Tom Bishop. P.E.
EASA Technical Support Specialist
It should not be assumed that because a motor can drive a running load, it also has the capability to accelerate the load up to rated speed. During starting, a motor must deliver the energy required to accelerate the load. To do this, the motor torque must exceed that needed to accelerate the load. The motor torque value in excess of the load torque requirement is termed the “torque available for acceleration,” as shown in Figure 1.
Though this explanation appears to be relatively simple and straightforward, there are some complex conditions. Namely, that the motor torque during starting is not constant, and unless the load is a pure inertia load (very rare), it does not have a constant speed-torque relationship. Therefore, the torque available for acceleration is the difference between the speed-torque curves for the motor and the load.
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