Softstarter/Motor troubleshooting 1

[JWellwood]

Hi all-

Long time lurker and while I could find quite a few similar problems, of the ones I've seen, none quite matched close enough for a solution. I am absolutely certain that I will miss important information, but I will try to be as descriptive of the equipment/environment as I can. Also, I have clips of operation from coupled/uncoupled operation if it would help anyone.

This is a centrifugal pump system. The motor in question is a Techtop GR3-CI-TF-256TC-4-BR-D-20, and is secondary in the system. The primary The softstarters are an ABB PSR30-600-70 and PSR37-600-70. I have attempted to attach a screenshot of the datasheets. If it does not work, I will type them out in a reply. The ramp time is set to max at 20s. Starting voltage at minimum 40%. The supply voltage is 475-490V. When the pump is started, it is being started at low load, approx. 16HP and after the main is powered on- approx. 5s delay, kicks up to 20HP. Using a clamp, the max draw measured was 83.21A measured above the softstarter. For this particular measurement, the incoming voltage was 484.56V and the current across the legs were 24.3A, 24.6A, and 25.5A. The chief complaint is not of the timing to reach speed, but rather "just turning", as if there is too much of a starting load.

Upon reading the datasheets, my initial thought was that it was just a sizing issue and recommended increasing to the PSR37-600-70. However, the problem persists. Is there any information that I'm missing that would be helpful in making an assessment?

Thanks in advance,
J

[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1695394773/tips/sofy_start_psr_dat_cje3ls.pdf[/url]

 

[waross]

Starting a 20 HP centrifugal pump with a soft start seems like overkill.
20 second ramp time seems like overkill. (By a factor of 6 or more.)
Are you starting on a generator?
Is there another reason for a soft start?

Using a clamp, the max draw measured was 83.21A measured above the softstarter. For this particular measurement, the incoming voltage was 484.56V and the current across the legs were 24.3A, 24.6A, and 25.5A.

Is the starting current 83 Amps or 24 Amps?
At 83 Amps a 20 HP motor should be up to speed in a few seconds.

How do you control the load on a pump from 16 HP to 20 HP?

It sounds as if you have overthought and over controlled this installation.
I would go DOL and move on to the next problem, unless there is a very good reason for a soft start.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[LionelHutz]

Turn up the initial voltage or current until it does begin to rotate immediately after being started.

 

[jraef]

I agree with LionelHutz, but I'm not entirely sure what you are complaining about here. So let's go over the typical procedure, then the expectations, because your description of "HP" ramping is possibly indicative of a misunderstanding.

First off, the "initial Torque" setting is, as LionelHutz indicated, a parameter INTENDED to be set for only as much initial voltage as is necessary to create enough torque to BEGIN rotating. Too low and the motor is just being warmed up with no actual work being performed, wasting the thermal time of the motor. Too high and you risk losing the mechanical benefits of having a soft starter in the first place. So as he said, adjust that value to where the motor JUST starts to turn.

Next is the ramp time, or the current limit, depending on the goal. Ramp time simply dictates the time it takes to get from the initial voltage to 100% voltage. What is important to note here however is that this does NOT mean it is going to ramp the SPEED in a purely linear fashion. That's because the torque will be increasing at the square of the voltage increase, and the load itself may not actually NEED all of the motor's available torque to accelerate. So the load can (and often does) fully accelerate BEFORE the ramp time is complete. Torque is going to follow current, so by using Current Limit (if it is available), you can artificially extend that acceleration profile in some cases, which is especially important if the desired EFFECT of soft starting is a specific amount of current. But again, it is the LOAD that still dictates the actual acceleration time.

There are some newer Soft Starter products that are capable of a more linear acceleration ramp profile, by combining the current limitation and voltage ramping along with a feedback loop that estimates the actual motor speed by looking at the back EMF from it. This is valuable on certain types of loads, but centrifugal pumps are not really that difficult.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[edison123]

Jeff, lps for clear explanation.

Muthu

http://www.edison.co.in

 

[Marke]

Well said Jeff.

In addition to Jeffs comments, there are major differences in start characteristics due to the variation in High Slip characteristics of different induction motors.
Typically, you will need to develop around 40% torque initially to start the pump, and usually you would look at 3 - 5 seconds ramp time.

To calculate the initial start current, you take the Locked Rotor Current of the motor and multiply by the square root of the required start torque divided by the square root of the Locked Rotor Torque of the motor.
You can convert this to the start voltage by dividing the start current by the locked rotor current.
Some modern high efficiency motors require as much as 450% current to start a pump.

To minimise start delay, increase start voltage till the motor begins to rotate as soon as voltage is applied.

Mark Empson
Advanced Motor Control Ltd