Jawaharlal Nehru Technological University Hyderabad B.Tech IV UTILIZATION OF ELECTRIC POWER Previous Question Paper (December - 2020 - R16)

The performance of a DC motor drive is defined by its torque-speed characteristics during starting and steady-state operation, and its ability to maintain speed under varying load conditions.

1. Starting Characteristics

At the moment of starting, the back EMF (E_b) is zero because the speed (N) is zero. According to the equation:

Since E_b = 0, the starting current is limited only by the armature resistance (R_a), which is typically very low. This leads to an excessively high starting current that can damage the commutator and windings. Therefore, a starter (such as a 3-point or 4-point starter) is used to insert external resistance into the armature circuit, which is gradually cut out as the motor gains speed.

2. Running Characteristics

The running performance is governed by the relationship between torque (T) and speed (N):

• DC Shunt Motor: Acts as a constant speed motor. The flux (Φ) is constant, so speed remains relatively steady as load increases, with only a slight drop due to armature resistance drop.
• DC Series Motor: The flux is proportional to the armature current (Φ ∝ I_a). At low speeds, torque is very high (T ∝ I_a²), making it ideal for high-starting-torque applications like traction. However, it must never be started at no-load, as the speed can reach dangerous levels.

3. Speed Control Methods

The speed of a DC motor is given by the fundamental equation:

Based on this, speed control is achieved via:

• Flux Control Method: By varying the shunt field current using a rheostat, the flux (Φ) is changed. Since N ∝ 1/Φ, reducing the flux increases the speed above the rated value. This is an efficient method but limited by commutation constraints.
• Armature Resistance Control: By inserting a variable resistance in series with the armature, the voltage drop (I_aR_a) increases, reducing the effective voltage across the armature. This controls speed below the rated value. It is simple but results in high power loss in the external resistor.
• Voltage Control Method (Ward-Leonard System): The armature voltage (V) is varied using a separate DC generator. This provides a wide range of smooth speed control in both directions, though it is expensive due to the requirement of an additional motor-generator set.

Industrial loads are classified based on their duty cycles, torque-speed characteristics, and operational requirements. These classifications are critical for selecting the appropriate electric motor to ensure efficiency, longevity, and performance.

1. Classifications of Industrial Loads:

• Continuous Duty Loads: These loads operate at a constant power level for long periods without interruption (e.g., centrifugal pumps, fans, and compressors). Motors for these applications are selected based on their rated power and thermal capacity to prevent overheating.
• Intermittent Duty Loads: These loads operate for short periods followed by rest periods (e.g., cranes, hoists, and machine tool positioners). Motor selection depends on the equivalent continuous rating, considering the heating and cooling cycles.
• Variable Duty Loads: These loads undergo frequent changes in torque and speed (e.g., rolling mills, mine hoists). Motors must be selected to handle peak torque requirements and frequent acceleration/deceleration without exceeding thermal limits.
• Short-time Duty Loads: These operate for a very short duration, insufficient for the motor to reach thermal equilibrium (e.g., valve actuators).

2. Impact on Motor Selection:

• Torque Requirements: The motor must provide sufficient starting torque to overcome static friction and breakdown torque to handle peak load demands.
• Speed Control: Depending on the load, motors may require constant speed (synchronous or induction motors) or variable speed capability (DC motors or VFD-driven induction motors).
• Thermal Rating: The motor's insulation class and cooling method must match the duty cycle to prevent insulation failure due to heat accumulation.
• Inertia Matching: High-inertia loads require motors with high thermal capacity during the starting phase to handle the prolonged acceleration time.

3. Requirement for High Moment of Inertia Loads:

Accelerating a load with a high moment of inertia (J) requires a significant amount of energy to be dissipated as heat within the motor windings, as the motor must operate at high slip for an extended period. The main requirements are:

• High Starting Torque: The motor must produce a torque significantly higher than the load torque to allow for acceleration.
• Thermal Capacity: The motor must be designed to withstand the high I²R losses generated during the prolonged startup period.
• Soft Starting Techniques: To prevent mechanical stress and excessive current draw, techniques such as Star-Delta starters, Soft Starters, or Variable Frequency Drives (VFDs) are employed to control the acceleration ramp.

Where T is the motor torque, J is the moment of inertia, and dω/dt is the angular acceleration.