Selecting for jams is less about enlarging the motor than about designing the system as a whole. The motor's high starting and breakdown torque, the gearbox's ability to withstand shock loads, a control that detects a jam and reverses the machine, and an arrangement that protects the motor from overload must all be considered together. Simply increasing power can put the gearbox and the cutters at risk.

DRG-225M-6 30 kW 1000 rpm DRG three-phase motor with cast iron frame – shredder motor selection for jams

What happens at the moment of a jam?

A hard piece (metal, a thick log, tyre wire) entering the low-speed, high-torque shredder shafts stops the cutters. Motor speed drops and current rises sharply. If this lasts a few seconds the winding heats up; if it goes on longer the protection trips. The motor shaft, coupling and gearbox are all subjected to a large torque shock at the same time.

What to take into account in selection

  • Material type and the hardest piece; plastic, wood, rubber and scrap give different load profiles
  • Single-shaft, twin-shaft or four-shaft design; on multi-shaft machines, the number of motors and how they are matched
  • The gearbox's shock load factor; DRG does not sell gearboxes, and the motor is selected to suit the input conditions of the existing gearbox
  • Coupling type; a flexible or slip coupling absorbs part of the shock
  • Starting method and the need for frequent reversing

Reversing and protection

On shredders, the control can be set up to reverse the shafts briefly when current rises, freeing the piece. Since this means re-accelerating the motor at every change of direction, heating and the number of starts must be assessed together. The protection concept is explained in the article on overload protection; the settings are a matter for qualified electrical personnel.

Pages to consult by application

The specific load conditions for plastic, rubber and scrap are covered on the plastic shredder motors and scrap and metal shredding motors pages. Once power and speed have been determined, cast iron models are listed in the shop.