I need help designing an automatic generator set start circuit to be run from a remote transfer switch. The switch sends a two wire "close" signal to start the generator and "opens" to stop it. The generator requires a momentary close on wire 1 and 3 to start and a momentary close on wire 1 and 2 to stop. Wire 1 is engine ground. The current required is only enough to run relays. 120VAC is available at the transfer switch when the generator is off. 12VDC is always available at the generator and 120VAC is available when it runs.

 

You have a cable between transfer switch and the generator set.

The important point to know first is how many cores(wires) of the cable is available for use by this intended circuit?

Two cores(1 wire + ground) or three cores(2 wires + ground) makes a big difference to the control scheme implementation.

 

Two wires, dry contact, no ground, thanks

Brian

 

Ok. Let state the requirement to see if it is correct.

1. The transfer switch has a dry contact consists of two states. Open and Close.

2. A cable of only two cores available between transfer switch and remote generator set

3. When transfer switch contact is close, a relay on the generator side closes a contact for a brief period, say one second, to start up the gen set

4. When the transfer switch contact is opened, a relay on the generator side closes for a brief period, say one second, to stop the gen set

5. A change of the transfer switch status during the period (3) or (4) above will be ignored until the current period terminated

Is that about right or you have something more to add?

 

Ok. Let state the requirement to see if it is correct.

1. The transfer switch has a dry contact consists of two states. Open and Close.

2. A cable of only two cores available between transfer switch and remote generator set

3. When transfer switch contact is close, a relay on the generator side closes a contact for a brief period, say one second, to start up the gen set

4. When the transfer switch contact is opened, a relay on the generator side closes for a brief period, say one second, to stop the gen set

5. A change of the transfer switch status during the period (3) or (4) above will be ignored until the current period terminated

Is that about right or you have something more to add?

1- yes, 2- yes. 3-might need to be longer to start. We could use 120VAC from the running generator or an oil pressure switch to sense running. 4-might need to be longer to enable the generator to spin down.4-no, a change in status of the transfer switch needs to be followed by a change in the generator.
The transfer switch is connected to mains power. If power fails, the transfer switch drops out to a neutral position, closing the contact in the "generator run " circuit. When the transfer switch senses generator power, it switches the load to generator until it senses a return of mains power. This circuitry is in the transfer switch itself. What I need to do is connect the NO/NC transfer contacts to the existing 3 wire "common- momentary run, momentary stop " circuit on the existing generator. The common is also 12VDC engine ground

 

The kind of AC transfer switch I'm familiar with usually does not make a physical load transfer "until there is proper voltage on the alternate bus". This makes sense as there is no point transferring a load to a bus with no voltage.

If you want to use the transfer switch contact for genset startup/shutdown, then it won't work. The genset should be started up first by detecting an under voltage condition on the mains, starts the genset and then the transfer switch, upon sensing voltage on the alternate bus, would automatically make a transfer. Perhaps some kind of transfer switches is working like a plain AC contactor with normal open and normal close contacts.

Can you clarify that?

 

Attach a 120V relay tapped off of one leg of your utility in your ATS. Then attach another relay off one of your generator legs. These relays need to have three sets of contacts. meaning you should have the ability to create a combination of three n/o n/c contacts per relay. Take wire 1 (ground) to your common terminal on the relay on the utility side. On the other relay attach wire 3 to the common terminal and attach another wire from the n/c terminal and run it up to the n/c contact on the utility side relay. What this will do is when utility power drops out, a contact between
1&3 will happen, cranking and starting the genset. Once the generator starts the other relay energizes breaking the connection from wire 3. This should get your generator up and running. In order to shut the unit down is simple. Attach wire 2 to the n/o terminal on the utility side relay. When power returns 1 and 2 will close together shutting engine down, and will remain closed until utility power drops out initiating the sequence all over again. I would suggest installing a timing relay tapped off the utility side interupting wire 2 before the start stop relay on the utility side. This way it will allow your transfer switch to go through its time delay and retransfer, and not shutting the generator down under load. I am assuming you are working with an Onan generator, probably a JB/JC model. I am in no way guranteeing this operation, and remember your completely bypassing much of the logic of your ATS. This is offered as a suggestion to making an incompatible system operate, and all responsibility is assumed by anyone who takes this suggestion and utilizes it.

 

It seems like you need 2 separate functions for your generator to work. The simplest is the stop function. Energize a dpdt relay coil (12vdc) from the battery and the transfer switch dry contacts. The relay's first set of C and NC contacts connect to generator lines 1 & 2. When the transfer switch opens, the generator shuts down; lines 1&2 are closed. Starting the generator, the second set of relay contacts (C & NO) now connect to a zero speed switch that is coupled to your generator shaft. The zero speed switch contacts (C & NC) will open when the generator comes up to about 75% rated speed. This gives you a momentary pulse start on lines 1 & 3 of the generator. It also is dependent upon the generator actually starting and running. It means having to install a ZSS (zero speed switch) to your generator, but it will be worth the cost.

Regards, DPW

 

m also working on this project....i've done tht stop thing before ,but waz havin bit confusion in starting the gen. ,this zero speed switch idea iz rlly nice ...thnx Duane but can plz elaborate how to connect it to the gen. shaft??? we're waiting..

 

Hi,

First you would need three relays and one timer, 2 dcr one acr and one dc timer.
Now the DC supply will connect directly to A1 of R1, A1 of T1.
The DC supply also passes through RI NO,R2 NO and R3 NC R3 been the AC relay.
If you are using 1 and 3 as open on R1 then tap the supply directly to the fuel solenoid.
R3 NC goes to the start and R2 gets it supply through T1 NC.
Hope you can understand.

Cheers Dennis.