Two motors on 1 soft start

[tish53]

I read this forum each week and always learn great new things.:happyyes:

Now I have a question.:? I work for an Aggregates producer and we are building a new plant. We will have 3 conveyors that the mech. eng. have decided to split the motors used to power the conveyor.
Example: large conveyor calculated to 700 hp needed. Mechanically we will have (2) two 350hp motors tied to a common drive pulley. Electrically our POCO requires us to soft start all of our larger motors. That brings me to my dilemma. One 700 hp soft start with dual overloads feeding the two motors OR twin 350 hp soft starters.

Does anyone have an opinion on which way is better or if there is any potential problems. My take is the common 700 hp with twin overloads will work but economically is bit more money. Control is easier and I only need one incoming CB.
The two soft starters can be programmed the same ( starting current, max current, ramp time etc.) and theoretically should work. They would get a common control start signal from a PLC. It involves more PLC I/o and 2 incoming CB's. Control wise I need to interlock the two softstarters so if one trips the other one also shuts down. I am also a little concerned that someone could adjust parameters on one, but not the other and we could get in a bit of a problem. I guess overloads should protect the motor in that case.
Would love to hear what anyone in the group has experience with or can comment on.

thanks

 

[iceworm]

... I am also a little concerned that someone could adjust parameters on one, but not the other and we could get in a bit of a problem. I guess overloads should protect the motor in that case. ...

Engineering is a constant struggle against the universe. We try to design and build foolproof equipment. The universe continues to build better fools. So far the universe is winning. **

Ignorant is curable. Stupidity is terminal.. **

Warning seen on machine: "This machine does not have a brain. You will have to use yours." **

I don't know any way to protect against stupid. Someone wants to go in during the dead of the nght and make undocumented adjustments, change overloads, wire around protective relays .... and on and on. They got you.

As for the interlocked trips: Good idea. Might consider putting in a "first out" relay so you can tell which one tripped first. Could help with trouble shooting.

ice
**None of these are original with me.

 

[Jraef]

Done this many many times.

If there is ANY chance that the conveyor will need to be re-started after say, a power line failure, then you will NEVER get the two separate soft starters to share the load evenly and one will trip out. Trust me on that. The only viable method is one soft starter and two overload relays down stream. You TECHNICALLY have a problem with the motor SCPDs as well, the SCPD for a 700HP soft starter might be too big for either single 350HP motor, but it's a numbers game that you have to play, coordinating with the minimum CB or fuse size the soft starter is listed to be used with. For example if the soft starter is listed to be used with 1200A fuses, it might be ONLY 1200A fuses, in which case they are going to be too big to protect a 350HP motor.

If there is NO chance that the conveyor will need to start loaded, i.e. 700HP would not be enough to start it loaded anyway so they MUST shovel the load off no matter what, then it's simpler. You do NOT need to soft start the second motor. You start the first motor with a soft starter and use a Bypass Contactor. The first motor will likely be enough to START the empty conveyor on it's own (in my experience). Then because they are connected together, the second motor is ALREADY at full speed when you start it and is NOT required to have Reduced Voltage starting. So when the Bypass Starter closes on the soft starter for motor #1, use an aux contact to trigger an Across-the-Line starter for motor #2 (i use about a 1 second time delay too, it never hurts). It's exactly the same as a "pony motor" starting method, which the utility will accept, the only difference is that your "pony motor" is the other 350HP one. Done it many times, never had an issue. But again, this ONLY works if there is NO CHANCE that the conveyor will have to restart with a load on it. If it does, choice one is the only thing you can do.

 

[tish53]

Thanks for the responses.
Jraef, I appreciate your insight. I am leaning toward the two soft start solution because of the SCPD problem you mentioned.
We may have a condition where a power failure or some other out of the ordinary condition shuts us down with a conveyor filled with rock. In that case we have two options, one give everyone and their brother a shovel, or two we can call the utility to alert them and then start the twin motors across the line simultaneously. This event ( in our humble opinion) is a once a year or more likely once every 2-3 years occurrence. So If I go that way, we will order the soft starters with a starting duty bypass contactor.
The control side is easier with one soft starter but the incoming power is cleaner and meets code easier with two soft starters. Also economically the two soft starters is around 10 % less expensive.

 

[Jraef]

... In that case we have two options, one give everyone and their brother a shovel, ...

Been there, got that t-shirt...

... or two we can call the utility to alert them and then start the twin motors across the line simultaneously. This event ( in our humble opinion) is a once a year or more likely once every 2-3 years occurrence. So If I go that way, we will order the soft starters with a starting duty bypass contactor. ...

If that option is open to you, I think that's a valid approach. But watch out for belt damage. Been there, got the t-shirt too...

Also economically the two soft starters is around 10 % less expensive.

I find that surprising actually. Who are you getting quotes from?