Adjustable Speed Drives,

Motor Torque and HP

Important considerations when

applying Adjustable Speed Drives

By Russ Safreed, PE

IEEE WV Section Meeting - held at

WVU Tech, Montgomery, WV

February 25, 2014

Abstract

The goal of this presentation is to present

practical considerations when applying

Adjustable Speed Drives in real world

applications.

A Real Life Project (things are never as easy as what they seem to be)

Project Basic Data

Difficult material to pump

Owner had experiences with their installation that did not work well.

Installation in Germany worked really well.

Wanted to copy it exactly.

Owner was very reluctant to change any part of the Germany design.

Fast tracked project… very short schedule… little engineering required.. Pressure to just copy the installation and get it installed!

Electrical Data provided from

Installation in Germany

Motor:

600hp,

1500rpm (4 pole)

driven by Adjustable Speed Drive

Initial Project Flow

Owner wanted to spec and buy motor

separately from pump

Process guys worked on pump spec which

was a long process

We began work to specify and obtain

quotations on motor and ASD.

Electrical Progress

Specified 600hp Motor

4 pole machine, 1800rpm in US

Can slow the motor down with the ASD –

so no problem there..

All this work was done without having the

manufacture data on our pump yet…

Pump Data Sheet Arrives

Pump requires 420HP at 1150rpm

OK… no problem… we’ve got a 600hp

motor… exactly what they used in

Germany.

Let’s move the project on… remember…

copy the design and keep the project on

schedule!!

Wait… Let’s think about this

Why a 600hp motor in Germany? The pumps are identical. Why did they use such a large motor??

Maybe it was an extra motor they had laying around??

Is the pump really the same?

Are the flow rates the same?

Maybe the process guys didn’t specify the exact same pump?

Maybe we need to understand this better before we move on…

Additional Data

Additional conference calls with Germany revealed:

The pumps they were using were obtained from the US on a project that had been cancelled.

Load data indicated that power consumption was in the 400hp range… not 600hp.

So why such a large motor???

Some Clarity begins to emerge

Germany’s power system is 50hz. US is 60hz (Humm?? Let’s think about that. How could that be affecting this… I mean, 420 hp of work in Germany will still be 420hp of work in the US)

Wait.. Here’s a thought. Since the pump was from a US project, it would be designed for a commonly available motor speed in the US.. which was about 1150rpm.

German’s available motor speeds are 3000, 1500, 1000. Maybe the German’s had no choice but to buy 1500rpm motor and slow it down to 1150 to match pump.

Could this have something to do with it???

Reviewed the Germany data again and found that they are operating the pump around 1150rpm… We may be onto something..

Do we really understand

HP and Torque?

Torque – a force measured in ft-lbs

HP is a measurement of Power

HP = (Torque X RPM)

5252

Torque

Torque is a measurement of force.

Torque can be multiplied or divided

through a gearbox to achieve any output

torque regardless of input torque available.

For motors, measured in ft-lbs

Notice that the unit of time is not involved

with torque

Horsepower

HP cannot be multiplied through a gearbox.

HP = (Torque X RPM)

5252

HP is always a consideration when “time” is involved.

ie. If you are trying to pump a liquid and trying to achieve a given GPM, then you are going to be concerned about HP.

The Motor Nameplate

Motor Nameplate

A motor’s nameplate will tell you the

horsepower the motor can provide given

certain input parameters.

It is basically the HP output at one point on

“the curve”.

If any of the input parameters change, the

horsepower will not be what the nameplate

says.

Let’s do some research…

Time to REALLY read the ASD manual!!

Not really exciting stuff!! …300+ pages…

But… we find this statement… “the ASD can provide constant torque for speeds below the baseline frequency and constant hp for speeds above baseline frequency”

Wow… that is easy to skim over and not really think about what is it saying…

Lets put that statement in a graph

Motor RPM

nameplate freq

Torq

ue

Motor RPM

nameplate freq

Hors

epow

er

HP = Torque x RPM

5252

We are on to something..

Remember that HP formula???

HP = Torque X RPM

5252

What happens to HP when you slow a motor

down?

Lost HP

So let’s evaluate Germany’s motor HP.

600hp = (T x 1500)/5252

T = 2100 ft-lbs

Now lets change speed to 1150 and plug the Torque back into the formula

HP = (2100 x 1150)/5252

HP = 460hp at 1150rpm

Same motor in US

4 pole motor = 1800rpm

600hp = (T x 1800)/5252

T = 1750 ft-lbs

AT 1150 RPM

HP = (1750 x 1150)/5252

HP = 383

Wow… this motor that we have specified isn’t even going to work!!

We are numb..

Let’s read the manual again… run the

numbers again… does this make sense…

Yes it does… Our conclusion is that

Germany had to oversize the motor

because they had to slow it down from

1500 to 1150 to satisfy the pump..

We would need to oversize even more!

So what would you do??

Changed mindset from “copy

design” to “lets engineer this right”

Since the pump requires 1150rpm, the obvious

solution is to provide a 1200 rpm motor.

Motor size will be 450hp

All of a sudden we are heros:

Avoided an installation that would not have

worked (this would have been really bad!)

450hp motor and ASD was significantly less

expensive than 600hp motor and 600hp ASD.

Win-Win situation… (those don’t happen often!!)

Let us think about this a little further

outside the project.

What concerns are there when applying

ASD’s to centrifugal pump loads.

Need to consider pump affinity laws and

the ASD’s torque-speed curves (loosely

follows the Torque-HP formula)

Pump Affinity Laws

HP1

HP2

RPM1

RPM2

3

FLOW1

FLOW2

RPM1

RPM2

FLOW RATE IS DIRECTLY

PROPORTIONAL TO RPM

HP REQUIRED INCREASES

EXPONENTIALLY (CUBED)

AS RPM GOES UP

Learnings

When operating a pump above design speed, the HP requirement goes up exponentially

Whereas, the ability of the ASD will not allow any additional HP output above motor nameplate.

If you plan to run a motor above nameplate speed, you will have to significantly oversize the motor and ASD

Not usually any issues when operating a motor below nameplate rpm because hp requirement will drop faster than the ASD output will drop.

Additional thoughts

How does this affect other types of loads.

Does this scare you away from ASD design.

Don’t let it… there are plenty of resources out

there to help you…

You just need to never quit asking yourself the

question… what am I missing here…

Donald Rumsfield made a quote during the

second Gulf War …. It also applies in

engineering…

Rumsfield Quote

There are known knowns; there are things we know we know.

We also know there are known unknowns; that is to say we know there are some things we do not know.

But there are also unknown unknowns – the ones we don't know we don't know.

Conclusions

How do we protect ourselves from the “unknown unknowns”

Don’t ever think you know or understand

Never stop participating in training

Get help from the experts (equipment manufacture in many cases)

Read magazines or web articles on subjects you are involved with.

Be careful you are not led down a path that doesn’t follow good engineering practice