Can a transformer run on low voltage ac?
Yes, you can. The transformer will consume a bit of power and the available output current will be reduced, of course, in keeping with conservation of energy. 

So if you had 0.1A at 0.5VAC you could get less than 10mA at 5VAC. 

In general, to be optimal, a transformer should be designed specifically for the voltages, currents and frequency or frequencies involved. 

Transformers get more difficult at extremely low and extremely high frequencies.