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Starting An Overhauling Load


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I'm trying to save a few five-thousand dollars by not using a regenerative VFD for a conveyor application. I'd also prefer not to waste all the energy as heat in a brake chopper.


When loaded, the 3~ 480VAC, 15KW conveyor load is 75% overhauling. If my motor theory is remembered correctly, across-the-line power will run the motor at slightly faster than synchronous speed and generate power back to the grid.


A soft-starter will be used to reduce mechanical and electrical stresses (of course). But the problem comes when the conveyor needs to be started with the overhauling load on-line.


To understand what will happen, let me think through a situation where the break is released with no power on the motor. In this case, the conveyor will accelerate freely up to and beyond the rated speed of the motor (a very bad thing, and will never be tested, but just for discussion). Just for example, lets say it takes 3 seconds to reach rated speed.


Next, a soft-starter is used to start the motor under the same situation as above. When the soft starter begins, the motor is going to be electrically driven towards rated speed, but at lower than rated power. It will continue to push the load downhill until the synchronous speed is reached at which time it will start to generate (this time will be less than 3 seconds because of the added motor power). Current is going to stop moving forward through the SCR and be stopped from reversing by the diode nature of the SCR (is this all correct?).


Looking at some common architectures for Soft Starters


http://www.lmphotonics.com/images/triac.gif TRIAC will allow current either direction when fired.


http://www.lmphotonics.com/images/SCR-DIODE.gifSCR with reverse parallel diode will allow reverse current through the diode


http://www.lmphotonics.com/images/SCR-SCR.gif2 SCR's reverse paralleled will only allow reverse current if they are both fired at the same time. However, if the forward SCR is only fired on positive phase voltage, there will be no current allowed to flow when the motor is generating.


So maybe I just answered my own question: I need to find a device with on of the former to architectures above.


Stopping the load is going to be complete chaos, though. The brake has 33% more torque than the motor rating, so it will stop with a load, but it will just take some distance to do it. I might have to use a VFD solely for stopping distance...


Comments, suggestions, corrections are all appreciated.

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Hello ETooothpaste


Welcome to the forum.


You are essentially correct in your comments.

In terms of architecture, I doubt that you will find a triac based soft starter as a potential candidate in todays market.

Likewise, the SCR diode option, is probably not an option either. They were much more common 20 - 30 years ago.


Nearly all 400 - 480 volt soft starters that you are likely to find today, would be SCR-SCR format. I believe that all will trigger both SCRs on a phase at the same time.


There are a number of other factors that could affect the performance. There are still a number of soft starters that use a single pulse trigger at the beginning of the current conduction only, and this pulse can be quite short. Other soft starters use continuous firing either as a hard fire technique or picket fence firing.


I would expect that most soft starters using either hard fire or picket fence firing would handle your situation, but you may have problems with the single pulse firing situation.


I have used soft starters for this type of application numerous times in the past with no problems.


I would recommend using a heavy duty rated starter as loaded conveyors can require a high start torque to break them away, and I would also recommend using a bypass contactor (internal or external).


Braking can be achived by applying DC to two terminals of the motor. I would expect that you will need around 24VDC to achieve good braking torque.

I would recomend a proper DC source rather than a half wave SCR based DC brake unit which may not cope with an overhauling load.


Best regards,


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