3 Phase Motor Overload Problem?

[gerard]

A conveyor system at work is powered by a 3-Phase Motor. It's a simple setup really. It is DOL start/stop
system with a telemechanique thermal overload. The problem is, when the conveyor becomes jammed-which is regularly does the overload does not cut out fast enough to prevent damage to one of the many drive shafts
on the system. So my question is - Is there an Electronic/Electrical or any other method of cutting the motor
more quickly and preventing this problem?

 

[adnarim]

Assuming that you have the correct overload and setting for the FLC of the motor,you could fit a slipping clutch coupling with a proximity switch fitted to it ,to indicate jam up.
The clutch can be set up to release at a certain torque to prevent damage,hope this helps

 

[gerard]

Thanks for your reply. The settings on the overload are fine.
Ok that sounds like a good idea. Have you any idea where I could get some information on
a slipping clutch and is it difficult to install?

 

[adnarim]

Sorry i don,t have any info,but search around on the net, i am sure you will find info,most machines i have worked on came with them fitted,or try RS Components or similar.

 

[stef]

Hi there,

we had similar problems at work and then decided to fit a PowerFlex inverter with torque limiting.
No problems since . . .
A bit pricy though

 

[SirKit Breaker]

The root cause of your problem would appear to be the conveyor jamming. Why cant this problem be addressed, instead of trying to engineer out another problem by installing extra bits and bats?

 

[gerard]

Thanks guys,
The conveyor runs well for the most part but becanse of the nature of the product
it is impossible to prevent the jam up when it does occur- unless you sit and look at it all day- and
then it would never happen!!
I have 2 good potential solutions to my problem:-
(1) Install a slip clutch
(2) Install an Inverter with torque limiting
I will look into these and see which I think is the best suited .

Much obliged!

 

[Quadrant]

A conveyor system at work is powered by a 3-Phase Motor. It's a simple setup really. It is DOL start/stop
system with a telemechanique thermal overload. The problem is, when the conveyor becomes jammed-which is regularly does the overload does not cut out fast enough to prevent damage to one of the many drive shafts
on the system. So my question is - Is there an Electronic/Electrical or any other method of cutting the motor
more quickly and preventing this problem?

Hi Gerard
You get programable electronic overloads where you can set o/l window,but the best would be to get your client to change to VSD control(every electrical fault feature is covered by a mere perameter setting)
Regards Roy

 

[Wazzajnr]

I would go down the inverter route myself. Most inverters have digital i/o and some basic programming ability. You could even set it to set off a siren at a certain torque just above normal running and then a stop somewhere slightlky above this.
That may allow the jam to be cleared before it gets too entwined in the rollers thus minimising downtime.
I find Parker SSD 690+ drives a good drive with free programming software. Also Control techniques are good as they allow ladder programming within the drive but these tend to be more expensive

 

[Griffin]

I agree with Sir Kitt, fix the route cause of the problem the jamming. If this cannot be sorted maybe a couple of photoelectric sensors maybe to detect when boxes jam and drop out the control circuit to the contactor.

A

 

[ACDC]

Have you considered fitting a load monitor? (Foxtam or lavato), they have adjustable tripping times on them. Tie it into the trip circuit and it should trip quicker than the overload and cheaper than inverter route. Good luck

 

[ericks]

Hi there, a bit late but if you fit the correct MCB then it should trip as soon as the motor "jams".
This is actually a standard by us (in the industry in South Africa). Yous MCB should protect your cable but also the motor against sudden and extreme overcurrents.

Thanks

 

[Darkwood]

Your motor which controls your conveyer belt is clearly too large for the mechanics of the machine as when the conveyor jams it damages the machine rather than trip the overload in time, the inverter although expensive route is a good suggestion but what i recommend is that during normal running you clamp test the current to the motor and reduce the overload 5% to 10% above it, if this cant be acheived than replace the overload with a lower rated.
It appears a poor design flaw and the overload is rated to a motor too large for the job its undertaking, it should have been rated to the machanical limits of the machine.
Due to you enquiring with this question i can only assume fitting and configuring an inverter will be too complicated for you to do and the simplest and cheapest option is as ive mentioned above.
I do this for a living, coming across poor design so hope this helps.

Also as mentioned above you clearly have an issue with loading or jamming of the conveyer and this needs to be addressed too the overload trip is there for the rare occasions something is wrong so if its common then corrective measures need to be employed.

Hi there, a bit late but if you fit the correct MCB then it should trip as soon as the motor "jams".
This is actually a standard by us (in the industry in South Africa). Yous MCB should protect your cable but also the motor against sudden and extreme overcurrents.

Thanks

Check you tripping curves in the regs if you double/tripple the current on most motor rated c type mcb's then the motor will burn out before the mcb trips through overload, mcb,s tend to trip with the short circuit created by overloads rather than the overloads themselves when motors jam thats why we have to have digital monitoring or extra overload protection after the contactor.

Your regs may differ in south africa but its a poor way of motor protection to rely on the front end mcb, remember this mcb allows for the inrush of the DOL startup so how can it be relied on to trip fast with a motor jam.

 

[gerard]

Thanks for the reply guys. I am going down the Inverter road on this one - I already have it ordered!
'Darkwood' hit the nail one the head when he suggested that the motor is too big for the mechanics of the machine. And if you could only see the design of the the drive system you would be amazed at how overly
complicated it is! If you can imagine - it's a cooling conveyor for pizza bases with 9 separate conveyors on top
of one another. One motor drives the 9 conveyors by means of chain and sprockets, each sprocket been driven by the one above it by means of one continuous chain. So the total torque required to drive the whole machine goes through each sprocket and shaft - hence the problem!
We have planned to install a second motor on the opposite side of the machine and split the load in 2.
So hopefully all will be well!!

There is an MCB fitted but this does not solve my problem. I need it to trip out instantaneously once a fault current has been detected and activate an alarm in the process

 

[Darkwood]

Inveter gives good control with options of connecting remote alarms or beacons etc without the the need of additional circuits, usually 24v dc pnp or npn devices or external circuit of various voltage can be run through volts free changeover contact that can asigned to close on alarm.

Additional if you got the correct overload set up for the running load of the conveyor then the n/o volts free terminal could have been used for the alarm and save you a lot of money.
I bet the conveyor is jamming because the conveyor cover / cloth is creeping into the side due to an out of balance/position roller, well just a stab in the dark.

 

[gerard]

The conveyor jams because of the nature of the product. It's just high volume food belting down the line and unless someone just stands there and polices the line all day, jam-ups will happen. I will be able to install the Inverter myself, I was just trying to see what you guys would do in a similar situation.

 

[ericks]

Hi Darkwood,
You might have misunderstood me.
We use MCB's and motor overload / protection devices.
Not just a MCB to protect the motor. The whole design is carefully done to match the motor to the mech. drive unit. We always!!! use motor overload devices whether it be thermal / electronic overloads. Depending on the application we would consider whether to use thermal / electronic or even vsd's.
We tend not to overdesign as cost is important. What i tried to say is that you should start with the MCB sizing as this gives a "degree" of protection. If your application is wrong, where your motor might be too powerfull for the application you should solve this first. We start and protect 0.55Kw to 2500Kw motors in our plant. I presumed having a overload device in the starter came without saying!!

Working in a plant with about 2000 motors we regulary have either conveyors and or pumps jamming. Where there are thermal overloads fitted we never have damage as the MCB always trips. I agree the MCB alone is def not an ideal motor protection device. When you have a steady overcurrent over a long period of time the MCB won't trip, that is the job of the overload device.

Thanks

 

[Quadrant]

Hi Guys,I am getting the same reply to a thread(3ph Overload problem) twice a week with no new content.Does any one know what im doing wrong?
Roy
Quadrant

 

[Darkwood]

Hi Darkwood,
You might have misunderstood me.
We use MCB's and motor overload / protection devices.
Not just a MCB to protect the motor. The whole design is carefully done to match the motor to the mech. drive unit. We always!!! use motor overload devices whether it be thermal / electronic overloads. Depending on the application we would consider whether to use thermal / electronic or even vsd's.
We tend not to overdesign as cost is important. What i tried to say is that you should start with the MCB sizing as this gives a "degree" of protection. If your application is wrong, where your motor might be too powerfull for the application you should solve this first. We start and protect 0.55Kw to 2500Kw motors in our plant. I presumed having a overload device in the starter came without saying!!

Working in a plant with about 2000 motors we regulary have either conveyors and or pumps jamming. Where there are thermal overloads fitted we never have damage as the MCB always trips. I agree the MCB alone is def not an ideal motor protection device. When you have a steady overcurrent over a long period of time the MCB won't trip, that is the job of the overload device.

Thanks

Was just a short post you put and it gave the wrong impression but your obviously putting all this into the design, sorry for the mix up but not everyone is fluent in motor control and protection but obviously you are.

 

[TF and the wire]

I think you are correct to go for an invertor.

Overloads are supposed to be coarse enough to avoid tripping on start up and short heavy loads.

I had one on a conveyor recently that was tripping after about 30 seconds. It was set at 4 amps, as the motor plate stated but was actually taking 15amps. The cause was that it had been wired in delta instead of star. So it was taking that long with almost 400% overload.

I usually use Allen Bradley powerflex 4 invertors. Dead easy to get hold of, wire up and program. You can select the current to be displayed so you can watch what it takes on normal loading. You can then set the overload trip current accordingly.