Experiment No. 12
Title of the Experiment: –
Speed control of three phase slip ring induction motor at variable rotor resistance.
Objectives: –
To be written by student.
Apparatus Required: –
| Sl. No. | Name | Specifications | Quantity |
|---|---|---|---|
| 1 | Three Phase Slip ring Induction Motor | 1 No. | |
| 2 | Three phase rotor resistance starter | 1 No. | |
| 3 | Voltmeter | (0-500)V, MI | 1 No. |
| 4 | Ammeter | (0-10)A, MI | 1 No. |
| 5 | Tachometer | Digital Type | 1 No. |
| 6 | Connecting Wire | PVC Insulated copper | As per required |
Circuit Diagram: –
Theory: –
Slip ring induction motors (SRIMs) allow external resistance to be added to the rotor circuit, making them well-suited for speed control applications. One common method for controlling the speed of an SRIM is by varying the rotor resistance.
This method is in fact similar to the armature rheostat control method of d.c shunt motors.
Precautions: –
- The motor input current should not exceed its rated value.
Procedure: –
- Connect all apparatus as per the circuit diagram.
- Keep the rotor resistance stator output as zero voltage & the external rotor resistance at minimum resistance position.
- Switch ON the supply & increase the input voltage to stator winding up to its rated value.
- Now increase the rotor resistance in steps & note the corresponding values of speed.
- The rotor resistance is varied and corresponding values of speed, voltage and current are noted down.
- Draw a graph of rotor resistance versus speed.
Observation Table: –
| Sl. No. | V (in Volts.) | I (in Amps.) | R (in Ohms.) | Speed (N) in rpm |
|---|---|---|---|---|
| 1 | ||||
| 2 | ||||
| 3 | ||||
| 4 | ||||
| 5 | ||||
| 6 |
Plot graph:
Speed vs resistance
Conclusion: –
To be written by student.
Some Viva-voce Questions:
Why external resistance is added in slip ring induction motor ?
Answer: Torque is directly proportional to rotor resistance, if we add external resistance in the rotor circuit the starting torque is improved.
When does the induction motor behave as induction generator ?
Answer: When the speed of the rotor is made to rotate above synchronous speed, induction motor will act as induction generator i.e when the slip is negative.
What is single phasing in induction motor ?
Answer: Single phasing is a fault condition in which a 3 phase motor is operating with one line open. The 3 phase motor will not start with one line open. If the motor is running when single phasing occurs, it will continue to run as long as the shaft load is less than 80% rated load and the remaining single phase voltage is normal; rotation of the rotor produces a quadrature field that maintains rotation. If single phasing occurs while operating at or near rated load, the increase in phase current will cause rapid heating of the windings, and therefore protective devices, must be provided to trip the machines from the supply lines, or severe damage to stator and rotor winding may occur.
What are the advantages of addition of external rotor resistance at starting ?
Answer:
- It decreases the starting current,
- Increases the starting torque and
- Improves the starting power factor
Why is an induction motor not capable of running at synchronous speed ?
Answer: When the motor speed reaches synchronous speed, no rotor emf, no rotor current and hence no torque is produced. Hence, the induction motor never attains synchronous
speed.
What are the different starters needed for three phase induction motors ?
Answer: (i)Direct On Line (DOL) starter
ii) Stator impedance (resistance/reactance) starter
iii) Autotransformer starter
iv) Star-Delta Starter
v) Rotor resistance starter (Only for slip ring induction motors)
The rotor core loss of an induction motor under running condition is usually
neglected. Why ?
Answer: During running condition, rotor frequency is equal to slip frequency (=sf) which is
very small. Hence rotor core loss will be very small which can be neglected.
Why are starters needed for induction motors ?
Answer: If rated voltage is applied, large starting current (5 to 8 times full load current) will flow. This causes appreciable voltage drop in the line and may affect other equipment’s connected to the same line. Also, if a large current flows for a long time it may overheat the motor and damage the insulation. In such case, reduced voltage starting must be used.
What are the various methods of speed control of 3 phase induction motor ?
Answer:
(i) stator voltage control
(ii) stator frequency control
(iii) V/f method
(iv) pole changing method
What will happen if the rotor circuit of a slip ring I.M. is kept open & electric supply is given to its stator winding.
Answer: If the rotor circuit of a slip ring induction motor is kept open and the stator is energized, the motor will not start. Since no closed path exists for rotor current, no rotor magnetic field is generated to interact with the stator field, preventing torque production. However, high voltage will be induced across the open slip rings due to transformer action, posing a risk of insulation failure and electric shock. Additionally, the motor may exhibit excessive heating in the stator windings due to high magnetizing current. To avoid damage, the rotor circuit must always be closed before starting the motor.
What is the constructional difference between a slip-ring & a squirrel cage I.M.
Answer: The primary constructional difference between a slip-ring induction motor and a squirrel cage induction motor lies in their rotor design.
- Rotor Construction:
- Slip-Ring I.M.: Has a wound rotor with three-phase windings connected to external resistances via slip rings and brushes. This allows speed control and high starting torque.
- Squirrel Cage I.M.: Has a squirrel cage rotor, consisting of short-circuited copper or aluminum bars embedded in slots and connected by end rings, making it rugged and maintenance-free.
- Slip Rings & Brushes:
- Slip-Ring I.M.: Equipped with slip rings and carbon brushes for external connections.
- Squirrel Cage I.M.: Does not have slip rings or brushes, making it more reliable and low maintenance.
- Performance:
- Slip-Ring I.M.: Suitable for variable speed and high torque applications.
- Squirrel Cage I.M.: Preferred for constant-speed applications due to its simple and efficient design.
Draw & explain the Torque-speed characteristic of a slip ring I.M. for different values of rotor resistance.
What is the effect of changing the rotor resistance on the slip at max torque SMT.
Answer: The torque in an induction motor is given by:
where:
- s= Slip
- E2 = Rotor induced EMF
- Rr = Rotor resistance
- X2 = Rotor reactance
Slip at Maximum Torque (SMT)
The slip at maximum torque is derived from the condition:
Effect of Increasing Rotor Resistance (Rr)
- SMT Increases: Since SMT∝R, increasing rotor resistance shifts the maximum torque point to a higher slip.
- Starting Torque Remains Constant: The maximum torque value does not change, as it is independent of Rr.
- Lower Running Speed: Higher SMT results in greater speed reduction, making the motor suitable for speed control applications.
Thus, increasing Rr increases slip at which maximum torque occurs, allowing better torque control at lower speeds.
Can a I.M. rotate at synchronous speed ? Justify your answer.
Answer: No, an induction motor cannot rotate at synchronous speed. This is because torque in an induction motor is produced by the interaction between the rotating magnetic field of the stator and the induced current in the rotor. If the rotor were to reach synchronous speed, there would be no relative motion between the stator field and the rotor, preventing any EMF induction in the rotor. Without induced EMF, no current would flow in the rotor, and consequently, no torque would be generated. Thus, to maintain torque production, the rotor must always lag slightly behind the synchronous speed, operating with some slip.