Inductive Reactance in Real Life

Ohm’s law states that voltage is equal to the product of amperage and resistance. However, Ohm’s law gets a bit complicated when we work with magnetic loads like motors.

For the demonstration, Bryan uses a contactor coil, a relay, and a stack sequencer (with bimetallic disks). The contactor coil and relay are inductive loads, and the stack sequencer is a resistive load. When you have a resistive load, you can get a more reliable ohm reading than on an inductive load.

Bryan takes the ohm and voltage readings on each load. He then takes the Ohm’s law equation (E = I x R) and replaces E with the voltage reading and R with the ohms. To get the amperage, you would divide the voltage by the resistance. The amperages on the relay and the contactor coil seem high compared to the stack sequencer.

When Bryan takes the amperage reading on the contactor coil, the amperage is much lower than expected. The calculation was off because there was additional resistance that the meter couldn’t pick up, which was actually something called “inductive reactance.” A meter can typically only measure resistive ohms, not the inductive reactance that happens when the electromagnetic fields expand and collapse. The relay’s amperage is similarly off, but the stack sequencer’s amperage is pretty close to what we expected (even though the resistance increases as the heater comes up to temperature).