Can I operate a 120v 60 hz sewing machine with a 220v 50hz power supply?
General rule of thumb is that transformers must be run at the proper frequency they were designed for. Professor CP Steinmetz worked out why AC transformers burned up and enabled Westinghouse and the world to enjoy the use of Teslas work on AC motors and power transmission because he determined that the alloy used in transformers and motor electromagnetic circuits was very critical in controlling hysteresis (the resistance to iron changing magnetic polarity) so they wouldn't overheat. 

Motors are just another form of transformer so any inductive type motors must also be run at the proper frequency to prevent overheating and run at the proper speed.

That being said, in a sewing machine, you usually have a special case as it typically is powered by a brushed universal motor which is a whole lot less sensitive to the frequency issue due to the design. Its speed is dependent on voltage and current allowed through the device and the magnetic path alloys are specifically designed for this.

So to sum it up, don't run your refrigerator on the wrong frequency (uses an inductive motor), you probably can run your sewing machine on the lower frequency (uses universal brushed motor). You will need a 50Hz transformer to step the voltage down!

The heating affects from magnetic path hysteresis are worse when running 60Hz equipment on 50Hz.