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Thread: Transfer Switch Start Contact

  1. #11
    Join Date
    Aug 2008
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    Quote Originally Posted by myspark View Post
    Notwithstanding the relevant responses. . . the specificity of the answers seem to not address OP's specific question which in part: “is that a momentary contact closure. . . ?”


    The answer is: it depends.

    The operative word here is: generator.
    To start an emergency source, the main focus would be the prime mover AKA the engine.

    The ATS is not meaningful if the prime mover is not working.

    Not all engines are the same. Most of the answers to OP's question point to gas driven engine similar to what we are familiar with. . .you fill it up regularly at a gas station (normally). They can be started with a battery or crank.
    Although crank type starting is quite rare now.

    Most modern emergency generators are diesel. They are totally different beasts.
    Not only that they don't need electricity for engines to operate since there is no need for spark plugs, they are also “tricky” to start and maintain.

    They need batteries to start and glow plugs (heater) to improve oil viscosity for easy cranking.

    Now, to address whether the starting switch (starter) is held close or momentary.

    Just like a regular car, there are contacts that are held close and some are momentary. It's the same for both diesel and gas.

    Both engines have alternator to charge batteries for starting. When you turn the ignition key, the first indentation provides power to the excitation winding of the alternator. The second momentary indentation provides power to starter motor through a high-current rated contactor (relay). When you let go of the switch it returns to its open position (you don't want the starter motor to keep turning) when the engine is running. Suffice to say this is a momentary contact.

    With these gas engines, the battery and alternator provide power for the high voltage coil for the spark plugs. So one contact is always closed.

    On diesel engines, the first key indentation turns on a fuel valve and pump and the second indentation turns on the glow plug and the starter motor.

    In both type engines the starter motor is disconnected once the engine is running.

    The normally closed contact to enable the generator control system has been around for some time and hasn't been mentioned in some regulatory agencies. This is a part of engineering principles that has been employed in practical application long before these agencies even thought of making this mandatory. ( based on your post. )

    Another tricky part is shutting down a diesel engine.

    Unlike a gas engine where pulling out the ignition key ensures a totally disabled machine, diesel will keep turning even with the key off because fuel could still be dribbling to the injectors that would keep it turning. Hence the word “dieseling”.

    Besides, diesels are engineered not to shut the fuel valve off right away because fuel lines could suck air that could make it difficult to start the next time around.

    I owned a diesel Mercedes Benz car for twelve years when the fuel shortage made it difficult to find a gas station open at certain times. Truck stops are always open with a lot of diesel fuel.

    Some guys running diesel service trucks probably know this.


    All the best.
    Thank you for your dissertation on emergency genset engines. However, you yourself have seemed to gone off subject regarding the OP's question. Your response was "it depends" and then continue on to compare an emergency standby system to a gas or diesel car. I think you are in error when comparing both systems. In my previous post I stated that the engine start in an emergency standby system is typically a "latched" signal and not momentary (whether NC, NO, 2 wire, 3 wire, etc.). Simply put, like a light switch: switch on, gen runs: switch off, gen stops. I will go as far as to challenge you to name a commercial or industrial application where this signal is momentary. With all due respect I am used to commercial and industrial systems employing diesel or gas turbines. So if there are other systems that fit your description....I am all ears.
    Am I in shape?? I get plenty of exercise pushing my luck!!

  2. #12
    Join Date
    Oct 2015
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    Quote Originally Posted by ATSman View Post
    Thank you for your dissertation on emergency genset engines. However, you yourself have seemed to gone off subject regarding the OP's question. Your response was "it depends" and then continue on to compare an emergency standby system to a gas or diesel car. I think you are in error when comparing both systems. In my previous post I stated that the engine start in an emergency standby system is typically a "latched" signal and not momentary (whether NC, NO, 2 wire, 3 wire, etc.). Simply put, like a light switch: switch on, gen runs: switch off, gen stops. I will go as far as to challenge you to name a commercial or industrial application where this signal is momentary. With all due respect I am used to commercial and industrial systems employing diesel or gas turbines. So if there are other systems that fit your description....I am all ears.

    For what its worth guys, I talked with ASCO, they say that the start contact on their ATS closes and stay closed for the duration of the generator run time. When it opens the generator goes into its cooldown mode.

    Also, apparently in their NATS of the same switch line (ASCO 7000 series) it does not have a start contact at all and you have to start the generator separately. I have a hard time beliving that because I thought that the difference between the ASCO 7000 ATS and the ASCO 7000 NTS is that the autotransfer was inhibited.

    But oh well.

  3. #13
    Join Date
    Nov 2014
    Location
    Miami, Florida, USA
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    185
    Thread is over but I feel compelled to mention a few things.

    The ATS doesn't really send a start signal to the generator. In larger installations, especially those with more than one generator, it sends a signal to the generator switch gear. The switch gear than issues the start command to the generator. In smaller stand alone generators the generator has a factory (usually) mounted control system which receives the ATS start signal/contact closure and then starts the generator.

    The ATS can be set up (programmed) to operate in certain ways and have certain (optional) features. These may include minimum times before re-transfer to normal power or the ability to "exercise" the generator on a time schedule (either under load or not). The ATS will maintain a start signal to switch gear until power has returned plus any "programmed" minimum delay to re-transfer takes place. The switch gear should be programmed for a five minute cool down additional run time after all ATS's are no longer calling for the generator to run.

    For example. Lets say we have a power outage of 10 seconds. At time of outage ATS sends start signal. Generator takes seven seconds to start, make rated voltage, close breaker, and assume load. Three seconds later normal power is restored. IMHO it would be a grave mistake to allow the ATS to open it's start contact and allow generator to only run five additional minutes in cool down mode. IMHO if the generator has started and assumed the load you should let it continue to do so for 30 minutes. This lets the generator get up to operating temperature, reduces possibility of wet stacking, and reduces the chances of multiple start/stops in a short period of time.

    Personally, I would never program an ATS to automatically "exercise" an unattended generator and to run said generator unloaded. Generators should (ideally) never be run with less than a 30% load.

    P.S. All the above nonsense assumes we are talking diesel generators.

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