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What happens to the DC motor when it loses its field current?
Since D.C. motor field loss can cause a hazardous runaway or overspeed condition, loss of field relays are connected in parallel with the motor’s field to sense field current. When the field current decreases below a set point, the relay will deenergize the motor’s armature.
How will the armature current and the output power of a DC shunt motor change with increasing load?
If the load on the motor is increased, the armature rotation slows and back EMF is reduced, since back EMF is proportional to speed. With less back EMF voltage and a constant supply voltage (E), the net voltage increases. The increase in net voltage results in an increase in armature current.
What will happen if both the field current and armature current of DC shunt motor are reversed?
When both (the direction of field as well as that of armature current) are reversed the direction of rotation will not change. So in case of DC shunt motor, if the supply terminals are interchanged, there is no change in direction of rotation. control the field flux.
What will happen with the increase in speed of a DC motor?
What will happen, with the increase in speed of a DC motor? a) Back emf increase but line current falls. 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.
What will happen if field winding is disconnected accidentally from rotating DC shunt motor?
If field winding is disconnected accidentally, the speed would dangerously increase in order to maintain the back emf of the motor. For a constant back emf, flux is inversely proportional to the speed of the motor.
Can we change the direction of rotation of DC shunt motor?
The direction of rotation of DC shunt motor can be reversed by reversing either field terminals or armature terminals but not both. The torque can be changed by changing either field flux or armature current. Hence the motor will rotate in the same direction by changing the supply connections.
What do you mean by zero load on a DC shunt motor?
Armature current of any DC motor depends upon load. At no load condition, load on the motor is zero hence armature current(Ia) is also zero. From equation (i) (If=Ia) hence field current(If) will also=0 at no load condition.
What happens when the shunt field is lost in a compound DC motor?
In compound DC motors, if the shunt field is lost, there is little to restrict the armature from reaching extremely high speeds. As a result of a lost shunt field, the armature and the series field see a tremendous increase in current.
How to calculate the speed of a shunt wound?
From a mathematical standpoint.. the motor speed (N) equals to the ratio of motor constant times current applied minus current lost to field flux. N = K (I-VR) / phi .. as the field is opened and the flux collapses the armature current is becoming increasingly high, and as such the motor speed N is increasingly proportionally.
What happens when you lose field current in a DC machine?
Losing field current in a DC machine 4. 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. The penalty for this speed increase is loss of torque – with such a low field magnetic field the motor will be quite weak.
How is a Shunt winding different from a series winding?
As you know a shunt motor has its field winding connected in parallel w/ the armature winding. It is a high resistance winding (about 10 ohms) and as such accepts a lower amount of the applied current. The shunt winding does not saturate as quickly as a low resistance heavy gauge series winding.. as in a series motor.