700 kw motor started with a soft start

[gaux]

We think on putting a more powerful pump, and we´´ll have a 700 kw motor 400 V, it´´s a pump with pitch control, and we want to start it up, and we got a power plant feeded with gen sets, and we have power enough to run the motor on full power, but in the moment of start up, maybe some circuit breaker trips... for this we are thinking on use two options:

VFD. with this we are sure we never overload the system cause we´´ll start from 0 to full hz.

Soft start with tyristors, and with the pitch control of the pump on 0º, and my question is "will the current on the start up be higher than nominal current"? what do you think.
From the point of view of technically and safety, maybe VFD is better, but from the price view the best thing is soft start, and when we raise to full voltage we by pass the soft start....

thanks for your kindly attention.

 

[itsmoked]

Have you tried starting with pitch at zero now?

A VFD would have no starting issues but certainly would cost a lot in that size. It would allow a reduced pumping rate if that is actually needed.

The softstarter with the pitch set to zero would be the least expensive and probably do a great job.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[gaux]

yes, i agree, and we´´ll start with pitch on zero, but my question is if the current for starting will be greater than nominal current... and if the start up time is too large?

 

[itsmoked]

ONLY a VFD will allow you start the motor with nominal current. A SS is going to be at least %400 of nominal current and a direct start is going to be at least 700% of nominal, even with no pump hooked to the motor.

Turning the pump to zero will likely not change the percentage of starting current at all, what it will do is reduce the starting time substantially. This is less wear and tear on the motor. The motor will last longer. And your plant voltage will dip for a shorter period.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Marke]

Hello gaux

The minimum start current is a function of the load (pump) speed / torque curve and the speed / current and speed / torque curves of the motor.

The minimum start torque is determined by the pump characteristics, and the motor converts electrical energy into mechanical energy to start the pump.
At any speed, you can determine the torque required to turn the pump from the speed / torque curve of the pump.
From the speed / torque curve of the motor, you can determine the full voltage torque available from the motor at that speed. From this you can determine the ratio between the full voltage torque and the load torque to get the minimum torque required to keep the pump rotating at that speed.
The torque developed by the motor reduces by the square of the voltage reduction when using a reduced voltage or soft starter. So if we take the square root of the torque reduction ratio, we will get the current reduction ratio and from that can determine the minimum current at all speeds up to full speed.
If the torque requirement is low, then the start current can also be low, but never as low as with a VFD.
I would expect that with the pitch at zero, the start current will be in the order of 250% of the rated current, but without seeing the motor and pump curves, it is not possible to be more accurate than that. (If the motor has very bad high slip performance, the current may be closer to 350%)

Best regards,

Mark Empson
L M Photonics Ltd

 

[DickDV]

One additional point to add to Mark's comments above: The lowest starting current is not going to be determined by the pump curve but by the breakaway torque generated by the bearings, seals, etc.

The lowest inrush I have ever acheived with a reduced voltage starter was just under 300% and that was on a load that ran on near-perfect ball bearings and no seals. As Mark points out, this is due to the available motor torque being a function of the speed reduction squared. Under 300% inrush, there is virtually no torque at all.

 

[LionelHutz]

A pump with pitch control running on genset power at 400V - is that a bow thruster?

I've seen bow thrusters at zero pitch start as low as 140% current and up to 350% current. It is very, very unlikely you can start with rated current.

If the application is intermittent duty then specifying a motor with high starting torque and low starting current is a good start - like a design D motor. It will likely be less efficient but that wouldn't matter.

 

[aolalde]

You will go to the motor and pump speed-torque curves to calculate the starting time required to reach full speed under the nominal or the reduced voltage applied to the motor. But you need the total inertia (motor+pump+coupling) to be accelerated (WK2) (Lb*Ftsq)

And yes, your motor torque will be reduced almost proportional to the squared voltage ratio, as compared to the nameplate or nominal voltage. It means at 80% voltage the motor torque will be reduced to 64% of the torque developed at nominal voltage, and the starting current is reduced only to 80% of the full voltage starting current.

The accelerating torque (Ta) is the result of (Tmotor – Tload) ( Lb*FT), and the accelerating time required ( t) (seconds)

t = WK2*(Speed change)/ (308*Ta)

 

[Keith1976]

I just testcom a 75kW clean water pump using SS, the issue is in fact during turning off the pump. The discharge check valve slams heavily and scare the sh*t out of me.

For the above application, is it safe to turn off the 700kW pump using SS?

Just my 2 cents

 

[itsmoked]

Yes! In your case it could reduce system wear and rapid failure from water hammer.

Next time you should start a new thread.

Welcome to Eng-Tips!

Keith Cress
kcress -

http://www.flaminsystems.com

 

[LionelHutz]

I just testcom a 75kW clean water pump using SS, the issue is in fact during turning off the pump. The discharge check valve slams heavily and scare the sh*t out of me.

You either have a very poor soft-starter or it was not set-up correctly. Most good soft-starters come with deceleration control which would stop the pump hammer.

 

[DickDV]

Keith1976, a very common use for a softstarter is to function as a "softstopper". By this I mean that the decel ramp is used to bring a motor and load down more slowly than it would if it was coasting.

This feature is very useful for eliminating water hammer in pumps and would be just what you need.

 

[Keith1976]

Itsmoked and DickDV;

I have tried many setting of the SS (its Danfoss MCD3000) and when I brought this upto Danfoss, they offer a so-called advanced contol module (an optional accesories for MCD3000).

Certain SS work well in certain application only, they said. Well I agree as 2 years ago, I installed the same SS at the same customer's place, but different pump station. The SS works nicely.

So this March2009 Danfoss will launch new SS, the MCD500 which they say has a kind of Artificial Intelegence to adapt to the application so softstop is not an issue anymore.

I'd better be frank upfront. I work in a company, distributor for Danfoss product. But this is not a campaign though.

Regards,
Keith

 

[waross]

Thanks for the inside tip, Keith1976.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Marke]

The problem with soft stopping, is that you are reducing the voltage and forcing the motor to stall. If the motor has a very sharp torque curve, the speed can collapse dramatically.
If the motor has a very smooth almost flat torque curve, you will get a much improved soft stop.
Applying soft stop to motors that are lightly loaded does not work particularly well.
There are major variances that are due to the load curve and the motor curves.
Advanced algorithms can alter the voltage profile and give improved performance, but these are not perfect and can result in instability.
The MCD3000 is a simple voltage ramp soft stop. At the time we designed that, the cost of more advanced processing power was quite high.

Best regards,

Mark Empson
L M Photonics Ltd

 

[Keith1976]

Marke;

I quote your line "At the time we designed that, the cost of more advanced processing power was quite high."

Are you saying that you were in the team who designed the MCD3000?

To TS, sorry is this is way out of topic.

Rgds,
Keith

 

[Shozza]

Keith,

The new (or not so new as this technology has been in sale for the past 6 months already) artificial intelligence that adapts the soft start and soft stop profiles that you describe does work very well indeed however each application still has to be judged on its merrits. I have seen a few applications where large pumps are used to lift water several hundred meters. When the pump is stopped the starter performs a soft stop smoothly until the point where it no longer produces enough torque to control the mass of water coming back at it. At this point, as Mark indicated it collapses and the valve slams shut. Anything short of a VSD is virtually useless in this application. But in 99.9% of applications it works very well.

Regards,
Shozza

 

[LionelHutz]

The OP has a pitch control application. It's very unlikely he has to worry about the deceleration or water hammer.

We got into an application where a water transient expert had modeled the pump and piping and he was positive that a soft-starter could not work because he'd tried other brands on less demanding stopping profiles without any luck. Our TruTorque deceleration worked great, as has continued to do so on a few other difficult pumping applications he's come across. We've been selling it as a standard feature of the present starter model for about 4 years now.