A relay generally allows a low power signal to control a high power load. They're usually smaller, enclosed versions of contactors.
Relays are composed of a coil, and a set of magnetic contacts. When energized, the coil will open and/or close some of those contacts, depending on how the relay is designed.
For example, a relay in a heat pump may have a coil rated for a 24 volts at 0.5 an amp, but the relay contacts may be rated for 30 amps at 240 volts. When the 24 volt control signal from the thermostat flows to the relay coil, it closes the contacts which allows 240v to energize the heat pump.
Re: What is the primary purpose of a relay?#65808 05/13/0605:13 PM05/13/0605:13 PM
The most common reason I install a relay or contactor is so I can switch a large circuit or many smaller circuits remotely with simple low cost low ampacity switch.
I might install a 200 amp 3 phase 600 volt contactor (relay) to control an entire lighting panel. By using the contactor I can use a small time clock to control the contactors coil and by doing that the entire panel.
Bob Badger Construction & Maintenance Electrician Massachusetts
Re: What is the primary purpose of a relay?#65809 05/13/0605:22 PM05/13/0605:22 PM
Simply put...it's a switch that is controlled by another switch.
Here is an easy example: You have parking lot lighting supplied by by a 3Ø 480v 40A circuit. How do you control it? - You could have the hired help turn it on/off at the desired times with a switch
- You could use a photocell (if you could find one that is rated)
Each of these methods has it limitations: - The store is closed on the Holidays, but you still want the lights on for security. Do you want to pay the help to turn the lights on/off?
- Still looking for the that rated photocell?
So you get yourself a contactor with a 120v coil and contacts rated at 3Ø 480v 40A. Now you wire the photocell to the coil. When it gets dark, the photocell does it's thing and completes the circuit to the coil. The coil closes, the lights come on. When dawn comes, the opposite happens.
Switch high voltage/current with safer/ more economic/ smaller/ lower voltage/ control circuit components, say in power transmission. Or, mixed voltage circuitry; controls= lvdc, power= higher vac.
Switch multiple circuits with no backfeeds. As for example a heating boiler may be fired by signals from multiple programmable zone thermostats or timers, without backfeeding the thermostats/timers themselves or operating all the solenoid valves controlling water flow or all the circulators in the water circuits.
Switch applied logic circuits in machine controls, say Guard open = machine stops but a keyswitched relay allows running during tool setting. Largely displaced by PLC controllers and ICs.
Mechanically, a relay would just take a switched signal current and use it to energise a coil on a solenoid or a motor to operate another switch.
Wood work but can't!
Re: What is the primary purpose of a relay?#65811 05/13/0608:52 PM05/13/0608:52 PM
All these years I understood what a contactor did. Hell, I've wired more than a few. But I never understood that the relay itself was the guts of the contactor and how it physically worked. Now I know.
Re: What is the primary purpose of a relay?#65812 05/13/0609:10 PM05/13/0609:10 PM
Relay, cintactor...the difference is only in scale!
Well....almost. Most 'contactors' have only "normally open" contacts. A relay typically will also have "normally closed" contacts. These contacts will let the relay operate something when the relay is "off". A common example is your typical alarm system; cut a wire, and you trigger the alarm.
Now...take a contactor, add overloads... and you now have a "starter." Add a disconnect switch, and it's now a "combination starter."
Re: What is the primary purpose of a relay?#65814 05/13/0609:43 PM05/13/0609:43 PM
Relays were really starting to go pretty fast when transistors took over. The basic wire contact relay had a 6ms operating time. The permissive make wire contact relay shaved that to 3ms. The reed relay got that down to about 1ms. You can do about as well with a mercury wetted relay. The slowest transistor gates were running about 1000 times that speed.