Urgent: AB PowerFlex40 Drives

Hi,

I have one pumping application where when the water level goes below 34 the pump have to work and when the level goes above 36 the pump have to stop.
The Tank have pressure sensor which converts the pressure to 4-20mA. I'm using PID control for this and feed back to this PID will be the 4-20mA from Pressure Sensor. I have configured dead band for the PID also.

We have tested it, the pump is getting stopped one it attains the level; Now the problem is, the Pump is getting heated to 120 Degree C.

Is it because of PID? Any solution for this?

Pumps overheat because they are not pumping enough liquid to cool them. Several causes insufficient NPSH, discharge valve or suction valve shut, pump is not bled from air.

At 120 C you are I bet boiling the water in the pump. Your shaft seals are either destroyed or are nearly destroyed. Get new ones. Do NOT run pump until you get problem resolved.

99.99 % sure this is not a control problem since you say pump is shutting down. What do number 34 and 36 refer to? Height you have to pump water in meters?? - does this pump have sufficient head to do this at your desired flow rate?

What is water temp at pump suction?

Need some numbers ie what type pump, ratings ie head, flow, HP, how long it takes to pump from 34 to 36.

Since using VFD to drive pump what is RPM of pump as you have it set up?
Dan Bentler

You will have to tell us a lot more about the system. I can't tell if you are trying to control pressure, or level, or something else.

It sounds like you are using the onboard PI loop of the PowerFlex 40, or the PID loop on a PowerFlex 40P, to control its speed.

When the drive stops, does it actually STOP, or does it just run at 0 Hz ? You could definitely warm up your motor if you're trying to magnetize it at 0 Hz indefinitely. 120 C is pretty darn hot for any motor to get !

Are the motor and pump closely coupled together ? Is the pump overheating, or just the motor, or both ?

If you want somebody on the Forum to diagnose your pump application, you are going to have to post a wiring diagram, a parameter list, and a more detailed description of the process you are trying to control.

Ken Roach said:

You will have to tell us a lot more about the system. I can't tell if you are trying to control pressure, or level, or something else.

It sounds like you are using the onboard PI loop of the PowerFlex 40, or the PID loop on a PowerFlex 40P, to control its speed.

When the drive stops, does it actually STOP, or does it just run at 0 Hz ? You could definitely warm up your motor if you're trying to magnetize it at 0 Hz indefinitely. 120 C is pretty darn hot for any motor to get !

Are the motor and pump closely coupled together ? Is the pump overheating, or just the motor, or both ?

If you want somebody on the Forum to diagnose your pump application, you are going to have to post a wiring diagram, a parameter list, and a more detailed description of the process you are trying to control.

Click to expand...

More good questions.
PIPING DIAGRAM also

Dan Bentler

no

My drive is Powerflex 40 (15HP, 3 Phase, 415V).

Wiring: 3 phase is connected to a ‘32A SDF’, its output is connected to an ‘Input Choke’ from there to ‘Drive’.

Our application is Level Maintaining of a WATER TANK using a Motor. The Water tank is a closed one, so according to Water Level the pressure variation will occur, thus taking level measurement from pressure variation. This pressure variation is converted to 4-20mA.

I don't know the capacity of the Water Tank, but for 35kg/cm2 we will get 15 mA. Our requirement is to operate the motor with in a band, i.e. we want to maintain Water level within 34-36kg/cm2. For this we configured PID (A123 settings). For 35kg/cm2 the Analog reading shown in the Drive is 70%, so I have set ‘70%’ as the set value for PID Set Point. The Analog current 4-20mA is given as Feedback (reference signal) to our drive.

@Dan Bentler:: You asked "What do number 34 and 36 refer to?". They are the Pressure level of the Water Tank.

@Ken:: You asked “When the drive stops, does it actually STOP, or does it just run at 0 Hz ?”

Here PID actions are running correctly, but due to continuous running of motor even in zero speed, motor taking 3A current and motor produce heat.

In start-up settings I have given Min Freq as ‘0’ Hz and Max Freq as ‘50’ Hz. Is the ‘0’ Hz giving me the problem? What is the ideal min Freq.?

To me it looks like you have no way to give the drive a stop / start signal. I'm not that familiar with the AB drives, but do you have the run input bridged or the drive in HAND mode to make it go?

I'm a Danfoss guy, so I can only speak for these drives. We have a Sleep speed setting, which can be used to stop the drive if the speed drops below the sleep speed when running in PID control. There are also some basic comparator / logic functions that could be used to start / stop the drive. You may have something similar with the Powerflex's?

However, leaving a drive in HAND mode just makes me feel pretty nervous...

Is the motor actually rated for inverter duty?
And is it the actual pump getting hot? or the motor?

Thanks,

Rockwell tech support told me to do 2 things,
1. Set min Freq to 20 HZ, now it is '0'Hz
2. Set PWM Frequency(A091)to 6Hz, now it is 4Hz. As Output current and PWM Frequency are inversely proportional.

@Saffa: I have talked to the Tech Support guys of Rockwell, they says Sleep options is no available for Powerflex40 model.

@rdrast: Only Motor is getting heated up and not the pump.

Hi,randheerp
What is frequency in most time, haw is cooling of motor (with own fan or external)

If it is under 25Hz motor will heat, because don't have enough ventilation.

Hi Bahobeti,

Thanks.

Normal frequency is 50Hz. And cooling is with own fan...

I think I see what may be much of your problem.
You stated the tank is enclosed AND
you are relying on pressure change in tank to determine level.

First we need to determine what you want to control for
volume of water in tank OR
pressure of water in tank

Dan Bentler

Ten seconds on Google found a workaround for the absence of a "Sleep Mode" in the PowerFlex 40:

http://www.eng-tips.com/viewthread.cfm?qid=275378

I am not a drives expert, but ignorance never stopped me before.

The "Rockwell Support" guys sound like they are addressing the focused question of "how to I minimize the heating of a motor driven continuously at low or zero speed" rather than the basic application question of "how do I stop a motor that doesn't need to be running".

PWM Frequency is sometimes called "carrier frequency", and is usually expressed in kilohertz (4, 8, 12, and 16 kHz are typical PWM freqencies for AC drives).

That is not the same as the "Output Frequency" of the drive (the 0 to 400 Hz that actually spins the motor). The carrier frequency affects motor heating only a small amount compared to the magnetizing current and the torque current.

Can we do a sanity check on units ?

By my math, 1 kilogram-force per square centimeter is roughly 98 kilopascals, so 35 kgf/cm2 pressure is about 3430 kilopascals, or 497 pounds per square inch.

This sounds less like a "water tank" and more like an air-over-water pressure vessel.

I agree with Ken about the PWM frequency being of little consequence and that stopping the drive when below minimum output is the more important solution.

As for the math:
34 to 36 kg/cm2 = 484 to 512 psi = 13399 to 14187 inH2O

That's one tall tank of water at atmospheric pressure (1182.25 feet or 360 meters)!

Okie

Apparently tank is not vented to atmosphere. If you and Ken numbers are correct at 500 psi I would for sure check pressure label on tank
AND
decide if it can be pressurized to that pressure
OR
stay a mile away from it.

ONE THING FOR SURE
motor is not able to shed heat
WHETHER
motor is overloadec
or undercooled because driven at too low RPM
remains to be determined.

I think we need to get valid numbers for head
and a look at pump curve.

Dan Bentler