overload relays and thermal relays

[gjo]

I was reading basic electricity by valkenburgh and on page 68 of volume five he says" in many circuits you will see overload contacts in series with the line. These contacts are normally closed, but if the temperature of the device becomes excessive, the thermal relays will close, causing the overload relay to become energized and opening the line."

The part i dont get is where he implied the thermal relays are acting like a switch to energize the overload relay. I thought the thermal relay is an overload relay, but here he years it like the overload relay and an electromagnetic relay are used as spate relays , the thermal feeding the electromagnetic relay, and that the thermal relay is normally opened in this case. Can someone clear this up.

 

[FvM]

A basic thermal or overload relays isn't but a circuit breaker, not involving a relay to be energized or something of that kind. If the book isn't more specific about the described design, I would simply ignore this detail.

 

[chuckey]

In motor start circuits, because of the large currents a contactor (sort of high power relay) doe the actual switching. Its coil current is controlled by the on/off switch. Also in series with the motor are a series of very low resistance "heaters", when one of theses get hot, they make a bi-metallic strip bend (like a thermostat) which breaks some contacts which are in series with the motor contactor coil. So if the motor takes a lot of current because of overloading or a fault, the heaters trip the motor off. Because the are a thermal device they allow a high starting current to flow without tripping for a few seconds.
Frank