motor torque vs. pump torque requirements

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GeorgeB

ElectroHydraulics engineer (retired)
Location
Greenville SC
Occupation
Retired
I'd like to learn more about motor torque vs. pump torque. anyone know any good publications to read?
+1 on Eric's response to what you probably meant ... but assuming direct coupled, pump torque IS motor torque.

What the book will give you is a guideline to understand, with centrifugal pumps, the relationships among construction, pressure, and flow. The impact of fluid characteristics will be addressed.
 

Designer69

Senior Member
George thanks for the replay, btw you hit the nail in the head for what I was asking.

I already purchased the book, but more specifically...

if I have a direct coupled motor/pump then you are saying the pump and motor will have the same torque characteristics.

so the pump manufacturer's torque-speed curve should match the motor manufacturer's torque-speed curve?

thanks
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
What George was saying is that the torque coming out of the motor must equal the torque absorbed by the pump, at the operating point.

The over-all torque characteristic of the motor must be different than that of the pump, otherwise the system could never accelerate up to the operating point. At start the motor must produce more torque than the pump consumes.

If you overlap a graph of the motor torque characteristic and the pump torque characteristic, the operating point will be at the intersection of the two. If there are multiple intersections, then there are multiple possible operating points.

-Jon
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
What George was saying is that the torque coming out of the motor must at least equal the torque absorbed by the pump, at the operating point.

The over-all torque characteristic of the motor must be different than that of the pump, otherwise the system could never accelerate up to the operating point. At start the motor must produce more torque than the pump consumes.

If you overlap a graph of the motor torque output characteristic and the pump torque requirement characteristic, the operating point will be at the intersection of the two. If there are multiple intersections, then there are multiple possible operating points.

-Jon
Minor edits to a great explanation.
Also just to be clear, the concept of multiple intersections would only come into play if you were selecting a pump/motor size without yet knowing the head and/or pressure. Once you know that for an application, i.e. once someone has already selected the right pump for the job, there will only be one torque requirement curve for it and so only one intersection.

torque-speed-curve-of-three-phase-induction-motor-b.jpg

On this graph, the pump torque requirement curve is the dotted line near the bottom. The motor would be selected so that at slip speed, it is supplying as much or more torque as the pump required, which happens at about 97% speed here.
 

Designer69

Senior Member
wow this is great stuff. question.. The motor is spinning the pump as soon as it is started (throughout starting torque and breakdown torque) What is it that makes the operating point different?

also is there a book specifically on this stuff, really interesting.

thanks
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
100826-1713 EST

Designer69:

First, there are a whole set of motor speed curves that are similar for a particular motor but vary as the supply voltage changes. These are not what the motor produces at steady-state, but rather are the limit capability of the motor at a specific supply voltage.

Second, in the illustration by Jraef the motor never had a steady-state torque load above the curve higher than its limit.

During acceleration of the motor from 0 RPM to its steady-state speed the motor torque limit curve would be followed in some manner close to what is on the graph. Inertia plus friction plus the pump load is what the motor torque is driving as it accelerates to the steady-state speed. So inertia and very little friction is the difference between the pump curve and the motor limit curve.

The accelerating force (torque) has an odd shape.

Remember that the sum of the forces is always zero. In this case part of that sum is the force to accelerate the mass.

.
 
Pump torque Vs Motor torque

Pump torque Vs Motor torque

After selecting the pump its torque speed characteristics should be sent to a motor manufacturer or supplier to quote for a suitable motor for that particular application.
 

jdsmith

Senior Member
Location
Ohio
After selecting the pump its torque speed characteristics should be sent to a motor manufacturer or supplier to quote for a suitable motor for that particular application.

And what is the motor supplier going to do to size the motor? We're trying to dig a little deeper than how to just solve the problem, we're trying to understand the physics of what's actually going on.

In the real world this is a moot point because 99% of pumps are purchased with a motor already attached, all selected and packaged by the pump manufacturer.
 
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