Soft startes vs VFD for intake gate
Soft startes vs VFD for intake gate
(OP)
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.





RE: Soft startes vs VFD for intake gate
RE: Soft startes vs VFD for intake gate
RE: Soft startes vs VFD for intake gate
But it does seem like overkill for a gate hoist.
RE: Soft startes vs VFD for intake gate
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.
RE: Soft startes vs VFD for intake gate
Also wont the softstarters be bypassed into a run contactor to run the motor direct across line ?
RE: Soft startes vs VFD for intake gate
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
RE: Soft startes vs VFD for intake gate
RE: Soft startes vs VFD for intake gate
I think that a VFD would be a better option if any of the answers are yes.
RE: Soft startes vs VFD for intake gate
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
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RE: Soft startes vs VFD for intake gate
RE: Soft startes vs VFD for intake gate
RE: Soft startes vs VFD for intake gate
RE: Soft startes vs VFD for intake gate
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Soft startes vs VFD for intake gate
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
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RE: Soft startes vs VFD for intake gate
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?
RE: Soft startes vs VFD for intake gate
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
RE: Soft startes vs VFD for intake gate
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.
RE: Soft startes vs VFD for intake gate
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.
RE: Soft startes vs VFD for intake gate
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).
RE: Soft startes vs VFD for intake gate
Also, you need to be concerned about the distance from the VFD to the motor. You may want to add some type of filtering on the VFD output if you have long runs. Check with VFD supplier.
The motors may last another fifty years on the VFD, especially if they don't run much. But the odds are that the motor windings will fail sooner on the VFDs than they would running across the line.
RE: Soft startes vs VFD for intake gate
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
RE: Soft startes vs VFD for intake gate
So I now understand that the problem with transient voltages may be severe, but could anyone comment whether an oversized VFD would make the situation better or worse. I was thinking of oversizing it as the breakaway starting torque for this application may sometimes happen to be high (winter low temperatures, high frication in the gear, increased water pressure (reservoir full), ice, debris, etc, but, on the other side will such move (VFD upsizing) make it even worse in term of various transient phenomena?
RE: Soft startes vs VFD for intake gate
But the voltage level and PWM waveforms will be the same on the larger drive, since this is determined based on the motor's rated voltage.
I agree with jraef that good filtering could make a big difference.
RE: Soft startes vs VFD for intake gate
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