Soft-start or Variable Speed Drive? 2

[sry110]

I am a mechanical guy in need of electrical expertise.
I have a motor-driven gear system that has a considerable about of backlash to be taken up before the motor+gear sees the load. When it sees the load, there is a large impact that can damage mechanical components in our system. The impact is due to the motor (3-phase, 1800 RPM) hitting the stationary load at full speed.

I need a way to softly and/or slowly run the motor+gear up against the load to reduce the impact effect. Keep in mind that the motor+gear is completely unloaded (except for the weights and inertias of their own components which is negligible) from the moment it is energized until the driven load appears. Also, once the load is engaged and we are past that point of impact, I need the motor to produce its full locked rotor torque (typically ~300% of Full Load Torque for the motors we use) in order to start driving the load.

Some specific questions:
1) Will a basic soft starter keep the current reduced when there is no load on the motor, such that when our unloaded motor suddenly encounters the load the electrical torque will still be reduced to the set value? For example, let's say I set the soft starter to ramp from 20% to 100% current over 8 seconds (10% per second), and let's say the motor will run unloaded for 1 second until it hits the driven load. Will the motor be at 20% current (and correspondingly reduced torque) when the load suddenly appears, or will the soft starter have already decided the motor is at full speed / no load and therefore bypassed the soft start function altogether?

2) How much torque will a typical VSD allow the motor to produce? I could use a VSD to slowly run the unit up against the driven load, but once the load is engaged I need the motor to produce high starting torque to get it moving. Does the VSD allow the motor to generate 150% torque, 200% torque, higher, lower? I would typically count on ~300% locked rotor torque to get the load moving, but if the VSD only allows %150-200 then I would need to increase the motor size accordingly.

Thanks in advance for any insight.

 

[ScottyUK]

I think you're mistaken iop.

The motor would stall when the clutch engages, but it will maintain torque to keep the coupling engaged. It just won't be able to provide sufficient torque to break the load away. The motor slip will be 100% and it won't be able to sustain this indefinitely because of rotor heating and lack of cooling, but it would handle it for the few seconds to bring the clutch into engagement.

 

[iop995]

If I understood corectly, SSS is a synchronous clutch, that mean it engage when drive shaft speed is equal with driven (load) shaft speed. Load is not at zero speed, so to engage need that drive shaft to reach load speed, so motor speed can't be zero when engage. Big problem is that motor and SSS will "see" a step torque when clutch engage and may be bad effects in both of them (mechanicaly mainly). If step torque value need to be below a limit, that need to close slowly motor speed to load speed and after full engaging, increase load speed at desired value by proper motor torque control. If step torque is allowed, both above mentioned methods by warros and ScottyUK may be used.

 

[mikekilroy]

I still think there is benefit to a vfd alone without the other equipment. It gives almost infinite choices on start/run methods. An S curve accel ramp even allows really slow engagement of sss then faster accel to speed. Since there is no accel ramp limit, using 10 seconds isn't an issue, even 20 sec is prob ok starting up such a big gadget. It has been shown around a 6-8 sec accel ramp will cause engagement around 300rpm even without the S curve ramp.

Look at its benefits over the softstart or resistors then relay DOL:

- similar hardware cost for much more capability
- reduced torque starting by adjusting the ramp to slow enough speed - easily adjustable
- no extra parts required (KISS)
- REDUCE YOUR TORQUE REQUIREMENT BY 39% or MORE

I think OP must understand very clearly what torque a vfd can produce compared to DOL: THE SAME. This is critical to understanding how the vfd can solve this. If one thinks the vfd cannot produce the SAME EXACT maximum torque as the DOL approach, then the resulting decision will be tainted. With about 250% of nameplate current, the vfd will produce the 225% or so of max motor torque that requires 600% current in DOL. Actually the vfd will produce that 225% torque at the 300rpm while I believe DOL will still be down at 150% at such a low speed (I am sure others will correct me if I am wrong on this statement).

OP calculates the max breakaway torque required, so KNOWS what is required max. In this case he uses a 20:1 worm gearbox in order to get maximum DOL torque from a 5hp motor at DOL speed of 1800rpm. But using the VFD he can run to 2500rpm or more with the same motor and now use a 30:1 worm gearbox. He effectively increased his max available torque again by 40-50% or more. This allows him to reduce the size of the motor or vfd to pay for the cost of the vfd. Or just add the vfd with its 200% max output capability and effectively get around 300% capability withi the higher gear ratio.

Until OP applies soft torque (no 1800rpm crash) to engaged SSS via a vfd or torque wrench, he will not know empirically how good his breakaway calculations really are and how much the stored rotating kenetic energy in his motor rotor really effects the equation. This vfd method allows him to gather that empirical data once and for all. With a good vfd's reporting capability on speeds, amps, torque, it can tell once and for all if a vfd is even required on future jobs or if a reduced voltage scheme is applicable.

I think the main benefits of the vfd over all other methods is allowing OP to run to 2500 or higher rpm on same motor and get more low end torque out of the system for SSS engagement, adjust speed of engagement, and adjust the torque available at that engagement.

http://www.KilroyWasHere<dot>com

 

[LionelHutz]

Something that should have been asked and could be very important in picking a solution - What kind of redundancy is required in case this new solution fails?

A smaller VFD with the controls and extra contactors to bypass it once the drive is engaged will cost more money and be more complex then just picking the correct size of VFD.

Resistors could work and might be cheaper, but there is no speed control of the motor using resistors. Use too much resistance and it may not accelerate while using too little resistance means it may still reach a higher speed and "bang" the clutch into engagement.

I don't believe worrying about the losses of a VFD is worthwhile. Say the motor takes 1000W input to keep turning (around 1hp on the shaft) and the VFD losses are absolutely terrible at 50% of the output power. That is 500W per hour - 0.5kW x 24h x 365d * $0.2/kWh = $876 to run continuously for a year. Then, I would hope the load is actually running greatly reducing this number. So, is it worth worrying about or not?

iop995 - you should re-read some of the posts. Load is at 0 speed.

 

[waross]

VFDs: Lots of features and advantages and possibly a few maybes.
Resistors: Cheap and simple.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[iop995]

If use SSS to start-up load from rest only, is somewhat strange. In this case, to keep clutch engaged add a small DC motor on same shaft and put DOL IM motor after power-up DC motor. May be a solution even it's more complicate (mechanicaly) than a good programmed VFD.

 

[sry110]

@ iop995: Our turning gear system is used both to start-from-rest (breakaway), as well as to catch on the fly during coast down.

 

[sry110]

@ mikekilroy: Thanks for the additional thoughts. I understand your concept of over-speeding the motor and increasing the gear ratio accordingly, but our gearbox manufacturer limits us to 1800 RPM maximum input speed (due to the type of seals used on the input shaft). Sorry for the piece-mealed info, but again this was something I didn't mention previously because I did not think it would be relevant.

 

[mikekilroy]

fair enough sry110 on your PRESENT gearbox mfgr. I understand you have a desire to stick with your PRESENT worm gearbox at its much lower efficiency than a spur gear box with same ratio, same price, and 97% efficiency. Sometimes the expertise of the supplier or supplier's sales engineer is well worth sticking with such a certain product at other expenses. Lionelhutz clearly stated that efficiency may not be a decision factor; indeed, you never answered HOW LONG this gidget runs once upto speed so we cannot even guess at efficiency costs.

Just so you realize there are other equally as good or better worm or spur boxes that CAN go 2500 or 3600rpm input speeds all day long; then you've made a proper value judgement call.

So in lieu of higher speed, I would go with the recommendation to THIS TIME go with a 2-3x oversized vfd to be COVERED and gather real life data from it to see how to maximize your company's profits on future jobs. If you do go this route, I hope you pick a vfd capable of recording real time current & speed and downloadable to a computer for analysis.

Glad you came here and posted; it has been a fun, & hopefully, educational, thread!

http://www.KilroyWasHere<dot>com

 

[ScottyUK]

Remember though - these things are typically sold to very conservative {small 'c') customers who just might not embrace a VFD on this equipment.

Power gen and oil & gas users tend to value reliability above pretty much anything else, and in my experience VFDs don't last as long as DOL starters and definitely are not as easy for a maintenance tech to repair, especially when that repair is probably needed at night, when all the suppliers are closed and when there's little or no engineering support. Sometimes the question is not whether the solution is technically sound, but whether the customer will accept the solution.

 

[jraef]

To Scotty's point, I have experienced similar attitudes toward VFDs in the power gen industry (which to me carries a certain amount of irony), but mainly on LARGE systems. I'm working on a project at a gas fired plant right now on retrofitting the 6 x 2500HP cooling water pumps with VFDs, but only because recent environmental regulatory changes are forcing them to curtail their water use. After 50 years of operating with full flow and bypass-to-waste of what they don't need, they are having to actually LOOK at matching their flow to their needs. The concept of doing this with VFDs is abhorrent to most of the people holding the purse strings, it's been very frustrating. Energy cost is nothing to them, but VFDs are big bad voodoo boxes. They want to do it with valves, but they would have to shut down completely for up to 6 months to install them because the way the piping was originally configured (having never thought this would be an issue). Somehow, that is still preferable to them compared to the voodoo boxes (which could be retrofitted one at a time) at this point.

But on this application, don't forget this is a 5HP motor. Carrying an identical VFD with identical programming as a spare, even if it is a 15HP VFD, would be relatively cheap redundancy if you ask me. I apply a lot of drives in that industry at the small end of the spectrum, even for critical equipment. There are also drives that can separate the controls from the power module, so swapping out a failed power section could involve zero programming and be done in 5 minutes or less.

Now getting into the fine details: Can a VFD be had with a built-in bypass and a timer so that the VFD and DOL starter could be wired in series? I'm picturing the VFD is set to run the motor at 300 RPM for 1 second. When 1 second passes the VFD times out and the bypass contactor closes allowing full line voltage to go to the DOL starter. Then the DOL start contact closes and sends full line voltage to the motor. Right/wrong?

To this issue, "bypassing" a VFD is not as simple as just closing a contactor around it when you no longer need it, like on a soft starter. A VFD must have it's output section completely isolated from line power. So you end up with an open transition and that open transition may result in the load slowing briefly, which may have very negative consequences for the clutch and motor. There is a concept called "Synchronous Transfer", but that is not something you would do on a 5HP motor drive, it would cost more than another drive, maybe two or three more drives... But as I said earlier and Mike has said again, there is no need to do that. A good quality VFD with Sensorless Vector Control can make the motor deliver its maximum rated torque at any point, on demand. So just engage the gears slowly, then tell the drive to punch it. Done.

"Will work for (the memory of) salami"