Three phase motors run at fixed speeds according to the frequency of the power supply. A 2-pole motor running on 60Hz will run close to 3600rpm, a 4-pole turns at 1800rpm. The fastest they turn on 60Hz is 3600rpm because you cannot have less than 2 poles.
You can use a magic box to increase the frequency, up to around 400Hz. But a motor designed for 60Hz will start flying apart from the centrifugal forces if you turn it too fast - plus the bearings won't last. Colombo, Perske, et al, are basically 2-pole 3-phase motors, designed for high speed, and have to have magic boxes to increase the frequency.
Tom, those speed controllers do give full torque, but they don't give full power. If you run a 2HP at half of its rated speed, you will only get 1HP out. If you had used pulleys to reduce the speed by half, the torque would have doubled - the electronic box cannot double the torque, only keep it constant.
