Electric motor as a damper

[Barry1961]

A customer has a 20 hp, 100 foot (30 m) bucket conveyor on a 45 degree incline. For mechanical reasons the conveyor chain will often surge or jerk. The speed of the conveyor is around 40 fpm (12 m/min) and the surges can be from 1hz to .25hz. The surge is most pronounced and causes the most trouble near the tail pulley.

Using pneumatic cylinders for chain tension reduces the surges as does proper lubrication and maintenance. The chain return supports could also be redesigned to provide better support.

The idea I have is to use a small motor or gear motor on the tail pulley to dampen the surges. The tail pulley motor would be trying to maintain a speed slightly slower the main motor. This would hopefully tend to isolate the surges to the return side where they do no harm.

Does this sound like a good application for an electric motor?
If so, what type of motor would be best, AC or DC?

The surges could cause cycling forward and reverse on the motor.

Barry1961

 

[waross]

If you subtract the rated speed of the motor from the synchronous speed you will get an idea of the speed range that you have to work with.
Common synchronous speeds are 1200, 1800, and 3600 in North America and 1000, 1500, and 3000 ind IEC land.

A motor rated at 1750 RPM will have a slip of 1800-1750=50 RPM
It will be happy anywhere from 1800-50 RPM to 1800+50 RPM or 1750 RPM to 1850 RPM.
For short periods or surges you can probably go to 200% slip safely. That would be 1700 to 1900 RPM.
(For those who want to know exactly how much momentary slip is safe, see RMS Horsepower in the Cowern papers on the Baldor motor website)
If you can arrange your motor to function in this range there is a good chance it may help you. If it works, but the overloads trip, that is a good indication that either your drive ratio should be adjusted, the motor is too small, or both the above. Properly set overload protection should give adequate motor protection.
Good luck
yours

 

[electricpete]

The purpose of the tail end motor is to apply a force (or torque) in direction opposite of motion. That amounts to absorbing energy... acting like a generator.

In order to accomplish this, the tail-end motor (if induction motor on fixed-frequency supply) must operate above it's syncronous speed, right?

The basic function desired is damping.. resistance to a change in position (let's say it creates a resisting force proportional to velocity). I wonder if a cylinder half-full of sand on the tail end would accomplish this? Or maybe some kind of hydraulic machine.. I'm sure this problem has been tackled many ways before. Might be worth a try on a mechanical forum such as "gear and pully engineering" as well.

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[electricpete]

Thinking again, a backwards directed torque proportional to velocity will consume power in steady state. That would be a negative aspect.

You are really interested in a backwards torque which is proportional to rate of change of velocity, i.e. acceleration.

A large flywheel might help do the job. (in addition to whatever spring tensioning devices you have).

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[electricpete]

Once again, my idea of damping action (cylinder of sand) was wrong. I think a flywheel or large rotating inertia would help do the job. Also if a little bit of friction in the bearings of that device it will be pulled by the belt and keep the driving end tight, trailing end slightly slack.

Another question to consider would be what kind of machine/control do you have driving this conveyor? Ideally you'd want a relatively flat torque-speed characteristic that does not increase sharply when speed decreases.

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[electricpete]

Another thing to consider is that the masses in your system together with the springiness of the belt may form a mass spring system. Flywheel can alter the situation. To better or worse I'm not sure.

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[waross]

Slow down, pete
You sound like you're drinking too much coffee.
If you over-drive the tail end pulley as you originally suggested, it will put a constant drag on the belt as you originally stated. It will be an added load on the head end pulley, but the tail end pulley will be generating and returning most of the energy that it consumes to help the head end pulley drive the added load.
The head end pulley motor will be working harder to supply the energy to the tail end pulley motor but, as I said, most of this energy will be returned.
respectfully

 

[Barry1961]

I was thinking of the tail pulley motor being around 1 hp AC and it would be applying about .5 hp of pull against the drive motor. This would put the slip around 1 to 1.5%. During a positive surge it could drive the motor speed past synchronous but hopefully the total swing in motor speed would be less the 2%. During the positive surge the motor will be helping to drive the motor speed to synchronous which it a bad thing.

Would it be possible to continuously mechanically drive an AC motor in reverse of the direction it is being electrically driven? Maybe a 5 hz (+150 rpm) command and an actual speed of -20 rpm. I would guess that any low voltage boost would be set to zero.

I have used DC motors in continuous stall and would assume that they can be mechanically driven in reverse with a low enough current/voltage limit.

Would a DC motor be able to hold a tighter speed?
Does anti-slip in a VFD affect overdriving?

A hydraulic motor would probably be better in this application but they do not want any hydraulic fluid near the process.

Barry1961

 

[waross]

There seems to be a misunderstanding here. I don't know if it is me or someone else.
If you put a 1750 RPM motor that is belted or geared to run at 1800 RPM when the belt is being driven by the main motor it will niether help nor hinder.
However:
When a surge tries to accelerate the belt the motor will resist by absorbing energy and through regeneration will return the energy to the power lines.
If the surge succeeds in pulling extra belt past the tail pulley, instead of stopping and waiting for the next surge, the tail pulley motor will start working and feed belt to the top of the conveyor to delay the next potential surge.
By changing the belt ratio slightly you can put a continous drag on the belt. This drag will increase with increased belt speed during a surge.
Your setup will be challanging. I suggest an adjustable pitch pulley, and an ammeter. At minimum current the motor will be idling. Make your adjustment based on motor current.
respectfully

 

[Barry1961]

I think the rpm of the tail pulley will be between 10 and 20 rpm. I would guess much closer to 10.

Is it possible to use a VFD driven AC motor in a constant stall or reverse driven mode?

I would guess that it would have to be at low hz/rpm and without voltage boost.

Barry1961

 

[macmil]

I have to say that i don't like what you are proposing. If there are mechanical reasons for the uneven speed, then presumably they can either be corrected or accomodated by a mechanical modification; without knowing the details it's a little tough but there are several items which spring to mind that could be causing this behaviour. i think it would be more apropriate to address them rather than introduce another complexity.