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…
Can DC machine run without field current?
The motor will try to speed up to develop a back-emf and with no field it requires a very large armature current. That is why one never starts a shunt motor without the field winding excited. … In this case the motor will accelerate and run away until the fuses blow.
What should happen if there is no back EMF?
If there is no back end then large current flows in starting of motors because initial speed is zero and back emf is zero thus winding gets damaged for this purpose only we use starters for all motors. motor will not start. when the motor runs,there has to be back emf. … This is the back emf.
What will happen when the field or armature current of dc motor is reversed?
If either the direction of the field or the direction of current flow through the armature is reversed, the rotation of the motor will reverse. However, if both of these factors are reversed at the same time, the motor will continue rotating in the same direction.
Can a DC motor run on AC power?
The AC series motor you describe is a universal motor, and will work fine on DC. The speed characteristics will be a little different, since when running on AC there’s a significant reactive drop in the field windings that reduces the available driving emf.
Why do we need a starter to start a DC motor?
Starters are used to protect DC motors from damage that can be caused by very high current and torque during startup. They do this by providing external resistance to the motor, which is connected in series to the motor’s armature winding and restricts the current to an acceptable level.
What will happen if the back EMF of dc motor?
a) Back emf increase but line current falls. … Explanation: In case of DC motor, the speed is proportional to the back emf (Ea ∝ N). So, with the increase in speed, the back emf also increases. Therefore, armature current is also decreased, in case of series motor, armature current is equal to the line or load current.
Why back EMF is used in DC motor?
The presence of back emf makes the d.c. motor a self-regulating machine i.e., it makes the motor to draw as much armature current as is just sufficient to develop the torque required by the load. When the motor is running on no load, small torque is required to overcome the friction and windage losses.
Is back EMF good or bad?
Back emf is a measure of energy conversion from electrical to mechanical in an electric motor. If you are able to make it zero no energy conversion to mechanical. … Eb is back emf, I armature current and R armature circuit resistance. So if you want energy conversion back emf is good.
When the speed of DC motor is increased?
The speed of a dc motor is changed by changing the current in the field or by changing the current in the armature. When the field current is decreased, the field flux is reduced, and the counter emf decreases. This permits more armature current. Therefore, the motor speeds up.
How do you reverse a DC motor?
The relative speed of the motor is controlled by adjusting the applied volt age to the series field and the armature. The motor can be reversed by either changing the direction of current in the series field or the armature.
Which DC motor has approximately constant speed?
Hence, a DC shunt motor always maintains a constant speed. Under no load condition, the back emf becomes nearly equal to the applied voltage.
Which is more efficient AC or DC motor?
Consequently, AC motors are less efficient than DC motors. In fact, the DC motor is 30% more efficient than AC motors due to the secondary magnetic field being generated from the permanent magnets rather than copper windings.
How do I know if my DC motor is bad?
To test your DC motor, touch the leads of the ohmmeter to the leads of the motor. The meter’s screen should indicate a low resistance (somewhere between 10 and 30 ohms), but if it reads an infinite ohms or an open circuit you should rotate the end shaft of the motor.
What happens if you plug DC into AC?
If we connect an AC supply to the DC devices and equipment: The positive and negative voltage will destroy (this is not always the case) some of electronic components such as transistors and electrolytic capacitors. In case of higher AC voltage, they may burn with blast and catch fire.