Resistance boxes are used to add resistance in to an electric motor circuit for modifying the performance characteristic of slipring induction motors of EOT cranes, rubber mills, steel rolling mills, cement mills, power plants, conveyors, coke oven, blowers etc. The basic types of resistors are stainless steel ( grade – 304) and punched steel resistors. Resistors are designed to meet requirements of both A.C. & D.C. applications. These are manufactured for current rating from 10 to 800 amps. Continuous resistors of higher ratings are manufactured using 2 or more parallel paths. Resistors with short time rating up to 3000 amps or even more for neutral earthling are possible which suitable for system voltage upto 11 kv and above. For speed control and developing starting torque with low starting currents. They are also used as Dynamic braking resistor for V.V.V.V.F.A.C. Drives, electric loading of ac alternators, degenerators and dynamometers.

Types of Resistors:

1. Stainless Steel ( grade-304 ) Resistors: These resistors consists of stainless steel wires or strip in form of grids. The current rating generally ranges from 7 amps to 400 amps continuous with multiple or single paths.
2. Punched Steel Resistors: The resistors consists of grids punched from correction resisting nickel chromium alloy sheet steels. The punched steel grids. The punched steel grids are completely immune to shock vibration.They function reliably under the worst operating conditions and are particularly suited for steel mill duty. These resistors are available in wide range of current rating from 8 amps to 800 amps continuous with single & multiple parallel paths.

The system of cooling resistors are of two kinds:

1. Air Cooled Type
2. Oil Cooled Type