Dear Readers, Welcome to AC and DC Motor Interview Questions and Answers have been designed specially to get you acquainted with the nature of questions you may encounter during your Job interview for the subject of AC and DC Motor. These AC and DC Motor Questions are very important for campus placement test and job interviews. As per my experience good interviewers hardly plan to ask any particular questions during your Job interview and these model questions are asked in the online technical test and interview of many IT & Non IT Industries.
Whenever a current carrying conductor is placed in an magnetic field it produce turning or twisting movement is called as torque.
DC Generators are classified into two types 1)separatly excited DC generator
2)self excited DC generator, which is further classified into; 1)series 2)shunt and 3)compound(which is further classified into cumulative and differential).
DC Series motor has high starting torque. We can not start the Induction motor and Synchronous motors on load, but can not start the DC series motor without load.
The motor which work or act on the applied input pulse in it, is called as stepper motor. This stepper motor is under the category of synchronous motor, which often does not fully depend of complete cycle. It likes to works in either direction related to steps. for this purpose it mainly used in automation parts.
A two phase motor is a motor with the the starting winding and the running winding have a phase split. e.g;ac servo motor.where the auxiliary winding and the control winding have a phase split of 90 degree.
Its Single Phase induction motor which mostly squirrel cage rotor and are capacitor start capacitor run.
1. By using flux control method:in this method a rheostat is connected across the field winding to control the field current.so by changing the current the flux produced by the field winding can be changed, and since speed is inversely proportional to flux speed can be controlled
2.armature control method:in this method a rheostat is connected across armature winding by varying the resistance the value of resistive drop(IaRa) can be varied, and since speed is directly proportional to Eb-IaRa the speed can be controlled.
The shunt connection in four point starter is provided separately form the line where as in three point starter it is connected with line which is the drawback in three point starter.
In simple, synchronous generator supply's both active and reactive power but asynchronous generator(induction generator) supply's only active power and observe reactive power for magnetizing. This type of generators are used in windmills.
Synchronous machines have capability to work on different power factor (or say different imaginary power varying the field EMF. Hence syn. generators r used for the production of electricity.
In DC motors, Voltage equation is V=Eb-IaRa (V = Terminal voltage, Eb = Back emf in Motor, Ia = Armature current,Ra = Aramture resistance). At starting, Eb is zero. Therefore, V=IaRa, Ia = V/Ra ,where Ra is very less like 0.01ohm.i.e, Ia will become enormously increased.
The main advantage of using the star delta starter is reduction of current during the starting of the motor. Starting current is reduced to 3-4 times Of current of Direct online starting.(2). Hence the starting current is reduced , the voltage drops during the starting of motor in systems are reduced.
Series motor cannot be started without load because of high starting torque. Series motor are used in Trains, Crane etc.
The different methods of starting an induction motor:
DOL:direct online starter
Star delta starter
Auto transformer starter
Series reactor starter
Motor Generator Sets are a combination of an electrical generator and an engine mounted together to form a single piece of equipment. Motor generator set is also referred to as a genset, or more commonly, a generator The motor generator set can used in the following different ways:
1.Alternating current (AC) to direct current (DC)
2.DC to AC
3.DC at one voltage to DC at another voltage
4.AC at one frequency to AC at another harmonically-related frequency
Dc series is in the trains to get high starting torque while starting of the trains and operating voltage is 1500v dc.
In thermal power plants dc motors are employed for certain control and critical emergency operations which are supplied by dedicated batteries. DC motors operate as backup drives for normal ac drive systems when ac power supply to the plant is lost.
In thermal power plant, the dc motors finds applications for performing control functions such as
Turbine governor motor
Governor limit setting
Motor operated rheostats
Emergency lubrication for the turbines (main, boiler feed pumps)
Generator (H2 oil seal).
DC motor operated valves
DC motors employed in thermal plants are classified in to two types based on the type of application.
DC motors carrying out Control function
Dc motors carrying out Emergency function
Control functions:This category consists of the turbine governor motor, governor limiting setting, motor operated rheostats, etc. These motors are small, about 1/8 hp or less. They are operated quite infrequently for short duration.
Emergency functions:This category consists of turbine-generator emergency (lubrication) bearing oil pumps and emergency seal oil pumps. Such pumps may also be provided for steam turbine drives of feedwater pumps, fans, and other large loads. The lack of lubrication during a shutdown without ac power will ruin the linings of the bearings and damage the shaft.
Hydrogen seal oil pump is provided to prevent the escaping of hydrogen (for large turbine-generators hydrogen cooling is provided for efficient cooling) from the casing by providing a tight seal with high pressure oil
Advantage or Merits:
One of the major advantage of using synchronous motor is the ability to control the power factor. An over excited synchronous motor can have leading power factor and can be operated in parallel to induction motors and other lagging power factor loads thereby improving the system power factor.
In synchronous motor the speed remains constant irrespective of the loads. This characteristics helps in industrial drives where constant speed is required irrespective of the load it is driving. It also useful when the motor is required to drive another alternator to supply at a different frequency as in frequency changes.
Synchronous motors can be constructed with wider air gaps than induction motors which makes these motors mechanically more stable.
In synchronous motors electro-magnetic power varies linearly with the voltage.
Synchronous motors usually operate with higher efficiencies ( more than 90%) especially in low speed and unity power factor applications compared to induction motors
Disadvantages or Demerits:
Synchronous motors requires dc excitation which must be supplied from external sources.
Synchronous motors are inherently not self starting motors and needs some arrangement for its starting and synchronizing.
The cost per kW output is generally higher than that of induction motors.
These motors cannot be used for variable speed applications as there is no possibility of speed adjustment unless the incoming supply frequency is adjusted (Variable Frequency Drives).
Synchronous motors cannot be started on load. Its starting torque is zero.
These motors have tendency to hunt.
When loading on the synchronous motor increases beyond its capability, the synchronism between rotor and stator rotating magnetic field is lost and motor comes to halt.
Collector rings and brushes are required resulting in increase in maintenance.
Synchronous motors cannot be useful for applications requiring frequent starting or high starting torques required.