Automatic Transfer Switch

[FaradayFF]

Greetings, fellas

Does an ATS has an internal UPS or battery that allows it to operate and make transfer decisions upon loss of primary power supply and before the 2nd source is transferred on?
Where does it get the juice from during the transition?

Thanks,
EE

 

[infinity]

The power for the transfer operation is provided by the emergency source. No EM power no transfer.

 

[hillbilly1]

Depends on the manufacturer, Onan switch’s controls are powered from the generator battery, (solenoids though are powered via available source) while Kohler is powered via whichever source is available to power the controller and solenoids (three wire control vs two wire control) the small Genercraps have a relay powered by the generator signal that allows the switch to transfer depending on the source available. (Again, solenoids are powered from available source)

 

[ron]

Different manufacturers have options.
ASCO has a super cap option that will allow the controller to stay energized for several minutes. You could also use an external UPS control power source.

 

[kwired]

Actual transfer mechanism might be powered by the generator output once it is up and running, but initiating the process after normal power loss occurs generally is done from the same battery that starts the generator prime mover.

 

[infinity]

Actual transfer mechanism might be powered by the generator output once it is up and running, but initiating the process after normal power loss occurs generally is done from the same battery that starts the generator prime mover.

With ATS being such a generic term I guess that we're talking about different things. My initial response was based on large ATS's which would have no reason to transfer if the EM source wasn't available. They also use a set of NC contacts to start the generator.

 

[hillbilly1]

Yeah, it varies widely among manufactures, but the two wire start as mentioned by Infinity is the most common. With Generac small units, the controller is built into the generator, and it makes the decision instead of the ATS. With the two wire, the ATS fails the start contacts closed or open depending on the generator, if it starts and provides output, it then powers the switch, which in turn makes the decision to transfer or not.

 

[infinity]

With the two wire, the ATS fails the start contacts closed or open depending on the generator, if it starts and provides output, it then powers the switch, which in turn makes the decision to transfer or not.

Yes I guess that it could be set up either way based on the gen set. We always use a close on normal power loss relay (NC contact) to start the generator.

 

[kwired]

With ATS being such a generic term I guess that we're talking about different things. My initial response was based on large ATS's which would have no reason to transfer if the EM source wasn't available. They also use a set of NC contacts to start the generator.

I might not been clear enough, initiation of the entire transferring process is powered by the prime mover starting battery in most cases. the actual transfer mechanism likely powered by the generator output, if there is no starting battery the initiating controls never are powered, the prime mover never starts, the generator doesn't produce output, the transfer mechanism never has any input power to make it transfer.

One could use a UPS or other alternate supply for controls, but you still kind of get nowhere if your prime mover never starts so that makes using prime mover starting battery a good source for the initial controls anyway.

 

[hillbilly1]

I might not been clear enough, initiation of the entire transferring process is powered by the prime mover starting battery in most cases. the actual transfer mechanism likely powered by the generator output, if there is no starting battery the initiating controls never are powered, the prime mover never starts, the generator doesn't produce output, the transfer mechanism never has any input power to make it transfer.

One could use a UPS or other alternate supply for controls, but you still kind of get nowhere if your prime mover never starts so that makes using prime mover starting battery a good source for the initial controls anyway.

Only on Onan, and maybe another manufacture or so. Kohler and Cat takes a dry contact from the transfer switch to initiate the start process. Once the generator is running, the voltage output powers the ATS controller to complete the transfer if everything is within spec. Somehow (I don’t know t-switches that in-depth) the controller isolates the two input sources to power it’s self, whether source one or source two, or both are energized, then makes the decision to transfer or not.

 

[kwired]

Only on Onan, and maybe another manufacture or so. Kohler and Cat takes a dry contact from the transfer switch to initiate the start process. Once the generator is running, the voltage output powers the ATS controller to complete the transfer if everything is within spec. Somehow (I don’t know t-switches that in-depth) the controller isolates the two input sources to power it’s self, whether source one or source two, or both are energized, then makes the decision to transfer or not.

And that dry contact is switching a 12VDC control circuit isn't it?

The first initiation step is loss of AC power to a control relay maybe, but next steps are all powered by the 12 volt battery. Generator doesn't start because AC power is lost, generator starts because loss of AC drops out a relay or other solid state equivalent component and the 12VDC system is what is powering the controls to start the generator. Once started it is possible the transfer operation is powered by generator ouput. Still the whole process was able to be executed because of the 12VDC system powered by the same battery that starts the prime mover.

 

[ATSman]

In general, all replies are giving the correct information. In a utility to gen configuration, after a power failure, the ATS initiates the starting of the gen usually by the closing of a contact from a relay or TDOD timer in the ATS. Typically, if you put a voltmeter across this start contact you will measure the battery voltage of the generator (whether 12VDC, 24VDC, etc.) The contact closure picks up a start relay in the gen to start the gen. It is the output voltage (proper voltage and frequency) of the gen that energizes the ATS controller or control relays in older units that completes the cycle to apply line voltage (480, 208, 120 etc.) to the solenoids in the transfer mechanism that transfers the power to the load. One key rule to remember is that an ATS will not transfer to a dead bus so there is no reason the keep power on the controller (or control relays in older units.) If the load shed option is used then a special circuit will allow the ATS to transfer from emergency to a dead bus (utility) or neutral position in a delayed type ATS.
The only battery in the controller is the watch battery that holds the memory settings for the time & date when the exerciser option is used.
The only place where I have seen terminals provided for an external UPS is on the GE-Zenith (now ABB) MX350 controller (see P. 4 on the UTA unit, attached. GE MX350 P. 4 UTA.doc )
The information provided here applies to commercial and industrial applications. For residential it's common for the ATS controller to be located in the gen enclosure.

 

[FaradayFF]

Thank you all for the responses! Much appreciated..

 

[ggunn]

Aren't some of them spring driven where when the grid is live it energizes a coil that pulls against the spring and keeps the switch in the grid position?

 

[kwired]

Aren't some of them spring driven where when the grid is live it energizes a coil that pulls against the spring and keeps the switch in the grid position?

Which will also allow you to put logic in the circuit ahead of the coil. Say you want to prove grid power or proper volts/phasing for a certain amount of time before transferring back to grid.

 

[ATSman]

Aren't some of them spring driven where when the grid is live it energizes a coil that pulls against the spring and keeps the switch in the grid position?

All of the ATS's made today I have come across use solenoids to transfer the contacts. Power to the solenoid is momentary and the contacts are mechanically latched into the source position. This applies to "stand-alone" ATS units. The only exception to this are the units made today by Russelectric and the older Westinghouse, ITE Telemand and Lakeshore Trans-O-Matic types that use motors and gear boxes.
You may be referring to the circuit breaker type transfer schemes in the large switchboards that use air frame or molded case breakers to transfer the power. An air frame breaker uses an internal motor to charge the closing springs, then a close coil releases the springs to close and hold the contacts closed. Molded case breakers use an external motor operator that is mounted on the breaker front cover that operate the breaker handle. In a molded case breaker the contacts are also held close by the closing springs.