Resistance issue in a triac-optocoupler switch
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For BT136 to switch on you must make a voltage difference (positive or negative) between the gate and A1 which happens, with this circuit, after mains zero crossing.

The triac triggers in quadrants I and III thereby avoiding quadrant IV triggering.

RG is calculated by considering the triac's Vgt and Igt parameters.
Assume a minimum mains voltage at which you want the triac to switch on, say 20V.

Then RG = (20 - Vgt)/Igt.

This circuit could be improved with the addition of a snubber network across the triac to guard against dv/dt turn on and ensure that the triac commutates correctly (switches off at mains zero crossing). - This is because the load is inductive.

EDIT

If too low a value of mains turn-on voltage is chosen then RG will be too small and the peak allowable gate current (IGM) data sheet specification will be exceeded.

To calculate the minimum value of RG assume the opto triac turns on when the mains voltage is at its peak. MOC3021 is a random phase (non-zero crossing) triac driver.

RG(min) = (Vpk - Vgt)/IGM = ((230V * sqrt(2)) - 1.5V)/2A = 162R (in the UK).

Allow a safety margin, let RG(min) = 200R.

Now it's possible to calculate the minimum mains turn-on voltage using this value of RG.

If Vgt = 1.5V and Igt = 35mA then...

Minimum mains turn-on voltage = (35mA * 200R) + 1.5V = 8.5V