Direct Current Motors and Generators

In Direct Current Motors and Generators , you'll learn ...

• The components and theory of operation behind a direct current (DC) generator
• The cause of armature losses in DC generators
• Three types of field connections to a self-excited generator
• Types of DC motors and how they operate

Overview

Preview a portion of this course before purchasing it.

Credit: 4 PDH

Length: 50 pages

A direct current generator, or DC generator, produces a voltage of constant polarity, which means the voltage and current do not change over time. DC generators are basically AC generators whose output voltage is switched the other way around at the proper moment, so that the direction of the voltage is always in a single direction. The AC generator uses slip rings to transfer the current to the electrical circuit, while the DC generator uses a split-ring commutator.

Generators can be very small or quite huge. Commercial DC generators are commonly found in traction applications like subways and trains. Factories that do electrolysis, electroplating and those that produce aluminum, caustic soda, chlorine, and some other industrial materials need large amounts of direct current and use DC generators.

An electric motor is very similar to a generator, except that power is provided to turn the rotors. They may, in fact, be described as generators "running backwards". When current is passed through the armature of a DC motor, a torque is generated by magnetic reaction, and the armature revolves. Adjustable speed is difficult to obtain with motors whose armatures are connected to fixed frequency power lines (AC motors). Here is where DC motors shine; their rotating field speed depends on the rotor speed itself. The speed of DC series motors varies with load, and torque varies inversely with speed. This makes them particularly suitable to starting high inertia loads such as railway trains.

Starting a DC motor often requires an external resistor or rheostat to limit the current. The value, in Ohms, of that resistor is reduced in steps as the speed of the motor increases, until finally that resistor is removed from the circuit as the motor reaches close to its final speed.

This 4-hour course discusses salient features of DC motors and generators in detail.

Specific Knowledge or Skill Obtained

This course teaches the following specific knowledge and skills:

• The principle by which generators convert mechanical energy to electrical energy
• The rule to be applied when you determine the direction of induced emf in a coil
• What component causes a generator to produce direct current rather than alternating current
• How field strength can be varied in a dc generator
• The three classifications of dc generators
• The term that applies to voltage variation from no-load to full-load conditions and how it is expressed as a percentage
• The factors that determine the direction of rotation in a dc motor
• The right-hand rule for motors
• The main differences and similarities between a dc generator and a dc motor
• The advantages and disadvantages of different types of dc motors

Certificate of Completion

You will be able to immediately print a certificate of completion after passing a multiple-choice quiz consisting of 25 questions. PDH credits are not awarded until the course is completed and quiz is passed.