Matching the Induction Motor Rating to the Driven Load Power

It is of utmost importance that an induction motor selected for a particular application to have sufficient power and torque to be able to function properly and efficiently. Starting high inertia loads (i.e. conveyors) require a high starting torque motor where a normal torque motor may fail to start or accelerate.

Table 1 provides a rule of thumb for the relationship of the driven load power to the motor rating.

Table 1 Margin of motor rating above the machine rating [1]
Driven Load Power (kW) Margin multiplier (per-unit)
Up to 15 1.25
16 to 55 1.15
56 and above 1.10

Belt-driven loads require higher margin factor than for direct coupled driven machines due to the lower transmission efficiency of belt drives. It is also advisable to obtain advice from the manufacturers of both the driven machine and the motor.

1. Handbook of Electrical Engineering: For Practitioners in the Oil, Gas and Petrochemical Industry
Alan L. Sheldrake
ISBN: 0-471-49631-6