Soft startes vs VFD for intake gate

[ters]

I asked the same question on Power Engineering forum and was told that the matter has been discussed here several times. Cannot seem to find any recent discussion though. I’m trying to make a comparison between soft starters and variable frequency drives for a hoisting application – a 40 kW motor will be operating a vertical water gate through a worm gear driving two large screws. What would be major advantages/disadvantages of both options for the given application? I know that VFD is much more expensive, but there seems to be some controversy as which solution might be more advantageous re overcoming mechanical problems with overloading by jamming, friction on screws, gear resistance, etc, or if the gate encounters and obstacle (such as ice). Thank you for any advice.

 

[dpc]

Why do you think you need either? What are you trying to accomplish?

 

[ters]

That is a good question.... A Client of mine decided to change old gate drives which are just standard induction motors. They would like to use VFDs for the reasons I'm not so sure of, but then somebody else who already implemented VFDs for the very same application commented that soft starters would be a better choice not only because of much lower cost but also because of torque characteristics which are supposedly a better match for a worm gear driving two long screws (approx. 30 ft - 10 m). Now, the dilemma (and my task) is not why either is needed, but to figure out which one would be a better overall choice for this specific application (where braking issues while lowering are seem to be non-existent).

 

[dpc]

If you need a lot of starting torque or torque control (or braking), the VFD is the best option. The RVSS starter provides reduced (effective)voltage and reduced torque to limit voltage drop and mechanical stresses during starting. It's just an electronic version of a reduced voltage starter.

But it does seem like overkill for a gate hoist.

 

[LionelHutz]

The worm gear drive means that the load should not back-drive the motor. So, I'd think you will be mostly concerned with reducing the shock loads on the system when it's started. This can be accomplished with a soft-starter.

Good soft-starters can also be programmed with a over-current trip so that they'll detect and quickly trip off if the motor current goes too high.

Now, the VFD could be used at a slower speed for more precise positioning of the gate if that's a requirement.

 

[shreesathe]

I would say use ASDs. They will provide the comprehensive protection desired through out the operation.

Also wont the softstarters be bypassed into a run contactor to run the motor direct across line ?

 

[waross]

Are they changing the motors as well or just the motor drives?
If they are changeing just the controls, and the operating parameters have not changed, then the best choice may be new standard DOL starters. They will probably give another 40 years of dependable service.
If they are going to replace old "U" frame or pre-"U" frame NEMA motors with new "T" frame motors the installation should be re-engineered.
You may want to consider that "T" frame motors may be less robust than the original motors. The allowable locked rotor time is less and I understand that the breakaway torque is less. You may find that you need a VFD to squeese enough breakaway torque and provide enough protection for "T" frame motors with the same HP rating as the original motors. You may have to use higher horsepower motors.
respectfully

 

[ters]

Thank you all very much for helpful comments. Warros, no, motors will not be replace, just old starters. As per the supplied info, motors are old GE frame size 505, enclosure TEFC, 900 RPM, 42 kW, primary volts 600 V, secondary volts 129 V.

 

[CJCPE]

Does this application require reversing for opening and closing? When there is a jam does clearing the jam involve reversing and then attempting to drive through the jam again? When closing is the gate driven against the stops to make sure it is tight?

I think that a VFD would be a better option if any of the answers are yes.

 

[jraef]

While it is true we have discussed Soft Starts vs VFDs in the past, your application is a little different. I have done several weir gate controls at dams, here is my take on this.

You already have DOL starting, so that tells me you don't have a problem of needing to reduce current, you just want to reduce the mechanical wear and tear for starting torque shock on the worm gear drive system. Very valid reasoning. Both the VFD and the Soft Starter will perform this function.

From my experience, there is no need for speed control on a weir gate, the gearing is usually very low and the gates move very slowly so as to avoid binding against the water pressure as they close. The duty cycle is also very low, you typically don't change the gate position more than a few times per day, so motor heating is not going to be an issue.

For those reason, I would say that the Soft Starter is a better choice that the VFD. A VFD will not offer any additional benefits to warrant the added cost and lower reliability. A Soft Starter is simpler therefore less prone to failure and will accomplish your task. Use a heavy duty rated Voltage Ramp Soft Starter because they will allow up to locked rotor current at start up, for instance in iced conditions where you need to break free. By heavy duty, I mean one that allows up to 600% current for at least 10 seconds. Some are not rated for this kind of capability and rely upon Current Limit to protect themselves. In your application, Current Limit would pose a potential problem. An alternative to that is a Soft Starter with what is called a "Kick Start" option, but I don't really like those because it somewhat defeats the purpose of having the Soft Starter. At 40kW (assuming 380 - 600V nominal), you should be able to find a soft starter with a "fins out" heat sink design that could go into a sealed enclosure and not need a bypass contactor, although a bypass contactor is not a bad idea as a backup starter. If you want it for that reason, just make sure you specify to the supplier that you need Across-the-Line bypass capability.

JRaef.com
"Engineers like to solve problems. If there are no problems handily available, they will create their own problems." Scott Adams
For the best use of Eng-Tips, please click here -> faq731-376

 

[DickDV]

Fellas, am I seeing things or are these wound rotor motors? The nameplate given includes "secondary voltage=129V"

 

[ters]

jraef Thank you very much for a very useful and detailed reply. But the more I think about this the more it looks to me that waross amy be right saying that the best option may be just to replace existing starters with someting of similar design(assuming they are giving them a trouble… what I’m unsure of). I have a minimum information about the existing system and the purpose of this rehab, but as Dick DV noted, it seems that the existing gate motors are with wound rotors (otherwise what else “secondary voltage” on a motor may mean?) with likely external resistor employed. So if the rotors are wound, will it affect, and how, the application of either, VFDs or SSs?

 

[ters]

CJCPE the mechanical system consists of a motor driven gear (speed reducer) driving two horizontal shaft which in turn drive two worm gears which then drive two vertical long screws with passing through “nuts” attached to each end of the gate. So, yes, for different operations (opening or closing) the motor rotation direction has to be changed.

 

[itsmoked]

You haven't said what these gates are for... But a lot of gates need to absolutely positively work when needed or else,(picture breaching dams, floating houses). I would be very hesitant about changing to less reliable technology and, face it, a soft starter is less robust then a mechanical starter. If the application was not associated with life/safety like an irrigation system I would go for the Soft Starter. Of course all this is probably a mute point with the wound rotor.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[jraef]

You must have posted that motor nameplate as I was typing. With WR controls you don't really have DOL starting, you undoubtedly have either a drum switch or step controller and a set of resistors for accelerating the WR motor. Repairing or replacing the control system may be problematic as a lot of the OEMs are out of that business. You could have a completely new system fabricated, but it will likely cost as much as a soft starter.

It's easy to retrofit a soft starter or VFD to a WR motor. I would worry more about the VFD however, the motor windings are not going to be designed for inverter applications.

To retrofit a soft starter, all you need to do is leave one set of resistors in place for use during acceleration. Then have a shorting contactor for full speed run.

If the existing control system took advantage of the variable speed capability of the WR motor and you need to maintain that, then you would need to use the VFD. You can just short the rotor bars when using a VFD (don't do that with a soft starter), but make sure you buy a good output filter for that VFD because it will kill you motor winding insulation in short order.

JRaef.com
"Engineers like to solve problems. If there are no problems handily available, they will create their own problems." Scott Adams
For the best use of Eng-Tips, please click here -> faq731-376

 

[ters]

itsmoked these are water level control gates (4) on a river, which either supply a small hydro power plant or just let the water run down the river if the plant is not operating or there is an excessive inflow. Gates are operated infrequently, probably less then once per day, could be just once per week.

jraef thank you again for very informative and helpful advice. When I posted my initial question, I didn't know about the WR, but later while reviewing available documentation I found a test report indicating the motor nameplate info. I couldn't find any info as what kind of starter they presently have (will go on site to assess it), however, if original (likely), it is probably something close to 50 years old. Given the age, would it be advisable to replace motors as well?

 

[jraef]

WR motors are generally good motors. I'm generally of the "if it isn't broken, don't fix it camp" so if you are not having trouble with the motors themselves, I would try to continue using them and just update the controls. WR motors are not terribly efficient, but that is almost totally irrelevant in your application. Being as old as they are on the other hand, they have a lot more iron in them than anything new you can buy, meaning they are capable of taking more thermal abuse such as might be the case under adverse conditions.

JRaef.com
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[LionelHutz]

A wound rotor motor basically operates at different speeds by changing the slip via resistors. So, if you give it a high slip then the breakdown torque of the motor occurs near 0 speed. Change the resistors to a lower value and move the breakdown torque to a different speed. In this manner, you can have the maximum motor torque possible at any speed and you can also operate at any speed.

If the customer is changing the motors from wound rotor to standard induction then I would expect they will require a VFD to generate the required torque. A VFD can operate an induction motor at it's breakdown torque from 0 speed to full speed. Because of this, the induction motor and VFD are the most likely candidates capable of replacing a wound rotor motor.

A full-voltage or soft-starter operated motor relies on the motor speed-torque curve so you will find that the torque availabe to accelerate is usually much lower than the breakdown torque. This means that a wound rotor motor can generate a much higher torque over most of the speed range compared to a induction motor operated across the line or via a soft-starter.

Because of this, it is generally not recommended to directly replace a wound rotor motor with an induction motor unless you do some engineering to determine what torque is required and if the new motor and motor controller will meet this torque requirement.

 

[DickDV]

LionelHutz, just one comment on your otherwise excellent post.

An induction motor and VFD sized by ordinary conventional rules will not come anywhere near providing breakdown torque levels. The drive would need to be increased in ampacity considerably to provide the higher current even for a few seconds.

This is sometimes done to take advantage of the motor's breakdown torque capability but standard sizing rules won't get you there.

 

[ters]

LionelHutz thank you very much for reply.

Just had a meeting with the Client. Their verdict is: they would like to retain existing WR motors and use VFDs. The motors are very old, used to operate at 25 Hz long ago but at some point (30+yers ago) they were rewound to 60 Hz (600 V) and the stator core was probably left as it originally was. So they should have much more iron than needed.

Presently, the Client is using an old scheme employing a 3-speed automatic acceleration with the drum switch and 2 sets of external resistors but with an unusual connection - two gates share the same resistors and associated circuitry (timers, contactors, etc) so only one gate can be operated at the time, i.e., there are only 2 starters for 4 motors/gates.

So the idea is to replace exiting 2 starters with 4 VFDs. Presently, a constant torque VFDs are specified, rated at 50 HP (motors are 42 HP), whereas they say that the starting current will be limited to 150% of nominal.

Gents, given the potential problems with the starting VFD torque versus presently used scheme, would it be advisable to upsize proposed VFDs to something higher, say 65 HP or whatever the next standard rating might be?

The price doesn't seem to be a concern, as proposed VFDs (Schneider) appear to be rather relatively cheap for some reason (less than $10,000 each).