# DC Machines Questions and Answers – Armature Reaction -1

This set of DC Machines Multiple Choice Questions & Answers (MCQs) focuses on “Armature Reaction -1”.

1. In a DC machine, the form of armature mmf waveform is __________
a) Triangular
b) Sinusoidal
c) Saw tooth
d) Rectangular

Explanation: All the conductors on the armature periphery between adjacent brushes carry currents (of constant value, UNcIc) in one direction and the current distribution alternates along the periphery. Because of commutator action, armature current distribution is in the steps of UNcIc. Thus, mmf waveform can be generalized by joining peak points to get triangular wave.

2. In a DC machine, the direct axis is __________
a) Axes of main poles
b) Axes perpendicular to axes of main poles
c) Not determined from the poles position
d) Can be drawn anywhere

Explanation: Direct axes is simply defined as the line passing through the axes of main poles. Maximum flux passes through this line. It’s also called as Direct Axis. Direct axes is always perpendicular to the geometrical neutral axis of machine.

3. In a DC machine, the form of flux density distribution (main field only) waveform is __________
a) Triangular
b) Sinusoidal
c) Saw tooth
d) Trapezoidal

Explanation: Flux density waveform is symmetrical and square wave with distortion at the zero points, causing wave to be in trapezoidal shape. The wave is flat topped, which get distributed due to armature mmf distribution, giving rise to the resultant flux distribution wave.

4. Due to the effect of armature reaction in DC machine, the flux per pole and generated voltage ______ and ________ respectively.
a) Increases, decreases
b) Decreases, decreases
c) Decreases, increases
d) Increases, increases

Explanation: The nature of armature reaction in a dc machine is cross-magnetizing with its axis (stationary) along the q-axis (at 90° elect. to the main pole axis). It causes no change in flux/pole if the iron is unsaturated but causes reduction in flux/pole (demagnetizing effect) in presence of iron saturation.

5. Armature reaction of an unsaturated DC machine is (in terms of magnetization) ________
a) Cross-magnetizing
b) Demagnetizing
c) Magnetizing
d) Cannot be determined

Explanation: Initially at unsaturated condition in a DC machine armature reaction lies along the q-axis. It will cause no change in flux/pole if iron is unsaturated. Now, when iron gets saturated axis gets shifted which will cause reduction in flux/pole.

6. What is the effect of demagnetizing component of armature reaction?
a) Reduces generator emf
b) Increases armature speed
c) Reduces interpole flux density
d) Results in sparking trouble

Explanation: When the armature of a dc machine carries current, the distributed armature winding produces its own mmf (distributed) known as armature reaction. The demagnetizing component acts in the opposite direction, reducing flux/pole in a machine, which will ultimately reduce generator emf.

7. What is the reason behind short circuit in armature?
a) Insulation failure between two commutator bars
b) Insulation failure between two turns of a coil
c) Two or more turns of the same coil getting grounded
d) Insulation failure between two commutator bars, two turns of a coil or the same coil getting grounded

Explanation: Armature short circuit may occur due to contact of two commutator bars or due to contact in of two coil turns as commutators are connected to respective coil sides. If two or more turns of coil are grounded then they have common end which again leads to short circuit.

8. What will happen at poles due to armature reaction in DC generator?
a) Demagnetisation of leading pole tip and magnetisation of trailing pole tip
b) Demagnetisation of trailing pole tip and magnetisation of leading pole tip
c) Demagnetising at the centre of all poles
d) Magnetising at the centre of all poles

Explanation: Leading pole tip (LPT) and trailing pole tip (TPT) are the two edges of the pole, they depend upon the direction of motion of the armature (in case of DC). While performing a motion the armature first saws an edge of the pole, that edge is called leading pole tip. Thus, at leading pole tip there will be demagnetization.

9. In DC generator, how armature reaction is produced?
a) Its field current
b) Armature conductors
c) Field pole winding

Explanation: When the armature of a dc machine carries current i.e. load current in armature, the distributed armature winding produces its own mmf (distributed) known as armature reaction. The tothe field ampere-turns (ATf) and armature ampere-turns (ATa).

10. In a DC generator, the effect of armature reaction on the main pole flux is to _________
a) Reduce it
b) Distort it
c) Reverse it
d) Reduce and distort it

Explanation: When non-zero load current is passed through the armature winding, the distributed armature winding produces its own mmf known as armature reaction. According to its nature cross-magnetizing and demagnetizing, it will distort or reduce the main flux distribution.

11. In a DC machine brushes are normally located along GNA.
a) True
b) False

Explanation: Brushes are generally located at 900 to direct axis. The axis 900 to the direct axis is called as quadrature axis (q-axis). Generally, q-axis is along the geometric neutral axis (GNA) of machine. The brushes in a DC machine are normally located along the q-axis.

12. Armature reaction at 900 to the main field is called as ____________
a) Demagnetizing mmf
b) Cross-magnetizing mmf
c) Anti-magnetizing mmf
d) Magnetizing mmf

Explanation: The armature reaction flux strengthens each main pole at one end and weakens it at the other end (cross magnetizing effect). Armature reaction with axis at 90° to the main field axis is known as cross-magnetizing mmf.

13. Increase in flux density at one end of the pole is less than the decrease at the other end. This is called as ____________
a) Demagnetizing mmf
b) Cross-magnetizing mmf
c) Anti-magnetizing mmf
d) Magnetizing mmf

Explanation: If the main pole excitation is such that iron is in the saturated region of magnetization (this is the case in a practical machine), the increase in flux density at one end of the poles caused by armature reaction is less than the decrease at the other end, so that there is a net reduction in the flux/pole, a demagnetizing effect.

14. Peak flux density in terms of total flux density is given by _________
a) ATa (peak) = ATa (total) /P
b) ATa (peak) = ATa (total) *P
c) ATa (peak) = ATa (total) /P*P
d) ATa (peak) = ATa (total) *P2

Explanation: Peak flux density is defined as ratio of total flux density given to the number of poles for a given DC machine. Peak flux density in terms of total flux density is given by ATa (peak) = ATa (total) /P.

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