1000 hp pumps (4160 volt each with itÆs own VFD)
1000 hp pumps (4160 volt each with itÆs own VFD)
(OP)
We have three 1000 hp pumps (4160 volt each with it’s own VFD) to control the clear well level of a water discharge station. There is one PID level controller giving an output to the three pumps. The third pump is seldom used and the first two pumps have to run at 100% for ten minutes before the third pump is allow to start. The pumps are rotated monthly as to lead, lag 1, and lag 2.
The biggest concern is that the third pump not start unnecessarily to keep the month’s power demand as low as possible. None of the pumps will run below 50% because they can’t open the check valve against the head pressure at low RPM’s.
Most of the time it takes 1 ½ pumps to maintain setpoint.
My question is:
Is it better to let both pumps run all the time and ramp them between 50% and 100% or is better that the lead pump runs at 100% and lag 1 pump runs from “off” to 100% as needed (this results in the second pump shutting down completely about twice an hour)?
The biggest concern is that the third pump not start unnecessarily to keep the month’s power demand as low as possible. None of the pumps will run below 50% because they can’t open the check valve against the head pressure at low RPM’s.
Most of the time it takes 1 ½ pumps to maintain setpoint.
My question is:
Is it better to let both pumps run all the time and ramp them between 50% and 100% or is better that the lead pump runs at 100% and lag 1 pump runs from “off” to 100% as needed (this results in the second pump shutting down completely about twice an hour)?





RE: 1000 hp pumps (4160 volt each with itÆs own VFD)
My opinion for what it’s worth.
Drives are harmonic generators. The harmonics must be dissipated as heat somewhere in your system. If you run one unit at 90 percent you will allow additional cooling (more iron than required) that will result in longer run life for your motors, cable and drive. You can control flow with the second unit. Based on your post the second unit should not need to run at 100 percent to keep up with your demand.
This is just my opinion and I’m sure some of the “EE” guru’s will smoke me on this.
Good Luck!
D23
RE: 1000 hp pumps (4160 volt each with itÆs own VFD)
Is it better to let both pumps run all the time and ramp them between 50% and 100%
///This operation will guarantee that both sets will run at least at 50% load or more. This will be better in a sense that both set will get the same wear. However, the efficiency will have to be analyzed with respect to the following item below.\\\
or is better that the lead pump runs at 100% and lag 1 pump runs from “off” to 100% as needed (this results in the second pump shutting down completely about twice an hour)?
///The second set will experience bigger electrical and mechanical stresses. However, efficiency of this operation is to be compared to the above mentioned operation. Then, a break even analysis of these two operations is suggested to select the better operation on the overall assessment of each operation.\\\
RE: 1000 hp pumps (4160 volt each with itÆs own VFD)
Just out of curiosity, how much additional heat (percentage) do you think that VFDs output harmonics contribute to the motor;
a) 0-5% ?
b) 6-10% ?
c) 11-15% ?
d) 16-20% ?
e) >20% ?
... and your answer is based upon what type of modulation:
a) Sine weighted (sine coded) PWM
b) SVPWM (Space Vector PWM)
RE: 1000 hp pumps (4160 volt each with itÆs own VFD)
Perhaps you will enlighten us as to what "bigger electrical and mechanical stresses" set 2 will experience, since Andy32821 stated that all three are on VFDs.
RE: 1000 hp pumps (4160 volt each with itÆs own VFD)
How are you controlling the three pumps ?
Are you using a Pump Controller ? PLC ?
And are you controlling level utilizing a level sensor in the well.... or are you controlling discharge flow or pressure ?
What is the set-point..... level ?.... flow ?.....pressure ?
Are all three pumps matched; i.e., same pump characteristics/rating ....? and do you have access to the pump curves...?
RE: 1000 hp pumps (4160 volt each with itÆs own VFD)
the company I work for has run heat rise tests in motors that run on VSDs. The ususl heat rise was less than 5%, for both PWM and VVI styles.
d23
since the water flow past the motors is sufficient, we assume, for cooling at 100% load the heat rise in the motors as demonstrated by tests conducted by the companies we both work for is under 5% why the concern about the heat rise? the other assumption, and that is such a loaded word, is that the motors were designed with VSD application in mind.
andy32821
my preference would be to allow the two pumps to run 100% of the time with the PID loop controlling the speed. I would rather run them both than start and stop large units like these. that is a personal preference but I believe it is preferable to continual stops and starts on large equipment.
all the best
dadfap
RE: 1000 hp pumps (4160 volt each with itÆs own VFD)
This will ensure even wear and tear, an important issue with lead/lag units. You still need to keep the rotation between pumps 1-2, 2-3 and 3-1.
RE: 1000 hp pumps (4160 volt each with itÆs own VFD)
Maraton Electric Motors has run heat rise tests on most major VFDs with their inverters.
VVIs or six-step type inverters produced the most heat rise (highest harmonic content). Next would be PWM sine-weighted/sine coded modulation. The space vector PWM types were the least offensive and indeed, were typically less than 5% rise.
BUT.... and here's the kicker..... rotor bar design has a lot to do with it too.
The testing led to the policy that motors (in general) with Class F insulation, were a class B rise and 1.15 s.f. on sine wave power.... and Class F rise and 1.0 s.f. when operated on inverter power.
That's a broad range for inverter user... but it covers the gamut of drives ... of which it can be said that all are not created equal.
The last drive I tested there employed voltage vector PWM and was less than 3% heat rise over sine wave.
By-the-by... would the company you work for be Reliance Electric ?
Not too many companies referred to six-step inverters as VVI which is what Reliance called them back in the late 60's.... I seem to recall also that the Reliance motors they provided for use with their VVIs were special design, heat being only one consideration.
jO
RE: 1000 hp pumps (4160 volt each with itÆs own VFD)
If the process is unstable and at some points during the billing period high load will be imposed on the pumps then it is purely efficiency. (ie rain, peak water load, etc) If properly setup the drives should have impose virtually no mechanical stress on the system (accel & decel curves), and the VFD drives should very little electrical impact on the system or motors from starting and stopping.
I would guess the outcome would be to run one pump full out and cycle the second to allow water level to fluctuate. And share roles for all three motors.
RE: 1000 hp pumps (4160 volt each with itÆs own VFD)
A condition not as severe but certainly potentially destructive is operating a pump very far to left of it’s curve. You have the potential to enter that situation with one pump running at 100% speed and 1 pump running at 50% speed, depending on how the 100% speed pump influences the system.discharge pressure seen by the 50% spped pump.
Operating far to the left of the curve (flow far below BEP) causes high vibration.
RE: 1000 hp pumps (4160 volt each with itÆs own VFD)
Your question can be answered with a generality as to energy consumption, "best to run multiple pumps at the same speed".
To get a better answer you need a system curve, the pump curves, and an energy analysis software porgram.
The system curve will provide the relationship of static head vs. friction head (you save energy in the friction head). If you were designing a pump station from scratch you would compare pumps with steep vs. flat performance curves with the goal of staying as close to the pump's BEP as possible during the majority of the operating time.
It sounds to me that you might like a suggestion on how to prevent the third pump from starting when your system needs a little more flow than is avaliable with two pumps running at full speed.
Check your pump curves and amp load (two pumps at full speed). If you have enough head room (BHP vs. nameplate HP), consider increasing the max frequency (speed) of the pumps. Suggest no more than 66 Hz, unless the motors are substantially oversized you may not even get that high in speed.
One caution, you may need to set the controls/programming to limit "one" VFD operation to 60 Hz, otherwise it may overload.
CB2
RE: 1000 hp pumps (4160 volt each with itÆs own VFD)
Depending on your VVVF drive design , the Motor may run cooler with the VVVF supply than on the mains .
Differences of a degree or so in favour of the VVVF have been recorded on large MV motors.
RE: 1000 hp pumps (4160 volt each with itÆs own VFD)
Nup, it ain't Reliance and I still refer to 6 step as VVI, purely out of a bad habit.
I work in the oilfield and the company I work for has tested Toshiba, Robicon, ABB et al and with each of the various waveforms in the rather unique motors that go into oil wells we saw around 5% heat rise.
all the best
dadfap
RE: 1000 hp pumps (4160 volt each with itÆs own VFD)
From a hydraulic point of view running two pumps at 75% is clearly more efficient than running one pump for 100% of the time and the second pump on and off for 50% of the time.
Pump power is proportional to QH and the head loss in the pipe is proportional to Q^2. Thus if you keep the flow at 75% over 24 hours you use less power than cycling between 100 and 50% flow.
brian
RE: 1000 hp pumps (4160 volt each with itÆs own VFD)
RE: 1000 hp pumps (4160 volt each with itÆs own VFD)
But I do want to mention that your concern about starting and stopping the pumps is somewhat reduced because of the Adjustable Frequency Drives. These will provide a very smooth start without the the high torque levels and heating associated with full voltage starting.
RE: 1000 hp pumps (4160 volt each with itÆs own VFD)
I think you have it right he is comparing one pump running 100% for 100% of the time and the second running at 100% for 50% of the time against the alternative of them both running at 75% for 100% of the time.
However since Andy32821 has only ever once logged into the forum and doesn't appear to be logging in to read the answers I don't suppose that we will ever know.
brian
RE: 1000 hp pumps (4160 volt each with itÆs own VFD)
RE: 1000 hp pumps (4160 volt each with itÆs own VFD)
For my own curiosity,
what was the methodology used the measure the temperature rise.......?
Thanks.
jO
RE: 1000 hp pumps (4160 volt each with itÆs own VFD)
JB:
Perhaps you will enlighten us as to what "bigger electrical and mechanical stresses" set 2 will experience, since Andy32821 stated that all three are on VFDs.
///The set 2 was obvious from the context to be:
"...or is better that the lead pump runs at 100% and lag 1 pump runs from “off” to 100% as needed (this results in the second pump shutting down completely about twice an hour)?"
Please, would you post for yourself in this Forum, namely, your request would read better as:
"Perhaps you will enlighten me (not us, who are those others?) as to what "bigger electrical and mechanical stresses" set 2 will experience, since Andy32821 stated that all three are on VFDs.\\\
\\\
RE: 1000 hp pumps (4160 volt each with itÆs own VFD)
You made the statement about set 2 experiencing "bigger electrical and mechanical stresses".
You offered no support for your statement.
Your statement is contrary to general VFD Pump Application knowledge.
... and so, I asked, on behalf of all of us who read your posts, for you to provide us all with some proof or evidence or documentation that supports your statement.
I think you owe it to ALL of US... who read this forum to not make claims that cannot be substantiated.
Remember JB, the people who open these threads in this forum are seeking answers, knowledge, and truth.
Posting statements that are inaccurate, misleading, or unsubstantiable helps no one.
When you are challenged to support your claims, I think you owe all of us validation thereof.... or retraction thereof.
I hold myself to the same professional responsibility.
So, would you please advise us as to what "bigger electrical and mechanical stresses" set 2 would incur....
and be specific if you can