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VFD or Soft starter?? Application question?

VFD or Soft starter?? Application question?

VFD or Soft starter?? Application question?

(OP)
So I have an application that I am currently stumped on.

We have a 150HP motor, 575V AC, that we have to start multiple times each hours, could be as high as 25 starts each hour.

The motor is driving a rotary plough discharger which is started under load.

I have been having a hard time getting information on the starting torque requirements from the vendor but I believe this will be a high inertia start.

I don't know if a softstarter will work for this application. Some have said no problem, others say there won't be a chance and the soft starter will die.

Would a VFD work better?? I know the price is a little higher but the last thing I want is to walk away and have either the motor burn up, starter burn up, or soft starter burn up.

Does anyone have some information that could possible help me out, anyone seen applications like this before?

Thanks
Hank

RE: VFD or Soft starter?? Application question?

25 starts per hour on a normal 150HP motor may only be contemplated with a VFD.
There is much less motor heating when starting with a VFD compared to a soft start or DOL.
As an example, a common cool down time for a soft started motor or a DOL may be 20 minutes. 25 starts times 20 minutes is 500 minutes. It's hard to fit that into an hours time.
BUT, is this a special motor designed to accept extreme starting duty?  

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

RE: VFD or Soft starter?? Application question?

(OP)
Nothing special about the motor,

150HP, WEG, TEFC - IEEE841 NEMA Premium Efficiency.

Being inverter duty (part of the IEEE841 I believe) I am sure it can handle the heat a little better but still not 25 starts an hours better

RE: VFD or Soft starter?? Application question?

Any scope for a mechanical clutch or a fluid coupling, and leave the motor running?
  

----------------------------------
  
If we learn from our mistakes I'm getting a great education!
 

RE: VFD or Soft starter?? Application question?

(OP)
I thought about the clutch or fluid coupling but I don't see it working in this situation as the motor is located inside a cone at the bottom of a silo and I see space issues coming up, and the fact that the silo is already manufactured and onsite.

If we would have known about the number of starts each hours from the beginning we may have been able to incorporate this into our design.

RE: VFD or Soft starter?? Application question?

Even with VFD, the motor is going to need an externally driven cooling fan running all the time. And I doubt whether normal S1 duty motor will actually work reliably here even with VFD. It looks like an S4 or S5 duty application.

RE: VFD or Soft starter?? Application question?

25 starts/hr on a 150 hp motor is way off the chart.  VFD is the only option. Even then, buy a couple of spare motors.

As waross said, reduced voltage starting does not really reduce heating the motor much, if at all.  A VFD can reduce heating because with a VFD, the motor isn't really being "started" as much as being run from 0 Hz up to desired speed.   

RE: VFD or Soft starter?? Application question?

Look at your motor's specs.  It will not tolerate that many starts in an hour.  Perhaps 4?  That is a continuous running frequency of use.

Keith Cress
kcress - http://www.flaminsystems.com

RE: VFD or Soft starter?? Application question?

If space is an issue, you may want to consider a hydraulic pump and hydraulic motor. With a variable displacement pump you should have no problems with breakaway torque or smooth starting.

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

RE: VFD or Soft starter?? Application question?

A couple of other options to consider:  First, you could run this motor on a drive and simply switch from zero speed to running speed without turning on and off.  This would still require an inverter duty motor with auxiliary blower.

I doubt that this second option will be totally satisfactory but some commodity motor manufacturers built TEFC motors and then supply an add-on kit for an auxiliary blower when used on an inverter.  It would be tempting to try this kind of motor with the auxiliary blower but run it across the line.  I think the repetitive inrush current would shorten the motor's life considerably but it wouldn't be as bad as running a plain TEFC motor that way.  And, it would eliminate the cost of the VFD.

RE: VFD or Soft starter?? Application question?

Another thing to look at is why the motor is starting/stopping 25 times an hour. In cases like this I generally advise people to look at the overall process and see if better control can be made of the primary process rather than focusing on the control of secondary processes.
 

RE: VFD or Soft starter?? Application question?

My 2 cents...

1st off, NO soft starter that I know of would be capable of 25 starts per hour. The BEST ones are maybe 10, probably 6, most are usually 2-4 unless they are seriously over sized. That is because the MOTOR is not rated for that, as others have pointed out. So in soft starter design, there is usually no point in designing a soft starter that is more capable than the motor it is connected to. So bottom line, the people who told you "no problem" with using a soft starter are ignorant of how they work and what they do. Pay no attention to them.

2nd. Starts-per-hour ratings of motors are based upon Across-the-Line (DOL) starting, wherein the excess energy pumped into the motor at low power factor (as it is when not moving much) is absorbed by the motor stator and rotor. It's rare to see a 150HP motor rated for more than about 6 loaded starts per hour for that reason. But when using a VFD, you are NOT always pumping extra energy into the motor at the same rate because the VFD can inherently match the energy put into the motor to the task at hand. So if you can accelerate slowly enough, you can even do it without exceeding FLA of the motor, which means there might theoretically be NO excess heat to consider. But here's another reality; 25 starts per hour, evenly spaced, means that each ENTIRE run cycle cannot be longer than 2.4 minutes! That has to include acceleration, operation and deceleration. Deceleration is not likely an issue for you on this application, but that is because it s likely a high friction load, which means ACCELERATION is going to be a much bigger deal. So even though a VFD is CAPABLE of alleviating some duty cycle issues, in your case you are still going to need to pump a lot of energy into that motor to get it moving. The VFD will help, but not completely solve this problem for you in my opinion.  Unless perhaps you only needed a 50HP motor and you used a 150HP to compensate.

You need to do more homework on this...


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln  
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies  

RE: VFD or Soft starter?? Application question?

The starting torque of an induction motor improves considerably at 15 Hertz. The load probably will not break loose until the drive reaches 10 Hertz.

For some of these applications you need a VFD that is sized and programmed to continuously deliver 200% of motor full load current and drive to motor wiring sized at 200% of nameplate current is needed so that drive does not trip out during starting or otherwise wears out too soon. An external overload relay is then needed in addition to a motor thermostat to protect the motor and gearbox. See National Electrical Code Table 430.22 for a continuous duty motor used in varying duty service.

What happens at 15 Hertz is that motor locked rotor current decreases to about 200% of full load and locked rotor current increases to about 200% full load for a design B or C motor. That allows the motor to break away heavy loads but the drive has to be sized to continuously take that or it will either nuisance trip or wear out prematurely or all of the above.

RE: VFD or Soft starter?? Application question?

25 starts per hour on a 150hp motor is a rediculous requirement to purposely build into new equipment. However, since it appears to have reached the point where it's too late to do anything about it, I would recommend installing a cooling blower on the motor and a VFD. Then, the VFD should be set-up to speed control the motor to avoid starting and stopping.

The starts/hour capability of the motor does depend on the load. The motor may be rated for 4 starts per hour but that is at the recommended load which I believe is an inertia load specified in North America in the NEMA MG1 motor book. If the load is lower then you might be able to get more starts per hour.

The soft-starter is a no-go. If you attempt to use anything except a VFD to start the motor 25 times per hour you will likely destroy the motor. The soft-starter itself isn't really the problem if the application is known. I could easily set you up with a soft-starter that would burn your motor to the ground.

There might be some advantage to investigating a lower speed smaller HP motor which is still the same frame size - for example a 150hp 1800rpm = 75hp 900rpm = 444TS frame. Running at 1/2 speed should mean about 1/2 the staarts/stops.

Yet another option might be some kind of intermediate hopper or larger hopper to store more material between cycles.
 

RE: VFD or Soft starter?? Application question?

Good points, Lionel. This issue may be best addressed by reviewing the entire setup and operating cycle.
Controlled motor speed, controlled feed rates, some type of clutch, magnetic or possibly air operated, modified operating cycles, it may be well to step back and look for a solution that avoids a high number of starts per hour.

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

RE: VFD or Soft starter?? Application question?

(OP)
Thanks for everybodys input,

The reason for the high number of starts/stops per hour is that this discharger is being used to load trucks.

The thought is a truck driver would pull up, start loading watching the scale reading till it hits about 80% max. Operator would then stop the discharger, wait till the dust settles and check the load, then either pull the truck forward a bit or continue loading. Some trucks are long and would probably require moving once, other trucks are pulling a trailer and might move 2-3 times, the are looking at approximatly 6-8 trucks per hour.

Again thanks, meeting with Client tomorrow and we will see what comes out of this.

 

RE: VFD or Soft starter?? Application question?

I would rather try to control the feed rate (down to even zero) into the discharger than start a 150 HP motor that often.

RE: VFD or Soft starter?? Application question?

Letting the motor run and bypassing the material back into the silo when the truck is moving seems like a much better setup.  You would be wasting some energy moving material to bypass but that would be far better that replacing a 150hp motor 4-5 times per year.

Of course, this mechanical arrangement may not be doable, either.

RE: VFD or Soft starter?? Application question?

Quote (mc5w):

The starting torque of an induction motor improves considerably at 15 Hertz. The load probably will not break loose until the drive reaches 10 Hertz.
Perhaps on an improperly set up V/Hz drive.  On sensorless vector you ought to be able to get stable full torque at near standstill.  Throw in an encoder and you can get stable full torque production at standstill.  The key is being able to have rated full load slip at standstill rather than having a much higher slip.

RE: VFD or Soft starter?? Application question?

The number of starts per hour that a motor can do, and the number of starts that a soft starter can do, are both thermally restricted.
Typically, we talk about a limit of say 6 starts per hour for a motor, but the rating is very dependent on the starting conditions.
Every time we start a motor, we dissipate the equivilent of the full speed kinetic energy of the driven load into the rotor as heat. The limit of six starts per hour is only meaningful at a defined start duration (under full voltage conditions) or load inertia. If the motor is rated at 6 starts per hour with a full voltage start time of 15 seconds per start, then we could get 12 starts per hour at 7.5 seconds per start and 25 starts per hour at 3.25 seconds per start. The rotor heating is the same, infact the increased number of starts for shorter periods will result in a lower rotor temperature at the end of each start and so it is thermally kinder to the rotor.
When the motor is started by a soft starter, the same rule applies in terms of rotor heating. The energy dissipated in the rotor is equivilent to the full speed kinetic energy of the driven load. The soft starter will reduce the rate of energy flow and will extend the start time except that if the start current is too low, there will be additional rotor heating due to the motor being held in a semi-stalled state.
Likewise, soft starters are rated at a number of starts per hour for a given start current, start duration and ambient temperature. Reducing the start time and/or the start current can increase the number of starts per hour. It is possible to thermally model the soft starter and predict the starts per hour depending on the start conditions.
I have installed motors and soft starter operating at greater than 25 starts per hour without problems, but the thermal design must be robust if a long life is expected.

Use the right motor and the right soft starter and I would say that it is possible.
From the description of the application, it sound as though there will probably be long periods of time where the motor will not be operating. With additional cooling during the off times, the prestart temperature would be lower and would further extend the starting capabilities.

If there was room, I would also investigate the use of a delay fill fluid coupling. This would allow the motor to start very quickly and the slip losses would be dissipated in the fluid coupling rather than in the motor.

Best regards,

Mark Empson
L M Photonics Ltd

RE: VFD or Soft starter?? Application question?

Actually that many starts even uncoupled can only be done with the use of a VFD.  Just the inertia of the motor's rotor assembly will cause enough heating in the motor for damage to result with the number of starts required by this application.

RE: VFD or Soft starter?? Application question?

In your truck loading application it may be possible to start a loading, and when the trailer is mounding enough the motor could be speed reduced to perhaps 10% so material is dribbling out.  The driver could then hop in and move forward.  Then turn up the motor speed for the second hump.

I am just wondering if some subtle usage mods like this might be workable while X-ing out a bunch of starts.

Could be too much to ask of a bunch of truck drivers...

Keith Cress
kcress - http://www.flaminsystems.com

RE: VFD or Soft starter?? Application question?

This is a perfect application for a hydraulic unit and a hydraulic motor.  Leave the motor running continuosly.  Resize your motor for 25 full load current operations per hour instead of 25 starts per hour.
 

RE: VFD or Soft starter?? Application question?

I agree totally. Then you just turn the motor off when no trucks are waiting.

Keith Cress
kcress - http://www.flaminsystems.com

RE: VFD or Soft starter?? Application question?

(OP)
So we had our meeting with the Client today and everybody came to an agreement that a VFD would be required.

Client had standard VFD sizes so we are planning on using a 200HP VFD to drive our 150HP motor.

Hopefully we can ramp the discharger down while the truck is being moved rather then stopping the motor. Won't know until the commissioning stages.

Thanks for everyone's input, greatly appreciated.
 

RE: VFD or Soft starter?? Application question?

I'd jettison the VFD and go DOL with hydraulics.  It would be fairly simple and more robust, cost less, and you will have to do it anyway, after the second motor you'll be frying.  We won't even go into the energy savings.

Keith Cress
kcress - http://www.flaminsystems.com

RE: VFD or Soft starter?? Application question?

Hi I am new, and hope you find my comments useful.
You can use a VFD for this application however you need to select a Vector duty, 1000:1 range, TEBC motor with encoder feedback. The drive needs to run in Full Flux Vector mode to get 200% torque at start.
To take this one step further you can go to a Water cooled PM Torque motor, this option also allows you to eliminate a gearbox since these motors run at 50 to 500 RPM. Obviously for this option you should choose a VFD that can handle PM Torque motors.
 

RE: VFD or Soft starter?? Application question?

Actually, I'm a little surprised at the size of the motor now that I think about it. Rotary Ploughs I've worked on usually have a very high gear ratio and rather smallish motors, i.e. 20-40HP. So if you have a 150HP motor, I'd wonder if someone has ALREADY over sized that motor for the high duty cycle. I remember one that was maybe 250TPH and had 2 x 20HP motors on it. We did those Across-the-Line, with a truck through about every 10 minutes. No problem for the motors but if you think about it, that's only 6 starts per hour.

You aren't planning on trying a direct drive are you? If so, I don't think it will work.

By the way, if you just slow it down, it still keeps discharging and they use RPDs typically because it is wet material. Translate: big mess between truck trailers. But you mentioned a lot of dust, so I guess it's a dry material? Anyway, still a mess that someone would have to clean up if you don't completely stop the RPD. But you could cut down the starts per hour by slowing it down as a single trailer is moved under the silo to evenly distribute the material.

 


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln  
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies  

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