Time-delay relay reduces inrush current
Edited by Bill Travis
P Seshanna, Assumption University, Bangkok, Thailand -- EDN, March 7, 2002
A transformer switching onto a line can sometimes cause a circuit breaker to trip or a fuse to blow. This phenomenon occurs even if the transformer presents no load, such as when the secondary is open. The problem arises because of the heavy magnetizing inrush current in the transformer. The amplitude of the current depends on the instant on the ac waveform at which the transformer becomes energized. The inrush current is at its maximum value if the transformer becomes energized when the ac waveform goes through its zero-crossing point. A similar situation exists when a capacitor in a power-factor-improvement bank switches onto the line. In this case, the inrush current is at its maximum when the ac waveform goes through its peak value. Normally, a mechanical contactor effects the switching without any control of the instant of switching. The inrush current dies down exponentially to the normal operating value of the load within a few cycles. If the breaker trips from the initial inrush current, you close the breaker to re-energize the circuit and hope for the best. You can limit inrush current by inserting a series resistor, R1, during switching and then shorting this resistor after the transient period (Figure 1).
Lately, designers have been inserting negative-temperature-coefficient thermistors in series with some loads, such as switch-mode power supplies. This device presents a high resistance at the instant of switching, thus limiting the inrush current. After a few cycles, the resistance of the thermistor drops to a low value, allowing normal operation of the load. In contrast, the circuit in Figure 1 physically inserts a resistor in series with the load to limit the inrush current and then short-circuits the resistor after a time delay. You can adapt the circuit to any size load by suitably selecting the series resistor and the relay-contact rating. A drawback of negative-temperature-coefficient thermistors is their limited joule heat-absorption capacity. The circuit in Figure 1 works directly from the ac line to which the load drawing inrush current is connected.
The steady-state dc-current requirement of the relay coil determines the values of the other circuit components. You select capacitor C2 such that the average value of the rectified current, IAVE, is equal to the current the relay coil requires. The coil resistance should be smaller than the capacitive reactance of C2 at line frequency. Under these conditions, the average rectified current is approximately IAVE=V(2πfC2)/1.11, where V is the rms value of the line voltage (220V), f is the line frequency (50 Hz), and C2 is the required capacitor value. Once you know the relay current, you can select capacitor C2 and the bridge diodes. The value of capacitor C1 determines the delay time.
The voltage across C1 rises exponentially with a time constant, τ=RLC1 (Figure 2). If you know the relay's pickup voltage and its coil resistance, RL, you can choose the required value for C1. It is easy to see that when you close the main switch, the circuit simultaneously energizes the load drawing inrush current and the time-delay relay. A constant average-current source drives the capacitor/relay combination, and the dc voltage rises exponentially. When this voltage reaches the pickup voltage of the relay, the relay's normally open contact across the series resistor closes, thereby short-circuiting the resistor. When you open the main switch, the voltage across the relay coil drops, again exponentially. When this voltage reaches the dropout voltage of the relay, the contact opens. The resistor is again in series with the load and ready for the next switching operation. The pickup voltage of the 12V relay in the test is approximately 6V, and the contact-closure time is 330 msec, as the dashed line in Figure 2 shows.
The important design considerations are as follows:
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The normal operating voltage of the relay must be less than 10% and must be lower than the ac-line voltage.
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C2 determines the average operating current through the relay.
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The relay's contact rating must be adequate to meet the load-current requirement. The relay in Figure 1 is small and has 12V-dc coil rating and 220V-ac, 5A contact rating. The measured coil resistance is approximately 160Ω.
The function of the 50Ω resistor, R2, in Figure 1 is to limit the switch-on surge current into the time-delay-relay circuit. The zener diode, DZ, limits the voltage rise across C1 to 15V in the event of a relay-coil open circuit. You can use the circuit in Figure 1 in the laboratory for energizing a 220V, 1-kVA transformer for use in experiments.
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