VFD or Soft starter?? Application question?

[hanksmith]

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

 

[waross]

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

 

[hanksmith]

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

 

[ScottyUK]

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!

 

[hanksmith]

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.

 

[edison123]

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.

 

[dpc]

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.

 

[itsmoked]

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

 

[waross]

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

 

[DickDV]

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.

 

[ozmosis]

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.

 

[jraef]

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

 

[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.

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.

 

[itsmoked]

You mean the torque increases to 200%?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[LionelHutz]

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.

 

[waross]

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

 

[hanksmith]

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.

 

[edison123]

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

 

[DickDV]

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.

 

[davidbeach]

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.