VSD constant torque application 3

[monjo]

Can anyone please tell me whether current limiting will be effective for reducing overcurrent trips on VSD's for constant torque applications such as feeder belts, conveyors? Have found it an effective solution to the same problem with variable speed drives for pumps.

 

[jbartos]

Comment: Generically, current limiting downstream of the protective device or arrangement is always effective for reducing overcurrent (short-circuit) trips. However, current limiting upstream of the protective device or arrangement is effective for reducing overcurrent (short-circuit) trips.

 

[cbarn24050]

Hi, your drive should have a current limit setting on it. Unfortunatly if the load demands more current than the set limit then the motor must slow untill the load demand falls below the set level.

 

[jbartos]

Clarification of my previous posting (I beg your pardon):
Generically, current limiting downstream of the protective device or arrangement is can be effective for reducing overcurrent (short-circuit) trips. However, current limiting upstream of the protective device or arrangement is not necessarily effective for reducing overcurrent (short-circuit) trips downstream of the protective device or arrangement since the downstream protection may be set for the limited overcurrent (short-circuit) level.

 

[aolalde]

Monjo:
Pumps have a typical variable torque demand and low inertia. So I assume that a pump is ease to start.

Constant torque loads are harder of start specially if the conveyor is heavy loaded, the inertia will increase too.

My opinion is that a Feedback Vector ASDS will provide the best option for breakaway and low speed Torque, so the current is torque effective during your accelerating process. As the load inertia and torque increase, the accelerating time has to increase too, if you want to limit the line current.

The Motor has to supply both the Torque to accelerate the inertia plus the load torque during the acceleration and within the available current from the control.

So, it is necessary to know the allowed acceleration time, the reflected total inertia at the motor shaft and the load torque.Then you should calculate the size of the motor and ASDS driver.

 

[jraef]

aolalde's post is right on with regards to starting issues, I voted him a star. However if your question was pertaining to running current, the same holds true. If you use a standard scalar drive (V/Hz), current limit during run will lower torque significantly, possibly to the point of stalling. Some drives allow you to choose to lower the frequency in order to facillitate limited current, an effect that may have other consequences. By using a Vector drive you can maximize the effective torque while in current limit during the run mode.

"Venditori de oleum-vipera non vigere excordis populi"

 

[aolalde]

I agree with jraef statements. I voted him a star.

 

[TomG33]

Monjo,
I wish to take a step back here and look at your whole statement.

Your question, and indeed the responses, have centered on whether current limiting is a good or bad thing.

There is no doubt that with a variable speed Drive, Current limiting is desirable and indeed effective way of controlling the current to the motor.

my question centers more on

"Why are you getting these overcurrent trips in the first place?"

From my experience with VSD's they have very good current limit control, and an inherent part of engineering the system is to specify and set the current limit function at a level that is appropriate to the Drive, the Motor and application. (taking into account the comments by Aolalde, and others)

Usually the Overcurrent trip level is set a reasonable margin, outside this Limit value, and as long an nothing fails the the question of trips has been a "Moot Point"

Maybe your Conveyor Application has some very sudden impact loadings, leading to fast motor decelerations, however the speed of most control functions in most modern VSD's that I have dealt with, are usually fast enough to handle this.

I think you should be asking questions such as

"Is the trip a thermal overload ( I^2 T ), a Timed overload or an instantaneous overcurrent (IOC)?

"What level is the IOC setting with respect to the Drive nameplate? (I would be aiming for the area around 2.2 PU)

"what level of current can the drive handle in an IOC situation"

"Is the Overload setting based on the Drive current capability or the motor capability ?"

"At What Value do you plan to put the Current limit?" (1.5 PU of motor rating is a good starting place)

"Can your drive impliment a 'Tapered Current Limit' that will give you high torque for breakaway from standstill and reduce as the speed builds up?"

I would base the IOC setting on the Drive rating, as motors tend to ride through IOC's better then Drives.

I would base the Current Limit and the thermal overload on the motor nameplate value. ( after checking that the Continuous rated current of the drive exceeds the continuous rated current of the motor)



Finding answers to these questions should lead to a proper engineered application that should give trouble free operation.

I am just concerned that there is some other outside influence causing the trips and that placing the Current limit function at a more conservitave setting would restrict your application, and hide a more insideous problem that will come back to haunt you at a later date(most likely at 2.00am new years day or some other inappropriate time like that).

Regards
Tom

 

[gsimson]

Conveyor is certainly not a CONSTANT TORQUE application. At zero speed the torque requirement is app. 160% of the rated torque which comes down to 100 % at full speed.

Also the torque required for horizontal movement is speed dependent- mainly friction load. Only the torque required for "lift" is constant and depends on load only.

Only a Crane "Hoist" is a Constant Torque drive ie., the load torque is constant independent of speed.

Conveyors operating without Fluid Coupling requires a running torque of app. 60% of the normal torque ( Current ) of the motor.Hence the current limit will work.

Conveyors with fluid coupling can operate with current limit as the shock loads are taken care of by fluid coupling.

 

[jbartos]

Comments: The original posting refers to "..reducing overcurrent trips.." rather than "..reducing overload trips.."" that are actually discussed in some of the above postings. The overcurrent is supposed to be linked to faults due to short circuits. The overload current is supposed to be in excess to rated current; however, it should be less than the short circuit current.
Reference IEEE Std 100 "Dictionary"
If there are true overcurrent trips, then TomsG33 statement "Usually the Overcurrent trip level is set a reasonable margin, ...." is relevant.
Also, TomsG33 question ""Is the trip a thermal overload ( I^2 T ), a Timed overload or an instantaneous overcurrent (IOC)?"" is linked to a statement in the original posting, namely "..whether current limiting will be effective for reducing overcurrent trips..".