Why is the trransformer on my ac keep blowing

You have a contactor that has shorted windings, a short in the control wiring, or another shorted coil somewhere. This usually happens with the main contactor in the condensing unit. Get a transformer with a circuit breaker built in. They have a little button that pops out when you have an overload, saving the transformer. You could also be buying too small of a transformer. You should have had one with at least an inline fuse if it was big enough. I have seen the control wires melted together to the outside unit when it has a shorted coil in the contactor. Check the wires with an ohm meter and use ohm's law (V/R=Amps; if R=3000Ω and V=24volts then I (amps)=.008, which is a good reading.) to calculate the amp draw which should be about .01 or below the transformer rating when the other loads are added. The coil should have around 500-3000 Ω .