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INTRODUCTION
DC Motor are widely used in adjustable speed drives and position control application, their speeds bellows base speed can be controllable by armature voltage control and the speed above base speed can be controlled by field flux control method . As speed controls method for DC motor are simpler and less expensive so DC motor are preferred where wide speed control range is required.
Phase controlled converter method or a drive provides an adjustable DC output voltage from a fixed AC input voltage. In AC circuit, the SCR or THYRISTOR can be turned ON by the gate pulse at any angle α with respect to applied voltage. This angle α is called the triggering or firing angle and power control is obtained by varying the triggering angle and is known as phase control converter method or drives.
DC drives consist of an SCR (Silicon Controlled Rectifier) bridge, which converts incoming three or single-phase AC volts to DC volts. During this conversion process DC drives then can regulate speed, torque, voltage and current conditions of the DC motor.
This is ideal for industrial processes such as tube mills, extruders, mixers, paper machines and various other controlled applications.
PRINCIPLES OF OPERATION
DC drives, because of their simplicity, ease of application, reliability and favorable cost have long been a backbone of industrial applications. A typical adjustable speed drive using a silicon controller rectifier (SCR) power conversion' section, common for this type unit, is shown in Figure . The SCR, (also termed a thyristor) converts the fixed voltage alternating current (AC) of the power source to an adjustable voltage, controlled direct current (DC) output which is applied to the armature of a DC motor
SCR's provide a controllable power output by "phase angle control", so called because the firing angle (a point in time where the
SCR is triggered into conduction) is synchronized with the phase rotation of the AC power source. If the device is triggered early in half cycle, maximum power is delivered to the motor; late triggering in the half cycle provides minimum power, as illustrated by Figure 3. The effect is similar to a very high speed switch, capable of being turned on and "conducted" off at an infinite number of points within each half cycle. This occurs at a rate of 60 times a second on a 60 Hz line, to deliver a precise amount of power to the motor. The efficiency of this form of power control is extremely high since a very small amount of triggering energy can enable the SCR (Silicon Controlled Rectifier) to control a great deal of output.
TECHNICAL INFORMATION
DC drive are available in four frame sizes up to 450kW, with a choice of two or four quadrant operation and the option of a unique electronic regenerative stopping facility on selected two quadrant versions. Steady state accuracy typically 0.01% when used in conjunction with an encoder and digital reference 0.1% with tacho and 2% with AVF up to 99 macro modes may be stored in the drive to be recalled at a later time. Making for easy interchangeability.
UNDERSTANDING DC DRIVES
DC motors have been available for nearly 100 years. In fact the first electric motors were designed and built for operation from direct current power.
AC motors are Now and will of course remain the basic prime movers for the fixed speed requirements of industry. Their basic simplicity, dependability and ruggedness make AC motors the natural choice for the vast majority of industrial drive applications.
Then where do DC drives fit into the industrial drive picture of the future?
In order to supply the answer, it is necessary to examine some of the basic characteristics obtainable from DC motors and their associated solid state controls.
• Wide speed range.
• Good speed regulation.
• Compact size and light weight (relative to mechanical
variable speed).
• Ease of control.
• Low maintenance.
• Low cost.
In order to realize how a DC drive has the capability to provide the above characteristics, the DC drive has to be analyzed as two elements that make up the package. These two elements are of course the motor and the control. (The "control" is more accurately called the "regulator").