Inverter Features to protect pump ...similar to shear pins??? 1

[cfordyce]

We have an Inverter driven motor driving a pump through a 40:1 gear reducer box. We keep breaking shear pins in the coupling between the gearbox and pump.

Is there a fearture in a typical invert which would protect the equipment in a similar fashion as the shear pins? (Inverter is Allen Bradley 1336 Pluss II.

I suspect you would see a drastic increase in motor load (.i.e. current) just before the shear pins break. What happens is the viscosity of the product changes and becomes so thick that it prevents the pump from turning. Or sometimes a fairly solid chunk of product comes downstream and gets caught in the pump. In all cases I would rather have the inverter stop on a programmed fault of some kind instead of replacing the shear pins each time (sometimes 4 -5 times in a row before the bad product is eliminated).

CAF

 

[gannu0716]

There is a feature of setting the Overload cutoff in the inverter which typically acts as an Electronics Shear pin.
You can set this from 100% of the nameplate or FLC to upto 200% (for a short duration) of the FLC/Name plate current.

 

[RadarRay]

If you find you can't do as suggested by Gannu there are torque limiting couplings. I forget the name of the manufacurers but I have used them before with great success. The ones I am familiar with are basically two disks which are held together with springs. There are half circle cutouts in each disk which line up to each other. In this space there are ball bearings. When the force exceeds the spring force the disks ride up the ball bearings and all torque to the load is lost. To reset the device all you have to do is run the motor in reverse. I haven't had to use one of these things in 15 years, but I always thought they were mechanically clever.

 

[GusD]

Hi cfordyce.

A month ago,I wouldn't think I would be sayng this.
Did you check the possibility of using a "Magnectic Coupling".They can be set for a given torque rate and stop transmiting torque everytime you exceed that rate.Once excess load is removed it will return to normal use on its own.I have not used one of these things myself,but I see where this technology can be a viable option.
Try their Website "magnadrives.com"

GusD

 

[jbartos]

Suggestion: To avoid Magnetic Couplings, Torque Limit Switches, and similar devices, a Hall current sensor might be installed in the motor feed between the inverter output and motor terminals. This sensor might be linked to the inverter via feedback loop to instantly turn it off on certain rush input currents in the motor feeder

 

[GusD]

hi cfordyce

Hello J. Bartos.I am curious to know why you caution the use of Magnetic Couplings.Do you have any reason for your concern regarding these couplings?We have never used this particular technology in our plant,but the idea is intriguing nevertheless.
As for stopping the breaking of shear pins,it requires a positive braking system that cannot easily be provided by the inverter itself.PHASE reversal would probably act fast enough,but the cure may cause some problems to the system
itself.I believe that in this application, you have to have positive braking at the motor or gearbox.Rapid Inverter Deaccel. or Dynamic Braking won't solve this problem.

GusD

 

[cfordyce]

The last few posts suggesting solutions outside of the inverter features, lead me to believe that the overcurrent limit setting on the inverter will not work correctly.. or fast enough to prevent the shear pins from breaking first?

CAF

 

[CB2]

cfordyce,

The type of pump, brake Hp of the pump, motor Hp, and VFD Hp rating would be very helpful.

The VFD, somewhat depending on size, may not be sensitive enough to protect the pump drive train from damage, lots of gear reduction and oversized VFD/motor I would bet. If you eliminate the shear pin, someone will program the VFD so that it will not "nuisance trip" and end up with a damaged pump. The shear pin is doing it's job, protecting the pump!

 

[gannu0716]

The sugesstions of installing magnetic couplings & others will require lot of Hardware modifications whereas my suggestion is a software one.

Please go ahead & try. You will get the required info. in the drive manaul.

 

[automatic2]

Seems to be a general misunderstanding about shear pins. The reason shear pins are used, is to prevent an overload from transmitting through the drive. The driver will see a rise in resistance up to the shear force, but not beyond. Your high current limit is just that. Set it below shear. Here's the concern, if your drive has mass, you'll want to disconnect it at a known location before it disconnects itself at the location of its choosing, typically with costly results. There is no electronic shear because by then, it's too late.

 

[peebee]

To take auto-2's statement one further:

I'm guessing that if you stuck a wrench in the pump impeller, there's no electrical or electronic solution available that would be fast enough to keep the pin from shearing. The inertia of the motor/shaft would probably shear the pin even if power was disconnected.

I suspect that mechanical couplings would prevent the wrench from shearing the pin, though.

For smaller overloads, electronic methods would probably be helpful.

 

[cfordyce]

Thanks folks.

CAF

 

[TheDOG]

There are two components of motor current:
1. Flux or magnetising current
2. Torque producing current

Torque producing current is in phase with the voltage (resistive), while flux current lags the voltage (inductive).

I would suggest using one of the digital outputs to "At Torque". This could then be used to shut down the drive. You will need to enter a value in the "Dig Out Current" setting of the drive that is of an appropriate value (dependant on motor and load characteristics).

I have used this feature many times with success.

 

[jbartos]

Clarification to GusD (Electrical) Jun 3, 2003 marked ///\\Hello J. Bartos.I am curious to know why you caution the use of Magnetic Couplings.
///I just mentioned a word "avoid" in a sense of "option" rather than "caution." Incidentally, this Forum offers eng-tips.\\Do you have any reason for your concern regarding these couplings?
///There might be some as already posted by gannu0716 June 17.\\\
We have never used this particular technology in our plant,but the idea is intriguing nevertheless.
As for stopping the breaking of shear pins,it requires a positive braking system that cannot easily be provided by the inverter itself.PHASE reversal would probably act fast enough,but the cure may cause some problems to the system
itself.I believe that in this application, you have to have positive braking at the motor or gearbox.Rapid Inverter Deaccel. or Dynamic Braking won't solve this problem.
///However, the electromagnetic brake could with some mechanical side effects.\\\
GusD