EMF Equation for DC Machine
DC machine may be either works as a dc motor or dc generator. EMF equation is important for both. In case of DC generator an induced emf is called generated emf and in case of DC motors generated emf is called Back or Counter emf The DC generator is rotate with the help of prime mover. Prime mover is directly coupled to the generator. When armature of dc generator rotates its conductor cuts by the magnetic flux that is produce in field winding and this results of an emf is induced in it. The induced emf depends upon the type of winding of dc generator whether it is wave or lap. In this article, we will drive the emf equation for dc machine.
Φ = flux per pole in weber
Z = total number of armature conductor
= number of slots * number of conductors per slot
P = number of parallel paths in armature
N = rotation speed of the armature in revolution per minute (r.p.m)
E = emf induced in any parallel path in armature
Eg = Generated emf
Flux cut by one conductor in one revolution = PΦ wb
Time to complete one revolution, t = 60/N seconds.
Average induced emf in one conductor,
The number of conductors connected in series in each parallel path = Z/A
The emf generated across terminals,
E = Average emf induced in one conductor * the number of conductors connected in series in each parallel path
Number of parallel paths in wave winding, A = 2
Number of parallel paths in lap winding, A = P
Conclusions from EMF Equation for DC Machine
- In above equation, poles remain constant.
- The emf induced in the armature is directly proportional to the flux per pole and speed.
- The polarity of induced emf in armature is depends upon the connections of field winding and the direction of rotation. If reversed the connection of field winding and direction of rotation, the polarity induced emf will change.
- The induced emf is fundamental phenomenon to all dc machines either it is operated as generator or motor.
- When the machine is operating as a generator, this induced emf is called the generated emf Eg.
- When the machine is operating as a motor, this induced emf is called back or counter emf, Eb. This emf plays an important role in dc motor. By using this equation we can find back emf.
- Wave winding is used for high voltage and low current applications and where as Lap winding used where large current at low voltage is generated.