Why can't an induction relay be used in DC controls?

An induction relay operates based on the principle of electromagnetic induction, which relies on alternating current (AC) to function effectively. The primary reason it cannot be used in DC controls is that direct current produces a constant magnetic flux. This constant flux does not change or vary over time, which is a crucial aspect for induction mechanisms.

In an AC circuit, the alternating current creates a varying magnetic field which induces a rotating or oscillating motion in the relay's disc, allowing it to respond to changes in the current or voltage levels effectively. However, in a DC circuit, the absence of this variability means that the induction relay does not receive the necessary changing magnetic fields to operate as intended.

Thus, an induction relay would be unable to respond properly to DC signals, rendering it ineffective for applications that rely on direct current. This understanding helps clarify why induction relays are fundamentally designed for use within AC systems.