Hello Spenceroo,
Generally they vary the frequency of the electricity, not the voltage, to slow, or speed up the Motor Here in US that frequency is 60hz, in Eu it is 50hz.
Some speed controllers, such as the VPI SDS also ramp down voltage to a pre-determined setting after initial motor start up at full voltage. This is said as well to reduce motor noise with the lower voltage, thus helping lower the noise floor with it.
Generally they vary the frequency of the electricity, not the voltage, to slow, or speed up the Motor Here in US that frequency is 60hz, in Eu it is 50hz.
Some speed controllers, such as the VPI SDS also ramp down voltage to a pre-determined setting after initial motor start up at full voltage. This is said as well to reduce motor noise with the lower voltage, thus helping lower the noise floor with it.