take the readings of voltmeter, ammeter and wattmeter.
Universal Motor with AC Source ο The universal motor, like a series DC motor, has a very high no-load speed that drops rapidly as the load increases. ο Unlike the induction motor variations, the universal motor is not limited to operating below synchronous speed. ο The speed of a universal motor is typically controlled by means of electronic devices. ο Universal motors are used in portable drills, saws, routers, vacuum cleaners, and similar applications.
Torque / Speed C/Cs
Related Documents
2l Single Phase Im.pdf
December 2019 25
Single Phase Meter Software
April 2020 17
Single Phase Induction Motor
May 2020 25
3 Single Phase Rectifiers
December 2019 21
2l
April 2020 16
L9 Single Phase Uncontrolled Rectifier
July 2020 12More Documents from ""
2l Single Phase Im.pdf
December 2019 25
Ramadan Preparation
May 2020 12
