Multiple motors and one contactor

[Shaneyj]

I am looking at a design that calls for a redundant 100 HP motor to back up the primary 100 HP motor.
I would like to use just one starter with an interlock that manually controls which motor is being energized. The problem I see is the switch would have to carry the load of the motor - unless some type of relay contact was used (at rated i and v).
Somebody has probably done this before. Do you know of a device that allows what I am looking for?

 

[masterinbama]

I would put interlocked breakers on the load side of the starter. Or interlocked motor starters so you would have a redundant starter also.

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[ptonsparky]

I am looking at a design that calls for a redundant 100 HP motor to back up the primary 100 HP motor.
I would like to use just one starter with an interlock that manually controls which motor is being energized. The problem I see is the switch would have to carry the load of the motor - unless some type of relay contact was used (at rated i and v).
Somebody has probably done this before. Do you know of a device that allows what I am looking for?

Use an external properly sized double throw switch. They can be arranged to switch between the two loads vs two power sources. You would want to make sure the starter is off before transferring loads but that can be done via an overlapping control interlock.

 

[paulengr]

You don’t need load rated disconnects if you can’t/don’t switch on the fly. If you want to do this via contactors use a reversing contactor setup but don’t bother wiring for reversing. Use a selector switch to swap coils. Unless you want the complication of two overloads put the overload ahead of the contactor. If you are building from scratch a manual motor starter with the overload built in solves that.

 

[Jraef]

I am looking at a design that calls for a redundant 100 HP motor to back up the primary 100 HP motor.
I would like to use just one starter with an interlock that manually controls which motor is being energized. The problem I see is the switch would have to carry the load of the motor - unless some type of relay contact was used (at rated i and v).
Somebody has probably done this before. Do you know of a device that allows what I am looking for?

You have a redundant motor for a reason. Using one starter for the backup puts your redundancy plan at risk. Lose the starter and you lose BOTH motors!

Don't cheap out on it right at the finish line, just get another starter and have full redundancy.

 

[texie]

You have a redundant motor for a reason. Using one starter for the backup puts your redundancy plan at risk. Lose the starter and you lose BOTH motors!

Don't cheap out on it right at the finish line, just get another starter and have full redundancy.

I agree. And the cost of materials and labor to do it by other means will approach the cost of just using another across the line starter and simply interlocking the control circuits.

 

[pjones]

Redundant means that if the first system burns up then you should be able to switch over to the other system and keep running.

Separate EVERYTHING should be installed, otherwise you are not supplying them with what they are asking for.

 

[paulengr]

Redundant means that if the first system burns up then you should be able to switch over to the other system and keep running.

Separate EVERYTHING should be installed, otherwise you are not supplying them with what they are asking for.

Kind of disagree here. It depends on reliability. As an example the PLC manufacturers have been pushing safety PLCs which contain redundant processors and some fail safe logic to test outputs for roughly a 300% markup.

So how often does a valve or motor fail compared to a PLC? How about a disconnect? How many times have you changed thermocouples, RTDs, or limit switches? How many times did you replace a PLC? See..,it’s the reliability of the component that matters. With motors if you have a top notch reliability program including lubrication, vibration, alignment, and balancing and you use 1.15 SF motors at or below name plate FLA, getting 20+ years of life is not unreasonable. A lot of plants would be happy with half that life. Conversely if the contactor is sized properly including number of starts getting 20+ years of life is not unreasonable either but since you don’t have to do anything to it other than eventually changing contact tips , it’s fairly normal to do that. So contactors will far outlast the motors. Only reason I’d use two contactors is double throw disconnects are far more expensive than another contactor.

 

[Shaneyj]

The requirement for a redundant motor is from our hydraulics engineer. He wants a back up just in case.
This is a skid mounted HPU that will operate on a drilling rig.
Its all C1D2 and its expensive.
The starter and enclosure costs more than the ex rated 100 hp motor.
This is not my specialty, hence the questions; my initial instinct was that a redundant contactor was unnecessary.
But I reserve the right to be wrong.

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[Jraef]

The requirement for a redundant motor is from our hydraulics engineer. He wants a back up just in case.
This is a skid mounted HPU that will operate on a drilling rig.
Its all C1D2 and its expensive.
The starter and enclosure costs more than the ex rated 100 hp motor.
This is not my specialty, hence the questions; my initial instinct was that a redundant contactor was unnecessary.
But I reserve the right to be wrong.

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Relevant details...
Anything you are going to do in explosion proof equipment is going to be expensive. So probably the lowest cost option will be to but a reversing starter and modify it to just have separate outputs for each motor. You will still have that single point of failure though.

 

[oldsparky52]

This is not my specialty, hence the questions; my initial instinct was that a redundant contactor was unnecessary.

Nothing wrong with asking for more customer input, or an engineered drawing (preferred).

 

[pjones]

Kind of disagree here. It depends on reliability. As an example the PLC manufacturers have been pushing safety PLCs which contain redundant processors and some fail safe logic to test outputs for roughly a 300% markup.

So how often does a valve or motor fail compared to a PLC? How about a disconnect? How many times have you changed thermocouples, RTDs, or limit switches? How many times did you replace a PLC? See..,it’s the reliability of the component that matters. With motors if you have a top notch reliability program including lubrication, vibration, alignment, and balancing and you use 1.15 SF motors at or below name plate FLA, getting 20+ years of life is not unreasonable. A lot of plants would be happy with half that life. Conversely if the contactor is sized properly including number of starts getting 20+ years of life is not unreasonable either but since you don’t have to do anything to it other than eventually changing contact tips , it’s fairly normal to do that. So contactors will far outlast the motors. Only reason I’d use two contactors is double throw disconnects are far more expensive than another contactor.

Failures can be anticipated but never predicted with 100% certainty. It all depends on how critical the equipment operation is as to what level they decide to make redundant. On critical equipment the level of redundancy required is achieved by completely duplicating the system so it will continue to serve its purpose even with the complete primary system in a total state of failure. In my line of work it’s quite standard to do this.

Having a single point shared anywhere compromises that redundancy. There are many different types of failures that can occur from many varying factors. Even reliable equipment can fail and that possibility should not be ignored if the equipment is critical.

Sounds like the OP needs to verify what level of redundancy the design requires. If the design engineer doesn’t have that answer then the OP needs to discuss the options, benefits, and costs with them so they can make an educated decision. Asking on here isn’t guaranteed to provide an answer to the OP that fits the customers requirements. Only the customer can provide that information.

 

[Shaneyj]

Nothing wrong with asking for more customer input, or an engineered drawing (preferred).

We're the OEM. Its my job to create the engineered drawing.

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[oldsparky52]

We're the OEM. Its my job to create the engineered drawing.

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So how much redundancy does the customer really want? Do they want it automatic or manual? Curious, the pump will be piped in parallel with the pump it's backing up?

 

[Shaneyj]

So how much redundancy does the customer really want? Do they want it automatic or manual? Curious, the pump will be piped in parallel with the pump it's backing up?

Great question... for my own curiosity I was going to ask how he plans on coupling 2 motors to1 pump... or the pump drive; its operating 3 pumps.

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[pjones]

We're the OEM. Its my job to create the engineered drawing.

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You still need to inquire what level of redundancy they need. What’s over engineered for one job will be under engineered for another.

Let them know you can design a product solution to achieve anything they require but they need to provide you with their requirements so you can be sure to meet their expectations.

 

[paulengr]

If efficiency/current isn’t a big concern, use one contactor sized for 200% and two overloads. Run two motors 100% of the time. Check valves on pump outputs. The relief valve will dump the excess fluid. Now you have 100% redundancy online. If they suspect problems they can uncouple shafts or unwire things.

 

[Shaneyj]

For those that were curious... I just got some more information.

The customer was in fact driving the request for redundancy. So we will quote for 2 of everything. I asked our hydraulics guy how he was going to couple 1 pump drive to 2 different motor shafts and he looked at me funny. He said he would have redundant everything; pump drive, pumps, multiports, etc.

The evolution of the sharing of information around here is fantastic...

Sales guy to hyd guy --> hyd guy to me --> me to internet forum --> questions back to hyd guy --> the rest of the pertinent details

I guess I didn't ask the right questions to be privy to all the information up front.

Thanks for the responses.

 

[texie]

For those that were curious... I just got some more information.

The customer was in fact driving the request for redundancy. So we will quote for 2 of everything. I asked our hydraulics guy how he was going to couple 1 pump drive to 2 different motor shafts and he looked at me funny. He said he would have redundant everything; pump drive, pumps, multiports, etc.

The evolution of the sharing of information around here is fantastic...

Sales guy to hyd guy --> hyd guy to me --> me to internet forum --> questions back to hyd guy --> the rest of the pertinent details

I guess I didn't ask the right questions to be privy to all the information up front.

Thanks for the responses.

 

[Shaneyj]

haha. Thats getting saved