PCI 264 Ch 8 Quiz- DC Drives

When the speed of a DC motor is reduced in an application where the horsepower required by the load is constant, the torque is increased. This is because torque and speed have an inverse relationship in a DC motor. As the speed decreases, the torque increases to maintain a constant power output. Therefore, reducing the speed of a DC motor results in an increase in torque.

Both permanent magnet and shunt DC motors are typically controlled by a DC drive. A DC drive is a device that regulates the speed and torque of a DC motor by adjusting the voltage and current supplied to it. Permanent magnet DC motors have a fixed magnetic field created by permanent magnets, while shunt DC motors have a separate field winding that generates a magnetic field. By controlling the voltage and current to these motors, the DC drive can effectively control their speed and torque. Therefore, both types of motors can be controlled by a DC drive.

When the current applied to the field coil of a shunt field type DC motor is reduced while the current applied to the armature remains constant, the motor speed will increase. This is because the field coil current directly affects the strength of the magnetic field in the motor. By reducing the field coil current, the magnetic field weakens, which reduces the counteracting force on the armature. As a result, the armature spins faster, leading to an increase in motor speed.

The speed of a DC motor that requires a constant torque is controlled by adjusting the armature voltage and keeping the field current constant. The armature voltage determines the back EMF (electromotive force) in the motor, which in turn affects the speed. By increasing or decreasing the armature voltage, the back EMF can be adjusted to control the motor speed. Keeping the field current constant ensures that the magnetic field strength remains the same, allowing for a consistent torque output.

All of the devices listed (switches and pushbuttons, keypads, programmable controllers, and computers) can be used as the operator control source for the DC drive. Therefore, none of the above options are correct.

The external feedback loop provides the most accurate signal for a DC drive. This is because external feedback involves measuring the output of the DC drive and comparing it to the desired input. By using external feedback, any discrepancies between the desired and actual output can be detected and corrected, allowing for more precise control of the DC drive. In contrast, internal feedback relies on measuring internal variables within the DC drive itself, which may not accurately reflect the actual output. Therefore, external feedback is preferred for obtaining the most accurate signal in a DC drive.

The effective operation of punch presses, high inertia machines, pumps, and centrifugal fans require variable torque load characteristics. This means that the torque required by these machines varies with the speed at which they operate. As the speed changes, the torque needed to maintain the desired performance also changes. Constant torque load characteristics would not be suitable as they would not be able to adapt to the changing torque requirements. Constant horsepower load characteristics would also not be suitable as the torque needed would remain constant regardless of the speed, which may not be optimal for these types of machines.

By varying the voltage from the drive to the field of a motor, the velocity above the rated base speed of the motor can be controlled. This is because changing the voltage alters the magnetic field strength in the motor, which in turn affects the motor's speed. By increasing the voltage, the motor can operate at higher speeds above its rated base speed. Therefore, varying the voltage allows for control over the velocity of the motor beyond its rated speed.