What is the primary purpose of braking methods in electrical machines?
A To improve efficiency
B To increase speed
C To reduce mechanical wear
D To stop the motor quickly
Braking methods are used in electrical machines to bring the motor to a stop quickly and safely. Common braking techniques include regenerative braking, dynamic braking, and mechanical braking, each serving to reduce the motor’s speed and halt its rotation.
Which braking method involves using the motor’s own energy to slow it down?
A Plugging
B Regenerative braking
C Mechanical braking
D Dynamic braking
Regenerative braking involves the motor acting as a generator. It converts the kinetic energy of the motor into electrical energy, which is fed back into the supply system or used to charge batteries, making it an energy‐efficient braking method.
What happens to the torque in a motor when the load increases?
A Torque remains constant
B Torque decreases
C Torque increases
D Torque fluctuates
In most motors, as the load increases, the torque required to maintain the desired speed also increases. The motor responds to the increased load by supplying more torque to maintain performance, especially in DC and induction motors.
What is the purpose of an equivalent circuit in electrical machines?
A To model the machine’s behavior
B To predict efficiency
C To reduce losses
D To increase torque
The equivalent circuit of an electrical machine is used to model its electrical and mechanical behavior. It helps analyze parameters like current, voltage, resistance, inductance, and efficiency, which are essential for machine design and performance prediction.
Which of the following is a characteristic of the load characteristics of a motor?
A Variable speed
B Constant current
C Constant speed
D Constant torque
Load characteristics of a motor refer to the relationship between the output torque and speed under varying loads. In many motors, the torque remains nearly constant as the load increases, especially in motors designed for constant torque applications.
What does “slip” in an induction motor refer to?
A Frequency deviation
B Speed difference
C Current change
D Voltage variation
Slip refers to the difference between the synchronous speed (the speed of the rotating magnetic field) and the actual rotor speed. Slip is necessary for the generation of torque in induction motors and increases as the load on the motor increases.
What is the effect of increasing the load on the efficiency of a motor?
A Efficiency remains constant
B Efficiency increases
C Efficiency fluctuates
D Efficiency decreases
As the load on a motor increases, the efficiency typically decreases due to additional losses, such as friction and heat dissipation. Higher loads increase the current, which leads to greater resistance losses and lower overall efficiency.
Which braking method involves reversing the motor’s direction to stop it?
A Plugging
B Dynamic braking
C Regenerative braking
D Mechanical braking
Plugging is a braking method where the motor’s direction is reversed by changing the phase sequence of the power supply. This method generates opposing torque, rapidly decelerating the motor. It is a quick but energy‐consuming braking technique.
What is the effect of load characteristics on the performance of an electrical motor?
A Determines the speed
B Affects voltage regulation
C Affects torque and speed
D Affects the efficiency
Load characteristics describe the motor’s ability to maintain speed and torque under varying loads. The torque‐speed curve is used to assess how the motor performs as the load increases. Different types of motors have distinct load characteristics that influence their applications.
Which of the following methods is used to reduce the starting current of an induction motor?
A Direct‐on‐line starting
B Autotransformer starting
C Slip‐ring starting
D Star‐delta starting
The star‐delta starting method is commonly used to reduce the starting current of an induction motor. By initially connecting the motor in a star configuration, the voltage applied to the motor windings is reduced, which in turn lowers the starting current.
What is the purpose of a dynamic braking system in electrical machines?
A To improve torque
B To stop the motor quickly
C To increase speed
D To reduce energy consumption
Dynamic braking involves applying a resistive load across the motor’s armature, which converts the kinetic energy into heat, rapidly decelerating the motor. This braking method is commonly used in applications where quick stopping is required, such as in elevators or cranes.
What is the primary cause of losses in the stator windings of an electrical machine?
A Hysteresis losses
B Eddy currents
C Mechanical losses
D Copper losses
Copper losses occur due to the resistance in the stator windings. As current flows through the windings, energy is dissipated as heat, reducing the overall efficiency of the machine. These losses can be minimized by using high‐quality copper and reducing the resistance.
In an electrical machine, what happens to the current drawn from the supply during startup?
A It increases
B It fluctuates
C It decreases
D It remains constant
During startup, an electrical machine typically draws a large current known as inrush current. This current is much higher than the rated current and is necessary to overcome the inertia and start the machine. This high inrush current can be managed using soft starters or star‐delta methods.
What is the primary function of the equivalent circuit in a motor?
A To calculate efficiency
B To predict motor behavior
C To represent energy losses
D To simulate load
The equivalent circuit of a motor helps predict its electrical and mechanical behavior under various operating conditions. It simplifies the complex relationships between voltage, current, resistance, and inductance, allowing for easier analysis of motor performance and efficiency.
Which braking method is most commonly used for high‐speed motors?
A Dynamic braking
B Plugging
C Regenerative braking
D Mechanical braking
Regenerative braking is most commonly used for high‐speed motors because it recovers energy during braking and feeds it back into the power system or stores it in batteries. This method is efficient and reduces energy waste, making it ideal for high‐speed applications.