Managing Inductive Loads: Arc Suppression with Snubbers
Relays or contractors are commonly utilized to control mains powered inductive loads like motors, valves, or electromagnets. However, a significant issue arises when these devices are turned off: an arc can form across the relay contacts upon opening. If left unaddressed, this phenomenon leads to premature relay failure. Moreover, apart from damaging the relay, the high-voltage spark also gives rise to interference and electromagnetic compatibility (EMC) problems. To counter this, a widely employed solution is connecting a snubber in parallel with the contacts.
This snubber consists of a series resistor and capacitor network. Now, when the contacts break, the energy stored in the inductor finds a pathway to dissipate through the snubber, generating a small amount of heat in the resistor. Typically, snubber components have values such as R = 1 to 100 Ω and C = 10 to 1000 nF. It’s worth noting that relay contacts open at unpredictable times during the mains AC cycle. When they open at the point of maximum current, the aforementioned induced ‘back-EMF’ and contact arcing issues are most severe. Proper snubber implementation is crucial to mitigate these challenges effectively.
Overcoming Challenges in Inductive Load Switching: The Snubber Dilemma
Semiconductor relays utilizing thyristors or triacs turn off precisely when the voltage across them hits zero. For loads with a significant reactive power component, the voltage and current become phase-shifted, aligning with the moment of peak current. In this scenario, a snubber becomes essential to handle the situation. However, snubbers introduce an issue: a small current continually flows through the snubber RC network and load when the contacts are open.
This wasteful current, especially with light loads like fans, can inadvertently keep the load running. Increasing the value of the snubber resistor (R) diminishes its effectiveness in suppressing arcs, complicating the solution. The author found a simpler alternative: ensure the inductive load can only be switched off when the current waveform, not the voltage, reaches zero. With no current flowing, there’s no stored energy in the load inductance causing problems. This insight led the author to design a zero-current switching electronic relay, eliminating the need for snubbers.
The Innovative Solution: Zero-Current Switching Electronic Relay
The circuit’s power-handling components include the bridge rectifier B1 and the DC base-emitter junction of T3. When turned off, T3 brings T1 into conduction, thereby activating the load. To deactivate the load, the TTL input is lowered to turn off the phototransistor. T3 can only turn on when T2 switches off. T2 remains conducting until the load current hits zero, indicated by the voltage drop across D1 and D2 reaching zero. Thus, after the TTL input goes low, the load stays on until the load current reaches zero.
Simulation results using inductive loads confirmed the circuit’s functionality. CH1 displays the voltage at T1’s collector, CH2 shows the voltage at the emitter of T1 and the base of T2 (representing the current in the load), and CH3 illustrates the control voltage applied to T1’s gate. A 20° phase shift between voltage and current waveforms was applied in the simulation, a condition validated by the author through observations of the finished circuit in action.