larger hp motor on a pump

[adamscb]

All,

We ran into an issue where the engineer running a project ordered a small pulsa feeder pump, except the motor was 120v single phase, 1/3hp. The wires we had going to it are 480v, three-phase. Not too big of a deal, except the only problem is that we don't have a 1/3hp, 480v three-phase motor in our plant. We do have a 2hp, 480v three-phase motor that has the same frame and almost the exact same RPM. Is increasing the motor size from 1/3hp to 2hp on this pump a reasonable idea? From my understanding doing this means the motor has the capacity to work harder than the 1/3hp, but it shouldn't because the pump stayed the same. Thoughts?

 

[Hv&Lv]

All,

We ran into an issue where the engineer running a project ordered a small pulsa feeder pump, except the motor was 120v single phase, 1/3hp. The wires we had going to it are 480v, three-phase. Not too big of a deal, except the only problem is that we don't have a 1/3hp, 480v three-phase motor in our plant. We do have a 2hp, 480v three-phase motor that has the same frame and almost the exact same RPM. Is increasing the motor size from 1/3hp to 2hp on this pump a reasonable idea? From my understanding doing this means the motor has the capacity to work harder than the 1/3hp, but it shouldn't because the pump stayed the same. Thoughts?

Somewhere around 6-8 times the kVA needed.
More energy needed.
Cant you get a small 1kVa dry pack and put there? Or move the wires to a 120 panel?

 

[adamscb]

The nearest 120v source is quite a ways away, plus this is in an explosion proof area, so we're wanting to avoid running new conduit. The existing electrical infrastructure can handle a 2hp motor (#10 wires, breaker is big enough, size 2 starter, overloads can be bumped up). So from what you're saying the amp draw will increase, but I think we may go with this 2 hp motor and just bump up the overloads.

Just curious - why does the amp loading increase? Does it require more energy to keep the shaft rotating, since this motor does weigh a bit more?

 

[Jraef]

Work is work. The pump will only use the energy it requires to do its job. There will however be a slight additional increase in the energy consumed because in any motor, a fractional amount of the energy it consumes is based on what it takes to magnetize all of the pertinent parts. Larger motor, larger parts, more energy to make them magnetic. It will however likely take years before that difference would equal the cost to run a new 120V circuit to that pump.

The economics however works here because you already own the 480V motor. If you had to go out and buy one, it might not.

 

[broadgage]

I believe that the larger motor will be fine. The pump will be driven at the correct speed and does not "know" that a larger than needed motor is installed. The pump will only absorb the needed HP to drive it, and thereby only slightly load the motor.
The running costs may be slightly increased, but only slightly.

Here in the UK I have deliberately upsized motors when there has been a history of failures. A moderately upsized motor may actually be MORE efficient than one run right at the limit, it will certainly be more reliable.
In this case the proposed motor is about SIX times the HP needed, and slight extra losses will result, but not that much.

 

[topgone]

The usual problem I see is when the replacement motor is a newer, high-efficiency motor that could sometimes go over the short-time settings of the previously installed unit. Their might be an EMC which can spuriously trip during starting of the bigger motor, IMO.

 

[PaulMmn]

All,

We ran into an issue where the engineer running a project ordered a small pulsa feeder pump, except the motor was 120v single phase, 1/3hp. The wires we had going to it are 480v, three-phase. Not too big of a deal, except the only problem is that we don't have a 1/3hp, 480v three-phase motor in our plant. We do have a 2hp, 480v three-phase motor that has the same frame and almost the exact same RPM. Is increasing the motor size from 1/3hp to 2hp on this pump a reasonable idea? From my understanding doing this means the motor has the capacity to work harder than the 1/3hp, but it shouldn't because the pump stayed the same. Thoughts?

Leave detailed documentation, both in the files and a sign at the pump, detailing the original configuration. Some time in the future someone will need to replace the larger motor, and knowing that a lower voltage, lower HP motor will work may save your successor lots of headaches!

 

[kwired]

though you won't have this issue with a pump, I once come across a fractional HP motor that was replaced with a 2 hp mostly because it was correct frame size and available at the time. (56C frame) This motor was driving an auger on a dual station bagging machine, it reverses direction every time a bag is filled to divert flow to the other bagging head. Reversing was done across line with standard reversing starter so whenever one bag got full you went from full speed one direction to full voltage and torque the other direction nearly instantly. Fractional HP motors had been on that machine for years, but this kind of torque on the shaft each time it reversed, maybe every minute and a half to two minutes, eventually stressed the shaft on the 2 hp motor. Don't know the words to describe it but it sheared the shaft of but not in a usual twisted looking manner, shaft was really brittle and kind of come apart like a stick of string cheese would.

 

[adamscb]

though you won't have this issue with a pump, I once come across a fractional HP motor that was replaced with a 2 hp mostly because it was correct frame size and available at the time. (56C frame) This motor was driving an auger on a dual station bagging machine, it reverses direction every time a bag is filled to divert flow to the other bagging head. Reversing was done across line with standard reversing starter so whenever one bag got full you went from full speed one direction to full voltage and torque the other direction nearly instantly. Fractional HP motors had been on that machine for years, but this kind of torque on the shaft each time it reversed, maybe every minute and a half to two minutes, eventually stressed the shaft on the 2 hp motor. Don't know the words to describe it but it sheared the shaft of but not in a usual twisted looking manner, shaft was really brittle and kind of come apart like a stick of string cheese would.

56C frame is the frame type we're dealing with here. That's crazy though how the shaft broke.

We found one motor in our warehouse that was fractional hp, but it had three p-lead wires instead of two (some kind of thermal protection that's in series with the windings??). All of the explosion proof motors we have here in our plant just have two-wire p-leads (thermal switch). That's why we had to bump it up to a 2 hp.

 

[zbang]

This motor was driving an auger on a dual station bagging machine, it reverses direction every time a bag is filled to divert flow to the other bagging head. Reversing was done across line with standard reversing starter so whenever one bag got full you went from full speed one direction to full voltage and torque the other direction nearly instantly.

Digression-
I'm working on a Covel surface grinder at the moment. The table motor also reverses ATL at the end of stroke; the manual mentions that it's a special motor and will run very hot.... when I can get close enough I'll look for the nameplate.

 

[RumRunner]

All,

We ran into an issue where the engineer running a project ordered a small pulsa feeder pump, except the motor was 120v single phase, 1/3hp. The wires we had going to it are 480v, three-phase. Not too big of a deal, except the only problem is that we don't have a 1/3hp, 480v three-phase motor in our plant. We do have a 2hp, 480v three-phase motor that has the same frame and almost the exact same RPM. Is increasing the motor size from 1/3hp to 2hp on this pump a reasonable idea? From my understanding doing this means the motor has the capacity to work harder than the 1/3hp, but it shouldn't because the pump stayed the same. Thoughts?

It is often seen as myth (or an outright assumption) that using a small motor in place of a bigger one would improve efficiency. This is not always the case.
In your situation it's the opposite.

Replacing the 1/3 HP with a 2HP would certainly work, as others had commented. . .it will pump as intended.
You mentioned that the pump is used as a "pulsa" pump which is ..to my understanding..is a PULSA PUMP designed for maintaining liquid pressure in chemical process and/or manufacturing that requires precise dosing of liquid ingredient.

In most pump-motor application, the relationship between the two components (pump vs motor) has an effect on efficiency should one parameter is changed. . . not to mention the effect on the precise metering requirement.

I don't know how meticulous you are in dealing with this issue--but as engineer like you, I tend to give too much emphasis on things like these. Maybe it's just me.
You did say that the two motors are "almost identical".
Sometimes even small difference can make a big difference.

Depending on the application. . .going through painstaking detail is boring, but sometimes it is necessary.
All things being equal, switching from a 5HP motor (for example) to a 10p HP motor will work.
The smaller motor will keep its efficiency at around 90% when operated at 50 % output. The usual efficiency rating of most AC motors are around 84%. (You can look it up).

So, by looking at those numbers you'd be better off using a small motor.

One comment says that "work is work" and the motor/pump combination will provide the outcome expected of it. It is true that the pump will provide the output as long as the parameters are met including GPM discharge.

However, there are two types of doing work. . .one is doing it efficiently and the other one inefficiently.
We can bail out a flooded basement for example by using a bucket or we can use a sump pump.

So, what does this have to do with bucket vs motor?

This where engineering comes to play. Engineers are hard at work to make us work smarter not harder and help us become more productive.

In this day and age, efficiency is everything. Even government regulators like DOE have guidelines on how machines should be designed in terms of efficient operation.

I'm sure the design engineer spent time in coming up with his setup and I'm not in a position to read his mind, however.

People reading this (including you) expect a solution, but, ( ceteris paribus) other things being equal. . .the final call being an engineer like you still rests upon you.

You are soliciting thoughts.

 

[Jraef]

1

 

[paulengr]

Well, here's the thing about that. If a motor's efficiency is rated for 84%, that is the Eff at 100% load. There is NO REQUIREMENT to list Eff at part load, and the part load Eff can very greatly from one motor to the next. But in any case, it is never the same as the full load Eff.

So in this case, if we assume that the original 1/3HP motor was going to be fully loaded (or close to it) and lets say the 2HP motor is 84% efficient, the small load of that pump means that motor is only going to be 16.5% loaded. The motor efficiency at only 16.5% load is NOT going to be 84% any more.

Here is a typical chart put on motor efficiency at part load. So from this you can see that at 17% load, the 2HP motor will be running at only around 50% Eff, not at peak (which in this chart was 80%). That's because, as I said, it takes more energy just to create FLUX in that larger motor, even though you don't really need or use in with a 1/3HP load.


So if you extend that out into a motor power consumption equation, the 1/3HP (.25kW) 1 phase motor, assuming similar 84% Eff, will consume;

.25kW / .84 = .296kW (absorbed) power.

2 HP at 16.5% load and 50% Eff will consume:

.25 / .5 = .496kW

The 2HP motor will consume 1.68x more energy running the same pump. It's still not a LOT of energy because the beginning amount is so low to start with, so it's a judgement call as to whether that's a problem, but everyone just needs to understand that it is NOT the same.

Agreed. But if we purposely adjust the voltage as in run it off a VFD or a continuous duty soft start we can make that excess flux disappear for the most part.

Efficiency at less than 100% load is largely due to flux as stated. One factor that affects it dramatically is poles. A 2 pole motor loses a lot less efficiency compared to 6 or 8 poles.


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