Although the relationship between torque and RPM is central to the behavior of any a-c motor, motor specifiers and users alike are sometimes uncertain as to what the motor speed-torque curve is -- and is not. It's a plot of the relationship between shaft output torque and RPM as the motor accelerates from standstill to "full speed." The speed-torque curve's purpose is to establish two criteria for motor application: 1. can the motor safely accelerate its load, and 2. do the variation of speed with load, including the pullout or stalling point, suit the operation of that load. The speed-torque curve of an induction motor can be calculated from equivalent circuit data developed during design. Efficiency, power factor, and temperature rise under load can be evaluated by test in a number of different ways, and certain procedures are sufficiently accurate to be the legal basis for enforcement of governmental efficiency regulations, for example.