Detailed Solution. For a constant back emf, flux is inversely proportional to the speed of the motor. If field winding is disconnected accidentally, the speed would dangerously increase in order to maintain the back emf of the motor.
Why is it dangerous to lose the field of a shunt motor?
Now cutting off the field winding is dangerous for shunt dc motor. As for shunt motor, speed is inversely proportional to field current, so cutting off the field winding results in dangerously high speed. Thus motor gets damaged.
What will happen if the field of the DC motor is opened accidentally in no load and full load?
The field will be reduced to only to the value of residual flux. The speed will be very high. The parts of motor may even fly apart.
Why DC shunt motor should not be started direct on line?
Direct-on-line (DOL) starting are not used for large DC motor because the starting current will be enormously high which will cause large voltage drop in the supply mains and armature may get damaged due to excessive heating. … DC shunt motor with armature resistance.What will happen if the DC shunt motor running on no load has its shunt field winding opened accidentally?
Ans: if a shunt motor, running at no load has its shunt field opened or burnt out, the back emf will fall to nearly zero . the armature current will continuously increase the speed of the motor until its parts fly apart.
Why do motor operators worry about losing the field current?
If the field current is reduced the magnetic field will drop causing the back EMF voltage to drop. This increase in the drive voltage to the armature will cause the motor to speed up until the back EMF is once again at its previous level.
What will happen if the field circuit breaks down?
Motor runs at dangerously higher speeds before its armature get melt due to higher currents. Originally Answered: What would happen if a field circuit opened during the starting up of a DC motor? The DC Motor does not run then there is no back EMF. Therefore there is no opposing force for input armature current.
What is the effect of inserting resistance in the field circuit of a DC shunt motor on its speed and torque?
In this method, speed variation is accomplished by means of a variable resistance inserted in series with the shunt field. An increase in controlling resistances reduces the field current with a reduction in flux and an increase in speed.What will happen to a DC shunt motor if by accident the field circuit suddenly loses supply?
speed will be dangerously high or run away condition occurred.
Why DC shunt motor should be started with load?When the motor is connected across the supply mains without load, it draws small current from the supply mains. … The increase in back emf weakens the armature current and hence the field current. This cause again increases the speed so the back emf.
Article first time published onWhich motor should not be started on load?
This can be seen from the speed-armature current and the speed-torque characteristics that on low armature current and low torque condition motor shows a tendency to rotate with dangerously high speed. This is the reason why DC series motor should never be started or Operated on light loads or No load conditions.
Why DC series motor should never be started on no load?
Thus from equation (2), it is clear that speed of DC Series Motor is inversely proportional to Armature Current Ia. … Therefore if the DC Series Motor is started at No Load then the speed of DC Series Motor will become dangerously high which will definitely damage the DC Series Motor.
What is the effect in a DC shunt motor if its supply terminals are interchanged?
When both (the direction of the field as well as that of armature current) are reversed the direction of rotation will not change. So in the case of DC shunt motor, if the supply terminals are interchanged, there is no change in the direction of rotation.
What will happen if DC shunt motor is connected across AC supply?
1. What will happen if DC shunt motor is connected across AC supply? Explanation: In case of parallel field connection, it won’t rotate at all and will start humming and will create vibrations, as a torque produced by positive and negative cycle will cancel out each other. DC motor will be heated up and it may burn.
What will happen if the back emf of a DC motor vanishes suddenly?
Detailed Solution. If back emf of a dc motor vanishes suddenly, motor circuit will try to retain back emf by drawing more current from supply. As the back emf vanishes zero, the whole supply voltage appears across armature and heavy current flows.
What is the function of the field loss relay?
Field Failure Relay, also known as Field Loss Relay protects the D.C. Shunt wound motor or main circuit, under open field conditions, or in the absence of the field supply. Speed of the Shunt Wound D.C. Motor increases as the field supply is weakened.
What would happen to the DC shunt motor if it is started with full load attached?
On adding a load to the shaft, the motor does slow down initially, but this is where the concept of self regulation comes into the picture. At the very onset of load introduction to a shunt wound DC motor, the speed definitely reduces, and along with speed also reduces the back emf, Eb.
What is field circuit in DC motor?
A field coil is an electromagnet used to generate a magnetic field in an electro-magnetic machine, typically a rotating electrical machine such as a motor or generator. It consists of a coil of wire through which a current flows.
How do you control the speed of a DC shunt motor?
In the shunt motor, speed can be controlled by connecting a variable resistor Rc in series with the shunt field winding. In the diagram below resistor, Rc is called the shunt field regulator. Figure: (a) Speed control of a D.C. shunt motor by variation of field flux.
What is the effect of a shunt motor when the field current is decreased?
Magnetic flux in turn is related to field current by the magnetisation characteristics. Hence speed of DC Shunt motor increases with decrease in field current.
When motor runs on no load then?
When the motor is connected across the supply mains without load, it draws small current from the supply mains. This current will flow through the series field and armature, the speed tends to increase so that back emf may approach the applied voltage in magnitude.
What is DC shunt generator?
A shunt generator is a type of direct current electric generator in which field winding and armature winding are connected in parallel, and in which the armature supplies both the load current and the field current.
What would happen if a series resistance is inserted in the field circuit of a DC?
In DC series motor, with increase in resistance of armature circuit increases the voltage drop in armature circuit and series field increases and therefore back emf decreases. Hence speed of DC series motor will reduce by inserting resistance in the armature circuit.
What would happen if a series resistance is inserted in the field circuit of a DC shunt motor at a constant supply voltage?
For a constant supply voltage, what are the effects of inserting a series resistance in the field circuit of a dc shunt motor, on its speed and torque? (a) Speed will decrease and the torque will decrease.
What would happen if a series resistance is inserted in the field?
If a resistance is added in series with field winding of d.c. shunt motor, then its speed increases, torque decreases.
Which type of DC motor should never be started with out connecting load?
A series motor should never be started at no load. With no mechanical load on the series motor, the current is low, the counter-EMF produced by the field winding is weak, and so the armature must turn faster to produce sufficient counter-EMF to balance the supply voltage. The motor can be damaged by overspeed.
Can we start the DC shunt motor and series motor without load?
You can start a DC shunt motor without a load, but never start a DC shunt motor with heavy load because, The change in field flux is very less due to the load, so we can assume the flux to be constant. So flux ɸ is constant, and the speed, N ∝ Eb where Eb is the back emf.
Which motor should always started with some value of load?
At no load, it will rotate at dangerously high speed 2. It will fail to start 3. It will not develop high starting torque 4.
Is the speed of a DC shunt motor is increased the back emf of the motor will?
So, the back emf is directly proportional to speed. Hence, when the speed of a DC shunt motor is increased, then the back emf of the motor will increase.
Why field winding of a DC series motor is provided with thick wire?
To elaborate: In DC series motor, the cross-sectional area of wire used for field coils has to be large because the in this case field coils carry the load or armature current. As the cross-sectional area is large, the resistance is minimum and series voltage drop is also minimum.
What happens if field is open in DC motor?
Answer: n d.c shunt motor if field winding supply is removed the motor back emf will drop to zero, hence the armature will draw more current to make up the emf. this results in blowing up of fuse. In dc series motor the motor will go to infinite speed (theoretically) as flux is inversely proportional to the speed…
