A DC machine—whether it's a generator or a motor—consists of several mechanical and electrical components. Each part plays a crucial role in its efficient operation. Let’s understand each component in a simplified manner.
🧩 Major Parts of a DC Machine
A typical DC machine consists of the following parts:- Yoke (Magnetic Frame)
- Pole Cores and Pole Shoes
- Pole Coils or Field Coils
- Armature Core
- Armature Windings or Conductors
- Commutator
- Brushes and Bearings
➤ Magnetic vs Electrical Circuit
- Magnetic Circuit: Includes Yoke, Pole Cores, Armature Core, and Air Gap.
- Electrical Circuit: Includes Field Coils, Armature Windings, Commutator, and Brushes.
🧱 1. Yoke (Magnetic Frame)
Purpose:- Provides mechanical support to the poles.
- Acts as the outer protective shell.
- Completes the magnetic circuit by carrying the magnetic flux.
- Small machines: Cast iron (cost-effective)
- Large machines: Cast steel or rolled steel (strong & high permeability)
- Rolled steel is bent and welded into a cylindrical shape.
- Feet, terminal box, and mounting base are welded afterward.
🧲 2. Pole Cores and Pole Shoes
Functions of Pole Shoes:- Spread magnetic flux uniformly in the air gap.
- Reduce reluctance (magnetic resistance) due to larger cross-section.
- Support the field coils.
- Solid Pole Core with Laminated Pole Shoe (old method)
- Fully Laminated Pole Core and Shoe (modern method)
- Laminations made of annealed steel (0.25–1 mm thick)
- Riveted and hydraulically pressed
- Attached to yoke via bolts or steel bars
🔁 3. Field Coils (Pole Coils)
Construction:- Made from insulated copper wire or strip.
- Former-wound to precise shape and then placed over pole core.
- When DC current flows through field coils, they magnetize the poles.
- These poles create the required magnetic field for EMF generation.
🧠 4. Armature Core
Functions:- Houses armature windings (conductors).
- Provides a low reluctance path for magnetic flux.
- Facilitates rotation for cutting the flux.
- Cylindrical structure made of laminated steel discs (0.5 mm thick).
- Laminations are slotted and keyed to the shaft.
- Perforations are provided for air ventilation and cooling.
- For large armatures, full circular laminations are hard to handle.
- Hence, laminations are segmented into 4, 6, or 8 parts.
- These segments are dovetail shaped and self-locking.
- Reduce eddy current losses by increasing path resistance.
- Thinner laminations → Lower losses.
⚡ 5. Armature Windings (Conductors)
Construction:- Made using flat rectangular former-wound coils.
- Coils are pulled into shape and placed into slots lined with insulation.
- Fixed in place using wooden or fibre wedges.
- Single-layer: One coil side per slot (rarely used).
- Two-layer (common): Two coil sides per slot (one in upper half, one in lower).
- Used in large machines (4, 6, or 8 coils per slot).
🔁 6. Commutator
Function:- Collects current from armature conductors.
- Converts AC induced in armature into unidirectional (DC) output.
- Made of wedge-shaped copper segments.
- Segments are insulated with mica.
- Each segment is connected to an armature coil via risers.
- V-grooves with mica insulation help withstand centrifugal force.
⚙️ 7. Brushes and Bearings
Brushes:- Made of carbon or graphite.
- Collect current from commutator.
- Fitted into box-type holders and pressed via adjustable springs.
- Pigtail wire connects brush to external circuit.
- Ball bearings – common due to low noise and wear.
- Roller bearings – used in heavy-duty machines.
- Sleeve bearings – lubricated via ring oilers.
🧮 Important Armature Winding Terminology
📏 Pole Pitch
- Definition: Slots per pole or armature periphery / number of poles.
- Example: 48 conductors, 4 poles → Pole pitch = 48/4 = 12
🔁 Coil & Winding Element
- Coil: Two conductors (sides) + end connection.
- Winding Element: Each side of the coil.
🧮 Coil Span or Coil Pitch (YS)
- Distance between two sides of a coil (in slots).
- Full-pitch winding: Coil span = pole pitch → Max induced EMF.
- Fractional pitch winding: Coil span < pole pitch → Reduced EMF.
📐 Winding Pitch (Y)
- Lap winding: Y = YB − YF
- Wave winding: Y = YB + YF
🔁 Back Pitch (YB)
- Distance between connected coil sides on back of armature.
🔁 Front Pitch (YF)
- Distance between connected coil sides on commutator side.
🧮 Resultant Pitch (YR)
- Distance between the start of one coil and the start of the next connected coil.
⚡ Commutator Pitch (YC)
- Distance between two commutator segments to which one coil is connected.
- For lap winding: YC = YB − YF (equals plex i.e., 1 for simplex, 2 for duplex etc.)
- For wave winding: YC = YB + YF
🔄 Two-layer Armature Winding (Most Used)
- Two coil sides per slot (upper and lower).
- Twisted at back end to connect distant slots.
- Odd numbers for upper conductors, even for lower (e.g., 1, 3, 5... and 2, 4, 6...).
✅ Summary
| Component | Purpose |
|---|---|
| Yoke | Supports structure & provides magnetic path |
| Pole Cores | Carry magnetic flux |
| Pole Shoes | Spread flux & reduce reluctance |
| Field Coils | Create magnetic field |
| Armature Core | Rotates to cut flux & supports windings |
| Armature Winding | Carry induced current |
| Commutator | Converts AC to DC |
| Brushes | Transfer current to external load |
| Bearings | Allow smooth shaft rotation |
✍️ Final Note:
This structure of the DC machine is fundamental for understanding both its electrical and magnetic behavior. Whether you are preparing for competitive exams like SSC JE, RRB JE, GATE, or learning electrical engineering basics, mastering these components is a must.
DC Machine – SSC JE Electrical MCQs
What is the function of pole shoes?
A. Conduct armature currentB. Spread flux & reduce reluctance
C. Support brushes
D. Insulate field coils
Answer: B. Spread flux & reduce reluctance
Explanation: Pole shoes widen flux path and reduce magnetic reluctance.
Which part confirms that the machine is a DC machine?
A. FrameB. Shaft
C. Commutator
D. Stator
Answer: C. Commutator
Explanation: The commutator distinguishes a DC machine by converting AC to DC.
Brushes in a DC machine are usually made of:
A. CopperB. Aluminium
C. Carbon or Graphite
D. Bronze
Answer: C. Carbon or Graphite
Explanation: Carbon/graphite brushes reduce sparking and wear on commutators.
Why are laminations used in the armature core?
A. Reduce weightB. Reduce eddy currents
C. Improve insulation
D. Reduce cost
Answer: B. Reduce eddy currents
Explanation: Thin laminations increase resistance and minimize eddy current loss.
Which winding is used to reduce ripple in DC generator output?
A. Filter windingB. Equaliser rings
C. More commutator segments
D. Series field winding
Answer: C. More commutator segments
Explanation: Higher commutator segment count smooths out the output waveform.
How is torque related to armature current in a DC series motor?
A. T ∝ IaB. T ∝ Ia²
C. T ∝ √Ia
D. T ∝ 1/Ia
Answer: B. T ∝ Ia²
Explanation: In series motors, flux increases with armature current, making torque proportional to the square of current.
Which rule determines the direction of force in a DC motor?
A. Fleming’s Right-Hand RuleB. Lenz’s Law
C. Faraday’s Law
D. Fleming’s Left-Hand Rule
Answer: D. Fleming’s Left-Hand Rule
Explanation: Used to determine direction of force in motors (Current, Field, Motion).
Function of commutator in DC machine?
A. Generate ACB. Collect voltage
C. Convert AC to DC
D. Reverse field current
Answer: C. Convert AC to DC
Explanation: It rectifies the alternating EMF into direct current at output.
Function of field coils in DC machines?
A. Reduce lossesB. Conduct load current
C. Create magnetic field
D. Connect armature
Answer: C. Create magnetic field
Explanation: Field coils, when excited, generate the magnetic field in poles.
Yoke in a DC machine serves the purpose of:
A. Generating fluxB. Holding commutator
C. Providing magnetic path & support
D. Insulating windings
Answer: C. Providing magnetic path & support
Explanation: The yoke supports the frame and completes the magnetic circuit.
What happens to back EMF if speed of DC motor increases?
A. DecreasesB. Remains same
C. Increases
D. Drops to zero
Answer: C. Increases
Explanation: Back EMF is directly proportional to speed.
Which of the following materials is used in small machine yokes?
A. Cast ironB. Cast steel
C. Laminated steel
D. Mild steel
Answer: A. Cast iron
Explanation: Cast iron is cost-effective and used in small-size machines.
Why are carbon brushes preferred over copper brushes in DC machines?
A. Better conductionB. Lower resistance
C. Self-lubricating and less wear on commutator
D. Easy to manufacture
Answer: C. Self-lubricating and less wear on commutator
Explanation: They are softer, produce less wear, and reduce sparking.
Which loss is NOT found in a DC generator?
A. Copper lossB. Hysteresis loss
C. Core loss
D. Synchronous loss
Answer: D. Synchronous loss
Explanation: Synchronous loss is found in synchronous machines, not DC ones.
Commutator segments are separated by:
A. PaperB. Air gaps
C. Mica insulation
D. Plastic washers
Answer: C. Mica insulation
Explanation: Mica is used for its good insulation and heat resistance properties.
Function of armature winding?
A. Excite the machineB. Convert mechanical to electrical energy
C. Generate magnetic flux
D. Control speed
Answer: B. Convert mechanical to electrical energy
Explanation: Armature winding cuts flux and induces EMF.
Ball bearings are preferred in DC machines because:
A. High costB. Reduce wear & noise
C. Allow brush movement
D. Insulate the shaft
Answer: B. Reduce wear & noise
Explanation: Ball bearings offer smoother and quieter operation.
What is coil span in a DC machine?
A. Slot widthB. Distance between adjacent poles
C. Distance between two sides of a coil
D. Air gap
Answer: C. Distance between two sides of a coil
Explanation: Also called coil pitch, affects EMF induced.
Why are air gaps kept small in DC machines?
A. To increase weightB. To reduce field strength
C. To reduce reluctance
D. To ease cooling
Answer: C. To reduce reluctance
Explanation: Small air gaps provide low magnetic resistance.
How many brushes are used in a 4-pole lap-wound machine?
A. 2B. 3
C. 4
D. 6
Answer: C. 4
Explanation: In lap winding, number of brushes = number of poles.