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]

Better get ready to rewind or replace the motors before too long. VFDs tend to put additional voltage stress on the motor windings. For new installations, "inverter-duty" motors are specified that have better insulation on the windings.

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.

 

[jraef]

I agree with dpc, as I stated early on. This is particularly true as the motor voltqge goes up, 600V being the worst case scenario in the low voltage world. Even though rewound in the '70s, the insulation used was most likely no more than 1500V rated. When you use a VFD, a phenomenon called "standing wave generation" (aka Reflected Wave) can occur for a number of reasons related to the high speed switching of the transistors used to created the PWM output. Because of these standing waves, voltaqe transients repeatedly seen at the first turn in the motor windings can exceed 3 to 5 times the line voltage. So in a 600V system the voltage transients can be 1800V or even as high as 3000V, either one well above even the most common insulation available in 1976. Filtering will help, but no not scrimp on the filter technology, you will need the best available. I would contact MTE and TCI with this project and get their engineers involved in quoting something to help allow those motor windings to survive.

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]

dcp and jraef thank you both very much for providing further useful advises.

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?

 

[dpc]

Oversizing the VFD will not make much difference as far as the voltage transients and winding insulation failure. Larger VFDs have higher current output ratings.

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.

 

[jraef]

Correct. Oversizing a VFD rarely poses a problem other than budgetary. In this case it may give you the extra umph you need some day. Standard sizes will be 60HP and 75HP, probably not much cost difference between them. Just be sure that at commissioning, you set the OL protection feature of the VFD for the actual motor nameplate FLA. That should not interfere with the over torque capabilities of the VFD.

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