Test: Effect of Brush Shift & Compensation Windings - Electrical Engineering (EE) MCQ

The brushes are placed along magnetic neutral axi so that in ideal conditions commutations take place without sparking. 

For the generator, the electromagnetic torque produced is in the opposite direction and so the induced current. So to eliminate the sparking the brushes should be shifted in the forward direction of rotation.

The armature reaction flux at the trailing edge weakens the flux and at the leading edge is strengthens the flux in the dc motor.

Since the time taken is more than the prescribed time limit for the commutation, it is under commutation.

For a practical DC machine operating under load, the correct answer is d) mna, mna.

Here's a detailed explanation of the concepts involved:

• Commutation: This is the process of reversing the direction of current in an armature coil as it passes under a brush. This reversal is necessary to convert the AC voltage induced in the rotating armature coils into a DC voltage at the output terminals (for a generator) or to maintain a constant torque direction (for a motor).

• Short-Circuited Coil: For commutation to occur, a brush must momentarily bridge two adjacent commutator segments. This action short-circuits the armature coil connected to these segments. The coil that is short-circuited is the one actively undergoing commutation. Therefore, the two phrases in the question—"coil undergoing commutation" and "coil getting short circuited"—refer to the same coil at the same point in its rotation.

GNA vs. MNA

• Geometrical Neutral Axis (GNA): This is a fixed axis of symmetry located exactly between two adjacent field poles. Its position is determined by the physical construction of the machine and does not change.

• Magnetic Neutral Axis (MNA): This is the axis where the magnetic flux density is zero. Along this axis, armature conductors move parallel to the magnetic flux lines, and therefore, no electromotive force (EMF) is induced in them.

Why MNA is the Correct Answer

In an ideal DC machine with no load, the MNA and GNA are in the same position. However, the question specifies a practical DC machine, which implies it is under load.

1. Armature Reaction: When a DC machine is loaded, the current flowing through the armature conductors creates its own magnetic field. This "armature field" interacts with and distorts the main field produced by the poles. This phenomenon is called armature reaction.

2. Shifting of MNA: The primary effect of armature reaction is the shifting of the Magnetic Neutral Axis (MNA). In a generator, the MNA shifts in the direction of rotation, and in a motor, it shifts against the direction of rotation.

3. Brush Placement: For commutation to be sparkless, the current reversal in the short-circuited coil must happen precisely when the induced voltage in that coil is zero. This zero-voltage position is along the MNA. If brushes were placed on the GNA in a loaded machine, the coil being short-circuited would still be cutting flux lines, inducing an EMF that leads to sparking and damages the brushes and commutator.

Therefore, in a practical DC machine, the brushes are always placed along the MNA to ensure smooth, spark-free commutation. Since the coil undergoing commutation is the one being short-circuited by the brushes, it must lie along the MNA.

 The interaction between the field excitation and armature supply create the mmf and the interconnected behaviour creates the emf.

The static emf induced inside gets reduced due to the armature reaction in large machines when there is no compensating winding present.

Compensating conductors per pole face is Z/A*Ia.

AT/pole for the compensating winding = 0.7*(Z*I/2P)
= 0.7*(860*100/2*10)
= 3010 AT.

All the options are valid for the interpoles except a because they are connected in series not in parallel.

d) none of the mentioned

Let's break down each option:

• a) using interpoles: Interpoles (or commutating poles) are used in DC generators to improve commutation by neutralizing the effect of armature reaction. This option is correct.

• b) using carbon brushes: Carbon brushes are used to ensure good electrical contact with the commutator. Using carbon brushes is a standard practice and can contribute to improving commutation, so this option is correct.

• c) shifting brush axis in direction of armature rotation: Shifting the brush axis in the direction of armature rotation can also help in improving commutation, as it compensates for the armature reaction and minimizes sparking. This is also correct.

• d) none of the mentioned: Since options a), b), and c) are correct methods of improving commutation, this option is incorrect.

Thus, the most incorrect option is:

d) none of the mentioned

By reversing the current direction in the MNA the current can be limited in the coils to avoid sparking.

The armature reaction is initiated by the loaded conditions of the machine because an unloaded dc machine does not armature flux to get distorted.

The compensating winding functions such that to nullify the cross magnetization effects in the armature.

In the saturation region the flux will be constant.