Like if you wanted the motor to only spin 1rpm (unrealistic but yeah just for examples sake) would it still have the same amount of torque as it would at its rated voltage? If not is there any way to achieve this goal of instant torque and variable speed?
For operation below normal voltage, an AC motor’s torque capability is approximately proportional to voltage squared. If the voltage is 1/2 of normal, the available torque will be about 1/4 of normal. Speed can not be controlled very well by reducing voltage. If the load torque is naturally reduced by reducing speed as happens with a fan or centrifugal pump, reducing voltage and motor torque provides some degree of speed control. Voltage reduction with a fan or pump load essentially allows the fan or pump to overload the motor and slow it down. The only way to have good variable speed performance is to use a variable frequency drive (VFD). A VFD controls the power to the motor and keeps the voltage proportional to the frequency as the frequency is reduced to reduce speed. VFDs work best with 3-phase motors. VFDs with limited performance are available for specific types of single phase motors. For more detailed information, look at the Wikipedia article.
It varies a lot with the voltage. A neat way around this is to use an electronic box called a drive. A drive gives pulses of electricity to the motor, keeps same torque all the way down to very slow speeds. It also keeps the motor from drawing excessive current when stalled. An inexpensive drive for a 3 horse motor is about $300.
