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VSD verses Soft Starter 3

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rdrago

Electrical
Nov 8, 2003
14
What is the point of installing VSD's on conveyor motors that will run only at 60HZ?

Would a "soft starter" be a better choice considering how VSD's can potentially damage non-inverter equipped motors even with the addition of dv/dt filters? Also, VSD's generate harmonics on the power source which can lower power factor along with other ill effects.

It seems to me that vendors are quick to push VSD's without considering their overall impact. Is it a cost issue? acceleration? braking?

Can anyone offer an opinion?



 
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Hello rdrago,

In the main you would be correct in suggesting a soft starter may be more beneficial (and less expensive) in these types of applications. However there are instances in which the acceleration profile (linear ramp) delivered by a VSD provides are far better result not only for the conveyor and its drive mechanism but also for the products being transported. Installing a VSD is also of particular advantage in those conveyor applications that exhibit a highly frictional starting characteristic.

As I am a soft starter man, I'll leave it to those involved in the selling of VSD's to sell you on the concept further.

Don't want to do myself out of a job!

Regards,
GGOSS
 
I think the primary thing that a soft-starter does is make the start less severe on the power system.

There may be slight benefit to the motor in terms of less mechanical stress. The thermal aspects of the start can in fact be much worse with soft starter than with d-o-l start, particularly when starting high inertia load with high torque over the course of the start.

VFD can tremendously reduce the heat generated in the motor during the start far below the heat produced during dol start.

There are many other aspects including good discussion by ggoss.
 
Suggestion: The soft starter can be justified since it produces often mentioned advantages to the motor and mechanical load attached to the motor (lower mechanical and electrical stresses, reduces input current inrushes, etc.). Depending on a type of soft starter, the damages done to non-inverter type of motor exist since the soft starter is using the same or similar power electronic switching devices as the VFD.
The VFD would not be justified, if there is no need for a speed reduction or speed increase. However, it appears that in some cases the conveyor speed is beneficial to increase or decrease. For example, the conveyor may be overloaded by heavy load thus eligible for the speed decrease or lightly loaded thus eligible for the speed increase. These speed variations could justify the VFD application.
 
If it's a belt conveyor, the starting torque of the motor must generally be limited to avoid damage to the belt. On the other hand, the motor must be capable of producing high starting torque to start a fully loaded conveyor. In olden days, we generally used wound-rotor induction motors for belt conveyors.

As long as the SS starter can provide sufficient torque through the speed range, it would appear to be a better choice than a VFD. But there may be some limitations in the performance of the SS starter on this high intertia load that would make a VFD necessary. (Or a wound-rotor motor :cool:)
 
The soft starter will reduce the current and torque produced by the motor at start. This will generally reduce electrical supply interference and mechanical wear and tear on the conveyor. If a soft starter is employed, it must be able to deliver sufficient current to start a fully laden conveyor. I usually recommend the use of a current ramp soft starter with the initial current set to provide breakaway torque for an empty conveyor and the maximum current set to provide enough torque for a fully laden conveyor. This will always start, but will minimise any transients and snatches on the belt.

A Variable speed drive will gradually increase the speed of the conveyor independent of load, at a much lower current than the soft starter, but will invariably cost a lot more than the soft starter, and will be much more inefficient during operation. - typically 5% losses.

Both can be set to provide a gentle transient free start.
Best regards,

Mark Empson
 
Suggestion: If filters are added to the VFD, then the efficiency will go down, e.g. to approximately 7% or so. The motor efficiency has to be added. So that the total efficiency of the system may be about 87% or so.
 
The starting torque requrement of belt conveyor is app. 160% of the normal load torque. Hence invariably larger rated motor is selected for conveyor applications. However, generally fluid couplings are provided in conveyors so that the motor can accelerate separately and the load is thrown on the motor only when the motor is operating in the stable region with much higher torque and less current. The motor rating will be only for running.
Conveyors with fluid coupling do not need any soft starter as the coupling takes care of all the stresses. Hence Sodt starter is of no use.
Conveyors without fluid coupling require much higher starting torque. And certainly Soft starter can not deliver the required torque, as only the supply voltage is reduced during starting and the torque is proportional to square of voltage.
As mentioned by "dpc" slipring motors with rotor resistance are provided in conveyors only to develop higher starting torque.
VSD is the correct option for conveyor drives as the torque even at zero speed is = FLT. The starting time comes down considerably, the stresses on the equipment is minimum with smooth starting. Also if the distance between the switchgear and the motor is < 100m , general purpose motor will serve the purpose as the no. of starts of conveyor is generally < 6 per day.
I am a practicing engineer and we maintain around 150 conveyoys with the total length of 150 km. Recently we have installed VSD for 160 kw motor in one of our conveyors. The system is working well with GP motor and the starting current (from zero speed to full speed)is always less than the FLC.
To our experience soft starter can be used stisfactorily only for pump applications, not for fan, not for cranes, not for crushers not for conveyors, etc.,
 
Hello grimson

I have succesfully used many soft starters for conveyors over the last 20 odd years.
You are correct in you statement that loaded conveyors can require 160% torque to start them, but empty conveyors often need in the order of 40% FLT.
The secret in using a soft starter for a loaded conveyor, is not to use an oversized motor, but rather to select a motor with good starting characteristics. If you select a motor with a LRT of say 240%, then this can be used to start and empty conveyor, or a loaded conveyor. The soft starter with current ramp (my recommendation) behaves like a delayed fill fluid coupling, except that it draws more current during start.
Best regards,

Mark Empson
 
Suggestion: The original posting questions the VFD application at 60Hz, i.e. motor run at constant speed. The previous posting does not address any speed variations in the VFD applications. It merely states that the soft starters have not been satisfactory. This may imply that some speed variation is being implemented.
 
Dear Empson,
Thank you for yor comments.
Soft starter with High starting current is not a Soft Starter
Conveyors may stop on load and our system should take care.

The GD2 value of conveyor is so much , kindly check your calculation of 40% FLT ( Empty conveyor)

With regards
 
Hello gsimson

I think that we will have to agree to differ on this one.
From my experience, the starting torque for an unladen conveyor can be as low as 40%, but not always. It is certainly common for a laden materials conveyor to require around 150% torque.
The advantage of a soft starter for a conveyor application, when used as a current ramp starter, is that the torque applied to the conveyor will slowly ramp up. If the conveyor load is high, then the conveyor will not completely breakaway until sufficient torque is developed, however if you watch a laden conveyor with a current ramp starter, you can see a wave passing along the conveyor as each section breaks away. When this wave hits the far end, the conveyor just gently accelerates to full speed. If the conveyor is started unloaded, the acceleration is still transient free and gentle with no snatch. The belt does not jump, and the damage is reduced.
This can certainly be achieved with a soft starter, but the motor must be a high torque motor. Essentially, the curent is limite to the minimum required to start the conveyor depending on the applied load.

There are a number of ways to start a conveyor, a delay fill fluid coupling, slip ring motor, VSD and soft starters are all options and the best option is dependent on the application. The fluid coupling and VSD are both lossy systems with upto 5% additional losses in the inverter or coupling under full load conditions.
The VSD and slip ring options offer the lowest start current, but the slip ring and fluid coupling require the most maintenance.
The fluid coupling requires DOL start current for a short time, the soft starter requires a lower current for a longer time.
It is really a case of engineering the system.

If you would like assistance engineering a soft start application, let me know.
Best regards,

Mark Empson
 
Dear Empson,
I really appreciate your views. All the systems are in use in some place or the other. I still vote for VSD considering
-Higher starting Torque
-Lower starting time
-Very smooth starting as per our need
-Much higher power factor
-Speed variation as per the actual load
-Running at 'ZERO&quot; speed when the conveyar is idle especially in tropical & humid ambients
Once again thank you for some wonderful discussions
I will contact you whenever I need some tips

 
Hello gsimson

I too am of the opinion that the brush used to paint your dark picture of soft starters is a bit too wide. Like Marke and GGOSS, I too have applied countless solid state soft starters on conveyors through the years. When properly applied they work fine, but the biggest caution is, NEVER let the lowest-price salesman do the applying! Incidentally, I have also had thousands of sucessfull applications on fans, cranes and especially crushers as well.

That said, your points about VFDs are nonetheless valid when taken on a case-by-case basis. I also have installed many VFDs to the same loads, even in fixed speed applications. For instance many conveyors are moving items that if accelerated too fast will sustain damage, i.e. glass bottles or stacks of appliances on pallets. If long slow consistant acceleration is the desired effect, by all means a VFD is the way to go. If a severely limited power source is at hand, that too can be a &quot;must have&quot; case for a VFD. Another great conveyor issue is the &quot;ruber band&quot; effect on long overland material belts. True linear acceleration, regardless of final speed, is the only answer and that is difficult to achieve with a soft starter.

As jbartos pointed out, the OP was questioning the application of VFDs when the final speed was 60Hz. In answer to that question, there may be valid reasons to use the VFD and if you are not sure, ask the engineer who designed it in. As you observed however, a soft starter is often a low cost viable alternative, and I cannot begin to count the number of applications I have seen where VFDs were used only because the engineer was unfamiliar with proper RVSS application. It never hurts to ask!



Quando Omni Flunkus Moritati

 
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