Soft Start vs VFD for 5hp motor

[rockman7892]

I am trying to decide weather or not to use a soft start or a VFD for a tripper car dirve motor which is 5hp at 480V. Right now we are just using a full voltage reversing starter (FVR) and when the tripper car gets to one end of the track the forward starter drops out, it pauses briefly (couple of sec) then the reversing starter pulls in to send the car the other way. It does the exact opposite at the other end of the belt.

To aovid wear and tear and the constant hammering of the starter and tripper car itself when the starter pulls in, we want to add a soft start or VFD to allow a smoother transition and startup when reversing direction. I'm not sure which one will work best for this application or which one is more economical.

I dont believe that we'd be varying the speed of the drive at all once it is up to full speed, so that why I'm not sure if a VFD would be worth it in this case. In other words we are just looking to ease the drive slowly up to speed then let it run at a certain speed to the end of the track and either ramp the speed back down or just let it come to a stop at the end of the track.

Has anyone had experience or know what will work best for this type of application?

 

[LionelHutz]

It sounds like a good application for a soft-starter.

 

[ScottyUK]

The VFD may make for an easier installation because reversing is inherently available with very basic signals, where it may take some extra effort to sequence the reversing contactors with the soft start. The cost difference between a soft start and a VFD on such a small drive won't equate to many hours of engineering time so the VFD might make better overall economic sense. Otherwise I agree with Lionel, the soft start sounds like a better match for the application.


----------------------------------

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

 

[waross]

I would do a reality check and look at the motor duty. You have not indicated whether the motor is running hot due to the numerous reversals.
If the motor heat is ok, then add a soft start behind the reversing starters with suitable interlocking.
If you would prefer that the motor run cooler, consider the VFD.

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

 

[rockman7892]

To my knowledge there is no evidence of the motor running hot and therefore no real alarming concern for getting to motor to run cooler.

Can a soft-start have a ramp-down function, or is it strictly only for ramping up the motor? It sounds like the costs for both the VFD and the soft-start are a wash.

 

[itsmoked]

Yes, most all soft starters have a built in soft stop function.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[macmil]

An alternative, if the configuration allows, would be to use a magnetic or fluid coupling in the drive. I put some on a stacker nearly 30 years ago, and it's still running !

 

[rockman7892]

In regards to heat buildup in the motor as mentioned above I read somwhere that a soft-starter is limited to successive restarts and that considerable dwell time in the power-off state must elapse before the drive can be restarted. What kind of elapse time generally needs to take place, and do I need to take this into consideration with my application that is constantly moving back and forth with the reversable starter?

 

[jraef]

Yes, since you already have the reversing starter, a soft starter would be the simpler solution.


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[jraef]

Sorry, I paused for a sandwich before hitting "Submit" and didn't see the subsequent posts.

Most soft starters are built to handle at least the same duty cycle as the motor it is connected to, so that should not be a problem. But for sure, mention it to the vendors you choose. There are some marginal products out there and if you never mention it to them, you have little recourse if it dies.

Do not use the Soft Stop function in this case, it adds no benefit here and would indeed cut the starter and motor duty cycle in half.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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[rockman7892]

Is the duty cycle of the motor determined by how many times is is run within a certain time frame if it is not continuous? Is there a formula for determining duty cycly or should this be spelled out on the motor datasheet for this motor?

 

[Thedroid]

I think that the VFD would be easier to wire and program, and also alows for more flexibility in the long run. Either one in the 5hp range will run close to the same price, I would go with the drive.

 

[itsmoked]

I would too at 5HP.

rockman; There is no set duty cycle for motors. Smaller ones can generally be started more often than larger ones but other than that it is up to the manufacturer to tell you.

I would guess that a motor not started with a VFD of 5HP size should not be started more than about every five minutes. With a VFD, as often as you desire - with caveats. Those being that your starts may need to take a little longer with the VFD.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[rockman7892]

itsmoked

The motor that I am referencing probalby starts about every 1-2 minutes. The motor starts in the forward direction and takes about a minute to travel to the end of the track. The motor then shuts off briefly for a period of 15s or so before it starts back up in the reverse direction. It the motor starting to frequent? Is this something to watch out for if using a soft starter, but does not matter with a VFD?

I also just discovered there are actually two 5hp motors on this drive that I am referencing. I dont belive that factors into anything but just wanted to mention that is is two motors we are controling.

 

[waross]

Look at the Cowern Papers on the Baldor web site. they explain and provide a method for determining the RMS loading of motors. This is most applicable to motors that are mostly lightly loaded but must contend with short term overloads. The information may be pertinent here.

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

 

[itsmoked]

Five minutes was only a guess. Lots depends on the load, the motor style, the ambient, etc.

The entire issue to the motor is temperature. If you can put your hand on the motor there is no issue with your particular application. If you cannot put your hand on the motor you need to gun the motor and see what the case temp is after its been running long enough to not see any further temp rise.

Add about 15C to your gun temp as that will be what the rotor temp likely is since it is taking the brunt of the starts.

If you are below the motor's temp limit then you are fine. If you are over it you are directly reducing the motors life by some amount. If you are over it you should probably consider a VFD. Or have spare motors on hand.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[LionelHutz]

Or, the OP can fairly safely assume that the motor is OK if there has been no service issues related to over temperature yet with the motor as of yet. If the motor has been in operation for quite some time it is most likely OK. It would likely die a quick death if the operation was overheating it.

 

[sreid]

Anothr option would be to use a VFD with built in PLC like the Mitsubishi A700.

 

[jraef]

I think that the VFD would be easier to wire and program, and also alows for more flexibility in the long run. Either one in the 5hp range will run close to the same price, I would go with the drive.

I think this s a valid issue if it were a new installation. But remember, he ALREADY has the reversing starter and it is ALREADY adapted into whatever control system he is using. To use a VFD he would have to remove and discard all of that, then completely reconfigure his existing controls to adapt them to the VFD's dry contact inputs.

In this particular case, adding a soft start controller downstream of the existing starter and just engaging it with aux. contacts of the reversing contactors (in parallel) would be the simplest install. No need to change anything in his existing control circuit.


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[jraef]

Is the duty cycle of the motor determined by how many times is is run within a certain time frame if it is not continuous? Is there a formula for determining duty cycle or should this be spelled out on the motor data sheet for this motor?

If you can get hold of a copy of NEMA MG-1 specifications there are design guidelines for motor duty cycles. I am not at the PC that has my copy on it of I would post the chart for you.

The duty cycle varies by HP size, but if I remember correctly, a 5HP motor frame like that could easily be in the 20-30 starts-per-hour range. But that also depends greatly on the connected load. So what happens at design time is that the Engineer determines the WK² (inertia) of the load, the starting duty, speed and the required torque at the shaft, then (hopefully) refers to those NEMA guidelines to pick out the proper motor.

So where I am going with this is that for all you know, the 5HP selection ALREADY took into account the duty cycle involved in the machine. In other words, it might have taken only 2 or 3HP to do the job, but the designer picked a 5HP motor in order to compensate for the duty cycle. Without that knowledge, assumptions avail you nothing.


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