What is the purpose of resistor to suppress EMI? Couldn't I just use a suppression capacitor alone between the coil of the relay or contactor? I have tried to use capacitor alone and it did reduce EMI in my project, but I'm just curious of the effect of adding resistor in series with it.
Snubbers are energy-absorbing circuits used to suppress the voltage spikes caused by the circuit's inductance when a switch,electrical or mechanical, opens. and may also cause false triggering of the device (relay or thyristor)
The snubber capacitance has to meet two requirements. First, the energy stored in the snubber capacitor must be greater than the energy in the circuit's inductance.
Secondly, the time constant of the snubber circuit should be small compared to the shortest on time expected, usually 10% of the on time.
consider the case When the transistor switch opens, the snubber capacitor looks like a short to the voltage change, and only the snubber resistor is in the circuit. Choose a resistor value no larger than the characteristic impedance of the circuit so that the inductive current to be snubbed can continue unchanged without a voltage transient when the switch opens. and R is also helps in power dissipation when the switch is open as C alone cannot do it.
hope this helps
Snubber circuit prevents arcing in relays or false triggering in SCRs, it can reduce EMI as well but this isn't the primary function of snubber. R is for damping the transients, a capacitor only can lead to ringing (oscilation bursts) thus increasing the EMI.
In AC circuit a capacitor also acts as a short, so adding R is the only way to minimize leaking current.