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Chapter 5: Problem 5

Name two methods that are used to vary the speed of a dc motor.

Short Answer

Expert verified
Two methods to vary the speed of a DC motor are: 1. Varying the armature voltage: This is done by adjusting the resistance of a series resistor or rheostat connected to the motor's armature. The speed is then given by \(N \propto \frac{V_a}{桅}\), where \(N\) is the speed, \(V_a\) is the armature voltage, and \(桅\) is the magnetic flux. 2. Varying the field flux: This is achieved by changing the field current or number of turns in the motor's field winding. The speed is still given by \(N \propto \frac{V_a}{桅}\), with the same variables as mentioned previously.

Step by step solution

01

Method 1: Varying the Armature Voltage

To control the speed of a DC motor, we can vary the armature voltage. The simplest method of doing this is through a series resistor or a rheostat connected in series with the motor's armature. By adjusting the resistance, we can change the voltage drop across the resistor, thus changing the voltage applied to the motor's armature and controlling its speed. The formula for the speed of the motor in this method is as follows: \(N \propto \frac{V_a}{桅}\) Where \(N\) is the speed of the motor, \(V_a\) is the armature voltage, and \(桅\) is the magnetic flux.
02

Method 2: Varying the Field Flux

Another method to control the speed of a DC motor is by changing the strength of the magnetic field. This can be done by changing the field current or number of turns in the motor's field winding. Using a controller, we can vary the amount of current going into the field winding, thus changing the magnetic field strength and, in turn, changing the speed of the motor. The formula for the speed of the motor in this method is as follows: \(N \propto \frac{V_a}{桅}\) As we can see, the speed is still directly proportional to the armature voltage and inversely proportional to the magnetic field strength.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Varying Armature Voltage
The speed of a DC motor can be altered by varying its armature voltage. Think of the armature voltage as the 'throttle' of the motor 鈥� just like a gas pedal in a car, more voltage means more power and consequently higher speed. To achieve this voltage change, resistors or rheostats are typically connected in series with the motor's armature.

When the resistance is increased, the voltage across the armature drops due to a larger voltage being dropped across the resistor. This reduction in armature voltage slows down the motor. Conversely, decreasing the resistance will allow more voltage to the armature, speeding up the motor. This method is convenient and straightforward for speed control; however, one should be aware that there might be power losses due to heat in the resistors.
Magnetic Field Flux Variation
Another method for controlling the speed of a DC motor involves the variation of magnetic field flux. The magnetic field within a motor is crucial as it generates the torque that drives the motor's rotation. By adjusting the field current, the magnetic flux density changes, which, in turn, affects the motor's speed.

A common way to vary the field flux is by using a field rheostat to alter the current flowing through the motor's field winding. Increasing the current intensifies the magnetic field, which decreases the motor's speed, and decreasing the current weakens the magnetic field, thus increasing the motor's speed. An important aspect to remember here is to maintain the stability of the motor鈥檚 operation as drastic changes in magnetic field strength can lead to irregular performance or even damage.
DC Motor Speed Formula
The relationship between a DC motor's speed, armature voltage, and magnetic field strength is captured in a simple formula:
\[N \propto \frac{V_a}{桅}\]
where \(N\) represents the speed of the motor, \(V_a\) denotes the armature voltage, and \(桅\) stands for the magnetic field flux. The proposition that speed is directly proportional to the armature voltage and inversely proportional to the magnetic field flux is central to understanding DC motor behavior.

From this equation, it's clear that if you increase the armature voltage while keeping the magnetic field constant, the motor's speed will increase. Conversely, if you increase the magnetic field strength while keeping the armature voltage constant, the motor's speed will decrease. This formula is a cornerstone for analyzing and predicting the behavior of DC motors and their speed control.

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Most popular questions from this chapter

A separately excited dc motor turns at \(1200 \mathrm{r} / \mathrm{min}\) when the armature is connected to a \(115 \mathrm{~V}\) source. Calculate the armature voltage required so that the motor runs at \(1500 \mathrm{r} / \mathrm{min}\). At \(100 \mathrm{r} / \mathrm{min}\).

If we reduce the normal exciting current of a practical shunt motor by 50 percent, the speed increases, but it never doubles. Explain why, bearing in mind the saturation of the iron under normal excitation.

What determines the magnitude and polarity of the counter-emf in a dc motor?

The following details are known about a \(250 \mathrm{hp}, 230 \mathrm{~V}, 435 \mathrm{r} / \mathrm{min} \mathrm{dc}\) shunt motor: nominal full-load current: \(862 \mathrm{~A}\) insulation class: \(\mathrm{H}\) weight: \(3400 \mathrm{~kg}\) external diameter of the frame: \(915 \mathrm{~mm}\) length of frame: \(1260 \mathrm{~mm}\) a. Calculate the total losses and efficiency at full-load. b. Calculate the approximate shunt field exciting current if the shunt field causes 20 percent of the total losses. c. Calculate the value of the armature resistance as well as the counter-emf, knowing that 50 percent of the total losses at full-load are due to armature resistance. d. If we wish to attain a speed of \(1100 \mathrm{r} / \mathrm{min}\), what should be the approximate exciting current?

Explain why the armature current of a shunt motor decreases as the motor accelerates.
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