Induction Motor Interview Questions and Answers- Q 41-50
Q.41 What is the condition for maximum torque in the induction motor?
The rotating magnetic field generated by the stator (the rotational speed is the synchronous rotational speed n1) and the relative motion of the rotor winding, the rotor winding cutting magnetic induction line generates an induced electromotive force, thereby generating an induced current in the rotor winding. The induced current in the rotor winding interacts with the magnetic field to generate electromagnetic torque that causes the rotor to rotate. Since the induced current gradually decreases as the rotor speed gradually approaches the synchronous speed, the generated electromagnetic torque also decreases accordingly. When the asynchronous motor operates in the motor state, the rotor speed is less than the synchronous speed.
Q.42 Why is a star delta starter preferred with an induction motor?
Star Delta Motors is preferred as a method of starting in Low starting Torque Applications. The starting current of Induction motors is six to seven times the full load current. Starters are employed to reduce the starting current.
A star Delta Starter reduces the starting current by connecting the motor winding in Star at the time of starting. This reduces the voltage across the winding. The voltage across the winding will be( 1/Sqrt(3))*Line Voltage 58.7% of the line voltage. After completion of starting time set in the timer, the winding will be connected in Delta. The voltage across the winding will be equal to the line voltage. Since the Torque is Proportional to the Square of voltage. A high starting torque is not possible.
Q.43 What is the difference between a squirrel cage motor and a wound-rotor (slipring) induction motor?
In a squirrel cage induction motor, the rotor has conductive bars embedded in slots in the laminations, running the length of the rotor. These bars are shorted at each end with shorting rings. There is no electrical connection between these bars and the motor terminal box. The current in the rotor bars is a function of the motor slip, and we have no direct control over it.
In a wound rotor induction motor, the rotor is wound as a 3-phase winding with magnet wire, the ends of which are brought out to three slip ring mounted on the motor shaft. Spring-loaded brushes connect to these slip rings and in turn, are connected to the motor terminal box. We can control the current induced in the rotor winding by using external resistors in the rotor circuit. This has the advantage of reducing starting current and increasing starting torque. Once the motor is up to speed, the external resistors are shorted out and the motor now has operating characteristics similar to those of a squirrel cage motor.
Q.44 Why does induction motor draw heavy current at starting?
An induction motor during start behaves like a short-circuited transformer. The stator of the I.M. is the primary where supply is given whereas the rotor is the secondary where current flows due to magnetic induction called Transformer Action.
Now, when supply is given to the stator winding, it draws a high current at constant voltage as the winding is short-circuited. This high current through the stator turns to generate a magnetic field that links with the rotor conductors. As the winding is distributive, the rotor begins rotation to oppose the air gap flux.
Q.45 What are the applications of a slip ring induction motor?
The slip ring induction motor is used for driving the loads which demand higher starting torque. The starting torque of the motor gets increased because of the increased rotor resistance.
The external resistance is added to the rotor conductor and the resistance is reduced as the motor is accelerated. When the motor accelerates up to its base speed the external resistance is short-circuited and the motor is equivalent to a squirrel cage induction motor.
The slip ring induction motor is used for driving the rotary kiln, bucket elevator, etc.
Q.46 What is cogging in an induction motor?
If the number of the stator slots is equal to the rotor slots or integral multiples of rotor slots, the motor may refuse to deliver the toque because of the magnetic locking between the stator teeth and rotor teeth caused by minimum reluctance.
That is why the number of the stator slots and the rotor slots are kept unequal. If the harmonic frequency coincides with slot frequency it causes torque modulation and it can create the condition of magnetic locking. The phenomenon of refusal of starting of the motor is known as cogging or the magnetic locking of the induction motor.
Q.47 What happens if an induction motor runs above its rated voltage?
Q.48 What will happen if we decrease the voltage and increase the frequency for an induction motor?
The flux in the motor is proportional to the ratio of volt/Hz. By decreasing the voltage and increasing the frequency in the same proportion keeps the V/f ratio constant, therefore flux in the motor remains constant. The speed will increase and power will also increase. The motor operates in the constant torque variable power mode.
Case 1: The single phasing when the motor is already running
The torque delivering capacity of the motor suddenly decreases with single phasing.
The net torque (T)= Tp-TN
Tp- Torque produced by the positive sequence current
Tn- Torque produced by the negative sequence current
If the load on the motor is more the motor can’t deliver the torque and it will trip with overloading.
If the motor is lightly loaded the motor may keep on running on single phasing. However, if the unbalance trip element(I2) is in the circuit the motor will trip with the current unbalance fault.
Case2: The motor is started on a single phasing
The motor will trip with overloading as the net torque produced is less because of the negative torque produced by the negative phase sequence current. Also, the motor draws a large current when it is started.
The operation of the motor on single phasing must be avoided as this case may cause the failure of the motor.
The motor will get tripped with I2( negative phase sequence fault). The heating in the motor will increase because of the negative sequence current caused by unbalancing voltage. The torque delivering capacity of the motor gets reduced with single phasing.
Q.50 If We use only 2 phases out of 3, will a 3-phase motor run?
This is the case of the single phasing of the induction motor. The balanced three-phase supply is a must for the idle operation of an induction motor. Any unbalance in voltage or missing of one of the phases causes unbalance stator current.
The unbalance current is the main cause of motor burning. The single phasing is the extreme case of voltage unbalances. The unbalance in voltage cause the production of the negative torque which counter opposes the main torque. Thus the negative torque draws current from the unbalance voltage and does not contribute to useful torque delivery. The increased current in the rotor causes additional heating and if the motor does not trip with single-phase detection through the protective relay, the motor is apt to fail.