Is it safe to use a transformer with a generator?

Yes It is safe to use generator with transformer!!

A generator is a DC item. an alternator is an AC item. If the transformer is designed to step the voltage down from 220 to 110 and at the proper frequency it will work, But 220 is mostly 50 Hz and 110 is mostly 60 Hz. so make sure the transformer is designed for ~(48 to 63 Hz). the output frequency will be equal to the input frequency unless you add an active filter to the output. and the current capability is high enough for the load.

YES, IT IS SAFE PROVIDED YOU BUY HIGH ENOUGH VA RATE TRANSFORMER TO MEET WHAT YOU NEED.

SUPPOSE THE GENERATOR PRODUCES NOT MORE THAN 1000VA POWER AND YOU NEED NOT MORE THAN 1000VA AT 110V. BUY A STEP DOWN TRANSFORMER HAS AT LEAST 1000VA RATE.

AND SUPPOSE YOU NEED JUST LESS THAN 500VA AT 110V, THEN BUYING A 500VA STEP DOWN TRANSFORMER REGARDLESS THE GENERATOR ABLE TO PRODUCE 1000VA POWER.

Yes.

Use a 220V/110V step down transformer capable of handling Volt-Amps greater than [1,500W/(pf of the 1,500W device)]

Of the two generators listed (3,500W or 2,000W) in your comments to Joe's answer select the lowest Watt rated generator that can supply 1,500 Watts at a pf of the 1,500W device. But I suggest selecting the 3,500 Watt generator unless the price prohibits it's selection. By doing so it would allow for an extremely low pf for the 1,500W device and possibly a little additional loading if the need arises.

It's fine as long as the transformer is made for that purpose.

Joe is correct. If the generator is a sine wave output, and if you connect the grounds properly and if the transformer has a good enough rating to handle the power requirements, then you should be ok.

That's not a simple answer.

Assuming that everything is operating within rated specs, and there's nothing funny about the generator (e.g., modified square wave output), it should be OK.

More details would let me give you a better answer.