Bonjour à tous
Simplest solution make an H field probe, ( magnetic field) zero cost:
Any coax, with a BNC con, coax other end to 2..9 cm dia. wire loop, 10 turns any insulated wire.
BNC to a scope or frequency counter
approach the motor or gen, position loop so one phase dominantes, thus a single phase sine wave, to get the frequency
RPM = 60X frequency
Jon