Relay Reversing
A method of changing direction by reversing the polarity of the motor terminals with two changeover-contact relays. It is cheap and the contact resistance while conducting is very small; however it offers no speed control, and contact life is noticeably shortened when switching under load.
The classic arrangement consists of two changeover (SPDT) relays: each relay connects one motor terminal either to the positive or to the negative rail. When both relays are in the same position the motor stops; when they are in different positions it turns in one direction. Logically it is the mechanical equivalent of an H-bridge.
Its strength is conduction loss: the contact resistance of a good power relay is typically a few milliohms and in practice it does not heat up at all. Its weakness is the moment of switching. A contact that opens under load produces an arc; the arc erodes the contact and radiates broadband noise. The inductance of the motor feeds that arc, which is why extinguishing an arc in a DC circuit is harder than in an AC one.
The practical rule is never to change the direction relays while the motor is turning: stop the motor first, wait a short time, then change direction. If speed control, ramps, current limiting or automatic current-sensing termination is wanted, a relay arrangement is not enough; those functions are provided by electronic driver modules. In the AXI product family the ST42 targets exactly this middle ground: with a 12V/24V DC supply it provides forward-reverse and on/off direction control, triggered by an RF receiver or a panel signal, with adjustable speed and ramp. When a higher current or automatic current-sensing termination is required you move to modules such as the KS250 or PT500.
Where this term is used: DC Motor Driver Selection Guide