Chapter 4 - Circuit Equations & Armature Reaction

Question 1

State the Armature Circuit Equation?

Answer 1

Vt = Eb + IaRa

• Vt = Voltage Total (Supply)
• Eb = e.m.f
• Ia = Armature Current
• Ra = Armature Resistance

Question 2

Using Kirchhoff’s Second Law derive the equation Eb, Vt, Ia, Ra for the speed of a shunt motor?

Answer 2

Speed formula.

Vt = Eb + IaRa

Vt = kNΦ + IaRa

Transposed to the following:

• N = Vt - IaRa
• kΦ

Question 3

Explain why the starting current is high for a DC motor?

Answer 3

Starting current is high due to the non existence of back e.m.f (Eb)

Question 4

Describe how a DC Motor responds to an increasing Load Torque?

Answer 4

When torque developed by the motor exacly balances the load torque the motor will run at a constant speed

When Load Torque increases the DC Motor will slow down.

Question 5

a. ) Describe Armature Reaction?
b. ) and its Effects?

Answer 5

a.)Armature Reaction is the Distortion of the Magnetic Field caused by the Armature Currents. b.) The effect of Armature Reaction causes the Magnetic Neutral Axis (M.N.A) to shift against the Direction Of Rotation (D.O.R). - If the M.N.A has Moved this may cause Sparking to occur Wearing the carbon brushes faster. - pole saturation which decreases total flux which in turn decreases Torque.

Question 6

a. ) How are the Effects of Armature Reaction corrected?
b. ) What Effect is not corrected?

Answer 6

a. ) Two way to correct armature reaction.
1. Shift the brushes to where M.N.A moves to
2. or use Inter-poles
b. ) pole Saturation is the effect which is not corrected.