Running conveyors at low speeds - seeking opinions

[steve708]

Lots of automotive plants running slow these days, consequently the large 6" chain conveyor systems are called upon to run at very slow rates. We have either AC or DC adjustable speed drives on many of them and are looking to convert many more of them from fixed speed to allow for maximum flexibility going forward.

We need between a 3 to 1 and 4 to 1 speed range on most conveyors and this is pushing the limits of motor capabilities, at least the AC ones. I would consider 20Hz the low limit, we may need to go slightly above 60 Hz in the future.

The AC motors are old Delco 7EH spec non-inverter duty NEMA Design B TEFC and are being run with Reliance or AB VF drives. The DC motors are old Reliance compound wound 240V types being run with old Reliance or AB DC drives.

I am looking for opinions on the long term effects of running AC and DC conveyor motors for 10 hours a day 4 days a week at 500 to 600 RPM. Changing the sheaves or reducers is not an option because we would like to get the right combination to achieve the 3 or 4 to 1 speed range and not have to change them again.

I also need to mention that all of these conveyors are mechanically oversized as far as motor horsepower goes. None of them ever draw more than half of their full load current in normal operation, most are at one third or less.

Thanks in advance!

 

[ianingram]

Hi,

In general terms, is it possible/practical for you to install external cooling fans (or systems) for each motor?

I have seen motors running successfully below 20Hz as long as they were externally cooled.

Also, if these conveyors are mechanically oversized, then what is going to happen if you encounter an overload or jam-up? The motor protection will not trip before something breaks, so do you have shear pins or something similar?

Ian

 

[JesperMP]

I'll guess that the chain conveyor drive motors will require almost the same torque to overcome the friction, even of the speed drops markedly.
If the AC motors have shaftmounted cooling fans, then the cooling will be drastically reduced at 1/3 speed. I would consider to install forced cooling if this is not already the case. With forced cooling you can go down to zero speed if you need to.

edit: Beaten to it by Ian !

 

[steve708]

Hi,

In general terms, is it possible/practical for you to install external cooling fans (or systems) for each motor?

I have seen motors running successfully below 20Hz as long as they were externally cooled.

Also, if these conveyors are mechanically oversized, then what is going to happen if you encounter an overload or jam-up? The motor protection will not trip before something breaks, so do you have shear pins or something similar?

Ian

Some conveyors have a spring loaded overload switch and a shear pin, others just a shear pin. When jams happen it is always the mechanical protection that trips.

Cooling fans would be an expensive proposition but would probably be added on an as needed basis. We have some AC motors running around 30 HZ now which are cool to the touch.

 

[OZEE]

Our experience has been that you're usually ok down to around 10Hz.

Why not just start switching them out for inverter-duty motors? I believe that the "standard" inverter-duty motor is 10:1. And for a few more $$ you can get 1000:1.

 

[milldrone]

We have either AC or DC adjustable speed drives on many of them and are looking to convert many more of them from fixed speed to allow for maximum flexibility going forward.

We need between a 3 to 1 and 4 to 1 speed range on most conveyors

steve708,

I do not know if this applies to your industry or not. In my industry (forest products) when converting an existing fixed speed drive to VFD. We use 90Hz as the planned top speed for the motor. This gives us the expanded turn down ratio we need. In your case that would give you a 4.5 to 1 turn down.

The AC motors are old Delco 7EH spec

If memory serves me correct that is a "modern" U frame motor.

and are being run with Reliance or AB VF drives.

You do not mention if these are sensorless vector or vector or just plain old scalar (V/Hz).

Changing the sheaves or reducers is not an option because we would like to get the right combination to achieve the 3 or 4 to 1 speed range and not have to change them again.

See my comment on 90 Hz.

I also need to mention that all of these conveyors are mechanically oversized as far as motor horsepower goes. None of them ever draw more than half of their full load current in normal operation, most are at one third or less.

I hope you guys don't pay powerfactor penalties!

This sounds like a perfect fit for 90 Hz operation because of the loss of torque above 60 Hz.

 

[leitmotif]

What size motors are you talking about here

To kind of encapsulate
1..You are running motors currently meeting your needs at half load.
2 Yes you are penalized on PF but VFDs will help greatly with this problem.
3. You want to run a 4:1 speed ratio on VFD to get speed variation but are concerned with motor cooling - good - I would guess that if you go with your minimum speed at 20 Hz and max at 80 or so you should be OK conveyers are constant torque load so the loss of torque over baseline should not bother you with underloaded motors.

4. I would suggest going at this one of two ways. Set one up as a trial in shop and juggle gearbox and chain ratios and motor speed til you get what you think will apply to 90%. OR take a look at you common setups and do it with two or three and try it out under production demands. IF you can get one criteria to fit all 100% you will be the first ever in American industry I think.

5. I do not think you will need braking resistors on conveyers due to the drag UNLESS you have one going downhill.

6. I agree with the thinking on shear pins but recognize many mechanics hate em and will replace brass with steel. I would check into setting the maximum current settings lower in VFD to allow protection for jams on conveyers.
Dan Bentler

 

[steve708]

Motor sizes range from 2 HP to 15 HP with most in the 7.5 to 10 HP range. As a design spec, motors 20 HP and above have capacitors installed to improve power factor.

The drives are Reliance GV3000 or AB 1336 or AB PowerFlex all with sensorless vector, but they are being used in V/Hz mode.

Thanks everyone for the helpful comments so far, I will probably size the sheaves to have 20 HZ as the absolute minimum and go up to 80 Hz if necessary. Fans would add peace of mind, but right now every penny counts in the auto industry.