A DC drive composes of a power module and base drive panels.
The power module contains the control electronics and power components necessary to control drive operation and the associated DC motor.
The base drive panel consists of the power module mounted on a base panel with line fuses, control transformer, and contactor. This design allows for easy mounting and connection of power cables.
CONVERTING AC TO DC
THYRISTOR
A primary function of a DC drive is to convert AC voltage into a variable DC voltage. It is necessary to vary to DC voltage in order to control the speed of a DC motor. A thyristor is one type of device commonly used to convert AC to DC. A thyristor consists of an anode, cathode, and a gate.
A thyristor acts as a switch. Initially, a thyristor will conduct (switch on) when the anode is positive with respect to the cathode and a positive gate current is present. The amount of gate current required to switch on a thyristor varies. Smaller devices require only a few milliamps; however, larger devices such as required in the motor circuit of a DC drive may require several hundred milliamps.
Holding current refers to the amount of current flowing from anode to cathode to keep the thyristor turned on. The gate current may be removed once the thyristor has switched on. The thyristor will continue to conduct as long as the anode remains sufficiently positive with respect to the cathode to allow sufficient holding current to flow. Like gate current, the amount of holding current varies from device to device. Smaller devices may require only a few milliamps and larger devices may require a few hundred milliamps.
The thyristor provides a convenient method of converting AC voltage to a variable DC voltage for use in controlling the speed of a DC motor. In this example the gate is momentarily applied when AC input voltage is at the top of the sinewave. The thyristor will conduct until the input’s sinewave crosses zero. At this point the anode is no longer positive with respect to the cathode and the thyristor shuts off. The result is a half-wave rectified DC.
The output of one thyristor is not smooth enough to control the voltage of industrial motors. Six thyristors are connected together to make a 3Ø bridge rectifier.
As we have learned, the gating angle of a thyristor in relationship to the AC supply voltage, determines how much rectified DC voltage is available. However, the negative and positive value of the AC sine wave must be considered when working with a fully-controlled 3Ø rectifier.
A simple formula can be used to calculate the amount of rectified DC voltage in a 3Ø bridge. Converted DC voltage (VDC) is equal to 1.35 times the RMS value of input voltage (VRMS) times the cosine of the phase angle (cosα).
VDC = 1.35 x VRMS x cosα
The value of DC voltage that can be obtained from a 460 VAC input is -621 VDC to +621 VDC. The following table shows sample values of rectified DC voltage available from 0° to 180°. It is important to note that voltage applied to the armature should not exceed the rated value of the DC motor.
The following illustration approximates the output waveform of a fully controlled thyristor bridge rectifier for 0°, 60°, and 90°. The DC value is indicated by the heavy horizontal line. It is important to note that when thyristors are gated at 90° the DC voltage is equal to zero. This is because thyristors conduct for the same amount of time in the positive and negative bridge. The net result is 0 VDC. DC voltage will increase in the negative direction as the gating angle (α) is increased from 90° to a maximum of 180°.
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