It doesn't matter to the armature, which usually works on DC anyway, all that has to be detected is the induced voltage, just as most 50hz transformers will work on 60Hz OK. and vice versa.
A growler is only usually used for a relatively brief time.
Max.
I was thinking more of the growler, it functions like a transformer, and if you lower the frequency in a transformer you risk saturating the core & burning it out.