Certain operating sequences for multi-speed motors may be necessary, or undesirable, depending on the nature of the drive. If, for example, the starting temperature rise is to be reduced or high inertia loads are to be accelerated, it is advisable to switch to low speed first and then to high speed.

It may be necessary to prevent switching from high to low speed in order to avoid oversynchronous braking. In other cases, it should be possible to switch each speed on and off directly. The operating sequence and indexing facilities of rotary switches allow for these possibilities. Multi-speed contactor starters can achieve these circuits by interlocking with suitable control circuit devices.

Fuse protection of the overload relay [t-head3]

When a common fuse is used in the supply line, it must not be larger than the back-up fuses specified on the rating plate of either overload relay, otherwise each relay must be protected by its own back-up fuse, as shown in the diagram.