Certainly not 'compteur de révolution' — that would tend to be something that counted how many revolutions had occurred. As Florence says, it is more colloquially know as a 'compte tours' (it is of course '...tours par minute', but that is taken as read).

However, more properly, a rev. counter is normally referred to in technical circles as a 'tachymètre' (tachometer in EN).

Strictly speaking, it isn't necessary to add 'électrique', since by definition, if it needs a generator, it MUST be 'electric' and not 'mechanical'; so depending on your wider context, that really may or may not be required.