Choosing Motors for Pumps

Motors like to be run in the most efficient part of the knee of their pump curve.

Or gat yourself a clamp=on AMP probe and measure the AMP draw of the motors at various flows.

Too many incomplete, and confusing concepts here to help you.

Are the electric motors wired in parallel with one motor controller (switch, VFD, variac) for control or is the piping of this fluid to two pumps piped in parallel? If the latter then are any check valves used?

What do you mean by the oxymoron "constantly variable"?

What type of pump mechanism; displacement (screw, gear, plunger, etc.), impulse, velocity (radial, axial, eductor, etc.), etc.

Is the pumped fluid compressible or not?

we have two motors driving two pumps, those two pumps are piped in parallel, or in other words, they both pump water in the same conduit.

those two motors are running at the same time, and there is a third motor in standby in case of a problem.

the flow of water is always variable (constantly variable), and there are valves opened and closed manually to control the flow of water.

the two motors that are running continuously have the following characteristics : 225KW, FLA 393A

they are running under 380 V, a frequency of 50 Hz and they are wired to a soft starter each.

the first motor draws a 260A and the second draws 290A both at full speed.

the standby motor is a 160KW with a FLA of 291A, and wired to a soft starter.

now, the current drawn by both motors is 550A. my question is the following:

based on this current draw, can we say that a 355KW motor with a FLA of 621A is enough to replace the two motors of 225KW, and by enough i mean that we'll have the same flow of water, and is this new motor going to consume less energy than the other two ?

Agreed. 100% The electric motors should be properly sized by the mechanical loads (pumps) they are expected to drive.

A critical thing the OP is overlooking in their plan to combine two pumps into one is that this system actually has three pumps not two pumps. This high reliability system is designed to work with any two out of three pumps working. Redesigning this system for only one pump to handle the load will mean a single larger pump must be in standby mode. There will also be significant changes in flow rate during switch overs. Once again we do not know if this is a critical attribute for operation or a trivial drawback.

"flow of water is always variable (constantly variable), and there are valves opened and closed manually to control the flow of water".

As I just explained to olivinak you can't have constant power consumption if you are changing the flow rate of the pumps.

It's ti,e to get professional help from your pump supplier.

We can't help you ask the right questions.

Homework?

Based on that description, the energy losses are taking place at the valves that reduce the flow.

• Why is constant flow required (rhetorical question - NNTR)? Consider leaving those valves wide open instead. If the flow is too high, regulate the flow based upon pump speed. For pumps in parallel, regulate the speed of both pumps simultaneously.

It is probable that sending two pumps' flow through one larger pump will mean increasing the pump size, and increasing the pipe diameter. The flow will not be the same, as the larger pump will have a different pump curve to the two/three smaller pumps and will be operating in a different piping system with a different system characteristic curve; so the operating point, which is where the pump curve and the system curve intersect, will be something other than that inferered from two smaller pumps' curves.

• In the end it is the process description that determines the configuration and use of the pump(s) and not the other way round.
• The best way to save energy is to switch them off when not in use.

The significance of the third motor to that description is lost here. If it is a cold spare, it is irrelevant. If the pumps are being operated as 3 x 50%, i.e. duty/duty/standby with auto changeover, it is also irrelevant; such things are process resilience questions that a control system is there to cope with.

Maybe i can try and make it clear,

2 motors, one left for emergency - do you mean one is the main/lead and the other the standby/Lag? or 2 motors runing in parallel, at the same time, with a third motor to serve as an emergency incase any of the other 2 pumps runing continousely fails?

If thats the case, i guess you can get one pump with the same rating as the 2 pumps and you have to get another pump to act as standby. About power consumption you may think of using a VSD, select this according to your system requirements.

Two small motors walk into a bar, they sit next to a bigger motor, the small motors order two 8oz beers each, and the bigger motor orders a 16oz beer, now there is a waitress motor that circulates the room passing the motor's position every 30 min, at which time each motor may get a free 8oz beer... there's a flower motor that walks by occasionally with capacitors that can be traded for 16oz or beer...er a what was the question again?

Two small 1 horse motors walk into a bar, bartender says "why the C face?"

One says, "Well, my love/joy, Flange, said..."

The unhappy feet on the T frame?

I believe your issue is about power saving initiative . those are pretty big motors. your process variable is manually controlled by closing valves. this can reduce the flow rate to your desired value, but your motor is running at full load and much longer. even if you replace the two smaller pumps with one pump, nothing shall change in regards to power saving.p the only way I may suggest is, check what power factor the motors are operating at, if low improve it with installing individual capacitor banks to each motor, here you will be controlling your reactive power hence reducing on wastege. Next suggestion is to use a VSD to control your process, it shall run at the necessary frequency depending on the system demand, hence u can have a pay back other than just manually throttling the valves.

This is misleading and actually incorrect!

You say,"[closing valves] can reduce the flow rate to your desired value, but your motor is running at full load and much longer".

Throttling the pump will indeed reduce the power consumed by the motor because it is doing less work.

And the pumps run continuously.

Lyn,I might need some correction here, take a scenerario if your using a VSD to achieve the same gaol, how shall the VSD do it?, throttle the valve or do some thing with the motor rpm?

First, the OP has NEVER mentioned a VSD.

His pumps run at the same speed, or so he says.

I don't believe that the OP has ever had contact with the Affinity Laws, either.

For you to suggest that a VFD can be used to change valve positions is a hoot.

I'll say this again.

The PUMP chooses its MOTOR.

Only a pump supplier/engineer/distributor with a complete selection of pumps can help the OP.

Changing to different motors on existing pumps is a waste of money.

I give up.

rayhud, you are a fool if you do not call a pump company!

Well thanks for sharing the artical.

I love this, about pump speed control and throttling of the valves in regards to energy saving.

From the link you just shared

Introduction

Pumping systems account for nearly 20% of the world's energy used by electric motors and 25% to 50% of the total electrical energy usage in certain industrial facilities. Significant opportunities exist to reduce pumping system energy con-sumption through smart design, retrofitting, and operating practices. In particu-lar, the many pumping applications with variable-duty requirements offer great potential for savings. The savings often go well beyond energy, and may include improved performance, improved reliability, and reduced life cycle costs.

Most existing systems requiring flow control make use of bypass lines, throttling valves, or pump speed adjustments. The most efficient of these is pump speed con-trol. When a pump's speed is reduced, less energy is imparted to the fluid and less energy needs to be throttled or bypassed. Speed can be controlled in a number of ways, with the most popular type of variable speed drive (VSD) being the variable frequency drive (VFD)

Pumps of that size are probably centrifugal; can you so verify?

If so, the flow is basically proportional to the rpm, and the pressure produced is basically proportional to the square of rpm. If any of your processes require at least a certain flow, this may dictate the minimum allowable rpm of the pump(s).

Dear Mr. rayhud,

It is better to have one properly sized single unit, rather than having 2 units for the same / similar output.

DHAYANANDHAN.S

Don't forget, a back-up pump is required.

With two smaller pumps+ a back-up of the same size the costs must be considered.

With one large pump, a large pump back-up will be required.

I have not worked out the cost vs reward details.

Thank you sir for the response.

I have another question if you don't mind.

Are the losses due to magnetization current in a 110KW motor greater or less than those in two 55KW motors running at the same time ?

Probably slightly less, but not enough so to be very relevant. It also depends on how heavily the motor is loaded as a fraction of its full capacity.

You can calculate this practically using the power factor triangle.

You see,, all inductive loads require active power (KW) to actually perform the work, and reactive power (KVar) to maintain the electromagnetic field. This reactive power is necessary for the equipment to operate but it imposses an undesirable budden to the supply system

You REALLY need to talk with a specialist pump supplier.

Your comment about constantly varying flow immediately leads me to wonder what the "turndown" ratio is between maximum flow and minimum flow, and what proportion of time is spent in any range of the flow rates.

The reason being that a large motor/pump combination will be HORRIBLE at the low flow options and VFD options become limited for low speed operation.

Your last line (in the original post) also concerns me. A bigger motor driving the same pump will not produce significantly more flow than the single pump, since it will not run faster than the mains frequency, so pump output will not be increased. You would also need a larger pump with suitably designed impeller and housing.

Seek professional advice. For items of this size, it's usually free and scientifically backed. They will ask you the correct COMPLETE set of questions and then provide cost options to suit different load conditions that are likely.

Motor type(NEMA-A,B,C,D etc) should match pump characteristics.

If water flow is critical to the process and it takes more than one pump to supply the correct flow of water, I would opt for two pumps in continuous mode and one pump in standby/emergency mode.

This allows regular maintenance of the pumps without negatively impacting the process.

If you go to one large pump the cost of maintenance and replacement parts will be significantly higher and the efficiency of the system will most likely decrease as the larger motor at times will not be optimally loaded.

Ideal operation would be to base-load one pump and vary the output of the other to meet process flow demand.

To equalize wear the pump lead/lag operation can be rotated so the pump life can be maximized.

This whole set-up sounds bizarre.

The OP knows very little about pumps, and then this: "the flow of water is always variable (constantly variable), and there are valves opened and closed manually to control the flow of water."

So, do we have some joker sitting at the valve all the time adjusting the constantly variable flow?

We still have no idea at all about the type of pump being used to move this water, too. (or was it a slurry?) For all we know the pumps could be two Archimedes screws used to fill an accumulating reservoir at the top of a water tower.

I think no one knows.

I'm assuming rotodynamic, because it makes sense.

I've always had a fondness for the elegantly simple rope pump.

Our neighbors, who didn't have an electric pump, used one of these.

Lehman's Own Galvanized Well Bucket

Nope, it's just skinny.

DANG!

When u say variable flow you need to study the flow variation over a selected period of time.. Both pumps always Run?how critical is the application ie how long can stoppage is acceptable.No doubt a bigger motor suitably selected will give a saving particularly the energy efficient types where eff of motor is almost constant from 100 to 60% of load.But a detailed study must be made and also to find out why system was designed with 2 pumps in first place.