balsaman, I'm not sure I agree totally with what you are saying.
First,
power (watts) is voltage (volts) times current (amps). To get horsepower (which is also a measure of power), divide by about 735 (or about 745 depending on if you are using SI horsepower or imperial horsepower - or better yet, stay with watts
).
Second, I believe that most servo drivers control current, and not voltage to the motor. It works something like this:
1) You tell the servo to go to a certain position (by sending it a number of step and dir pulses, for example). If the servo sees that it's at some different position than the commanded, it calculates what speed it should use to get to the right position "in reasonable time". This is called the "position loop". So, input to this loop is the commanded position and the actual position, and from this a speed is calculated.
2) Now the servo knows how fast it should turn the motor. It compares this speed reference with the actual speed, and if it sees that it is off, it calculates how much torque will be needed to reach the commanded speed. This is called the "speed loop". Here, input is commanded and actual speed, and output is a torque command.
3) Since motor torque is proportional to motor current, the servo drive applies the voltage needed to get the commanded current. It is the current that is controlled, but the only way to control current is by adjusting the voltage. This is usually done with PWM.
Since it is the current that is controlled, it's usually ok to use a somewhat higher input voltage to the driver than the motor's rated voltage. The drive should have a current limit, and if this is set correctly (by you) the current will not have any chance to become high enough to damage the motor, even if the PWM peak voltage is higher than the motor's rated voltage.
Hope this makes things clearer, instead of the opposite
Arvid