With frequent starts and stops, what stresses the motor is starting rather than running. With direct-on-line starting the motor briefly draws several times its rated current; on the catalogue's 7.5 kW 4-pole model this ratio (Is/In) is 7.5. If the heat from this current cannot be dissipated between starts, the winding gets a little hotter with every cycle.

Information to collect before selecting:

  • Starts per hour and the duration of each run
  • Whether starting is loaded or light, and the machine inertia
  • Whether the motor stops fully at each stop, and whether it reverses
  • Starting method

With this information, the following options are considered:

  • One power or frame size up: provides more thermal capacity.
  • PTC or PT100 winding sensor (ordering option): monitors the actual winding temperature that an overload relay cannot see.
  • Soft starting or speed control: reduces starting current and mechanical shock.

Our standard motors are rated for S1 continuous duty; a duty involving intensive starting and stopping must be stated clearly in the order. The protection side is covered in our motor protection devices article.