Well, you're going to be in for a surprise if you think you're going to see a nice sine wave to begin with 
Any load you place on your AC transformer (i.e., current draw) is going to distort the signal, either in shape or in amplitude, unless you have a "perfect" AC transformer (don't bother looking for one). The bigger (higher power) the transformer the less you will distort the wave.
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The Rugged Motor Driver: two H-bridges, more power than an L298, fully protected