DC Machines

Introduction and Context

A DC machine is an electromechanical energy-conversion device that can operate either as a DC generator, converting mechanical energy into electrical energy, or as a DC motor, converting electrical energy into mechanical energy. The same fundamental machine can perform both functions because generation and motoring are reciprocal processes.

Although power-electronic drives and AC machines dominate many modern installations, DC machines remain important in laboratories, traction, rolling mills, cranes, elevators, battery systems, and variable-speed industrial drives. Their principal historical advantage was straightforward speed control and high starting torque. Understanding DC machines also provides a strong foundation for electromagnetic energy conversion, feedback control, and electrical-machine selection.

Learning Objectives

After studying this chapter, the reader should be able to:

• describe the construction and function of the main parts of a DC machine;
• explain electromagnetic torque production and generator action;
• derive and use the EMF and torque equations;
• distinguish separately excited, shunt, series, and compound machines;
• interpret the characteristics of DC generators and motors;
• explain starting, speed control, and braking methods;
• calculate losses, efficiency, and operating performance;
• select a suitable DC motor for an industrial mechanical load.

1. Construction of a DC Machine

A conventional DC machine consists of a stationary field system and a rotating armature. The major components are listed below.

Yoke

The yoke is the outer frame. It provides mechanical protection, supports the poles, and offers a low-reluctance path for magnetic flux. In small machines it may be made of cast iron; larger machines commonly use cast steel because of its greater strength and improved magnetic properties.

Poles and field windings

Each pole has a pole core and pole shoe. The pole shoe spreads the magnetic flux over the armature surface and reduces magnetic reluctance. Field coils wound around the poles produce the main magnetic field. Depending on the machine, these coils may be connected separately, in parallel with the armature, or in series with the armature.

Armature core

The armature core is mounted on the shaft and contains slots for the armature conductors. It is built from thin, insulated steel laminations. Lamination reduces eddy-current loss caused by changing flux as the armature rotates. The core provides a low-reluctance path for flux and supports the conductors mechanically.

Armature winding

The armature winding consists of insulated conductors placed in the core slots. The two common winding arrangements are:

• Lap winding, generally used for low-voltage, high-current machines;
• Wave winding, generally used for high-voltage, low-current machines.

The number of parallel paths in a simplex winding is:

Here, A is the number of parallel paths and P is the number of poles. These relationships apply to simplex windings; multiplex windings have different values.

Commutator

The commutator is a cylindrical assembly of copper segments insulated from one another by mica. Armature-coil ends are connected to the segments. In generator operation, it mechanically rectifies the internally generated alternating EMF into a unidirectional terminal voltage. In motor operation, it reverses current in a coil as that coil passes through the neutral zone, maintaining unidirectional torque.

Brushes and brush gear

Carbon or electrographite brushes collect current from, or supply current to, the commutator. Brush holders maintain pressure and position. Correct brush alignment is essential: poor alignment causes sparking, heating, commutator wear, and electromagnetic interference.

Bearings, shaft, fan, and interpoles

Bearings support the shaft, while a fan may remove heat. Interpoles, placed between the main poles, improve commutation by producing a corrective flux. Compensating windings embedded in pole shoes may be used in large machines to counteract armature reaction under heavy load.

2. Operating Principles

Generator action

When a conductor moves through a magnetic field, it experiences an induced EMF. The direction is determined by Fleming’s right-hand rule. In a DC generator, mechanical rotation causes conductors to cut magnetic flux. The alternating EMF induced in individual armature coils is converted into a unidirectional output by the commutator.

Motor action

When a current-carrying conductor lies in a magnetic field, it experiences a force. Fleming’s left-hand rule gives the force direction. Forces on many armature conductors combine to produce electromagnetic torque. The commutator reverses conductor current at the correct instant so that the torque remains in the same rotational direction.

The energy conversion is reversible. In generator operation, electromagnetic torque opposes the prime mover; in motor operation, electromagnetic torque drives the mechanical load.

3. EMF Equation of a DC Generator

Let:

• P = number of poles;
• Φ = flux per pole in weber;
• Z = total number of armature conductors;
• N = rotational speed in revolutions per minute;
• A = number of parallel paths.

Each conductor cuts flux at a rate proportional to Φ and speed. The generated EMF is:

where E_g is generated EMF in volts. The equation assumes a uniform average flux distribution and neglects brush-voltage effects and detailed commutation transients.

For a simplex lap winding, A = P, so:

For a simplex wave winding, A = 2, so:

The terminal voltage of a generator is approximately:

where V is terminal voltage, I_a is armature current, R_a is armature-circuit resistance, and V_b is the total brush voltage drop. For a separately excited or shunt generator, field-circuit effects and armature reaction may also cause voltage reduction.

4. Torque Equation of a DC Motor

The electrical power converted into mechanical power in the armature is approximately:

where E_b is back EMF and I_a is armature current. Since mechanical power is torque multiplied by angular speed,

and

Combining these expressions with the EMF equation gives:

Here, T_e is electromagnetic torque in newton-metres. Thus, for constant flux, motor torque is directly proportional to armature current. If the magnetic circuit is unsaturated, flux is approximately proportional to field current; therefore, torque may also vary approximately with the product of field current and armature current.

The motor voltage equation is:

The back EMF is:

Back EMF opposes the applied voltage and automatically regulates armature current. At standstill, N = 0, so E_b = 0; this fact makes direct starting dangerous for most DC motors.

5. Types of DC Machines

A shunt motor has nearly constant flux over its normal operating range. A series motor has flux that increases with load current until magnetic saturation. It must never be operated without a mechanical load because excessive speed can result when current and flux become small.

6. Characteristics of DC Generators

Magnetization characteristic

The open-circuit characteristic plots generated voltage against field current at constant speed. Initially, voltage rises approximately linearly because flux increases with field current. At higher excitation, magnetic saturation causes the curve to flatten. Residual magnetism produces a small voltage even when field current is zero.

A self-excited shunt generator builds up voltage only when residual magnetism exists, the field connection reinforces the residual flux, field resistance is below the critical value, and the machine rotates above the critical speed.

Internal and external characteristics

The internal characteristic relates generated EMF to armature current and includes the effect of armature reaction. The external characteristic relates terminal voltage to load current. Terminal voltage falls with load because of armature resistance, brush drop, and armature reaction.

7. Characteristics of DC Motors

Shunt motor

A shunt motor develops nearly constant flux and therefore approximately constant speed. Its torque increases nearly linearly with armature current until saturation and heating become significant. It is suitable for machine tools, fans, pumps, and conveyors requiring moderate starting torque.

Series motor

Before saturation, flux is approximately proportional to armature current. Since T_e ∝ Φ I_a, torque is approximately proportional to I_a². After saturation, flux is nearly constant and torque becomes approximately proportional to I_a. A series motor provides excellent starting torque but poor no-load speed regulation.

Compound motor

A cumulative compound motor combines the desirable features of shunt and series motors. It develops higher starting torque than a shunt motor while maintaining better speed regulation than a series motor. The degree of compounding depends on the relative strengths of the field windings.

8. Starting of DC Motors

At starting, E_b = 0. Therefore:

Because R_a is small, the starting current may be many times the rated current. Excessive current can damage the armature, commutator, supply, and mechanical transmission.

A starter inserts external resistance in series with the armature. As the motor accelerates, back EMF rises and resistance is progressively removed. Three-point and four-point starters also provide no-volt and overload protection. Modern drives use controlled rectifiers or DC choppers to limit current and apply a programmed voltage ramp.

A starter should provide:

• current limiting during acceleration;
• protection against supply failure;
• overload protection;
• controlled acceleration;
• safe disconnection during faults.

9. Speed Control

From the motor voltage equation and the relationship between speed and back EMF:

This expression assumes a fixed machine construction and approximately constant proportionality between back EMF and ΦN. Speed can therefore be controlled in three principal ways.

Armature-voltage control

The applied armature voltage is varied while flux is held approximately constant. This provides efficient control below base speed and is common in modern controlled drives.

Field control

Field current is reduced to weaken flux. Since speed varies inversely with flux, speed rises above base speed. However, excessive weakening reduces torque capability and can cause commutation problems or overspeed.

Armature-resistance control

External resistance is inserted in the armature circuit. Speed decreases as the voltage drop across the resistance increases. This method is simple but inefficient because substantial power is dissipated as heat. It is mainly used for short-duration or low-power control.

10. Braking of DC Motors

Braking converts the motor’s mechanical energy into electrical energy or heat.

Regenerative braking

The motor operates as a generator and returns energy to the supply. This is efficient when the supply can accept regenerated power. It is useful in electric traction and lowering loads.

Dynamic or rheostatic braking

The armature is disconnected from the supply and connected to a braking resistor. Generated electrical energy is dissipated as heat. Braking torque decreases as speed falls unless resistance or field conditions are adjusted.

Plugging

The armature connections, or the effective polarity, are reversed while the motor is running. The supply and rotational EMFs act together, producing a large reverse current. A resistor must be inserted to limit current, and the supply must be disconnected at zero speed to prevent reverse acceleration. Plugging is rapid but inefficient because energy is dissipated in the motor and resistor.

11. Losses and Efficiency

Losses in a DC machine are classified as follows:

• Copper losses: armature, field, interpole, and compensating-winding losses;
• Core losses: hysteresis and eddy-current losses;
• Mechanical losses: bearing friction, brush friction, and windage;
• Stray-load losses: additional load-dependent effects caused by leakage flux and nonuniform current distribution.

Armature copper loss is:

Field copper loss is:

where I_f and R_f are field current and field resistance. The total loss is the sum of relevant losses:

Generator efficiency is:

Motor efficiency is:

For a generator, output is electrical power delivered to the load. For a motor, output is mechanical shaft power. Efficiency is always less than 100% in a practical machine.

Worked Example 1: Generator EMF

A simplex wave-wound generator has 4 poles, 480 armature conductors, flux per pole of 0.025 Wb, and speed of 1,200 r/min. Find the generated EMF.

For simplex wave winding, A = 2. Thus:

The generated EMF is 480 V, neglecting brush drop and armature-reaction effects.

Worked Example 2: Motor speed

A shunt motor operates from a 220 V supply. Its armature current is 40 A, armature resistance is 0.20 Ω, and brush drop is 2 V. At this operating condition its speed is 1,000 r/min. If the armature current becomes 60 A while flux remains constant, estimate the new speed.

Initially:

Finally:

For constant flux, N ∝ E_b. Hence:

The speed falls to approximately 981 r/min. The calculation neglects armature reaction and assumes constant brush drop and flux.

12. Industrial Selection and Application

Motor selection must begin with the load, not with the motor nameplate alone. Important load requirements include rated torque, starting torque, speed range, acceleration time, duty cycle, reversals, braking, overload duration, environment, maintenance access, and available supply.

For a rotating load:

where P is mechanical power in watts, T is torque in newton-metres, and ω is angular speed in radians per second.

A crane or hoist requires high starting and overload torque; a series or cumulative compound motor may be appropriate. A machine tool generally requires nearly constant speed and controlled adjustment; a shunt or separately excited motor is preferable. A fan or centrifugal pump has relatively low starting torque and a torque that increases strongly with speed; a shunt motor or modern variable-speed drive is suitable.

Continuous-duty machines should be selected so that normal operation does not exceed rated temperature rise. Intermittent-duty applications may exploit short-time overload capability, but thermal calculations and manufacturer duty classifications must be respected.

13. Illustrative Industrial Case: DC Motor for a Hoist

Background

A workshop hoist must lift loads repeatedly. It requires high torque at zero speed, controlled acceleration, frequent stopping, and reliable lowering of loads.

Problem

A simple shunt motor may not produce adequate starting torque, while a series motor may overspeed when the hook is unloaded. The drive must also prevent uncontrolled descent.

Analysis

The required characteristics are high starting torque, controlled low speed, regenerative or dynamic braking, and safe operation during load lowering. The duty is intermittent, with repeated acceleration and deceleration.

Approach and solution

A cumulative compound motor or separately excited DC motor with a controlled converter can satisfy these requirements. The series component of a compound motor increases starting torque, while the shunt component improves speed regulation. A separately excited drive offers more precise control, current limiting, plugging protection, and regenerative braking. A mechanical brake should remain available as a holding and emergency brake; electrical braking alone should not be treated as the sole load-holding system.

Outcome

The selected system provides controlled acceleration, adequate lifting torque, and safe stopping. Regeneration during lowering can reduce energy consumption if the supply and converter are designed to receive returned energy. Thermal evaluation must include the repeated duty cycle rather than only the average load.

Learning points

• Load torque and duty cycle determine motor selection.
• High starting torque does not eliminate the need for current limiting.
• Electrical braking and mechanical holding brakes serve different safety functions.
• A modern drive may outperform a basic resistor-controlled starter in efficiency and controllability.

14. Challenges, Limitations, and Contemporary Practice

The principal disadvantages of conventional DC machines are brush wear, commutator maintenance, sparking, limited high-speed operation, and sensitivity to dust or explosive atmospheres. Armature reaction can distort the main field and worsen commutation. Heating limits continuous current, while field weakening may create overspeed risk.

Modern industrial systems often replace large DC motors with induction or permanent-magnet motors supplied by variable-frequency drives. Nevertheless, DC-machine principles remain directly relevant to drive design because torque-current relationships, back EMF, braking, current control, and thermal limits are common to many electromechanical systems.

Best practice includes regular brush inspection, commutator surface inspection, insulation-resistance testing, bearing lubrication, ventilation checks, overload protection, and verification of field continuity. Maintenance personnel should isolate the supply and follow electrical-safety procedures before inspection. Machine guarding, earthing, emergency stopping, and applicable electrical codes are essential.

Digital drives can estimate speed and torque, detect abnormal current, and schedule predictive maintenance. Such systems improve availability but do not replace correct protection, calibration, or engineering judgment. Safety functions must be independently validated where failure could injure personnel or release a suspended load.

Chapter Summary

A DC machine converts energy reversibly between electrical and mechanical forms. Its yoke, poles, field windings, armature, commutator, brushes, and auxiliary components each have distinct electromagnetic or mechanical functions. Generator EMF depends on flux, speed, conductors, poles, and winding parallel paths, while motor torque depends on flux and armature current.

Shunt motors provide nearly constant speed, series motors provide very high starting torque, and cumulative compound motors combine useful features of both. Because a stationary motor has no back EMF, starting equipment is required to limit armature current. Speed control is achieved through armature voltage, field flux, or armature resistance, with modern electronic drives generally offering the best efficiency and control. Regenerative, dynamic, and plugging braking have different energy and safety consequences.

Copper, core, mechanical, and stray-load losses determine efficiency and heating. Correct industrial selection requires consideration of torque-speed characteristics, starting requirements, duty cycle, braking, environment, and safety. Although brushless technologies have reduced the use of conventional DC machines, DC-machine theory remains fundamental to electrical drives and energy conversion.

Key Terms

DC machine: A reversible electromechanical energy-conversion machine.

Armature: The winding and magnetic core in which EMF is induced and torque is produced.

Commutator: A segmented mechanical rectifier and current-reversing device.

Brush: A stationary contact that transfers current to or from the commutator.

Yoke: The outer frame and magnetic return path of a machine.

Flux: Magnetic field quantity, measured in weber.

Back EMF: EMF generated in a motor that opposes the applied voltage.

Armature reaction: The effect of armature flux on the main field.

Lap winding: An armature winding commonly used for high-current, low-voltage machines.

Wave winding: An armature winding commonly used for high-voltage, low-current machines.

Shunt motor: A motor whose field winding is connected in parallel with the armature.

Series motor: A motor whose field winding is connected in series with the armature.

Compound motor: A motor having both series and shunt field windings.

Regenerative braking: Braking that returns generated electrical energy to the supply.

Dynamic braking: Braking in which generated energy is dissipated in a resistor.

Plugging: Braking by reversing the effective torque while limiting current.

Armature copper loss: Resistive loss proportional to armature-current squared.

Critical speed: The minimum speed at which a self-excited generator can build up voltage for specified conditions.

Duty cycle: The pattern of loading, operation, rest, and braking over time.

Review Questions

Short Answer Questions

1. What are the principal functions of the commutator in generator and motor operation?
2. Why is a starter required for a DC motor?
3. State two differences between lap and wave windings.
4. What is the purpose of interpoles?
5. Why must a series motor not be operated without load?

Long Answer Questions

1. Describe the construction of a DC machine and explain the function of each major part.
2. Derive the EMF equation of a DC generator and state its assumptions.
3. Explain the characteristics and applications of shunt, series, and cumulative compound motors.
4. Discuss armature-voltage, field, and resistance methods of speed control.
5. Explain the mechanisms, advantages, and limitations of regenerative, dynamic, and plugging braking.

Analytical / Application Questions

1. A 240 V shunt motor has an armature resistance of 0.15 Ω and draws 50 A. If the brush drop is 2 V, calculate its back EMF and explain how the value affects speed.
2. Select a suitable DC motor type for a crane requiring high starting torque, controlled lowering, and moderate speed regulation. Justify the selection and braking method.
3. A generator has an internal generated EMF of 250 V, armature current of 30 A, armature resistance of 0.20 Ω, and brush drop of 2 V. Calculate its terminal voltage and identify two practical effects that could make the measured voltage differ from the ideal result.

Multiple Choice Questions with Answers

Question 1

Which component mechanically rectifies the internally generated alternating EMF of a DC generator?

A. Yoke

B. Commutator

C. Bearing

D. Pole shoe

Correct Answer: B. Commutator

Question 2

For a simplex wave winding, the number of parallel paths is:

A. P

B. P/2

C. 2

D. 2P

Correct Answer: C. 2

Question 3

A DC motor draws excessive current at standstill mainly because:

A. Flux is maximum

B. Back EMF is zero

C. Armature resistance is infinite

D. The commutator is open-circuited

Correct Answer: B. Back EMF is zero

Question 4

For constant flux, electromagnetic torque of a DC motor is approximately proportional to:

A. Armature current

B. Speed squared

C. Field resistance

D. Armature voltage squared

Correct Answer: A. Armature current

Question 5

Which motor generally has the highest starting torque?

A. Shunt motor

B. Series motor

C. Separately excited motor with zero field

D. Synchronous motor

Correct Answer: B. Series motor

Question 6

Which speed-control method is generally inefficient because power is dissipated in external resistance?

A. Armature-voltage control

B. Field control

C. Armature-resistance control

D. Regenerative control

Correct Answer: C. Armature-resistance control

Question 7

In regenerative braking, energy is primarily:

A. Returned to the supply

B. Destroyed in the commutator

C. Stored permanently in the bearings

D. Converted only into friction heat

Correct Answer: A. Returned to the supply

Question 8

A four-pole simplex lap-wound machine has how many parallel paths?

A. 2

B. 4

C. 6

D. 8

Correct Answer: B. 4

Question 9

If a shunt motor operates at constant flux and its back EMF decreases, its speed will generally:

A. Increase proportionally

B. Decrease proportionally

C. Remain exactly unchanged

D. Become zero immediately

Correct Answer: B. Decrease proportionally

Question 10

Which loss is proportional to the square of armature current?

A. Windage loss

B. Hysteresis loss only

C. Armature copper loss

D. Bearing friction loss only

Correct Answer: C. Armature copper loss