# DC Machine Commutation MCQ || DC Machine Commutation Questions and Answers

1. In DC generator on load, the air-gap flux distribution in space is

1. Sinusoidal
2. Triangular
3. Pulsating
4. Flat-topped

Explanation:

• The inter poles are narrow poles placed exactly midway between the main poles. The inter poles are fitted to the yoke and are also known as commutating poles.
• The inter poles cause demagnetization at the trailing pole tip, magnetization at the leading pole tip there by the air gap flux distribution nearly becomes a flat-topped wave. The flattening becomes more pronounced as the flux is increased
• Inter poles are placed along the interpolar axis (Q – axis), and its field winding is connected in series with the armature winding.

2. To speed up the commutation process ______ must be neutralized.

1. Reactance Voltage
2. Armature current
3. Field Current
4. Armature Reactance

Explanation:

• To speed up the commutation process, the reactance voltage must be neutralized by injecting a suitable polarity dynamical (speed) voltage into the commutating coil.
• For neutralization of reactance voltage at all loads, the interpoles must be excited by the armature current by connecting them in series with the armature.
• The interpolar air gap is kept larger than that of the main pole so that their magnetic circuit is linear resulting in the cancellation of the reactance voltage (a linear derivative term) at all loads.

3. In DC machine time of commutation decreases with:

1. Increase in width of commutator segment
2. Increase in peripheral velocity of commutator in m/s
3. Decrease in diameter of the commutator
4. Decrease in speed of the machine in RPM

Answer:2. Increase in peripheral velocity of commutator in m/s

Explanation:

The commutation period is the time required for commutation to advance a distance of one commutator segment.

Commutation time TC = (wc/Vc) sec

Where,

wc = width of the commutator segment = width of the brush(wb) – width of mica insulation (wm)

Vc = Peripheral velocity of commutator in meter/sec. = π DN / 60

Here D = diameter of the commutator and N = speed of the machine in RPM.

• Commutation time is directly proportional to the width of the commutator segment. So, the time of commutation increases with an increase in the width of the commutator segment.
• Commutation time is directly inversely proportional to the diameter of the commutator, speed in RPM, and the peripheral velocity of the commutator in meter/sec.
• Hence, the time of commutation decreases with an increase in the diameter of the commutator, speed in RPM, and peripheral velocity of the commutator in meter/sec.

4. In a DC generator, sparking at the brushes occurs due to:

1. Distributed winding
2. Armature reaction and commutation
3. High resistance of brushes
4. Improper connection of winding

Answer: 2. Armature reaction and commutation

Explanation:

• Due to armature reaction, the MNA is shifted in the direction of rotation in a generator and against the direction of rotation in a motor
• As a result, rotational EMFs are induced in the coils undergoing commutation which leads to sparking.

Sparking at the brushes can be minimized with an improvement in the commutation by providing special commutating poles (interpoles), placed midway between the main poles and wound with comparatively few turns and connected in series with the armature winding.

5. Sparking at the brushes can be minimized by providing ________

1. Interpole
2. Main Pole
3. Compensating winding
4. Distributed winding

Explanation:

• Due to armature reaction, the MNA is shifted in the direction of rotation in a generator and against the direction of rotation in a motor
• As a result, rotational EMFs are induced in the coils undergoing commutation which leads to sparking.

Sparking at the brushes can be minimized with an improvement in the commutation by providing special commutating poles (interpoles), placed midway between the main poles and wound with comparatively few turns and connected in series with the armature winding.

6. The MMF developed by the interpole must be stronger than the _______ in the neutral zone.

1. Field MMF
2. Compensating MMF
3. Armautre field MMF
4. Armature MMF

Explanation:

The MMF developed by the interpole must be stronger than the armature MMF in the neutral zone because this MMF has to cancel the armature MMF and in addition induce an emf in the short-circuited coil which opposes the reactance voltage and the voltage drop at the brushes.

If the MMF on the interpole is too weak, it would lead to retarded commutation and if it is too strong, accelerated commutation is achieved.

7. Commutation can also be improved by using ______

1. Low resistance Brush
3. High Resistance Brush

Explanation:

Commutation can also be improved by using high resistance brushes. Carbon has a contact resistance as high as approximately 12 times that of copper. As such carbon brushes should be used compared to copper brushes.

8. Interpoles or Commutating Poles can be used equally effectively in _______

1. DC Motor
2. DC Generator
3. DC Generator and DC Motor
4. None of the above

Answer: 3. DC Generator and DC Motor

Explanation:

The interpoles can be used equally effectively in motors as well as in generators. When the mode of operation of the machine changes from the motor to the generator, the currents in the armature and the interpoles are reversed in direction. Therefore, their voltage effects cancel each other out.

(a) In a generator, interpoles must have the same polarity as the next upcoming pole.

(b) In a motor, interpoles must have the same polarity as the previous main pole.

9.  The equélizing conductors, which are in the form of Cu rings at the armature are called _______.

1. Equalizer Conductor
2. Equalizer Ring
3. Armature Ring
4. Equalizer Armature

Explanation:

In DC Machine, multiple parallel paths exist across the terminals of an armature. Due to reasons such as long usage of the machine, the emf induced in one path is often different from that in other paths. This causes an undesirable condition of circulating current through the brushes, which in turn causes winding and brush overheating and frequent sparking at the brushes.

The equélizing conductors, which are in the form of Cu rings at the armature back and which connect such points are called Equalizer Rings.

The circulating current due to the slight difference in the e.m.fs. of various parallel paths, passes through these equalizer rings instead of passing through the brushes.

10. The equalizing current flows in the ______ due to potential differences in the alternating current.

1. Closed Circuit
2. Open Circuit
3. Both Close and Open circuit
4. Short Circuit

Explanation:

• In DC Machine, multiple parallel paths exist across the terminals of an armature. Due to reasons such as long usage of the machine, the emf induced in one path is often different from that in other paths.
• The equélizing conductors, which are in the form of Cu rings at the armature back and which connect such points are called Equalizer Rings.
• It should be noted here that the equalizing current flows in the closed circuit due to potential differences in the alternating current.
• This alternating current is produced by irregularity of flux distribution as according to Lenz’s law the magnetic effect of this induced current reacts with the main field to reduce the irregularity.
• The circulating current due to the slight difference in the e.m.fs. of various parallel paths, passes through these equalizer rings instead of passing through the brushes.

11. The function of the equalizer ring is to ______

1. Achieve unequal distribution of current at the brushes thereby helping to get sparkless commutation
2. Avoid equal distribution of voltage at the brushes thereby helping to get sparkless commutation
3. Avoid unequal distribution of current at the brushes thereby helping to get sparkless commutation
4. Achieve equal distribution of voltage at the poles thereby helping to get sparkless commutation

Answer: 3. Avoid unequal distribution of current at the brushes thereby helping to get sparkless commutation

Explanation:

• In DC Machine, multiple parallel paths exist across the terminals of an armature.
• Due to reasons such as long usage of the machine, the emf induced in one path is often different from that in other paths.
• This causes an undesirable condition of circulating current through the brushes, which in turn causes winding and brush overheating and frequent sparking at the brushes.
• The equélizing conductors, which are in the form of Cu rings at the armature back and which connect such points are called Equalizer Rings.
• The circulating current due to the slight difference in the e.m.fs. of various parallel paths, passes through these equalizer rings instead of passing through the brushes.

Hence, the function of equalizer rings is to avoid unequal distribution of current at the brushes thereby helping to get sparkless commutation.

12. The armature of a DC machine is placed on the rotor to _______.

1. Reduce losses
2. Save iron
3. Support commutation
4. Decrease armature reaction

Explanation:

• Commutation is the process in which generated alternating current in the armature winding of a D.C machine is converted into direct current.
• The armature winding of a dc machine is placed on the rotor to facilitate commutation.
• A good commutation means no sparking at the brushes.
• For good commutation, the coil short-circuited by the brushes should have zero emf induced in them
• In under commutation, the commutation does not complete before the brush has moved from one commutator segment to the next. This leads to sparking at the trailing edge of the brush.

13. The commutation is the process of in which  _______

1. AC converted DC
2. DC converted AC
3. AC converted to Smoothed AC
4. DC converted DC

Explanation:

The commutation in DC machine or more specifically commutation in DC generator is the process in which generated alternating current in the armature winding of a dc machine is converted into direct current after going through the commutator and the stationary brushes.

14. Under-commutation results in

1. Sparking at the middle of the brush
2. Sparking at the leading edge of the brush
3. Sparking at the trailing edge of the brush
4. No sparking

Answer: 3. Sparking at the trailing edge of the brush

Explanation:

• The cause of sparking at the commutator is the failure of the current in the short-circuited elements to reach the full value in the reverse direction by the end of the short circuit. This is known as under commutation or delayed commutation.
• It causes sparking at the trailing edge of the brush.
• The commutator is an assembly of bars or segmental sections, insulated from each other, to which the coil ends of an armature winding are attached. Commutation is the process of current reversal in the armature coils that occurs when they are shorted by the brushes contacting the segments to which the coil ends are attached.
• Moreover, the MMF induced on the interpoles must be sufficient enough to neutralize the effect of armature reaction and to produce enough field in the interpole winding to overcome the reactance voltage) due to commutation. It should be noted that the field produced by the inter pole winding should be neither too weak nor too strong.
• Weak field results in under commutation accompanied by arcing and a strong field into over commutation with arcing in the reverse direction. Therefore, the value of ampere-turns per pair of interpoles must be selected judiciously.
• The cause of sparking at the commutator is the failure of the current in the short-circuited elements to reach the full value in the reverse direction by the end of the short circuit. This is known as under commutation or delayed commutation.

15. The methods used to improve the commutation process are _____

1. Resistance Commutation
2. Voltage Commutation
3. Both 1 and 2
4. None of the above

Answer: 3. Both 1 and 2

Explanation:

The following methods are used to improve the commutation process.

Resistance commutation:

• Resistance commutation is done by providing high carbon brush resistance.
• The brush resistance is in series with the coal under the commutation process.
• If the resistance of the brush is high then the time constant of the current in the coil is reduced i.e the current reversal becomes faster.
• This method will be suitable in small fractional kW machines.

Voltage Commutation:

• In this method, the arrangement is made to induce a voltage in the coil undergoing the commutation process.
• Voltage Commutation will neutralize the reactance voltage.

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