Softstarter ramping Down

Hello Everyones

I know it's not a plc question but i have seen some good knowledge overall in here so i would like to ask the question...

I have softstarter to a squirrel motor using a pump stop feature to avoid water hammer but when i enable that feature, i did see a huge rise of amperage during the stopping time. Starting at 3x the motor nominal current and keep it for few seconds until it decrease gradually to 0 after 20-30sec of the stop command

The application absolutely need this long delay but my problem is overheating the motor because that much amount of current.

So Is it normal to experience that 3x motor fla in a controled stop? And if yes why ? Because I don't understand why it need it at first when the speed is close to nominal speed and lower amperage at the end even if at this point the motor slip is the highest...

Jeff,

For a soft starter or VFD to perform a controlled ramp-to-stop, it must "drive" the motor to stop, so it has to produce counteracting force to overcome the motor speed. In other words, your soft starter is acting as an electrical brake, using current to drive the motor to stop. You can often reduce the current during this phase by increasing the stopping time. Many soft starters have an adjustable stopping time. Check your manual to see what options are available for the controlled stop.

thanks for the answer but if you could go a little further in the motor/phase algorytmh i would like to understand the whole thing...

Actually when i increase the delay to stop, it just create more heat as the initial current remain for just a longer time. I don't have much more parameter to set.

I understand with a VFD that increasing delay will redure the stopping force and probably the stopping current but what about a softstart that remain with the 60hz rotating speed ?

Actually when I increase the delay to stop, it just create more heat as the initial current remain for just a longer time. I don't have much more parameter to set.

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What makes you think that? Do you have some type of high-speed oscilloscope to measure the current during the first few seconds? I think you are just wrong about this. My experiece with soft starters says that the longer the shut-down ramp, the less total heat on the motor. The reason is that the soft starter does not have to apply as large a reverse EMF because it has a longer time to stop. The max current is reduced. I have a good example. It was a large fan that my company installed. We used a soft starter. At first, it tripped on overload during the shutdown phase. I increased the ramp time and reduced the maximum current enough that the breaker no longer tripped on stopping.

...but what about a softstart that remain with the 60hz rotating speed ?

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If you have a softstarter that stays at 60 HZ speed, then it is malfunctioning. The entire reason to use a soft starter is that it DOES NOT stay at 60 HZ speed. It can ramp the motor up for a soft start, and ramp it down for a soft stop.

If I remember, there is also a soft-starter option to "coast to stop", where the soft starter just cuts the power and allows the motor to free-wheel to a stop. But if you want to avoid water hammer on a pump, then coasting to a stop would not be a setting for you to use.

If increasing the deacceleration time to the largest possible time does not reduce the current enough to keep the motor cool, then you may have to add a braking reactor, with a contactor that kicks it in during stopping. For most large motors (200 horsepower and up), there is a time limit on how often they can be started and stopped. I have seen some motor-and-controller combinations that were rated to be only started 1 time per hour. It makes troubleshooting a long process!

Are you sure that you have the correct size for your soft starter? Pumps generally require a much heavier-duty soft starter than fans, because of the large starting and stopping currents.

Lancie1 said:

What makes you think that? Do you have some type of high-speed oscilloscope to measure the current during the first few seconds? I think you are just wrong about this......

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Yes i do, the softstarter software allow to read a graphical of the current and i double checked it with a fluke 434 scope and another scope to make sure it was real.
I can see the initial current level going 3x fla when the stop is initiated regardless of the time ajusted. It just keep it longer.

Lancie1 said:

If you have a softstarter that stays at 60 HZ speed, then it is malfunctioning. The entire reason to use a soft starter is that it DOES NOT stay at 60 HZ speed. It can ramp the motor up for a soft start, and ramp it down for a soft stop

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A drive would change the frequency going to the motor starting from 0hz but a softstart just reduce the voltage going to it like an electronic autotransformer and keep the fundamental frequency (50 or 60hz) all the time...I don't know what it really do to stop a pump and this is my fundamental question here...

In my case i know that running with a VFD would have solved the problem but i have to tell to the engenneer why the softstart need so much amperage to stop and if it is correct or not etc...

A drive would change the frequency going to the motor starting from 0hz but a softstart just reduce the voltage going to it like an electronic autotransformer and keep the fundamental frequency (50 or 60hz) all the time

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Please tell me what brand of soft starter you have that does this, so that I can avoid EVER buying one. All the ones I have ever used were just stripped-down VFDs, that did indeed reduce the voltage AND Hz.

My old memory is foggy and fuzzy, but if I remember my basic AC Motors course, it is theoretically impossible to run a 3-phase AC induction motor from a device that reduces the voltage but not the cycles per second of the alternating current.

EDIT: Wait! Is this a single-phase motor, or what? If it is single-phase, then all bets are off. You could indeed have some type of soft starter that only reduces the single-phase voltage to the single-phase motor. In which case you have a bad problem. The best solution then may be to replace the soft starter with a VFD.

it's a standard 3 phases squirel motor but i would suggest that you double check your believing about the difference of a softstart and a drive, other peoples may comment also

As the drive cost 3times a softstart, (depending on hp) it should do something more...

You probably need to go up to at least a soft start with dynamic brake (Baldor has one). A VFD would be best overall to allow better stopping.

Lancie1 said:

You probably need to go up to at least a soft start with dynamic brake (Baldor has one). A VFD would be best overall to allow better stopping.

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And some soft starts only allow a max number of starts & stops per hour (or time period). Many the times I've had to remind the customer of that fact.

Some of the above statements are just plain wrong

First, a softstart does not change the frequency of the power to the motor. All it does is reduce the voltage to the motor by not turning on the SCRs, that are in each line, for the complete cycle.

Second, the softstart does not provide any braking to the motor. It cannot stop the motor any faster than if the power was cut off and the motor coasts to a stop. That is why many softstarts have a DC Injection Brake option for stopping motors quickly. This applies DC voltage to the motor windings which brakes the motor. However it is completely different from ramping the motor to a stop.

There is no difference between starting a motor by ramping up the voltage and stopping a motor by ramping the voltage down. Since the voltage frequency is always 60 Hz, if the motor is turning at less than its normal speed the slip will cause the corrent to be higher than full load. The softstart reduces the voltage to keep the current lower than you would have with a full voltage start. When the softstart is ramping the motor to a stop, it gradually ramps down the voltage causing a soft stop. The effect on the motor current will be exactly the same as when the motor is started. You will see about the same current in a 20 sec. stop as you would see in a 20 second start.

I know this is not a direct answer to your question but if the goal is to reduce water hammer would installing a damper of the discharge side of the motor not be suitable for your application.

In my experience this is an effective and simple way to reduce water hammer and would keep you from revisiting this issue should someone come behind you and screw up you parameters

Jeff23spl said:

I have softstarter to a squirrel motor using a pump stop feature to avoid water hammer but when i enable that feature, i did see a huge rise of amperage during the stopping time. Starting at 3x the motor nominal current and keep it for few seconds until it decrease gradually to 0 after 20-30sec of the stop command

The application absolutely need this long delay but my problem is overheating the motor because that much amount of current.

So Is it normal to experience that 3x motor fla in a controled stop? And if yes why ? Because I don't understand why it need it at first when the speed is close to nominal speed and lower amperage at the end even if at this point the motor slip is the highest...

Click to expand...

I have soft stop enabled on some pump soft starters to stop water hammer and pipe line damage just as you do, and yes the current does rise to atleast twice FLA.

A pump doing a run down stop would stop quite quickly in 2 to 5 seconds and hence water hammer.
So in soft stopping it is actually trying to keep the the pump running in your case running for 30 seconds as the slip is increasing and also doing a ramp down of the current and voltage, to control the stop.

If the period before restart is a few minutes the heat generated will in the stop will soon disapated

Vic is correct about volts / hertz relationship with a soft start - soft starts do not change the frequency, simply the conduction angle so that you progressively get more and of each AC cycle applied as the drive ramps up. This is why you can't use a soft start to run continuously at a reduced voltage - your V/Hz relationship would not be maintained, slip would increase and your machine would run very poorly at higher than rated amps until the motor overheated.

Soft stopping, as Vic said, is simply a gradual reduction in applied voltage to the motor. There is no reversal of direction or application of reverse current or anything like that with the standard "soft stop". The motor simply slows down because it doesn't have sufficient torque to meet load requirements.

I have certainly seen short increases in current when stopping pumps on a soft start. I've had to whip out the old uni notes for a quick refresher on squirrel cage motors, and I think (could be wrong) the short current increase on stop is simply due to increased slip (since your stator frquency remains fixed but your rotor frequency starts to drop) whilst still at a reasonably high motor voltage. This will nudge you "back to the left" of the standard induction motor characteristic current curve, i.e. increased current. essentially the reverse of starting, at least initially.

what make / model of SS are you using, and what size motor?

Some soft starts have a "Step Down" Voltage setting. This essentially reduces motor voltage on stop command to whatever you've got it set to, then ramps from there. This (may) be useful to overcome those first few seconds of high current. I'd start with setting step down voltage to 90% of rated volage and slowly decrease it to see if this makes any difference. Some SS's, like the Danfoss MCD series we use, don't let you directly change these settings but hide them behind a couple of different "Pump Stop" algorithms.

how often are you stopping / starting the motor? 3 seconds of higher current on stop probably isn't the end of the world, provided you're sticking within your recommended starts per hour for the motor.

Wouldnt braking by DC injection actually shorten the stopping time, rather than increasing the time ? So it will not be a remedy against water hammer.
If the pump is not very large, how about replacing the softstarter with a VFD ?
Small VFDs have dropped a lot in price the last few years.

What type of pump is it ?
How many kW ?

Jesper Its a 30kw pump, running on a Class 10 56kw 3RW44 Siemens softstart. I had to raise the class to 30 to overcome the thermal model overload obtained by the ramping down with class 15
It's a torque control pump stop algorythm used so not really fast DC braking...But something as to retain the water and i don't understand exactly in fine details how it does that and why the current raise so much. The raise doesn't appear on a standard softstop but the hammering is there.

I understand motor slipage and frequency and stuff but i was asking some help to convince Lancie that he was wrong about softstart without shocking him. a softstart not changing the frequency isn't a bad one...

And i also need help to convince engeneer about the motor overheat issue not being the cause of bad paramterers or bad softstart algorythm but process load as we have pushed Siemens instead of what they were requesting at first...They come back to me saying it's not normal to see 3times motor FLA when ramping down as it doesn't need to brake but just decelerate...

For others with nice suggestion, unfortunately we are stuck with a softstart option as someone decided it was what we need to do that job...