Need more pump speed 4

[prmmel]

I have a Centrifugal pump that peaks at 70psig @ 60Hz. The pump is doing no work at 70psig as the flow is almost zero. I need to increase the pump speed to get about 80psig. I think that going to about 70Hz or 4000RPM will give me the impellor tip speed needed to generate additional pressure at a low flow condition. The pump is on a drive, so speeding it up is not a problem. If anyone cares to chime in, advise would be appreciated. We will consult with the pump mfg (Myers) but I am sure they will conservatively say 60Hz max to stay in warranty. I am trying to figure out what the real effects of going 4000+RPM. The motor is a high end Al metric motor frame that I actually have made overseas and imported in..much higher quality bearings that most US NEMA motors.

The reasoning for the above is I really need the ~80psig. currently i have the pump on PID drive control from a 0-100 psig transmitter that is set at 75psig. pump only does 70, so it runs at max speed but is limited to 60hz by drive default.....ultimately this will just boil water in the pump. If I can set the drive to attain the 75psig and max speeed goes from 60Hz to 70Hz, then the pump will reach 75 and shut off in sleep mode.

Advise here is great!!!


All advise is appreciated.

 

[BigInch]

If you increase the maximum speed by 10%, this results in an increase in load horsepower of 33%. This may well exceed the capacity of the motor or cause nuisance VSD tripping.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[prmmel]

Actually centrifugal pumps use HP based on flow, not speed. We did run the unit up to 70Hz last night which did acheive the additional pressure and boosted the unit to 80psig. The HP use or load on the pump did not change with the additional speed. Our conerns are more on the speed, not loading as we know the load is not increased.

 

[Unotec]

You must have an oversized motor then.
Where abouts are you in the pump curve now?

<<A good friend will bail you out of jail, but a true friend
will be sitting beside you saying ” Damn that was fun!” - Unknown>>

 

[BigInch]

Didn't know that. I've been thinking centrifugal pumps draw power as a f([&rho;] * V_SQUARED) for all those years.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[wimple]

Hi

Have you checked the material of the impeller? you could have some limitation with regard to the peripheral speed (stress). Is the impeller trimmed or are you operating with max diameter available within Mfg range?

But if I understand well you are going to operate at left of BEP in continous opeartion right?

Rgds

 

[ScottyUK]

Bear in mind that your motor will be in the constant power operating region once it is above base speed, i.e. the motor torque will drop off as frequency / speed increases because the drive can not maintain constant V/Hz. You will run out of torque pretty quickly once the pump develops enough head to establish forward flow.

BigInch:


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If we learn from our mistakes I'm getting a great education!

 

[BigInch]

Scotty, "run out of torque pretty quickly"
You guys have a special way of using understatement.

Falls off faster than AIG stock.


**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[ScottyUK]

Hi BigInch,

It won't actually run off the end of the curve into the motoring region shown in red, instead the whole curve would get stretched out so 100% would be based on the higher frequency.

The problem occurs because the motor is really a current-operated machine although we normally control the voltage to it: to a first approximation if we halve the speed / frequency, we must halve the applied voltage to maintain rated current and torque, and at 25% speed / frequency the required voltage will be 25% to maintain rated current and torque. The ratio of voltage to frequency is a constant; more often it's expressed as the V/Hz ratio. Above base speed the motor needs additional voltage in order to maintain the V/Hz ratio, so at 150% speed / frequency the motor would need 150% voltage to produce rated torque. In the real world the drive can't deliver the additional voltage, so at 150% speed and with nominal voltage the available torque is down to 67% of rated, at 200% speed the torque is down to 50%, etc. The upshot is that the available torque from the drive running above base speed is actually falling away while the torque demand from the pump is rising.

Oh, and that graph looks like Enron stock. The top line is the stock price and the bottom one is the number of lies per second Ken Lay was telling. I assume the sudden drop is when he escaped to South America died...


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If we learn from our mistakes I'm getting a great education!

 

[Artisi]

It seems that the OP's question was related to running the pump at CV or near to it and the effect when operating at a higher than normal speed ie, 4000 RPM as a means of increasing pressure to trigger shut-down.

My question is, why not change the set point? If this is not possible then I don't see any great difficulty in operating at the higher speed / pressure for a short period to activate the shut-down sequence.

Of course all the normal factors need to be considered; recirculation, shaft deflection / seal suitabilty, bearing loads / adequate lubrication etc etc.

The Meyer's pump I don't know, but assuming it is a heavy duty process pump I wouldn't expect any major problems for short priods of operation at the increased speed but checking with the manufacturer for advice is always the best bet.

 

[BigInch]

Artisi, looks like it wouldn't be for a short term,

SCOTTY, I see what you're saying. I got the diagram off of a power point presentation sharing website instead of calculating it myself (see attached)
Does that bottom Torque vs Speed diagram look more realistic? From what I've just seen at Reliance Electric, its not a bad approximation.


**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[ScottyUK]

Hi BigInch,

Don't confuse the motor's inherent characteristics with those introduced by the drive. If the motor were perfect (oh, if only!) then the response would be as shown in the attachment.

For this example I've taken a 200kW 750rpm 50Hz 8-pole motor.

Below rated speed the power output is limited by the torque rating of the shaft, and thus available torque is constant until the motor reaches base speed at 750rpm and 50Hz. The drive reaches full output voltage at 50Hz.

Above rated speed the drive is unable to increase the output voltage and as a result the torque falls away. The falling torque is compensated by the rising speed to give a constant power output above base speed.


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If we learn from our mistakes I'm getting a great education!

 

[BigInch]

I always like to start out perfect and .. only then .. introduce a little bit of reality at a time. A lot of little pills are easier to swallow than one big one.

Thanks for having a look.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[wimple]

Scotty

Just a question with regard to the VSDS motor. If you change the frequency, you get the motor running at higher rpm speed - lets say it is the 100 % speed for the given frequency.
I thought that the motor develop the max torque capability always at 100 % speed whatever is the frequency which is applied in the VSDS range. But the torque, as absolute value, change when there is a slip in speed rpm. Is it correct?

Is there a rule of thumb formula to derate the rated power versus the % change in frequency (60Hz +/- Margin)?

Thanks
Wimple

 

[waross]

Hi, may I say the same thing in different words?
As the VFD ramps the speed up, the current, the maximum allowable torque and the V/Hz ratio are all roughly constant. As the frequency increases, the impedance increases and the voltage increases to maintain the same current (and allowable torque). This is the constant V/Hz ratio.
When the motor reaches base speed the voltage is at rated voltage. As the frequency is increased further it is not possible to increase the voltage above base voltage. The power is now limited by Volts x Amps. When both Volts and Amps are at rated values, the HP may not be further increased. As a result, increased speed must be at reduced torque.
However I understood that the pump was doing so little work at base speed that it may well be able to run over speed without overloading

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ScottyUK]

Wimple,

There are two characteristics you need to separate:

The motor has a torque / speed characteristic which normally looks much like the one BigInch posted and which I've repeated here:

That curve applies when a motor is connected to a fixed frequency supply - 50Hz or 60Hz - and has to accelerate from standstill to full speed. The shape of the curve is a result of how the magnetic and electrical circuits of the stator and rotor interact when the rotor speed is lower than the synchronous speed. It's covered in better detail and more lucidly than I can manage here by any good electrical machines text if you want to delve into the maths behind it. There are some excellent ones listed in faq238-1287.

If you plotted the characteristic curve at a number of different frequencies, and plotted frequency instead of percent synch speed on the X axis you would see a number of curves of basically the same shape but those at low frequencies would appear 'compressed' and those at higher frequencies would appear 'stretched'.

With a VFD the motor is effectively operating somewhere on the red portion of the curve all the time, right from standstill: it does not operate in the high slip region to the left of the peak marked 'breakdown torque'. The VFD allows the curve to be 'stretched' as it increases frequency but the motor is always at (more-or-less) the same operating point on the curve.

Torque is related to current and the VFD can modulate its output to maintain the current at rated value all the way down to zero speed, so rated torque is available right down to zero speed too.


Hope this is adding clarity and not confusion - apologies if it is vice-versa!



nanostructure,


Your formulae are so totally wrong that you should Red Flag your post and ask the administrators to delete it. Here are the correct formulae, image courtesy of the Wikipedia entry for 'Torque'.

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If we learn from our mistakes I'm getting a great education!

 

[BigInch]

Scotty, yes very clear.

Just like the curve above, except replace the point marked 100 on "Percent Synchronous Speed" with whatever Frequency is being feed to the motor (from the VFD) at any given time.

I shaped a 3 Phase Torque (attched) vs % Sync_Speed curve from this eq.,

Torque = 3 * V^2/s * (R2/s) / [(R1+R2/s)2+(X1+X2)^2]
R1= stator resistance
X1= stator reactance
R2 = rotor resistance
X2 = rotor reactance
s = angular speed of the stator
V = line voltage

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[BigInch]

Torque vs Synch Speed with Voltage Variations
and
Torque vs Speed with Voltage Variations

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[waross]

Nice graph, BigInch.
Thanks.
LPS

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[BigInch]

Thank's to you all too! Your explanations really cleared up how those little black boxes do their magic.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/