The motor is selected by looking at three quantities together: the equivalent load, which determines the average heating over time; the highest short-term torque; and the load at starting. Sizing for the average strains the motor at peak moments, while sizing only for the momentary peak usually means an unnecessarily large motor and a low power factor.

DRG-200L-4 30 kW 1500 rpm cast iron frame DRG three-phase motor – motor sizing for variable load

Building the load profile

The first step is to record a load cycle representative of a working day. Readings taken at regular intervals with a clamp meter or an energy analyser show at which hours the load rises and how long it lasts. For example, on a pellet drying line the load may be high during the morning warm-up and low in the afternoon, while a press produces peaks lasting a second or so on every stroke.

Equivalent load from a heating point of view

The temperature of the motor winding is related to the square of the current. That is why, with variable load, the square of each load level is weighted by its duration, averaged, and the square root is taken. This root-mean-square value should not exceed the rated power of the motor. If the cycle is short, in the order of minutes, the thermal inertia of the motor smooths out the peaks; periods of high load lasting hours, however, must be assessed on their own, without relying on the average.

Peak torque and starting check

  • The highest short-term load must stay safely below the breakdown torque of the motor; otherwise the motor approaches stalling.
  • If the machine starts under load, the starting torque is checked separately.
  • If the number of starts per hour is high, the additional heating is taken into account.

If information is missing

If the load cannot be measured, the power recommended by the machine builder, the pulley ratios and the operating temperature of the existing motor provide guidance. If the existing motor has run trouble-free for years without overheating, the same power is generally sufficient; if the overload relay trips frequently, the next power size up is considered.

To narrow down the suitable range by entering power and speed values, you can use the motor selection wizard, then compare the candidate models in the three-phase motor shop.