This can best be described as a magnetic switch. It comprises a small solenoid coil around a sleeve and an iron core. Inside the sleeve is a plunger to which the switch contact bridge is attached; the contacts are normally open.
When the coil is energized, a strong magnetic field of force is created because the current will be high during the starting phase. The magnetic force will move the plunger upwards and bridge the switch contacts, completing the circuit to the start winding. The run winding is wired through the relay so that it is always in circuit. Figure 34 shows a typical motor and control arrangement for a current relay, and Figure 35 shows the completed relay circuit.
A high starting current is drawn when the compressor motor starts. The current reduces as the motor gathers speed; the magnetic field through the relay then becomes weaker so that it can no longer hold the contact bridge on to the switch contacts. The plunger then drops down by gravity to open the circuit to the start winding.
It is not uncommon for a start capacitor (see later) to be fitted when a current relay is employed. This is wired in series with the start winding (Figure 36).
I want to create a circuit, so my refridgerator and hot water heater are never running at the same time. When the refrigerator switches on, I want it to switch a relay to interrupt the flow to my hot water heater. When the refrigerator is done with its cycle and switches off, the relay controlling the water heater is closed and power to it restored.
I thought I could do this by simply replacing the relay with a two position relay, but apparently this is not a simple relay circuit, but one that simply controls power to the starting coil to get the motor going, which switches off due to gravity once the motor has gained speed. I don’t understand what switches off power to the motor though because in these diagrams power is always going to the running coil.
Where would I put my detection circuit.. into the running coil circuit, and is there a simple straightforward way to do this… to insert a second relay that would switch power to the hot water heater on and off depending on whether the refrigerator motor was running.
This also doesn’t seem optimal, because apparently the refrigerator has a defrost circuit that could be running with the motor not running. What is the best way to detect that the refrigerator is drawing a load / power, and trigger a relay based on that to switch some other 120v AC plug device (a water heater) off.