Electric Drives MCQ Quiz - Objective Question with Answer for Electric Drives - Download Free PDF

Explanation:

Main Purpose of the Load in a Drive System

The main purpose of the load in a drive system is to consume power and perform the desired task. In any drive system, the load is the component or system that uses the mechanical power provided by the drive to carry out a specific function. This is an essential aspect of the drive system's operation as it directly relates to the system's efficiency and effectiveness in performing its intended purpose.

Understanding Drive Systems:

A drive system is designed to provide controlled motion to a load. It typically consists of an electric motor, a power supply, a control unit, and the load itself. The motor converts electrical energy into mechanical energy, which is then used to move the load. The control unit regulates the motor's operation, ensuring that the load moves in the desired manner.

Key Components of a Drive System:

• Electric Motor: Converts electrical energy into mechanical energy.
• Power Supply: Provides the necessary electrical energy to the motor.
• Control Unit: Manages the motor's operation, ensuring precise control of speed, torque, and direction.
• Load: The component or system that consumes the mechanical power to perform a specific task.

Role of the Load:

The load is the ultimate recipient of the mechanical power generated by the drive system. It is the part of the system that performs the actual work, whether it be lifting, rotating, moving, or any other form of mechanical task. The performance of the drive system is often evaluated based on how effectively the load performs its function. Here are some examples of loads in different drive systems:

• Conveyor Systems: In these systems, the load could be the conveyor belt that moves materials from one point to another.
• Pumps: In pumping systems, the load is the pump that moves fluids through pipes and channels.
• Fans and Blowers: In ventilation systems, the load is the fan or blower that moves air to provide cooling or ventilation.
• Machinery: In industrial machinery, the load could be any component that performs a specific mechanical operation, such as cutting, drilling, or pressing.

Power Consumption:

One of the primary functions of the load is to consume power. The efficiency of the drive system depends on how effectively the load uses the mechanical power provided by the motor. Inefficient loads can lead to wasted energy and reduced overall performance of the drive system. Therefore, it is crucial to design and select loads that match the capabilities of the drive system and operate efficiently.

Performing the Desired Task:

The load is responsible for carrying out the specific task for which the drive system is designed. This could involve a wide range of activities, from simple movements to complex operations. The nature of the task determines the type of load and its requirements in terms of power, speed, and control. Ensuring that the load performs its task effectively is a key objective in the design and operation of drive systems.

Correct Option Analysis:

The correct option is:

Option 4: To consume power and perform the desired task.

This option accurately captures the main purpose of the load in a drive system. The load is the component that uses the power generated by the motor to perform the specific task for which the system is designed. Without the load, the drive system would have no purpose, as there would be no work to be done.

Additional Information

To further understand the analysis, let’s evaluate the other options:

Option 1: To process and analyse data.

This option is incorrect because processing and analyzing data is not the primary function of the load in a drive system. These activities are typically associated with control units or computer systems that manage the operation of the drive system, rather than the load itself.

Option 2: To regulate the control unit's operation.

This option is also incorrect. The control unit's operation is regulated by sensors, feedback mechanisms, and control algorithms, not by the load. The load's primary role is to consume power and perform the desired mechanical task.

Option 3: To provide input signals to the control unit.

This option is incorrect as well. Input signals to the control unit are usually provided by sensors and other monitoring devices that detect the operating conditions of the system. The load does not typically provide input signals to the control unit.

Conclusion:

In summary, the main purpose of the load in a drive system is to consume power and perform the desired task. The load is the component that utilizes the mechanical power generated by the motor to carry out specific functions, making it a crucial part of the drive system. Understanding the role of the load and its interaction with other components of the drive system is essential for designing efficient and effective drive systems.

Explanation:

When selecting an electric motor for a specific application, several factors must be considered to ensure optimal performance and efficiency. Among the options provided, the most important factor governing the selection of an electric motor is the torque-speed characteristics of the motor. Let's delve into a detailed explanation of why this factor is crucial and analyze the other options to understand their relative importance.

Torque-Speed Characteristics of the Motor:

Electric motors are used in a wide range of applications, each requiring specific performance characteristics. The torque-speed characteristic of a motor describes how the torque produced by the motor varies with its speed. This characteristic is fundamental in determining whether a motor is suitable for a particular application. Here are some key points to consider:

• Matching Load Requirements: Different applications require different torque and speed profiles. For instance, in applications like conveyor belts, constant torque is needed across a wide speed range. In contrast, fan and pump applications may require variable torque. The torque-speed characteristic helps in selecting a motor that matches the load requirements precisely.
• Starting Torque: Some applications, such as those involving heavy loads, need a motor with high starting torque to overcome initial inertia. The torque-speed characteristic provides insight into the starting torque capabilities of the motor, ensuring it can handle the load during startup.
• Operational Efficiency: The efficiency of a motor varies with its operating point on the torque-speed curve. Selecting a motor with the right torque-speed characteristic ensures that it operates at its highest efficiency for the given application, reducing energy consumption and operational costs.
• Stability and Control: Motors with specific torque-speed characteristics offer better stability and control in dynamic applications. For instance, in precision machinery, maintaining a constant speed under varying loads is crucial, and the right torque-speed characteristic facilitates this control.

In summary, the torque-speed characteristic of an electric motor is the most critical factor in ensuring that the motor meets the specific demands of the application, operates efficiently, and provides the necessary performance. Now, let's analyze the other options to understand their importance relative to the torque-speed characteristic.

Ambient Temperature of the Place Where the Motor is Mounted:

While the ambient temperature is an important consideration, it is not the most critical factor. The ambient temperature affects the cooling and thermal management of the motor. Motors are designed to operate within specific temperature ranges, and exceeding these limits can lead to overheating and reduced lifespan. However, proper cooling mechanisms and insulation can mitigate the impact of ambient temperature, making it a manageable factor rather than the primary consideration.

Maintenance Cost of the Motor:

Maintenance cost is an important factor from an operational and economic standpoint. Motors with lower maintenance costs are preferred as they reduce downtime and operational expenses. However, maintenance costs are secondary to ensuring that the motor can perform the required tasks efficiently and reliably. A motor with the right torque-speed characteristic but higher maintenance costs may still be preferred over one with lower maintenance costs but unsuitable performance characteristics.

Appearance of the Motor:

The appearance of the motor is generally the least important factor in its selection. While aesthetics might be considered in some applications where the motor is visible and design continuity is desired, it has no bearing on the motor's performance, efficiency, or suitability for the application. Functional characteristics such as torque-speed performance take precedence over appearance.

Conclusion:

In conclusion, when selecting an electric motor for a specific application, the torque-speed characteristic is the most important factor to consider. This characteristic ensures that the motor can meet the performance requirements of the application, operate efficiently, and provide reliable service. While other factors such as ambient temperature, maintenance cost, and appearance are important, they are secondary to the fundamental requirement of matching the motor's performance characteristics to the needs of the application.

Additional Information

To further understand the analysis, let’s evaluate the other options:

Option 1: Ambient Temperature of the Place Where the Motor is Mounted

The ambient temperature affects the cooling and thermal management of the motor. High ambient temperatures can lead to overheating and reduced lifespan. While it is an important factor to consider, motors are designed with cooling mechanisms and insulation to handle specific temperature ranges. Therefore, it is not the most critical factor in motor selection.

Option 2: Maintenance Cost of the Motor

Maintenance cost is important for reducing operational expenses and downtime. However, it is secondary to ensuring that the motor can perform the required tasks efficiently and reliably. A motor with the right performance characteristics but higher maintenance costs may still be preferred over one with lower maintenance costs but unsuitable performance.

Option 3: Appearance of the Motor

The appearance of the motor is generally the least important factor in its selection. While aesthetics might be considered in applications where the motor is visible, it has no bearing on the motor's performance or suitability for the application. Functional characteristics take precedence over appearance.

Conclusion:

Understanding the relative importance of various factors in selecting an electric motor is essential. The torque-speed characteristic is the most critical factor, ensuring that the motor meets the specific demands of the application, operates efficiently, and provides the necessary performance. While other factors like ambient temperature, maintenance cost, and appearance are important, they are secondary to the fundamental requirement of matching the motor's performance to the needs of the application.

Explanation:
The control system in a load drive system is responsible for regulating and managing the operation of the load drive. It ensures that the load, which could be a motor or any other device, operates in a controlled and desired manner. The control system monitors parameters such as speed, torque, or position, and adjusts the input signals to the load drive to maintain optimal performance. By providing feedback and making real-time adjustments, the control system enhances precision, stability, and efficiency in the operation of the load drive system.

Continuous Duty with Intermittent Periodic Loading:

• It consists of periodic duty cycles, each consisting of a period of running at a constant load and a period of running at no load.
• Pressing, cutting, shearing, and drilling machine drives are the examples.

Intermittent periodic duty with starting and braking:

• It consists of a starting period, a running period; a braking period & a rest period are being too short to reach their steady state temperature value.
• This type of duty can be accomplished by single phase/ three phase induction motors and DC shunt motors, DC series motors, DC compound motors, universal motors.
• Examples: Billet mill drive, ingot buggy drive, screw down mechanism of blooming mill, several machine tool drives, drives for electric suburban trains and Mine hoist.

Continuous Duty with Starting and Braking:

• Consists of periodic duty cycle, each having a period of starting, a period of running at a constant load and a period of electrical braking; there is no period of rest.
• The main drive of a blooming mill is an example.

Intermittent periodic duty without braking:

• The duty type is defined as a sequence of identical duty cycles, each cycle consisting of a time of operation at constant load and a time of operation at no-load.
• There is no time de-energized and at rest. The motor runs at no-load, without actual stopping.

Concept:

• The coefficient of adhesion for a straight DC locomotive is typically around 27%. This means that the maximum tractive effort that the locomotive can develop is equal to 27% of its adhesive weight.
• The coefficient of adhesion can vary depending on a number of factors, such as the condition of the rails, the weather, and the type of locomotive. However, a value of 27% is a good general rule of thumb for straight DC locomotives.

Four quadrant operation:

• The torque and speed coordinates for both forward (positive) and reverse (negative) motions of Four Quadrant Operation of Motor Drive. Power developed by a motor is given by the product of speed and torque.
• In quadrant I, developed power is positive. Hence, the machine works as a motor supplying mechanical energy. Operation in quadrant I is, therefore, called forward motoring.
• In quadrant II, power is negative. Hence, the machine works under braking opposing the motion. Therefore, operation in quadrant II is known as forward braking.
• Similarly, operations in quadrant III and IV can be identified as reverse motoring and braking respectively.

 

Concept:

• Since the primary limitation for the operation of an electric motor is its temperature rise, hence motor rating is calculated on the basis of its average temperature rise.
• The average temperature rise depends on the average heating which itself is proportional to the square of the current and the time for which the load persists.

 

For example, if a motor carries a load L₁ for time t₁ and load L2 for time t₂ and so on, then

Average heating ∝ L₁² t₁ + L₂² t₂ + ..........+ L_n² t_n, and the rating of the motor is given as

∴ size of the motor \(= \sqrt {\frac{{L_1^2\;{t_1} + L_2^2\;{t_2} + \ldots + L_n^2\;{t_n}}}{{{t_1} + {t_2} + \ldots + {t_n}}}} \)

Calculation:

Let the full load L₁ = 100 kW operated for time t₁ = 10 min

Half-full load L₂ = L₁ / 2 = 50 kW operated for time t₂ = 20 min

No-load L₃ = 0 KW operated for t₃ = 20 min

∴ Rating of the motor\( = \sqrt {\frac{{{{100}^2} \times 10 + {{50}^2} \times 20}}{{10 + 20 + 20}}} \)

∴ The rating of the suitable motor is = \(\sqrt {3000} \;kW\)

Plugging:

• Plugging braking of induction motor is done by interchanging connections of any two phases of the stator with respect of supply terminals
• Plugging is used to reduce the speed of motors
• The supply voltage V and the induced voltage Eb (back emf) will act in the same direction
• The effective voltage across the armature will be (V + Eb) which is almost twice the supply voltage
• During plugging external resistance is also introduced into the circuit to limit the flowing current
• Thus, the armature current is reversed and high braking torque is produced
• It provides the highest braking torque in comparison to rheostatic and regenerative braking systems.

Fan laws:

The fans operate under a predictable set of laws concerning speed, power and pressure. A change in speed (RPM) of any fan will predictably change the pressure rise and power necessary to operate it at the new RPM.

Flow: Flow is directly proportional to the speed.

Q ∝ N

Varying the RPM by 10% decreases or increases air delivery by 10%.

Pressure: Pressure is directly proportional to square of the speed.

SP ∝ N2

Reducing the RPM by 10% decreases the static pressure by 19% and an increase in RPM by 10% increases the static pressure by 21%.

Power: Power requirement is directly proportional to cube of the speed.

P ∝ N3

Reducing the RPM by 10% decreases the power requirement by 27% and an increase in RPM by 10% increases the power requirement by 33%.

Permissible overloads in Generator and Motor:

• A generator after attaining a temperature rise corresponding to continuous run on full load should be capable of withstanding 50% overload for 15 seconds.
   

A motor shall be capable of withstanding the overloads of:

• Up to 50 hp – 100% for 15 seconds
• Above 50 hp – 75% for 15 seconds
• Above 500 hp – 60% for 15 seconds
• Motor with short-term rating – 100% for 15 Seconds

Size and Rating of Motor:

• Since the primary limitation for the operation of an electric motor is its temperature rise, hence motor rating is calculated on the basis of its average temperature rise.
• The average temperature rise depends on the average heating which itself is proportional to the square of the current and the time for which the load persists.

 

For example, if a motor carries a load L1 for time t1 and load L2 for time t2 and so on, then

Average heating ∝ L12 t1 + L22 t2 + ..........+ Ln2 tn, and the rating of the motor is given as

∴ size of the motor \(= \sqrt {\frac{{L_1^2\;{t_1} + L_2^2\;{t_2} + \ldots + L_n^2\;{t_n}}}{{{t_1} + {t_2} + \ldots + {t_n}}}} \)

The RMS value of motor rating is given by,

\(RMS\;HP = {\left[ {\frac{{\sum H{P^2} \times Time}}{{Time}}} \right]^{1/2}}\)

Calculation:
Continuous rating of a machine is given by = \(\sqrt {\sum \frac{{{{\left( {kw} \right)}^2} \times time}}{{total\;time}}}\)

Rating = \(\sqrt {\frac{{{{\left( {20} \right)}^2} \times 10 + {{\left( {10} \right)}^2} \times 20 + {{\left( {30} \right)}^2} \times 10}}{{10 + 20 + 10 + 10}}} = \sqrt {300} \;kW\)

Transformer:

• A transformer is an electrical device which transfers electrical energy from one circuit to another
• It works based on the principle of electromagnetic induction
• Power is transferred through the magnetic flux in transformer.
• It is used for increasing or decreasing the amount of voltage or current as per our requirement based on these transformers are classified into two types step-up (increasing) or step-down (decreasing) transformer.

• A transformer can be used to either increase or decrease the voltage of a circuit.
• In other words, it can either step up (increase) or step down (decrease) the voltage.
• A transformer is necessary because sometimes the voltage requirements of different appliances are variable.

 

Mechanical gear drive:

• Mechanical power is transmitted from one shaft to another through belts, chains and gears and type is selected according to application.
• In mechanical gear drive, there is a perfect ratio between the number of teeth and the speeds of the two gears.

 

Comparison of belt, chain and gear drive

Particulars	Belt drive	Chain drive	Gear drive
Main elements	Pulleys, belt	Sprockets, chain	Gear
Slip	Slip may occurs	No slip (positive drive)	No slip (Positive drive)
Suitability	For large centre Distance	For moderate centre distance	For short centre distance
Space requires	Large	Moderate	less
Design	Simplest	Simple	Complicated
Failure	Failure of belts does not cause the further damage of machine	Failure of chain may not seriously damage the machine	Failure of gear may cause serious break down in the machine
Life	Less	Moderate	Long
Lubrication	Not required	Require	Require proper
Use	For low VR	For moderate VR	For high VR
e.g.	Use as a first drive in transmission	Bicycle, Automobile	gear boxes, automobile

 

Concept:

Continuous and constant loads: The loads on the motor operate for a long time under the same conditions

Continuous and variable loads: The load on the motor operates repetitively for a longer duration but varies continuously over a period

Pulsating loads: The load on the motor which can be viewed as constant torque superimposed by pulsations

Impact loads: The load on the motor having regular and repetitive load peaks or pulses, i.e. load increases to a maximum level suddenly

Short time loads: The load of the motor occurs periodically remains constant for short time and then remains idle for longer time

Short time intermittent loads: The load on the motor occurs periodically in identical duty cycle, each duty cycle having period of application of load and rest

Application:

Cranes have the characteristics of short time intermittent loads.

Electrical drive:

The system which is used for controlling the motion of an electrical machine, such type of system is called an electrical drive. In other words, the drive which uses the electric motor is called the electrical drive.

Parts of Electrical Drive:

1.) Power Modulator:

• The power modulator regulates the output power of the source.
• It controls the power from the source to the motor in such a manner that the motor transmits the speed-torque characteristic required by the load.
• The power modulator converts the energy according to the requirement of the motor e.g. if the source is DC and an induction motor is used then the power modulator converts DC into AC. It also selects the mode of operation of the motor, i.e., motoring or braking.

2.) Control unit:

• The control unit sends the signal to the power modulator.
• The control unit controls the power modulator which operates at the small voltage and power levels.
• It also generates commands for the protection of the power modulator and motor. 

3.) Sensing Unit:

• It senses certain drive parameters like motor current and speed.
• It is mainly required either for protection or for closed-loop operation.

Concept:

• Since the primary limitation for the operation of an electric motor is its temperature rise, hence motor rating is calculated on the basis of its average temperature rise.
• The average temperature rise depends on the average heating which itself is proportional to the square of the current and the time for which the load persists.

 

For example, if a motor carries a load L1 for time t1 and load L2 for time t2 and so on, then

Average heating ∝ L12 t1 + L22 t2 + ..........+ Ln2 tn, and the rating of the motor is given as

∴ size of the motor \(= \sqrt {\frac{{L_1^2\;{t_1} + L_2^2\;{t_2} + \ldots + L_n^2\;{t_n}}}{{{t_1} + {t_2} + \ldots + {t_n}}}} \)

Calculation:

Let the full load L1 = 200 kW operated for time t1 = 10 min

Half-full load L2 = L1 / 2 = 100 kW operated for time t2 = 20 min

From above concept,

∴ Rating of the motor\( = \sqrt {\frac{{{{200}^2} \times 10 + {{100}^2} \times 20}}{{10 + 20}}} =\sqrt{\frac{60\times 10^4}{30}}=100\sqrt {2}\) kW