3Φ induction AC motor with VFD.

Narlin

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[FONT=&quot]If I design a fill line with a VFD & appropriately sized induction, inverter rated, AC motor that is using a gearbox to drive a conveyor, then is it reasonable to program the PLC to start and stop the motor 6 to 10 times per minute? I normally play with pumps instead of conveyors and I worry about overheating the motor with frequent on/off cycles of a contactor so I am naturally concerned that this VFD cycling from Off during a fill to On going the next set of bottles will not work out very well. In this case, I am considering leaving the contactor On all the time and just reducing the VFD speed such that the motor will not run. Would someone suggest how to do this correctly?[/FONT]
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[FONT=&quot]narlin[/FONT]
 
Motor heating is not usually a problem with VFDs because they limit current. Ten times a minute is quite a bit, though. Conveyors are often constant torque loads, which also increases overheating potential. Part of the answer depends on hp, since a large motor is more likely to overheat than a small one.

If you want to be sure get your motor with built in thermal protection. This can be as simple as a Klixon temp switch for a small motor or winding RTDs for a larger one. Then you are protecting based on actual motor temperature, not an arbitrary estimate.
 
I can't speak for every brand of VFD as I mainly use Allen Bradley but I suspect what you are suggesting will be OK with most VFD's.

Starting a motor Direct On Line with a contactor 6 times a minute will cause overheating because a motor started this way is likely to draw 6 times it's full load current. Doing the same thing with a VFD the current will be limited to a max of 1.5 x full load and if you have a slower acceleration time you will often not see anywhere near full load current.

That is the beauty of a VFD. However I would still stop and start the VFD, if you just reduce the speed to zero that is not the same thing as you will still be keeping power on the motor.

Definitely leave the power on the VFD between starts and check if you really need contactors on the VFD input, most of the time I just use a Safe Torque Off card in the VFD to handle emergency stops and don't fit any contactors in the 3 phase supply.

Depending on size of motor compared with load and the speed you are running the motors you may need force cooled motors, I would take advice from your motor manufacturers.
 
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This application will use a 5HP motor that is probably oversized for the job, since the line originally came with a 3HP. The gearbox must reduce the speed a lot because the line just creeps, going no more than 3 or 4 feet per minute. I don't know the gear ratios. 10 fills/min is probably on the high side, as each fill is about 1 liter of wax times however many bottles across there are.

I'll check out Klixon switches. I surmise that you think the control scheme is reasonable !
 
Andy, thanks. I understand turning off the motor with the VFD and leaving the VFD on. That is the usual ctrl scheme. I didn't know that the VFD limited the start up current so much, and also, i don't use a contactor in conjunction with the VFD and i do use mostly Allen Bradley VFDs.
 
I think that you are convinced that you shouldn't start and stop a drive with a contactor, but I just want to make sure. Doing that will shorten the life of a drive from years to hours or minutes. Keep the drive power on all the time, unless you need to shut it off as part of the E-Stop system. STO is the better choice here. Drive power on and drive running are 2 different things. Drives can be started and stopped. What happens during a stop command can usually be configured. Stopping the drive and taking the command signal to zero are 2 very different things. If a drive is running and you remove the start command, then the drive will either shut off and allow the motor to coast to a stop, or ramp down to a stop then turn of the drive(stop modulating). Starting and stopping a drive this way is normal. If you take the command to zero, the drive will ramp down to zero, but will still be modulating. It may or may not be able to hold position(zero speed, really) If you need it to hold, then DC hold(if available) will help. Keep in mind that you will be pumping current into a motor that isn't turning. Be sure the motor is designed for that.

What VFD are you using?
 
This would be a good application for a TENV motor. They cool themselves thru their own skin and fins without any fan so speed is of no consequence. Check Marathon and ABB-Baldor for motors in that size.
 
You could also install an xternal fan to cool the motor, by continuosly running the fan
 
If acceleration is not a big deal then it sounds like a good application for a clutch. Lot's of energy wasted starting and stopping this motor. Check out warnerelectric.com
 
If acceleration is not a big deal then it sounds like a good application for a clutch. Lot's of energy wasted starting and stopping this motor. Check out warnerelectric.com

Regenerative braking unit will pay itself very quick. Don't think lot of energy us wasted for starting, and with the regenerative brake each braking cycle puts money in your pocket
 
The beauty of a VFD with regard to starting is that it is possible to accelerate a load without exceeding the rated FLC of the motor, which means you can do it as often as possible without causing any significant excessive heating of the motor. The one caveat to that is that you trade the peak power for time. At 100% FLC you will get 100% FLT (Full Load Torque), but when starting Across-The-Line you get LRT (Locked Rotor Torque), which is usually 160% of FLT. So for example if it takes 2 seconds to fully accelerate the motor ATL, it might take 4 seconds with a VFD limited to 100% FLC. For starting and stopping 10 times per minute, you may run into a wall like that. But even if you must allow the current to go a little higher than FLC to accelerate fast enough, it will STILL be much much easier on the motor than ATL starting, thermally and mechanically. I seriously doubt that 10 starts per minute using a VFD will present a thermal problem in the motor at all.

Do not EVER use a contactor in a VFD circuit for anything other than planned and coordinated safety purposes. If used on the load side you can kill the transistors, if used extensively on the line side you can kill the capacitors. No need for it, just use the start-stop control of the VFD.

Side note: Line regen braking saves so little actual energy that it almost never makes sense to go that way for that reason alone. The only energy it can recoup is the excess energy it took to accelerate the load, meaning you cannot recover any more than you used. That is typically not as much energy as the purveyors of that technology want you to believe. Centrifuges? Yes, absolutely. Small conveyors on a bottling line? Pennies per day, maybe. Amortized over the DOUBLED cost of the line regen VFD and it typically comes out taking 10 years to pay back the difference. The VALID use of line regen is to avoid the thermal limits to high duty cycle applications using Dynamic Braking resistors. Again, small bottling line conveyor? Not going to be an issue.

Narlin, I am an AB drives support person for your area, send me a PM if you want to discuss this further.
 
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speeding up with a VFD is good, and as the current is limited to 1 normal. Now stopping will need a special VFD with a braking circuit. The motor will get hot as there is current flowing but motor is not running its normal rpm. so extra cooling will be needed.
In old days we used a cam and the motor was normally ruuning, maybe a speedchange to get in sync with other machine.
A servo motor is even better for this job.
the VFD must be 5 HP as the mass of the motor is higher.
 
Again, I don't use a contactor with a VFD ever. That was a gum chewing error as i wrote the question. My location, phoenix is correct, but without a state, who would have guessed maryland? I am going to look into the TENV motor idea and most especially, i am going to get the customer to commit to a maximum number of fills (start/stops) per minute. Overall, this sounds like an easier solution than some. I often wondered why high speed fill lines would sometimes move the filling nozzles. I think i know now!
 
Am thinking belt conveyer - there are others of course.

One advantage of VFD is reduced wear and tear on driveline components ie belts chains reducers etc.

On a conveyer especially if start stop is 6 times per minute you can just do STOP and let motor coast down. This will help cool motor, reduce tipping of product on belt and eliminate any worries on dynamic braking.

I think one important number needed is how long is run time on conveyer six times per minute is 10 second. It may be best to just let it run at one speed continuous.

VFD will allow for setting best speed on conveyer to "synchronize" with other steps / components on the production line. I think that is the main advantage compared to DOL starting.

Dan Bentler
 

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