Electric matress pad - issues with square wave inverters.

First: What makes up a square wave? Then you can see get an idea of why they cause problems. A square wave is made up of odd harmonic frequencies. 60 cps is what you need. Odd harmonics includes 3rd-180 cps, 5th-300 cps, 7th-420 cps, 9th-540 cps, 11th-660 cps etc depending on the inverter. I suspect the higher or more odd harmonics the more problem it causes.