Electronic – Is Inductive Kickback Protection not necessary for AC

1) An inductor's current remains flowing in the same direction when open circuited and the only way this can be achieved is by reversing its polaity so you can't have both.

2) When the voltage is zero the current flow is at a maximum but this energy feeds back into the voltage source hence no spike - the spike only happens with non zero-ohm voltage sources with discontinuous signals.

3) Generally there are diodes but they are probably on the circuit that drives the motor and might easily be inside some chips.

4) No an LED won't withstand much in the way of reverse voltage (maybe 5V) so any motor supply greater than 5V might destroy it but ostensibly it will work.

You are correct that the relay coils are inductive. The snubber protects any contacts that switch that relay coil.

Figure 1. There are four contacts in the circuit to K135 but only one can turn the circuit off and that is K415. To turn K135 on K415 must turn on. This will turn on K136 or K138 and K135 will then turn on and latch through K135/14. Now the only way to turn it off is to open K415.

As explained below Figure 1, the only way to interrupt the supply to K135 is to open K415 so really that is the only contact protected by Z135. Z136 and Z137 protect the contacts of K139 (a star-delta starter timer?) and K415.

The relays are all switching a 7.5 kW motor so these are probably "contactors". Wikipedia's Contactor article states,

Unlike general-purpose relays, contactors are designed to be directly connected to high-current load devices. Relays tend to be of lower capacity and are usually designed for both normally closed and normally open applications. Devices switching more than 15 amperes or in circuits rated more than a few kilowatts are usually called contactors. Apart from optional auxiliary low current contacts, contactors are almost exclusively fitted with normally open ("form A") contacts. Unlike relays, contactors are designed with features to control and suppress the arc produced when interrupting heavy motor currents.

Because arcing and consequent damage occurs just as the contacts are opening or closing, contactors are designed to open and close very rapidly; there is often an internal tipping point mechanism to ensure rapid action.

Rapid closing can, however, lead to increase contact bounce which causes additional unwanted open-close cycles. One solution is to have bifurcated contacts to minimize contact bounce; two contacts designed to close simultaneously, but bounce at different times so the circuit will not be briefly disconnected and cause an arc.

So, to answer your question, the currents are so large in the motor circuit that snubbers wouldn't make any real difference. Instead the contactor itself is designed to switch the loads quickly.

Star-delta

Figure 2. Star-delta (wye-delta) motor start circuit.

For anyone not familiar with the power circuit the motor is a three-phase type.

• There are three windings U1/2, V1/2, W1/2.
• K135 connects one end of each to the three phase supply.
• Initially K138 will connect the other ends together to create a star (wye) connection. This will reduce the starting current by \$ \sqrt {3} \$.
• The motor should start running and after a time delay K139 will switch.
• Notice the interlock contacts in Figure 1: when K139/18 is powered K136 can not energise until K138 has dropped out. This prevents a disastrous short circuit between the phases.
• When K138 is energised the motor is running in delta mode and full phase to phase voltage will be applied to each winding.