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, but it offers no speed profile. When speed and current limiting are required you move to switching modules such as the KS250 or PT500.
Where this term is used: DC Motor Driver Selection Guide