AC and DC Motors: Introduction

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DC motors are commonly used to operate machinery in a variety of applications on the factory floor. DC motors were one of the first types of energy converters used in industry. Recall that the earliest machines require speed control and DC motors could have their speed changed by varying the voltage sent to them. The earliest speed controls for DC motors were nothing more than large resistors.

DC motors required large amounts of DC voltage for operation. In other words, a source for the DC voltage is needed at the factory. This creates a problem because DC voltage can't be generated and distributed over a long distance, so AC voltage is the industry standard. One way to provide the DC voltage is to use generators that are set up at the factory site where large AC motors are used to turn them to produce the amount of required DC voltage. This system uses a large AC motor to drive a DC generator directly at a constant speed. The field current in the generator is regulated to adjust the level of DC voltage from the generator, which in turn is used to vary the speed of any DC motor that the generator powers. This system, the Ward-Lennard system, was popular until solid-state diodes became available for rectifying large amounts of AC voltage to DC for use in motor-driven circuits. Once solid-state diodes and SCRs became available, DC motors became more usable in industry.


above: A permanent split-capacitor-motor,_5-inch. Commonly used in HVAC for Enclosed Fan/Blowers.

During the 1950s and 1960s the use of DC motors became more prevalent in machinery control because their speed and torque were easy to control with simple SCR controllers. The SCR could rectify AC voltage to DC, provide current and voltage control at the same time, and were capable of being paralleled for larger loads up to 1000 A. As solid-state controls became more reliable in the late 1960s and the 1970s, a wide variety of low-cost AC motor speed controls became available.

During these years transistors could handle larger loads, and microprocessors became relatively inexpensive so that they could be used to make variable-frequency AC motor controls. At this time, one had a choice of using good-quality AC or DC motors for all types of special speed and torque applications.

In this section we will introduce the concepts of controlling a DC motor’s speed, its torque, and being able to reverse the direction of its rotation. It's also important to be able to recognize the features that make the series, shunt, and compound DC motors different from each other. One should also have a good understanding of the basic parts of the DC motor so that when one must troubleshoot a DC motor circuit, one will be able to recognize a malfunctioning component and make repairs or replace parts as quickly as possible.

It's also important to understand the methods of controlling the speed of AC motors, their direction of rotation, and the amount of torque they can develop, since these are the principles that motor drives use to control motors. Operation of special drive controllers is easy to understand if you know what electrical principle they are trying to alter to provide control for the motor. If one doesn't understand the motor principle, it's doubtful that one will fully understand the motor control device, and this will make the system nearly impossible to troubleshoot. If one understands these concepts, one will easily be able to understand the next generation of controls that will be produced during the next ten years.

In this section, magnetic theories are provided with a discussion of basic DC motor components. Additional information will explain the differences between series, shunt and compound motors. Diagrams and schematics are also provided to explain the methods of controlling speed, rotation, and torque. These diagrams and schematics are useful for making field wiring connections and for testing motors during troubleshooting procedures.



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