Torque limiting / Tension losses- Urgent! 2

[pmblotta]

Hello,
I'm with a problem: I have a 150 [CV] ; 440 V drive. But unfortunaly the available power supply for this is a bit far, and there is to much tension loss during the starting of this equipment (a conveyor belt), higher then our customer would aprove, requiring an huge investment in cables.
The solution would be changing the tension, to medium (13300 V), but, due to maintance logistics, stantandardization, whatever, the motors on the plant can only be of 13300 V for more then 200 CV motors....
The solution now could be changing the motor, but due to the fact that in this kind of equipment the reducer, the hidraulyc / elastic coupling has to be dimensioned to 1,5 the nominal power of the motor (to avoid higher losses in case the belt become stuck for any reason), we would then have to change every thing (wich has already been bought).
We are now to purpose a torque limiting system...but the available ones seems to be not to reliable / available...


I would appreciate any sugestions, mecanical or eletrical.

Thank's!

 

[Barry1961]

I assume you are already using a fluid coupling and not trying to start the motor under load.

Can you run 13300V close to the motor and then step the voltage down?

Barry1961

 

[Marke]

Hello pmblotta

I assume that this motor is being full voltage started.

You can reduce the voltage drop during start by reducing the start current. This can be achieved by the use of a reduced voltage starter such as a soft starter but, there are some constraints.

Reducing the start torque and continuing to full voltage start, will not alter the start current. The start current is a function of the motor design and the starting voltage.

If you use reduced voltage starting, the minimum start voltage is determined by a) the starting torque required by the driven load and b) the efficiency of the induction motor during start, or it's ability to convert amps into newton meters.

For the best results, you need to have a motor with very good start characteristics and also as low a start torque requirement as possible.

Conveyors can be difficult because they can be required to restart when loaded and start torques as high as 150% are often needed for loaded materials conveyors.

If you use a controlled delayed fill fluid coupling, you can get the motor to full speed before the load torque is applied to the motor. This enables the motor to start under no load conditions. You then add a soft starter to minimise the actual start current.

You need to both reduce the start torque and reduce the start voltage.
The alternative is to use a variable speed controller, but this is a much more expensive option.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[Drivesrock]

pmblotta,

me being biased towards drives I would suggest a VFD. You get the starting torque you need at motor nominal current or perhaps with some overload limited to 150% on the drive output, can do a nice controlled ramp up, program in some protection features, can have variable speed. However, as mentioned previously there are considerations - like is this a downhill conveyor (in full or part)? As there could be motor overhauling by the load and regeneration of electrical energy back to the VFD in operation which has to go somewhere. Perhaps wasted in a cheap resistor bank or pumped back into the mains via an expensive regeneration unit. Also, where are you going to put the VFD, are the environmental conditions adequate (temperature, dust, etc)?

VFD - certainly more expensive than a softstarter but should do what you need to do.

Drivesrock

 

[jraef]

Hello pmblotta,
Marke offered you the best advice. Assuming from your post that this is an outdoor conveyor (because of the distance issue), a VFD or fluid coupling would not be the best choices if maintenance is already a problem for you. Both will require a lot of attention. A soft starter on the other hand is a perfect choice for this. If properly selected, you should have a reliable solution that will reduce your voltage (tension) drop on startup and not add to your maintenance procedures.

Look for a soft starter that provides "dual ramps". With this feature, one ramp profile can be set up to soft start your motor under normal conditions. If however, your belt becomes "stuck" as you put it, or power fails when the belt is loaded, you can select a 2nd ramp profile that can be DOL only if necessary to get the load moving. This is a very common application for soft starters, you should be able to find plenty of help from reputable vendors in your area.

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[aolalde]

To me the VFD option is the best. So you can start with very good torque and very reduced current, as reduced as full load current. This way the voltage drop will improve a lot and the motor acceleration will be smooth and safe.In adition you will be able to optimize the conveyor speed.

 

[Marke]

Hello pmblotta

A VFD option is a godd option for an unloaded conveyor, but in many instances with materials conveyors, when they are stopped loaded (due to a trip of power loss) they can require 160% start torque or more to get them to breakaway.
If a VFD is used, it should be a vector drive capable of greater than 160% torque at standstill or you will need to empty the conveyor before a restart.
Another factor with the VFD is the losses which are on a par with a fluid coupling. The start current will be lowest with the VFD, but the harmonics will be higher and continuous due to the low distortion power factor of the drive. Also be careful of downhill conveyors with VFDs as you need to dissipate the energy somewhere.

Best regards,

Mark Empson

http://www.lmphotonics.com