Soft Starter on 125 HP blower

[nhee]

Can anyone offer suggestions for configuration of an ATS48 softstarter in a 125 hp centrifugal blower application? Specifically, beginning settings for startung torque, current limit and ramp time?

With ramp time of 15 secs, my current limit needs to be as high as 400% to allow motor to get up to speed (otherwise we go out on overload). We are starting with a closed inlet damper on the blower to reduce load.

I thought I might be able to reduce the starting current requiremetn to less than 400%.

Thanks

 

[DickDV]

While I am not familiar with the ATS48, I am familiar with many softstarters and the principles behind softstarting.

The minimum starting torque is determined by the breakaway torque of the load. Going any lower than that simply makes the motor stall until the voltage ramp brings the motor torque up to the point where it is larger than the load breakaway torque. At that point, the load begins to move. Clearly, there is no point in going lower than the load breakaway torque.

As for the ramp time up to full speed, the lowest possible overall inrush current is acheived by observing the current needed to provide the load breakaway torque and lengthening out the ramp time so the current stays near but does not exceed that current. Of course, the current will drop down dramatically as the motors nears full speed and "hooks up".

You state that you need 400% of nameplate current to get the 15 seconds accel time which it seems you also need. If that is the case, I know of no tricks to still get 15 seconds accel time with lower current. The fact is, longer accel time equals lower current and shorter accel time equals higher current.

There is no point in setting current limit below 400% if you want 15 minutes accel time.

The problem with all softstarters is that available motor torque at full inrush of 600-800% produces only 160% rated torque in the motor and this torque value falls as a function of the SQUARE of the voltage reduction while the inrush current falls only in direct proportion to the voltage reduction.

The bottom line is that the minimum inrush you can hope for on a NEMA B motor is about 300%. At that point, the inrush has fallen to 50% while the available motor torque is down to only 25%, barely enough for the motor to start itself, much less the load.

Softstarter sales people often leave out these important limitations and you find out about them only after installation.

Now, you've got the "facts", as they say.

 

[Marke]

Hello nhee

The start current is dependent on the motor characteristics and the load characterisitics. There are considerable differences between motors so it is difficult to apply rules for starting characteristics.
The start time is dependent on the inertia of the load and the acceleration torque applied to the load.
The acceleration torque is the difference between the "load torque" and the motor torque at all speeds.

If you can get the speed torque curve of the load, and the speed torque curve and speed current curve of the motor, you can calculate the start torque curves for the motor at different start currents and superimpose them on the load torque curve. The motor will only start if the motor torque (at the reduced start current) is higher than the load torque for all speeds from zero to full speed.
In the case of the fan, you will have an initial breakaway torque of perhaps 30%. The torque will then drop low and increase with the square of the speed.
It is common with reduced voltage starting for the torque to run out at around 70% speed unless the start current is increased, but it is very dependent on the motor and load characteristics.

If you download the Electrical calculations software from

http://www.lmphotonics.com/busbar32.zip,

you can load in your motor curves and load curves and see the effects of different types of starters and start currents on the start characterisics. If you know the load inertia, you can also get an indication of the start time.

From my experience, it is not uncommon for a fan to take in excess of 30 seconds to start due to the inertia of the fan.

With modern high efficiency motors, a start current of 400% commonly develops a relatively low start torque and high start torque machines often need a much higher start current in order to develop sufficient torque.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[nhee]

Thanks for the responses.

It looks like I need to obtain the curves for both the motor and fan, set a starting motor torque higher than breakaway torque for the fan, and adjust my acceleration ramp time as needed. 15 seconds was simply the default on the starter - the process can handle a longer ramp up.

 

[jraef]

nhee,
Although I am not directly familiar with that brand of starter, many of them are designed such that the Current Limit setting will override the Ramp Time anyway. With Current Limit, the ramp time seting acts simply as a slope angle adjustment. Once you hit the Current Limit point, it just stays there until it aceelerates, or trips out on Overload or Accel Time Limit (stall protection). Try just lowering the Current Limit to what you want.

Also, with a low current limit setting it is sometime necessary to have as little actual ramp time as possible, because if you are running into the overload curve you will want to maximize the work done by the motor. This method of starting is called Current Step or Pedestal Starting.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[raisinbran]

nhee,
It almost sounds like you are not getting much benefit out of the soft-starter, and may be damaging the motor if a large number of starts are required. It sounds like you have a class 10 overload if 400% for 15 seconds is causing you to trip. That may be a bit light for a high inertia fan load. If your power system can stand it, you may be better off across-the-line starting the motor.
Regards,
Raisinbran

 

[jraef]

I fail to see how Across-the-Line starting is EVER going to benefit a motor over soft starting. Assuming he gets the soft starter set up correctly so that it no longer stalls, what damage is going to happen raisinbran?

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[LionelHutz]

In my experience, the overload heating effect on the motor does not change very much with a reduced voltage start until you get close to stalling the motor. The motor heating will just increase slightly with more current limit until you reach this point. In other words, as long as you have some decent acceleration torque it doesn't make much difference on the overload or motor heating.

So, this tells me that you're either very close to tripping the overload with 400% current and reducing the current puts it over the edge or that you're stalling the motor when you reduce the current.

I'd suggest you find the motor stall-time damage curve and then set the overload to protect the motor while also providing a longer trip time. In other words, increase the overload class setting.

For a typical fan and motor combination, I will usually start at around 100% current and ramping up to about 300% current limit in about 15 seconds. I usually pick the actual parameters after examining the motor and load data if I can get it. Sometimes the source capability will come into play too. Otherwise, it's an educated guess usually based on the fan inertia. Sometimes 300% current isn't enough and increasing it to 350% or 400% is required.

The ATS48 manual is so vague about it's settings and operation that it's about next to useless.

 

[nhee]

Thanks for everyone's follow-up.

Just to confirm - at 15 seconds and 400% I don't overload. I had played around with lower current settings and higher initial torque settings and was seeing overload faults. Prior to playing around too much more, I was looking to get some suggestions for operating settings.

 

[raisinbran]

jraef,
Rather than prolong the agony of drawing high current for a significant time, I think it is easier on the motor/system to draw a slightly higher current for a much shorter period of time. If you need 400% current to get the job done, you are not that far away (voltage-wise or current wise) from an across the line start. Also the additional complexity of the soft starter and risk of single-phase or phase unbalance, especially if it does not have a bypass contactor. Just my opinion - that is all.
Raisinbran

 

[Marke]

Hello raisinbrain

The power dissipated in the rotor of the motor during start is a function of the full speed kinetic energy of the load. Provided that the motor develops enough torque during start to spin up freely, there will be no additional heating of consequence in the rotor. I could argue that the prolonged time when using a reduced voltage starter is actually kinder on the motor as the heat is able to dissipate and disburse during the start reducing the maximum spot temperature in the rotor.

Many modern high efficiency motors have a locked rotor current in the order of 750 - 800% of the rated curent of the motor, so a start current of 400% is a significant improvement over full voltage starting.

The soft starter reduces the start current, reduces the start torque and eliminates the switching transients.

If you full voltage start an induction motor, you will have a highstart current (typically 600 - 900%) preceded by a very high inrush current (typically 1.5 - 3 x LRC).
The soft starter can eliminate the inrush current and associated torque transient, and reduce the start current. Definitely does offer advantage.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[DickDV]

The best evidence for the value of an electronic softstarter is that motor repair shops usually hate 'em!!

Sure cuts down on their rewind business!!!!!!!!