North American micro-wave ovens in 220 Volt envioronments

You probably mean 220 Volt, 50 Hz unless you are talking about one hell of a big microwave oven.

There are several issues here that will be difficult to determine and may influence your decision.

1. Will the oven operate safely and effectively at 50 Hz vs. 60 Hz? A step-down transformer will convert voltage, but not frequency. Power systems for microwave ovens are extremely touchy. They are fused for so much primary current and no more. Many inductive type appliances (and most microwave ovens have a large step-up transformer for the magnetron) will draw more current at 50 hz than at 60 hz. Typically this transformer is designed cheaply to operate within very narrow parameters of current-limiting inductive reactance and the 50 Hz power may upset these tolerances.
2. Even if the frequency turns out not be be a consideration, your step-down transformer will have to be large and heavy to provide the equivalent of a 120V. 15A circuit. (I'm assuming you have one of the large 1000 watt oven models that actually draws 1250 watts or so from the line).

Recommendation - Purchase a 220 V. 50 Hz model from Britain, France, or Germany and have it shipped there. In some US cities with high numbers of ethnic immigrants like Chicago or New York, there are specialty appliance stores that sell 220 V. appliances. This might be another option.

Beachcomber

Also, you'll find a better selection and more features available in Europe. Many microwave oven models are not made available in the US because they draw too much power for a standard US socket.