Electrical Engineering: Armature Conductors

Questions and Answers

What is the impact of armature reaction in a dc machine?

It reduces armature current.

It increases voltage output.

It stabilizes the magnetic field.

It causes neutral-plane shift. (correct)

In the frog-leg winding, what does P represent?

The resistance of the winding material.

The length of each winding coil.

The number of poles on the machine. (correct)

The total number of turns in the winding.

What characterizes the rotor winding arrangement of the described four-loop machine?

It is a wave winding configuration.

It has a total of four conductors.

It contains four coils, each with one turn. (correct)

It has two coils with two turns each.

Which of the following is a major effect that disturbs the commutation process in real dc machines?

Armature reaction. Signup and view all the answers

The function of the commutator in a dc machine is primarily to:

Convert ac voltage to dc voltage. Signup and view all the answers

What is the effect of L di/dt voltages in the commutation process?

They cause voltage fluctuations. Signup and view all the answers

What does the term 'neutral-plane shift' refer to in dc machines?

A shift in the magnetic neutral axis. Signup and view all the answers

What type of winding does the four-loop machine utilize?

Progressive lap winding. Signup and view all the answers

How many parallel current paths are present in the winding of the described generator?

2 Signup and view all the answers

What is the total number of conductors on the rotor of the generator?

2880 Signup and view all the answers

What is the effective armature resistance when considering the two parallel paths?

3.96 Ω Signup and view all the answers

If a 1000-Ω load is connected to the generator, what is the current that flows?

2.88 A Signup and view all the answers

What is the value of the constant K as calculated in the problem?

2750.2 Signup and view all the answers

What does the torque experienced by the generator primarily depend on?

Induced voltage Signup and view all the answers

What components make up the physical structure of a DC machine?

Stator and rotor Signup and view all the answers

What role do the pole pieces serve in the stator of a DC machine?

Distribute flux evenly over the rotor Signup and view all the answers

What is the primary advantage of using commutating poles in a dc machine?

To cancel out the voltage in coils during commutation Signup and view all the answers

What happens to brush sparking if the voltage during commutation is made zero?

There is no sparking at the brushes Signup and view all the answers

Where are the commutating poles located in relation to the main poles?

Midway between the main poles Signup and view all the answers

What is the effect of having a component of armature magnetomotive force opposing the poles' magnetomotive force?

It reduces the net magnetomotive force in the machine Signup and view all the answers

Why was a new solution developed for brush sparking in electric machines?

Existing methods required constant adjustment of brush positions Signup and view all the answers

How do interpoles contribute to the function of a dc machine?

They help in canceling the voltage during commutation Signup and view all the answers

In the context of a dc machine, what does the term 'neutral plane' refer to?

The point where voltage is canceled during commutation Signup and view all the answers

What is indicated by a reduction in the net magnetomotive force of a dc machine?

Decreased risk of sparking at the brushes Signup and view all the answers

What is the primary purpose of adding compensating windings in a DC machine?

To cancel the effects of armature reaction Signup and view all the answers

In which section of the DC machine are the compensating windings placed?

In slots carved in the pole faces Signup and view all the answers

What happens to the current in the compensating windings when the load on the rotor changes?

It changes proportionally Signup and view all the answers

How does the net flux in a DC machine with compensating windings compare to the original pole flux?

It is equal to the original pole flux Signup and view all the answers

What major issue do compensating windings help mitigate in heavy duty cycle motors?

Flux weakening Signup and view all the answers

What does the compensating winding's flux do in relation to the armature flux?

Is equal and opposite to it Signup and view all the answers

Which figure illustrates the concept of compensating windings reducing flux weakening?

Figure 7-30 Signup and view all the answers

What is a negative consequence of not using compensating windings in a DC machine?

Distortion of the electromagnetic field Signup and view all the answers

What does the variable Z represent in the formula for the number of conductors on a machine’s armature?

Number of conductors on rotor Signup and view all the answers

What is the primary characteristic of a full-pitch coil?

Spans exactly 180 electrical degrees Signup and view all the answers

How does the electrical angle relate to the mechanical angle in a given machine?

Electrical angle is proportional to the number of magnetic poles Signup and view all the answers

What type of coil spans less than 180 electrical degrees?

Fractional-pitch coil Signup and view all the answers

Which of the following best describes a rotor winding with fractional-pitch coils?

It is called a chorded winding Signup and view all the answers

In a full-pitch coil, the voltages in the conductors on either side of the coil are characterized as:

Same in magnitude and opposite in direction Signup and view all the answers

What is typically insulated within a coil to prevent electrical shorts?

Each turn from others in the coil Signup and view all the answers

What defines the maximum value of electrical angle (θe) relative to mechanical angle (θm)?

It is multiplied by the number of magnetic poles Signup and view all the answers

Study Notes

Armature and Coils

The number of conductors (Z) on a rotor is defined by Z = C × Nc, where C is the number of coils and Nc is the number of turns per coil.
A coil typically spans 180 electrical degrees, positioning one side under a magnetic pole and the other under a pole of opposite polarity.
The relationship between electrical angle (θe) and mechanical angle (θm) is θe = P × θm, where P is the number of magnetic poles.
Full-pitch coils have equal voltages on both sides in magnitude but opposite in direction.
Fractional-pitch coils span less than 180 electrical degrees and contribute to chorded windings in rotors.

Current Paths in Windings

The number of current paths in a frog-leg winding is determined by P and the plex of the lap winding.
A four-loop machine with four coils, each containing one turn, results in a total of eight conductors and employs a progressive lap winding.

Commutation Challenges

Armature reaction occurs when current through the rotor windings produces a magnetic field that distorts the original magnetic field, leading to issues like neutral-plane shift.
Neutral-plane shift necessitates brush position adjustments as loads change, complicating effective commutation.
Commutating poles (interpoles) are added midway between main poles to provide flux that cancels voltage during commutation, reducing brush sparking.

Compensating Windings

Compensating windings help mitigate armature reaction by being integrated into pole pieces to counteract load-induced magnetic distortions.
These windings are parallel to rotor conductors and adjust their current with load changes to maintain voltage cancellation during commutation.

DC Machine Structure

A dc machine consists of two main parts: the stator (stationary) and rotor (rotating).
The stator includes the frame, which supports the machine, and pole pieces that provide a magnetic flux path, fostering even distribution of flux over the rotor surface.

Example Calculation

For a generator with ( Z = 2CNc = 2880 ) conductors, the effective resistance with two paths is calculated as ( R_A = 3.96 , \Omega ).
Under a 1000-Ω load, the generated current is ( I = 2.88 , A ), yielding a countertorque of 396 N-m on the generator shaft.

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Description

This quiz covers the calculation of the number of conductors on a machine's armature. It explores the relationship between the number of coils, turns per coil, and electrical degrees affecting the rotor's performance in magnetic fields.