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An essential tool for determining if a motor will accelerate its load safely
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" (more later about what that means).
Three points on the curve (and only three) represent steady-state conditions. First, the zero RPM point denotes the torque that the motor can supply indefinitely with the shaft locked against rotation, with full voltage applied (until the stator or rotor winding overheats and fails or some protective device operates). The second steady-state point is at full speed - the RPM and torque values that apply for 100% of the load mechanically connected to the motor shaft.
The third of these points is the RPM at which the motor runs with no connected load. Practically speaking, the torque at this point is zero, and the speed is the synchronous RPM. In plotting the curve, the maximum or "100%" speed value is that synchronous RPM - not the operating point corresponding to rated full load.
All other points on the curve represent transient conditions. The motor torque and speed values pass through those points more or less rapidly during acceleration. What happens during deceleration, depending upon whether the motor is simply coasting or is being braked to a stop, is represented by a different set of points along a different curve.
What the curve can and can't do
The speed-torque curve is not a basis for estimating steady-state performance. It cannot be used to evaluate losses and efficiency, power factor, or temperature rise. Its purpose is to establish two criteria for motor application: 1 ) can the motor safely accelerate its load, and 2) does the variation of speed with load, including the pullout or stalling point, suit the operation of that load.
As with any graph relating two variables, a speed-torque curve can be plotted in two ways - see Figure 1. Neither can be termed "correct" or "incorrect." Some engineers prefer version (a), others version (b), but...