Motor current, when drive in sensorless vector and scalar mode 1

[PaulBr]

We have pump driven by 37kW motor, which is controlled by PowerFlex 70 drive. When drive is configured as scalar, V/Hz mode, motor cannot run faster than 15-17Hz. Drive is displaying about 67-69Amp and going in current limit mode protecting motor.
When drive is configured as sensorless vector, motor will run at required frequency 42Hz, and drive will display about 57 Amp.
I always thought that motor current is dictated by the load. Why we are not able to operate same pump in scalar mode, but can do it sensorless vector.

Thank you,

PaulBr.

 

[cswilson]

Consider the standard torque vs. speed curve of an induction motor. It starts with low torque at zero speed ("starting torque"), quite flat, then gradually increasing until it reaches a peak at a substantial fraction of the synchronous speed, then decreasing to zero at the synchronous ("no-load") speed. The horizontal axis on this plot is usually presented as speed in RPM for off-the-line operation, but for our purposes, we should think of it as (1 - slip frequency).

Normal operation of the motor needs to be on the right side of the peak. In this range, torque and current are pretty much proportional. The (fairly constant) negative slope of the curve here means that extra load will automatically be balanced by extra generated torque.

If you are on the left side of the peak, you are in the starting zone of the motor. Here torque and current are not proportional. Torque can be very low, with current very high.

It appears to me that your drive, when operating in open-loop (V/Hz) mode, is not able to control the slip well enough to get you "over the hump" into the standard operating range, so the motor is stuck in the starting range, with its very high currents. In sensorless vector mode, the slip is constantly controlled, so you can generate enough torque to get into the normal range of operation.

 

[PaulBr]

Thank you. Now it make sense.

PaulBr.

 

[itsmoked]

Great description cswilson. Thanks.

PaulBr; This may also be because the ramp speed is not matching how the load builds up. A much slower or faster ramp up may work in the scalar mode.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[PaulBr]

This is new installation. Because this is pump I decided to configure drive in V/Hz mode. Actually we have tried to change ramp time in both direction. Default time is 10 seconds. We have tried in both direction - 3 seconds and 60 seconds. Result was the same, drive was stack at 15-17Hz speed at current limit. Someone suggested sensorless vector mode. We have tried and it was working.

PaulBr.

 

[iop995]

V/Hz control method have problems at small speeds even worse with non-liniar loads; if driver allow, increase V/Hz ratio at low speed (below 10Hz) to overcame stator resistance and assure enough flux for starting load; also I thing load have a very "vertical" curve load (very high increase in torque load for small increase in speed), so check, maybe more powerfull motor will overcome this probleme even in standard V/Hz control mode. Sensorless vestorial control assure almost nominal flux even at small speed (even zero for sensor vector) so is able to drive this load.

 

[bass1mj]

Are all the other parameters set correctly i.e Min Speed for instance. Sometimes we overlook the stupid things.

 

[jraef]

This is new installation. Because this is pump I decided to configure drive in V/Hz mode. Actually we have tried to change ramp time in both direction. Default time is 10 seconds. We have tried in both direction - 3 seconds and 60 seconds. Result was the same, drive was stack at 15-17Hz speed at current limit. Someone suggested sensorless vector mode. We have tried and it was working.

One thing that isn't clear here is what you mean by "pump". If it is a centrifugal pump I would be surprised at there being a problem at low speeds, because basically a centrifugal pump is unlikely to be doing any real work at 17Hz. I would suspect it is some sort of Positive Displacement pump or other form of Constant Torque type of "pump" and if so, cswilson's response is likely the exact reason. In a PD pump, you need full torque as soon as you turn it on, in V/Hz mode that is very difficult to attain at the lower end of the speed range.



"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
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[PaulBr]

I was on vacation and could not verify and respond earlier. It is centrifugial pump.

 

[jraef]

Shouldn't be happening on a centrifugal pump. I'd say there is something else wrong here.

Top of my list: you have connected the motor for the wrong voltage, i.e. you have a dual voltage motor and your VFD / line voltage is the lower of the two, but you have the motor strapped internally for the higher voltage. Example, you have a 230/460V motor, the line voltage is 230V so the VFD can only put out 230V, but you have the motor strapped for 460V. The result is, you have almost no torque to speak of and as soon as a load begins to couple to the motor (as it does when you increase speed in a centrifugal load), the slip increases so much that the drive has to pump more current into it. In SVC mode, the drive is capable to tweal\king it beyond it's normal capability so you can get more out of it temporarily, but in Scalar mode, no dice so the Current Limit feature of the drive is not allowing the speed to increase above 17Hz.

Double check your motor connections. If that's not it, double check your VFD programming. Someone may have programmed it to never exceed 230V because maybe they used it on a 230V motor before, same net result.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376