1). What is the 3 Phase Induction Motor’s Working Principle?
A three-phase induction motor, like any other electric motor, consists of a stator and a rotor.Â
The stator has a three-phase winding (stator winding), whereas the rotor has a short-circuit winding (rotor winding). Only the stator winding receives power from a three-phase source.
The rotor winding receives its voltage & power from the externally powered stator winding via electromagnetic induction, hence the name.
The induction motor functions as a “transformer-type” alternating current machine, converting electrical energy into mechanical energy through a revolving secondary.
2). Why is it necessary to Start Three-Phase Induction Motors?
The induction motor is essentially a transformer, with the stator serving as the primary & the rotor as the secondary.Â
At the start, the voltage induced in induction motor rotor is at its maximum (s = 1).
Because the rotor impedance is low, the current is abnormally high. The enormous rotor current is reflected in stator due to transformer action.
This produces a large starting current (4 to 10 times FLA-full-load current) in stator at a low power factor, resulting in a low starting torque value.
Because of short duration, this high current value does not hurt the motor while it accelerates regularly.
3). What effect does the number of poles have on the Speed of the Motor?
The greater the number of poles, the slower the motor, and vice versa.
When a three-phase supply is applied to the stator winding, a spinning magnetic field of the constant magnitude is generated. This revolving field generates currents in the rotor via electromagnetic induction.
4). What is mean by Squirrel Cage Induction Motors?
Induction motors with squirrel cage rotors are known as squirrel cage induction motors.
Most 3-phase induction motors employ a squirrel cage rotor because it is remarkably simple and strong, allowing it to function under the most adverse conditions.
5). What is the effect of starting an induction motor on the connected line?
A strong starting current will result in a large line voltage drop. This will have an adverse impact on the operation of the other electrical devices connected to the same lines.
As a result, it is desired and required to lower the size of stator current during startup, and numerous methods exist to do this.
6). How do Slip Ring Motors start?
Rotor resistance starting is used to start slip ring motors without fail.
7). Does Starting Torque is higher than Full Load Torque?
No, the starting current can be up to 5 times the FLA-full-load current, but the starting torque is only equal to the full-load torque.
As a result, the starting current is quite high, while the starting torque is rather low.
If this high starting current is maintained for an extended period of time, it may cause the motor to overheat causing damage to the insulation.
8). What is “Direct-On-Line (DOL) starting of induction motor”?
The following method of starting is exactly what the name Direct-On-Line (DOL) implies: the motor is started through connecting it directly to a three-phase supply.Â
The motor’s impedance at standstill is quite low, and when connected directly to the supply system, the starting current is considerable (4 to 10 times the full-load current) and has a low power factor.
As a result, this way of beginning is appropriate for relatively modest machines (up to 7.5 kW).
9). What are advantages of Autotransformer starting?
Autotransformer starting provides various advantages, including low power loss, low starting current, and less radiated heat.
This way of starting is commonly utilized for large equipment with more than 25 horsepower.
This method can be applied to both star-delta connected motors.
10). Why is the exciting current of an induction motor so high when compared to a power transformer?
Unlike a power transformer, the magnetic circuit of an induction motor has an air gap.
As a result, the induction motor’s exciting current (3O to 4O% of full-load current) is significantly higher than of the power transformer.
As a result, correct results need the usage of an exact equivalent circuit.
11). Why is Stator Resistance Starting not recommended?
This method has two downsides.
- First, the reduced voltage provided to the motor during starting period reduces the starting torque, which extends the accelerating time.Â
- Second, an extensive amount of power is lost in the early resistances. As a result, this method can only be used to start small motors.
12). What is the advantage of slip-ring induction motors over squirrel cage motors?
- High starting torque and low starting current.
- Smooth acceleration with heavy loads.
- No abnormal heating occurs at startup.
- Good running characteristics once external rotor resistances are removed. Adjustable speed.
13). What are the advantages & disadvantages of three-phase induction motors?
Advantages of Three-Phase Induction Motor:
- The construction is simple and rugged.
- It is quite inexpensive.
- It needs minimal maintenance.
- It has high efficiency & a reasonable power factor.
- It has self-start torque.
Disadvantages of Three-Phase Induction Motor:
- It is essentially a constant-speed motor whose speed cannot be easily adjusted.
- It has a lower starting torque than a direct current shunt motor.
14). What are the primary types of rotor winding used in three-phase induction motors?
The winding placed in such spaces (called rotor winding) can be one of two types:
- Squirrel cage typeÂ
- Wound type
15). How does the transformation ratio of an induction motor differ from that of a power transformer?
The windings in a transformer are concentrated, but those in an induction motor are distributed. This impacts the transformation ratio.
16). What is the Stator Resistance Starting Method?
External resistances are connected in the series with each step of the stator winding during the starting process.
This produces a voltage drop between the resistances, reducing the voltage available across the motor terminals and thus the starting current.
As the motor accelerates, starting resistances are progressively removed in steps (two or more) from the stator circuit.
When the motor reaches its rated speed, the resistances are entirely eliminated & full line voltage is applied to rotor.
17). What material is used to make the rotor of a three-phase motor?
The rotor, which is positioned on a shaft, is a hollow laminated core with slots on the exterior edge.
18). What material is used to make the three-phase induction motor’s stator?
The device has a steel frame and a hollow, cylindrical core built of thin silicon steel laminations to minimize
- Hysteresis Losses &Â
- Eddy Current Losses.Â
Slots are equally distributed throughout the inner perimeter of the laminations.
Insulated conductors are inserted in stator slots and linked to create a balanced 3-phase star (or) delta circuit.
The three-phase stator winding is wound for a specific number of poles based on the required speed.
19). How does a three-phase induction motor produce a rotating magnetic field?
When a 3-phase winding is powered by a 3-phase supply, a spinning magnetic field is generated.
This field’s poles do not remain in a fixed place on the stator, but instead shift about it which is known as a revolving field.
20). Why is magnetic flux stated to rotate at a Synchronous Speed?
If the motor and alternator have the same number of poles, the speed of the revolving magnetic field matches that of the alternator generating electricity. Thus, the magnetic flux is said to rotate at the synchronous speed.
21). Why is the starting torque of the Squirrel cage induction motor is low?
It has the disadvantage of the low starting torque. It is due to the rotor bars are permanently short-circuited, and there is no way to add external resistance to the rotor circuit to provide a high starting torque.
22). What is Synchronous Speed?
The speed at which the revolving flux rotates is known as synchronous speed (NS). Its value is determined by the number of poles & supply frequency.
The formula for calculating revolutions per second (NS) is
NS = (120F)/P
Where
f – Frequency and
NS – Revolutions per Minute.
23). What are the disadvantages of Slip-Ring Motors?
The initial & maintenance expenditures are higher than for squirrel cage motors.
The speed regulation is weak when running with resistance in the rotor circuit.
24). Which motor has both stator & rotor windings connected to the voltage source?
In a direct current motor, both the stator winding (and field winding) and the rotor winding (or armature winding) are connected to the voltage source.
25). How are the rotor windings connected to the supply, and why are wound rotors chosen over squirrel cage induction motors?
The rotor winding distributed uniformly throughout the slots and is often star-connected.
The open ends of the rotor winding are extended and connected to three insulated slip rings installed on the rotor shaft, with one brush resting on the each slip ring.
The three brushes are connected to a 3-phase star-connected rheostat.
External resistances are incorporated in the rotor circuit during startup to provide a high starting torque.
The motor steadily reduces resistances to zero as it gains speed. External resistances are only used during the start-up period.
When the motor reaches normal speed, the 3 brushes short-circuit, causing the wound rotor to operate similarly to a squirrel cage rotor.
