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

 

[Marke]

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

 

[lrumley]

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.

 

[itsmoked]

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

 

[sundep]

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.

 

[itsmoked]

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

Keith Cress
kcress -

http://www.flaminsystems.com

 

[hanksmith]

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.

 

[itsmoked]

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

 

[RPM57]

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.

 

[jraef]

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.




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