6.3 DC Motors

6.3 DC Motors

Introduction to DC Motors

A DC motor is an electromechanical machine that converts direct current (DC) electrical energy into mechanical energy. It operates on the principle that a current-carrying conductor placed in a magnetic field experiences a mechanical force (Lorentz force). While AC induction motors dominate modern industry due to their simplicity and ruggedness, DC motors remain indispensable in applications requiring precise speed control, high starting torque, or operation from DC power sources (batteries, solar systems). Their ability to deliver excellent torque characteristics over a wide speed range makes them ideal for traction systems, cranes, machine tools, and robotics. This section delves into their operating principles, types, control methods, and applications.

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1. Operating Principle and Construction

1.1 Operating Principle (Lorentz Law and Commutation)

1. Fundamental Principle: When a current-carrying conductor is placed in a magnetic field, it experiences a mechanical force. The force magnitude is: $F = B \ I \ l$ where $B$ is flux density, $I$ is current, and $l$ is conductor length. The force direction is given by Fleming's Left-Hand Rule (motor rule).

2. Role of the Commutator: In a DC motor, the armature is supplied with DC. However, for continuous unidirectional torque, the current direction in each coil must reverse as it passes the magnetic neutral axis. The commutator serves as a mechanical rotary switch that automatically reverses the armature coil connections to the DC supply as the rotor turns.

3. Back EMF ($E_b$): A crucial phenomenon in motors. As the armature conductors rotate in the magnetic field, an EMF is induced in them according to the generator action. This induced EMF, called Back EMF, opposes the applied voltage (Lenz's Law). Its presence limits the armature current. $E_b = \frac{P \phi Z N}{60 A} = k \phi \omega$ The net voltage driving current through the armature resistance is $(V - E_b)$.

1.2 Construction

The construction is almost identical to that of a DC generator, consisting of a Stator (field system) and a Rotor (armature). The main difference is in the purpose: the motor uses electrical input to produce torque, while the generator uses mechanical input to produce electricity.

A. Stator Components

1. Yoke: Provides structural support and a path for magnetic flux.

2. Field Poles: Made of laminated steel, carry field windings to produce the main flux.

3. Field Windings: Energized by DC to create the stationary magnetic field.

4. Interpoles (Commutating Poles): Improve commutation by reducing sparking at brushes.

5. Brushes and Brush Gear: Supply DC current to the rotating armature via the commutator.

B. Rotor (Armature) Components

1. Armature Core: Laminated silicon steel cylinder with slots, mounted on the shaft.

2. Armature Windings: Coils placed in slots, connected to form a closed circuit via the commutator.

3. Commutator: Segmented copper cylinder that rectifies the current in armature coils for unidirectional torque.

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2. Types of DC Motors

DC motors are classified based on the connection between the field winding and the armature.

2.1 Separately Excited DC Motor

1. Connection: Armature and field windings are supplied from independent DC sources.

2. Equations:

   • Armature Voltage: $V_a = E_b + I_a R_a$

   • Field Voltage: $V_f = I_f R_f$ (independent)

3. Characteristics: Speed and torque can be controlled independently by varying $V_a$ and $I_f$. Used in applications requiring precise control (e.g., machine tools).

2.2 Self-Excited DC Motors

A. Shunt Motor

1. Connection: Field winding (shunt field) is connected in parallel with the armature across the DC supply. It has high resistance with many turns.

2. Equations:

   • Supply Voltage: $V = E_b + I_a R_a$

   • Field Current: $I_{sh} = V / R_{sh}$ (approximately constant)

   • Line Current: $I_L = I_a + I_{sh}$

3. Characteristics: Nearly constant speed from no-load to full-load (small speed drop). Moderate starting torque. Used for constant speed applications: lathes, fans, blowers, conveyors.

B. Series Motor

1. Connection: Field winding (series field) is connected in series with the armature. It has few turns of thick wire (low resistance).

2. Equations:

   • $I_a = I_{se} = I_L$

   • $V = E_b + I_a (R_a + R_{se})$

3. Characteristics:

   • High starting torque (torque ∝ $I_a^2$ at low saturation).

   • Speed varies widely with load: High speed at light load, dangerously high at no-load (must never be run without a mechanical load).

   • Used for traction, cranes, hoists, electric locomotives.

C. Compound Motor

1. Connection: Has both shunt and series field windings. Can be cumulative (fields aid) or differential (fields oppose).

2. Types:

   • Long Shunt: Shunt field across supply, series field in line with armature.

   • Short Shunt: Shunt field across armature, series field in line with supply.

3. Characteristics: Combines features of shunt and series motors.

   • Cumulative Compound: Good starting torque (from series field) and reasonable speed regulation (from shunt field). Speed drops with load but not as drastically as a series motor.

   • Differential Compound: Speed increases with load; rarely used due to instability.

4. Applications: Cumulative compound motors are used where high starting torque is needed but runaway at no-load is unacceptable (e.g., elevators, rolling mills).

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3. Output Characteristics (Speed-Torque Characteristics)

These curves show how the motor's speed ($N$) varies with the load torque ($T$) at constant supply voltage.

3.1 Fundamental Speed Equation

From the armature circuit equation and back EMF formula: $V = E_b + I_a R_a = k \phi N + I_a R_a$ Rearranging for speed: $N = \frac{V - I_a R_a}{k \phi}$

For a given motor, speed depends on:

1. Applied voltage ($V$)

2. Armature resistance drop ($I_a R_a$)

3. Field flux ($\phi$)

3.2 Characteristics of Different Motors

1. Shunt Motor:

   • Flux $\phi$ is approximately constant ($I_{sh}$ constant).

   • Speed drop is small due to $I_a R_a$ drop and slight armature reaction.

   • $N$ vs $T$ curve is slightly drooping, nearly constant speed.

   • Equation: $N \propto (V - I_a R_a)$

2. Series Motor:

   • Flux $\phi \propto I_a$ (before saturation).

   • At high loads (saturated), $\phi$ is nearly constant.

   • Before saturation: $T \propto I_a^2$ and $N \propto 1/I_a$. Speed falls sharply with increase in torque.

   • At no-load ($I_a \approx 0$): $N$ becomes dangerously high (runaway condition).

   • Curve is hyperbolic, showing high torque at low speed and vice versa.

3. Cumulative Compound Motor:

   • Characteristic lies between shunt and series motors.

   • Due to series field, speed drop with load is more pronounced than in a shunt motor but less than in a series motor.

   • Provides a compromise: better starting torque than shunt, better speed regulation than series.

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4. Speed Control Methods

The speed equation $N = \frac{V - I_a R_a}{k \phi}$ indicates three primary control methods.

4.1 Flux Control (Field Weakening)

1. Method: Increase speed by decreasing the field flux ($\phi$). Achieved by inserting a rheostat in series with the shunt field to reduce $I_{sh}$.

2. Applicability: Above base speed. Only for shunt and compound motors (Not for series motors as it would cause further speed increase).

3. Characteristics:

   • Speed is inversely proportional to flux: $N \propto 1/\phi$.

   • Constant Power Drive: Since $T \propto \phi I_a$, reducing $\phi$ while keeping $I_a$ constant reduces torque. Power ($\propto T \times N$) remains roughly constant.

   • Maximum speed is limited by magnetic stability and commutation.

4.2 Armature Resistance Control

1. Method: Insert an external resistance ($R_{ext}$) in series with the armature circuit. This increases the $I_a R_a$ drop, reducing the back EMF and thus the speed.

2. Applicability: Below base speed. Used for all types (shunt, series, compound).

3. Characteristics:

   • Speed reduction: $N \propto (V - I_a(R_a + R_{ext}))$.

   • Inefficient: Losses in $R_{ext}$ are high ($I_a^2 R_{ext}$), especially at low speeds. Efficiency drops proportionally with speed.

   • Used for short-period or intermittent speed reduction (e.g., during starting).

4.3 Armature Voltage Control

1. Method: Vary the voltage applied directly to the armature terminals while keeping the field at full strength.

2. Applicability: Below base speed. Requires a separate adjustable DC source (Ward-Leonard system, chopper, or controlled rectifier).

3. Characteristics:

   • Speed is directly proportional to applied voltage: $N \propto V$ (since $\phi$ constant).

   • Constant Torque Drive: For constant armature current, torque remains constant as speed varies.

   • Efficient: No extra resistors, losses are minimal. This is the preferred method for wide, smooth speed control.

4.4 Ward-Leonard System (Historical/Advanced)

• A classic system for precise speed control using a motor-generator (MG) set.

• A DC generator, driven by an AC motor, supplies variable voltage to the armature of the main DC motor.

• Provides smooth control over a wide range in both directions with regenerative braking capability. Largely replaced by solid-state thyristor/chopper drives.

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5. Starting and Braking Techniques

5.1 Starting of DC Motors

1. The Starting Problem: At standstill, back EMF ($E_b = 0$). If full voltage is applied, the starting armature current would be dangerously high: $I_{a(start)} = \frac{V}{R_a}$ (can be 10-20 times full-load current). This can damage the motor (commutator, windings) and cause excessive line voltage dip.

2. Solution - Use of a Starter: A starter is a device that inserts external resistance in series with the armature at start and gradually cuts it out as the motor speeds up and $E_b$ builds up.

3. Types of Starters:

   • 3-Point Starter (for Shunt & Compound Motors): Has three terminals: Line (L), Armature (A), Field (F). Provides no-volt release (NVR) and overload release protection.

   • 4-Point Starter: Improved version that prevents the NVR coil from being affected by field rheostat settings.

   • 2-Point Starter (for Series Motors): Simpler, but less common.

5.2 Braking of DC Motors

Braking is used to stop the motor quickly or to control the speed of a descending load.

1. Regenerative Braking:

   • Condition: Motor acts as a generator when its speed exceeds the no-load speed ($E_b > V$).

   • Action: Armature current reverses, feeding power back to the supply.

   • Application: Used in electric vehicles and elevators for energy recovery during downhill travel or lowering loads.

2. Plugging (Reverse Current Braking):

   • Method: Reverse the polarity of the armature (or field) supply while the motor is running. This creates a torque opposing rotation.

   • Action: Very rapid braking, but kinetic energy is dissipated as heat in the armature and resistors. Requires a relay to disconnect the motor at zero speed to prevent reversal.

   • Application: For quick stopping (e.g., in presses).

3. Dynamic (Rheostatic) Braking:

   • Method: Disconnect the armature from the supply and connect it across a braking resistor ($R_b$). The motor acts as a generator, dissipating kinetic energy as $I_a^2 R_b$ heat in the resistor.

   • Action: Braking torque decreases as speed decreases.

   • Application: Common for shunt and compound motors; requires modification for series motors.

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6. Applications, Losses, and Efficiency

6.1 Applications of DC Motors

• Shunt Motors: Machine tools, centrifugal pumps, fans, blowers, conveyors, lathes (constant speed applications).

• Series Motors: Electric traction (trains, trams), cranes, hoists, elevators, winches (high starting torque, variable speed).

• Compound Motors: Rolling mills, shears, punching machines, elevators (where high starting torque and moderate speed regulation are needed).

• Separately Excited Motors: Precision machine tools, robotics, servo drives (where independent control of speed and torque is required).

6.2 Losses and Efficiency

Losses are identical to those in DC generators (see Section 6.2, 4.1).

1. Loss Categories:

   • Copper Losses: $I^2R$ in armature, field, and brush contacts.

   • Iron (Core) Losses: Hysteresis and eddy current in armature core.

   • Mechanical Losses: Friction and windage.

   • Stray Load Losses.

2. Efficiency Calculation: $\eta = \frac{\text{Mechanical Output Power}}{\text{Electrical Input Power}} \times 100\% = \frac{P_{out}}{P_{in}} \times 100\%$ $P_{out} = T_{sh} \omega$ $P_{in} = V I_L$ Maximum efficiency occurs when variable losses = constant losses.

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7. Introduction to Brushless DC (BLDC) Motors

7.1 Why BLDC?

DC motors with brushes (commutators) have limitations: brush wear, sparking, RFI generation, and maintenance. BLDC motors overcome these by using electronic commutation.

7.2 Construction and Operation

1. Construction:

   • Rotor: Permanent magnets (usually rare-earth like Neodymium).

   • Stator: Three-phase windings (similar to an induction motor).

   • Sensors: Hall-effect sensors detect rotor position.

   • Electronic Controller: Contains an inverter (power transistors) and logic circuits.

2. Operating Principle:

   • The electronic controller uses rotor position feedback from sensors to sequentially energize the stator phases.

   • This creates a rotating magnetic field that "drags" the permanent magnet rotor.

   • Electronic commutation replaces the mechanical commutator and brushes.

3. Advantages:

   • High efficiency, high power density.

   • Long life, low maintenance (no brushes).

   • High speed operation, better speed-torque characteristics.

   • Low electrical noise.

4. Disadvantages:

   • Higher cost due to electronic controller and magnets.

   • Requires a complex controller.

5. Applications: Computer hard drives, drones (UAVs), electric vehicles, HVAC fans, industrial robots, cordless power tools.

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8. Testing and Direction Reversal

8.1 Testing of DC Motors

1. Brake Test (Direct Loading): For small motors. A belt wrapped around a pulley applies a mechanical load (brake). Torque is measured using a spring balance. Simple but inefficient due to energy loss as heat.

2. Swinburne's Test (Indirect Method, for Shunt Motors):

   • Run the motor on no-load at rated voltage to measure no-load current ($I_0$) and speed.

   • Calculate constant losses (iron + mech + shunt field loss) from no-load data.

   • Estimate efficiency at any load using these constant losses and calculated armature copper loss. Disadvantage: Does not account for stray load losses and assumes flux is constant.

3. Hopkinson's Test (Regenerative or Back-to-Back Test):

   • Two identical motors are mechanically coupled and electrically connected in parallel.

   • One acts as a motor driving the other as a generator. The generator's output feeds back to the motor, drawing only losses from the supply.

   • Highly efficient for testing, allows full-load testing with minimal power input from mains.

   • Measures all losses accurately under load conditions.

8.2 Direction Reversal

The direction of rotation of a DC motor is given by Fleming's Left-Hand Rule and depends on the relative direction of magnetic field and armature current. To reverse direction:

1. Method 1: Reverse Armature Leads: Swap the connections of the armature terminals to the supply. This is the standard and preferred method, especially for series and compound motors.

2. Method 2: Reverse Field Leads: Swap the connections of the field winding terminals. Caution: In a series motor, this reverses both field and armature current, resulting in no change in direction. In shunt motors, it can be used but is less common due to the high inductance of the field winding, which can cause dangerous voltage spikes.

• General Rule: Reverse either the armature connections OR the field connections, but never both.

Conclusion: The DC motor's versatility in speed and torque control has secured its place in modern engineering despite the simplicity of AC induction motors. From the fundamental principles of back EMF and commutation to advanced control techniques and the evolution towards brushless designs, a comprehensive understanding of DC motors is essential for designing and maintaining a wide array of industrial and consumer motion control systems.