Motors Book - Chapter 7

A discharge resistor is used to release or dissipate any AC potential that accumulates in the amortisseur winding of a rotor in a synchronous motor. This helps to prevent any unwanted or excessive build-up of electrical charge in the winding, ensuring the motor operates efficiently and safely.

A brushless exciter is an AC generator and rectifier mounted on a motor shaft. Unlike traditional exciter systems that use brushes and commutators to transfer power, a brushless exciter uses electronic components to control the flow of current. This design eliminates the need for brushes, reducing maintenance and improving reliability. The brushless exciter is commonly used in applications where a stable and efficient power supply is required, such as in power plants or industrial machinery.

An out-of-step relay is used to protect a synchronous motor from damage caused by the rotor falling out of step with the rotating stator field. When the rotor loses synchronization with the stator field, it can induce high currents that can damage the motor. The out-of-step relay detects this condition and initiates protective measures to prevent further damage.

Synchronous motors that typically run faster than about 450 rpm are considered high-speed synchronous motors. The term "high-speed" implies that these motors operate at a faster rotational speed compared to their counterparts. This indicates that any synchronous motor running at a speed lower than 450 rpm would not be classified as a high-speed synchronous motor.

Permanent magnets can be used to generate a magnetic field without the need for any external excitation. Unlike other options such as static exciters, brushless exciters, or motor-generator sets, which require an external power source to excite the rotors, permanent magnets have a fixed magnetic field that remains constant. This makes them a convenient and efficient choice for applications where a continuous and reliable magnetic field is required without the need for additional power or control systems.

The exciter generator is typically a shunt- or compound-wound DC generator with the armature in parallel with the shunt field and its shunt-field rheostat. This type of generator is commonly used to provide the field current for the main generator in a power system. It helps to regulate the voltage output of the main generator by controlling the amount of field current supplied.

The correct answer is "all of the above" because a common type of relay used with synchronous motors can include polarized field frequency relay, loss-of-excitation (field-failure) relay, and out-of-step relay. These relays are used to protect synchronous motors from various issues such as incorrect field frequency, loss of excitation in the field, and being out of synchronization with the power system.

Amortisseur windings, also known as damper windings, are squirrel-cage conducting bars placed in slots on the pole faces and connected at the ends. These windings are used to reduce rotor vibrations and provide damping effect during transient conditions in electrical machines such as generators and motors. They help in improving the stability and performance of the machine by reducing mechanical stresses and preventing rotor oscillations.

The stator is constructed of laminated iron sheets to minimize eddy current losses. Eddy currents are induced currents that circulate within conductive materials, causing energy losses in the form of heat. By using laminated iron sheets, the stator reduces the path for eddy currents to flow, minimizing these losses and improving the efficiency of the device.

The correct answer is "all of the above" because all three types of torque - reluctance torque, pull-in torque, and pull-out torque - are important to understand for synchronous motors. Reluctance torque is the torque produced due to the magnetic reluctance of the motor's rotor and stator. Pull-in torque is the minimum torque required to start the motor and bring it into synchronism with the applied voltage. Pull-out torque is the maximum torque that the motor can deliver while remaining in synchronism. Understanding all these types of torque is crucial for proper operation and control of synchronous motors.

The correct answer is direct online (DOL). In direct online starting, the synchronous motor is connected directly across the line conductors without any additional starting devices or reduced voltage. This method allows the motor to start at full voltage and full speed.

Pull-in torque is the maximum torque required to accelerate a synchronous motor into synchronization at the rated voltage and frequency. This torque is needed to overcome the initial inertia and magnetic reluctance of the motor, allowing it to reach synchronous speed and lock onto the rotating magnetic field. Once the motor is in synchronization, it can operate efficiently and maintain a constant speed.

Synchronous torque is provided by the interaction of the stator field and the rotor field windings. In a synchronous motor, the stator and rotor magnetic fields must be perfectly synchronized for efficient operation. The stator field is generated by the stator windings, while the rotor field is created by the rotor windings. The interaction between these two fields produces the synchronous torque, which is responsible for the motor's rotation. This torque allows the motor to maintain a constant speed and operate in synchronization with the power supply frequency.

The power factor of a synchronous motor is determined by the amount of AC power applied to the rotor. The power factor is a measure of how effectively the motor converts electrical power into mechanical power. In an AC system, the power factor is influenced by the phase relationship between the voltage and current. A synchronous motor operates on AC power, and the power factor is important for assessing its efficiency and performance.

Permanent magnet rotors are simpler than excited rotors and have minimal maintenance requirements. This is because permanent magnet rotors do not require any external power source to generate a magnetic field. The magnets are permanently magnetized, eliminating the need for additional components such as brushes or slip rings. This simplicity in design results in lower maintenance requirements compared to excited rotors, which rely on external power sources to create a magnetic field.