full load amp runout 1

[tulum]

hello folks,

I am getting this info second hand so bear with me...

An electrician asked me whether I knew what the term "full load amp runout" meant? The electrician had sized a cable for a motor load (60m away) at 300mcm from the motor spec sheet. The engineer on the job said he wanted 400mcm to cover the FLA runout.

The only thing I could think of was that the engineer might be using this term to refer to the voltage drop in the cable and oversizing it to ensure that a proper motor terminal voltage is present.

Anyway ever heard of this term before?

Thanks in adavance
TULUM

 

[jraef]

I think he has his terminology skewed a little.
thread237-96961

Electrical Runout, while somewhat indirectly connected to motor leads, appears to be a specific phenomenon related to Eddy Current sensing probe signal losses, and I cannot see how the size of the motor lead conductor would change that. He may have heard the term used one day when someone was talking about a large motor and incorrectly attributed it to voltage drop, then repeats it because nobody like you has come along to challenge him.

"Venditori de oleum-vipera non vigere excordis populi"

 

[mc5w]

There is also the matter that if the motor has a service factor of 1.25 or greater you should use larger wire on the basis that the motor has more capability for sustained overload. Some machines also turn out to need a 1 size larger motor and I have encountered that a lot. I like EXTRAS but I also like for extras to be really easy. 1 size larger conductor will also make the motor more efficient.

I have had an application of an alledgedly 3 Horsepower pump made by Dunham-Bush that requires a 5 HP motor to run it, particularly on 120Y208 volts. This was how my customer ended up with a 5 HP motor on a 3 HP controller. The replacement pump when coupled to a 5 HP motor showed a full load current consistent with 3.5 HP of mechanical load.

At one point Dunham-Bush was able to find 3 HP motors with a 1.5 service factor - essentially a 4.5 HP motor. When those started to become hard to find they started sticking in 5 HP motors.

There were also a few other rather hilarious details such as that the salesman accidentally ordered an oversize pump just because the motor was 5 HP. To make things worse, the boiler room was ojn a second floor loft accessible only by ladder and hand winch.

Mike Cole, mc5w@earthlink.net

 

[buzzp]

Not sure why anyone would run a motor at SF amps if they don't need too. This is not where it runs most efficient. Cable size should not affect efficiency. The only thing that will be affected is the voltage. If the voltage is below nameplate then you could have problems. Other than voltage drop, cable has no affect. I agree on upsizing the cable if there is any chance the motor size will have to be increased and running new cable is difficult.

Never heard of a 3HP pump needing a 5HP motor. Sounds like the pump design is most inefficient.

 

[Mstrvb19]

Tulum,
We had this exact situation at our plant. Here is what the engineer was saying: The mechanical load for the pump (there is no doubt this is a pump motor) has been calculated under "run-out conditions", which (I am no mechanical engineer) is basically comparable to a situation where the pump was feeding water into a broken pipe (or some other very minimal flow resistance). This load can often be well above the motor service factor, as it is considered to be a short term condition. The engineer calculated the required electrical horsepower for the mehanical pump load under these conditions and has sized the cable accordingly. Why? These types of design considerations are common in high-reliability/single time safety operation conditions, such as in Nuclear power plants. For example, at our plant, we had postulated a level 4 LOCA (Loss of Coolant Accident), and in this situation, all loads must be considered at their worst case loading, and all contingencies considered. A large-break LOCA is basically a double-ended pipe break, and some pumps, particularly those that supplied coolant to the reactor core, could possibly be driving a delta-P similar to a run-out situation. A nuclear plant is required by the NRC (Nuclear Regulatory commission)to demonstrate that loads such as these are capable of performing their design basis function for the entire postulated accident time (i.e. your pump/motor at runout must operate successfully for a certain number of minutes) and to ensure that the motor can most easily supply such a large load without too much voltage drop, your engineer has sized the cable for motors required, one-time accident performance. In other words, no one cares if this motor craps the bed, as long as it runs for the required time and supplies coolant before it does.
So, as long as we are talking about an application similar to the one I just described, the engineers philosophy is sound. However, I have not heard of any other industry that places equipment one-time performance at such a high priority. Most other applications, the motor protection should be set to trip the motor under "run-out" conditions.

 

[jraef]

Come to think of it, I HAVE heard that term used in the mechanical context descibed above by Mstrvb19. It was on a 1000HP water supply pump for a small town, and their concern was a pipe break during a fire. They had proposely oversized the pump and motor for the worst case scenario, and wanted the motor protection system set for "run-out" conditions, meaning the mechanical / hydraulic issues that happen when the output of a pump is suddenly opened up with little or no head pressure, and the corresponding motor current that would result from that.

"Venditori de oleum-vipera non vigere excordis populi"

 

[tulum]

Thanks for the all the replies.

Mstrvb19 your response makes alot of sense. Thanks for the clarification and the sharing of your knowledge. I will past the info on to the electrician.

Best Regards,
TULUM

 

[mc5w]

I have a feeling that someone at Dunham-Bush designed the pump and somehow understated the horsepower required on say 50% of applications because the other 50% only needed 3 HP.

If you are the one that has the 3.5 hp flow and pressure a 3 hp motor will get to be like the kind of aircraft ehgine tuneup that is good for 2 years penal servitude in Leavenworth.

Fire pumps are also an application where you do not want to skimp on wire size. Besides maximum flow overloads a fire pump will likely dislogde iron bacteria clumps which are a precursor to iron ore. The pump would need to digest these and be subject to jamming. The motor only needs to last an hour until a volunteer fire department sets up the siamese connection.

Mike Cole, mc5w@earthlink.net

 

[jraef]

Huh?

"Venditori de oleum-vipera non vigere excordis populi"

 

[mc5w]

I was just trying to tell buzzp that the particular Dunham-Bush pump is not necessarily inefficient just that somebody dropped a decimal point and hoped that nobody would notice.

A lot of our older water mains have iron metabolizing bacteria in them. There is a British company named The Tate Pipelining Process Company that goes all over the world and removes iron bacteria and then relines the pipe with sand mortar to keep the bacteria from growing back. If you merely flush the mains the bacteria grow back with a vengence.

A fire pump tends to do a maximal flush of water mains.

Most of the iron ore deposits were laid down by giant colonies of iron metabolizing bacteria that convert soluble bivalent iron oxide into insoluble trivalent iron oxide.

Mike Cole, mc5w@earthlink.net

 

[buzzp]

In any case, someone screwed up somewhere when sizing the motor.

 

[rmw]

As a mechanical engineer reading this thread, and one who has had to size motors for pumps, and has had to account for 'runout' conditions on the curve, (not only broken pipes, but a control valve opening suddenly, can cause a pump to 'runout' on it's curve, well past the design operating point,) I interpeted the question to be a pump, and the provisions for operating well to the right hand side of the curve. The engineer was wise to size for a realistic condition that could occur.

rmw