Switching Ability of Automatic Transfer Switch

Thread Starter

eCaleb

Joined Feb 11, 2022
20
Do Automatic Transfer Switch make switches from mains supply to household generator without any interruption in supply ?

If yes ?
considering the time it would take to start the gen ; How's switching done from mains to gen(by the ATS) without interruption in supply ?
 

Reloadron

Joined Jan 15, 2015
6,581
The normal sequence:
Mains power fails.
Generator waits a predetermined time (mine was set for 10 seconds) just incase it's only a fast power "burp".
Generator Starts and runs a few seconds getting up to speed and regulated voltage.
Generator sends a pulse which can be 2 or 3 seconds to solenoid in transfer switch.

Below is a typical 100 Amp service transfer switch.
Transfer Switch.png

The large black coils are the solenoids which transfer. There is both electrical and mechanical interlock.

When mains power returns the system will notice it and begin a timing cycle and a wait to make sure power has returned and is remaining. Then a pulse is sent to the transfer switch to switch back to mains power. When switching back the change is not noticable. After switching the generator will normally run for a short cool down period.

This is typical, some systems may have a different scenario.

No, there is an interruption is supply when mains fail. Personally I have UPS units on things I do not want to shutdown. Computers, phone lines. cable systems and things like that. My old system about a 15 second total delay between mains power loss a genset online. New unit is about the same.

Also using a mechanical lever the transfer can be accomplished manually. This all depends on the ATS design scheme.


Ron
 
Last edited:

Thread Starter

eCaleb

Joined Feb 11, 2022
20
Wow, this is well detailed and practical.
Thanks Reloadron, thanks for using a real life practical scenerio, for taking your time to explain the sequence of how this really works and for uploading the image of your ATS that's helpful, I'm glad you did so.

Alright, I actually need build one for a some course mates, a school project though, but I needed to be crystal about the sequence so I don't find myself worrying about the unnecessary.

You're good Reloadron .
 

Thread Starter

eCaleb

Joined Feb 11, 2022
20
The normal sequence:
Mains power fails.
Generator waits a predetermined time (mine was set for 10 seconds) just incase it's only a fast power "burp".
Generator Starts and runs a few seconds getting up to speed and regulated voltage.
Generator sends a pulse which can be 2 or 3 seconds to solenoid in transfer switch.

Below is a typical 100 Amp service transfer switch.
View attachment 262456

The large black coils are the solenoids which transfer. There is both electrical and mechanical interlock.

When mains power returns the system will notice it and begin a timing cycle and a wait to make sure power has returned and is remaining. Then a pulse is sent to the transfer switch to switch back to mains power. When switching back the change is not noticable. After switching the generator will normally run for a short cool down period.

This is typical, some systems may have a different scenario.

No, there is an interruption is supply when mains fail. Personally I have UPS units on things I do not want to shutdown. Computers, phone lines. cable systems and things like that. My old system about a 15 second total delay between mains power loss a genset online. New unit is about the same.

Also using a mechanical lever the transfer can be accomplished manually. This all depends on the ATS design scheme.


Ron
Is the ATS in charge of directly Auto starting the generator, or there is a separate circuit taking charge of that ?

#Not all gen has can be started electronically, but I can try figure out a circuit for gen type that needs cranking to get into operation .
 

Reloadron

Joined Jan 15, 2015
6,581
Well alrighty then. One of the most important things to consider is while not obvious in the image posted is the transfer actual contacts are mechanically coupled and electrically coupled using two small micro switches. This is done to make absolutely sure that one is fully disconnected before the other side is connected. No unwanted surprises. The actual ATS box is independent of the actual genset. The ATS is located at power entry. Upper left side you see two fuses (well ok one is missing) those lines are AC mains at building entry. In my area they sense the 240 VAC mains power. Bottom left on the terminal strip that 240 VAC exits the ATS. That goes to the actual genset located outside. All the ATS does is a transfer function between AC Mains power and Generator power. So the generator (genset) is sensing mains power. All of the thinking and timing functions happen in the generator. The generator senses a loss of mains power and does a pause just in case mains power is lost (a glitch or commonly called a burp) when the loss is sensed it starts a timer function waiting to make sure mains power is lost. Following a delay the genseator starts. Again, once the system knows the engine has started and up to speed and voltage it sends a signal to the ATS to initiate transfer. These time intervals may be set by the user (adjustable) or in some cases fixed time intervals. A 12 VDC pulse is what the genset sends to the ATS for transfer.

Now there is also a safety factor to consider. At my location each residence is fed by a mains power transformer. In my area the high voltage is about 7.0 KV. That 7.0 KV is stepped down to 240 VAC what we call "split phase". Transformers work both ways. If 240 VAC were applied to the mains line from a generator the 7.0 KV line would be hot. This is known as backfeeding. A lineman thinking the line is dead could face electrocution. Additionally if the generator and mains power met face to face things get very ugly very fast. This is why good transfer switches meet certain codes and specifications. Really bad, very bad, things can happen if things are not done right.

Some time back a forum member had questions along the lines of yours. He was building his own transfer and had a 7.0 KW genset with electric start. The "thinking" and timing was done using a simple uC (microcontroller) like an Arduino or similar. Detecting mains power you want some form of galvanic isolation. For a simple project modules like this are plentiful and inexpensive. Today most if not all gensets rely on a uC for all the transfer operations. Not that discrete components can't be used but a single small uC can do just about everything.

My guess here is this is a school project because a ready made turn key off the shelf solution can be bought. DNA Robotics points out a good example above.

Ron
 

BobaMosfet

Joined Jul 1, 2009
1,979
Wow, this is well detailed and practical.
Thanks Reloadron, thanks for using a real life practical scenerio, for taking your time to explain the sequence of how this really works and for uploading the image of your ATS that's helpful, I'm glad you did so.

Alright, I actually need build one for a some course mates, a school project though, but I needed to be crystal about the sequence so I don't find myself worrying about the unnecessary.

You're good Reloadron .
@eCaleb Don't forget about sine sync to the ILEC.
 
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