phase angle
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The phase angle technique is often used in the control of transformer loads. At the start of each on period the thyristors are ramped to full conduction in phase angle mode, in numbers typically over ten to twenty cycles. This eliminates transformer inrush. Significant harmonic distortion and electromagnetic interference is created during the phase angle ramp. This technique was developed to minimise these effects of phase angle operation. The technique is applicable to single phase and three phase operation, all three lines are controlled.
The phase angle mode of control is traditionally associated with Thyristors. The load current is controlled by only switching the thyristor on at a delayed point in each half cycle. The thyristor automatically switches off as the cycle passes through zero. This technique is sometimes known as cycle chopping and everyday applications are light dimmers, portable tool speed controls and motor soft starts. In heating applications it should be avoided unless absolutely necessary. This is because it is associated with poor power factor, harmonic distortion, electromagnetic interference and potentially high neutral currents in three phase and four wire systems. All of these effects are likely to be very significant because of high currents associated with heating loads.
This phase angle technique has to be used where control over the load voltage or current is required, particularly for current limit control. This may be necessary if controlling a heater with very low cold resistance. It will be necessary to limit the current until the heater has warmed up.
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