Inverter driven pump application... 6

[cfordyce]

Question. Are there any problems assoc with running a motor-pump too slow or too fast? Someone in the past has told our production staff that a particular inverter driven motor shouldn't be run at less that 20Hz or greater than 45Hz to protect the motor. ???

Right now there is a bypass line tee'ing off just after the pump outlet which routes back to the tank that the pump is feeding from. In order to have the material delivered at the proper rate, the valve in this bypass line is adjusted so that some of the pumped material returns back to the tank.

I would like to do away with this bypass line arrangement and just have the pump operate at a slower speed.. to pump just the required amount of material.

 

[RonWB]

The bypass line is to prevent pump cavitation in the event that the pump load were to drop off. If you eliminate this bypass, there could be progressive pump damage, particularly to the impeller, if there is a varying load on the system.

In my experience, when a vfd is operated below 20 Hz, the resultant reduction in torque can cause the motor to stall. I've never used a VFD on a pump, but I have experienced this phenomenom on a mixer.

RonWB

 

[SteveKW]

Others may have more experience on VFD's... but a centrifugal pump is a variable torque load... so it would not stall.

 

[mehranektesabi]

One of the factors for the lower frequency operation of the motor , as well as its low speed torque vaule, depends upon the type of inverter.
Many of the VFDs (inverters) may provide higher frequency operation, but in practice the higher frequency operation of motor (or pump) is restricted to the designed values.

 

[abeltio]

The pump manufacturer has surely supplied the pump curves at different speeds.
The limitation for the curves is low efficiency (same as limitation for curves for varying diameters).

What RonWB stated, kind of confuses me, I have seen pumps cavitate when the flow is too high (very low resistance to flow) because the operating point is so far to the right that the NPSHRequired is too high. When the flow is to low but due to a restriction in the suction yes there is cavitation. That is why I'm kind of confused, with a bypass at the discharge if the flow increases way beyond the design flow the pump may cavitate because the NPSHR becomes too high.

The limitation for low flow is defined by the low flow limit set by the pump manufacturer (overheating of seals/packing) the high flow is limited by the cavitation characteristics of the pump (NPSHR) as well as velocity limitations in the piping, system pressure drop etc.

I have seen VFD pumps running at very low speeds for quite some time with no problems (water injection for NOx control in Gas Turbines).

The line at the discharge may help avoid the low flow condition of the pump, e.g. if the low flow limit specified by the manufacturer (Qm) is bigger than the lower flow limit required by the process (Qp)then the bypass is used to send back to the suction tank the excess flow.
I've used 1.25 times the min recomended flow as a rule of thumb.
So if Qm > Qp
The min flow used will be: 1.25 x Qm (this flow defines a min motor speed) and ensures that the pump operates above its min flow as recommended by the manufacturer.
To service flows Qp
To tank we bypass 1.25Qm - Qp

HTH
Saludos.
a.

 

[as66]

Hi,

What about the effects of slowing down the motor - surely this will affect motor temperature a sthe cooling is not what it is at normal motor speed?

 

[jbartos]

Suggestion to cfordyce (Electrical) Sep 17, 2002 marked ///\\Question. Are there any problems assoc with running a motor-pump too slow or too fast? Someone in the past has told our production staff that a particular inverter driven motor shouldn't be run at less that 20Hz or greater than 45Hz to protect the motor. ???
///This depends on the motor design. Some motors are designed that they can be in stalled position indefinitely. The inverter duty motors may be designed for their continuous operation at different speeds.\\
Right now there is a bypass line tee'ing off just after the pump outlet which routes back to the tank that the pump is feeding from. In order to have the material delivered at the proper rate, the valve in this bypass line is adjusted so that some of the pumped material returns back to the tank.

I would like to do away with this bypass line arrangement and just have the pump operate at a slower speed.. to pump just the required amount of material.
///This would require a feedback loop and sensing the pump output parameters, e.g. flow, etc., to control the motor speed.\\\

 

[PUMPDESIGNER]

Wow, opinions all over. Opinions are like armpits, everyone has a few.

Using VFDs on centrifugal pumps is an art, and there are not many artists in this category.

Without loads more information no real opinion can be offered.

Here are some of the information that must be obtained.

1. More information must be obtained than flow rate. Centrifugal pumps have a nasty non-linear relationship between pressure and speed. Slow the pump down a little, pressure drops way off to an unusable amount. How much pressure is required is just as important as flow required.

2. That by-pass would irritate me also. I have seen this on VFD centrifugal pump stations, I personally do not like it, it smacks of covering your butt because you do not know what you are doing.

3. Is this by-pass feeding back to a pressurized supply or into an "open" tank from which the pump withdraws fluid? Big difference between the two.

4. 45 Hz? Sounds wrong in relation to the motor speed. Normally that motor would run well at any speed up to 60 Hz. Perhaps that 45 Hz. limite was arrived at as a pump speed limit.

5. By the way your message reads, I would go get some real expert to help out. If your company does not mind experimenting then learn it yourself by experimentation, with everyone agreeing about that. Then you will know for sure.

6. If you experiment, you must learn the hydraulics of the system, the pump, etc. This is why there are so few people that know this stuff. VFD applications on centrifugal pumps is definitely a cross discipline area involving sensors, electrical, hydraulics, pumps, etc.

Richard Neff Richard Neff
Irrigation Craft

 

[PUMPDESIGNER]

OK, so I have more than 2 armpits, 'cause I got one more thing to say, I needed to go into that by-pass system a little more in depth.

Compare the pump curve to the system resistance curve combined with the system pressure requirement. If the pump must operate at relatively high speeds to overcome system resistance under low flow conditions then the centrifugal pump may encounter re-circulation and volute heating, especially if those conditions persist for long periods. The by-pass circuit would be one method of correcting that problem.

Other methods of overcoming the problem are listed as follows:
1. Select pumps that have steeper curves and/or higher shut-off pressures.
2. Selection of jockey pumps that can operate continuously at the low flow rate without re-circulation or heating problems.
3. Eliminate the low flow rate condition.
4. Eliminate the VFD and use a control valve with energy storage in an accumulator / pressure tank. VFD systems without control valves cannot store energy which is a valuable tool in controlling pumps. Some manufacturers are even offering a control valve with their VFD systems in order to achieve this energy storage. At low flow rates the VFD is driven to store higher pressures in an accumulator upstream of the control valve. The control valve then becomes the pressure controller to the system reducing this higher pressure downstream to the system. This obviously begs the question, why use a VFD anyway?

Richard Neff Richard Neff
Irrigation Craft

 

[jbartos]

Suggestion: There may or may not be any boundary to the engineering and design ingenuity.

 

[Mike4255]

The bypass line back to the tank was to probably prevent overheating when the pump is running at shut-off (no flow) and to minimize shaft deflection. Cavitation occurs at excessive flow rates, not minimum flow rates. If left on overnight the pump can make the entire water tank warm during the summer if the tank is small (2000 gallons or so) and not in an air conditioned environment. What type of system is this on?

Usually a VFD would be installed to provide a constant pressure on a system such as a potable water system. You have to be careful not to run the pump at its shaft critical speed or severe vibration can occurr. Is your motor rated for inverter duty? A mixer would be a constant torque application, centrifugal pumps, fans blowers, etc would be variable torque applications and should not stall.

 

[jbartos]

Suggestion: Visit

http://www.electrolink.co.nz/CC256A...012C8A6/78376DC76A4A1BAACC256A1C0082CEC8!Open

etc. for more info

 

[GusD]

Hell cfordyce

VFDs controlling centrifugal pumps are a very common piece of equipment in any plant today.From 20 hp to 5000 hp and probably higher.
All the VFDs used in our pumps are 60hz and VTorque.The motors are designed to run at least 65hz.No problem there
mechanical or elecrtrical.It also all have a minimum speed that is dictated by
1 -Pump curve (very little pumping capability below a certain spped)The pump manufacturer knows what there is .
2 -Even Variable Torque systems have some minimum speed limitations.
Our pump systems operate at a minimum of 30Hz and a max of 65HZ.THese parameters work well enough for the pump function as well as the VFDs performance.
Our main concern with these systems is to Block the Freqs.that may cause resonant conditions with the particular systems.

GusD

 

[GGOSS]

Hello jbartos,

Can you please add to your comments "This depends on the motor design. Some motors are designed that they can be in stalled position indefinitely. The inverter duty motors may be designed for their continuous operation at different speeds".

I am not aware of any ac induction motors that can be in stalled position indefinitely or inverter duty motors designed for continuous operation at different (I presume LOW speed).

Alternatively please refer me to web site that provides these details. I am certainly interested to learn more about them.

Thanks & Regards,
GGOSS

 

[jbartos]

Suggestion to GGOSS (Electrical) May 23, 2003 marked ///\\Can you please add to your comments "This depends on the motor design. Some motors are designed that they can be in stalled position indefinitely.
///Some tool motors are designed such that when stop, they can withstand considerable amount of time in the stalled position.\\ The inverter duty motors may be designed for their continuous operation at different speeds".
///This is meant within design basis speed range for continuous operation.\\I am not aware of any ac induction motors that can be in stalled position indefinitely or inverter duty motors designed for continuous operation at different (I presume LOW speed).
///The induction motor for ship, and vehicles have to be designed such a way that they can operate the ship or vehicle at speeds from 0 to max. Else, there would not be electrically propelled ships and vehicles.\\

 

[jbartos]

Suggestion to GGOSS: Visit

http://www.pscad.com/Showcase/Studies4.html

for the electric ship propulsion system description.

 

[jbartos]

Suggestion to GGOSS: Visit

http://www.bodine-electric.com/Catalog/K-2ACIN.asp

for: Bodine motor with Standard Features:
Totally enclosed, non-ventilated IP-20 rating.
Permanently lubricated, noise tested ball bearings.
Three-wire winding simplifies reversing circuitry.
Impedance protected to prevent current overload (motor can be stalled indefinitely without overheating).
High slip versions for increased starting torque.

 

[mdonner]

I have experience with many water pumping systems in all sorts of applications (building water supply for hotels, hospitals, high riser buildings. City water supply. Industrial. irrigation. Chemical industry.)
Most of them work with PID pressure control, some with flow control.
I have learned that VFD control is useless with pump that have a pressure/flow caracteristic curve which is not steep "enogh". Check your pressure difference at minimum and maximum flow rates. If it is very small, flow control may help.
Almost all the systems run at very low speed with no demand: this keeps the seal wet with no overheat (power demand at few Hz is very low and motors life span is long enough).
Some project engineers insist on a low flow bypass, but this proved to be redundant.
Another point: VFD control saves energy when compared to bypass control of flow rate.