What matters is not just power, but how often the motor starts in an hour and the mass accelerated each time. During start-up an induction motor takes a current several times higher than its nameplate value; if this heat cannot dissipate during short pauses, the winding warms up even when the load is light. This duty pattern must be written into the order so that the motor is selected accordingly.

DRG-801-6 0.37 kW 1000 rpm DRG three-phase motor with aluminium frame – motor selection for frequent starts

What each start costs the motor

When the motor brings the belt, rollers, pulleys and product from standstill up to rated speed, it leaves in the rotor at least as much heat as the kinetic energy of those masses. The larger the load inertia, the longer the start and the greater the heat. On a self-ventilated (IC411) motor the fan also stops when the motor stops, so cooling becomes weaker. The relationship between inertia and acceleration is explained in the article on inertia and acceleration time.

Information to include in the order

  • Starts per hour or per shift
  • How long the motor runs and how long it stands still in each cycle
  • The load inertia referred to the motor shaft (pulley, drum, product)
  • Whether stopping is by free coast-down, a mechanical brake or a drive
  • Whether the motor reverses; plugging (reverse-current braking) generates much more heat

Arrangements that ease selection

On some lines, interrupting the product flow with a clutch or a mechanical intermediate element while the motor keeps running, instead of stopping and restarting it, greatly reduces the number of starts. Soft acceleration with a frequency inverter also limits the current surge; the parameters depend on the drive documentation and the judgement of qualified electrical personnel. If precise positional stopping is needed, a brake motor or servo may be required; these are not in DRG's standard range and should be discussed with the machine builder.

When choosing the power rating

In frequent-start applications, even if the rated load is small, the next power rating up or a larger frame can provide a thermal margin; this decision should rest on the start frequency and the inertia data. Application examples are on the shrink wrapping machine motors page, and you can compare low-power models in the online motor list.