How Transformers Extend the Capabilities of SCRs, Part 2
Last month, I began a series on how transformers extend the capabilities of SCRs by looking at SCRs in the secondary. This month, I'll look at SCRs in the primary and three-phase systems.
SCR in the PrimarySCR in the primary means that your SCR is rated for line voltage and usually a lower current. The firing must be phase-angle with soft start, for several reasons:
- With fast-cycling, you risk a current inrush at switch-on and at every on cycle. This can be up to eight times full load current.
- The initial primary current aims to reach V/R -- a very high value because R is the very low primary winding resistance.
What limits primary current? Back EMF, which acts in opposition to the line voltage. It appears when the primary current causes a flux change in the transformer core. If you are lucky, the flux can undergo a large swing when the SCR switches on. The result in this case is a large dose of back EMF that limits the current inrush. But what if the last current turn-off left the remanent flux positive (figure 1)? Further, suppose the next switch-on causes the current to push the flux also positive?
The polarity of the remanent flux in relation to that of the next pulse of primary current is unpredictable. So, as a general rule, do not use fast-cycling control when you have the SCR in the primary. Use phase-angle control with soft start.
Figure 2 shows the result of a step application of control signal when you use phase-angle firing and soft start. The SCR ramps the power to the new value over several cycles. The transformer flux performs a succession of growing hysteresis loops like that in figure 1 and the controlled buildup of back EMF holds off any abnormal current inrush.
The load on the secondary may have a low cold resistance or may be abnormally low anyway. In this case, you will use current limiting and soft start to give it time to get to work.
Three-Phase SystemsAll the considerations of single-phase systems apply equally to three-phase systems. Of the various primary/secondary combinations of wye and delta, the delta/four-wire wye shown in figure 3 has become the most frequently used. The reasons?
- The delta primary provides the best primary current balancing for a given secondary unbalance.
- Third harmonics readily circulate round the delta, thereby reducing harmonic distortion of the supply.
The wye secondary offers you two choices of secondary voltage, e.g., 480 V line to line and 277 V line to neutral. Grounding of the secondary neutral conductor gives the least voltage-to-ground. This is a safety issue. You can run fast-cycling loads and even the most difficult phase-angle loads.
Transformer Voltage and Current Relationships Under Sinusoidal Conditions. Say the transformer in figure 3 has a 1:1 voltage ratio in respect of line-to-line voltage (that is, 480/480 V). At rated load, the secondary voltage will be 480 V line-to-line. (In practice, it typically would be 500 V unloaded and 480 V fully loaded.) The secondary voltage line to neutral will be 480 divided by the square root of 3, or 277 V. To calculate the primary phase current, use this equation
The three secondary currents converge on the neutral conductor and return to the neutral point of the transformer secondary. All three secondary currents are assumed sinusoidal, equal in magnitude and mutually 120o apart. They add up vectorially to zero, so there is no current in the neutral conductor. Any unbalance results in a nonzero neutral current. This fact provides a handy, cheap way to detect a break in a heater circuit: Run the three conductors through a small donut-type current transformer (the kind whose output feeds an LED). LED on indicates lost heater. If there is already some small unbalance (which is likely) and the LED indicates, reduce its sensitivity.