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VFD In-rush current 3

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Nat_Lortie

Electrical
Jan 8, 2021
4
I have a Chain belt conveyor that is controlled by a VFD.
It lacks the input current to get going, here is scenario.

Motor is 1800RPM FLA = 20Amps

Across the line it starts up fine and pulls 12Amps after start-up.
On the VFD it hmms but doesn't have sufficient In-rush current to start.
We actually use a 2*4 to help turn the pulley and the motor finally goes from 0 to 60HZ with our help.
Once it is turning, we can vary the speed (30-60hz) with no problem and the amps vary from 8 to 12 amps.

I was under the impression a Constant torque VFD would give me the necessary torque to get going.

Any suggestions or ideas?

Thanks,
 
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Just throwing this out there...the drive could be undersized. For example if you have a VFD sized for a motor in a variable torque application, it may not have enough "juice" to get going under a constant torque application. Manufacturers typically recommend upsizing to prevent problems.

Mike
 
Your drive is most likely improperly configured.
Check your current limiting, and if that is suitable, then check the drive manual for "Torque Boost"settings.
I was under the impression a Constant torque VFD would give me the necessary torque to get going.
Yes it will, unless the drive is undersized or the current limit or torque boost are improperly configured.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!
 
A chain drag conveyor has a lot of sticktion and needs a lot of torque to start.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!
 
The motor current is a 20FLA and the VFD current limit is 40Amps. I purchased 1 size up from the constant torque HP recommend.
In my scenario, the motor is a 20HP 575V motor.
I purchased a 30HP 575V constant torque VFD

I doubt my VFD is undersized... im just wondering if to start the motor might need something like 400%FLA to GO.. or something like that if that is the case I may need to buy a 50HP vfd?
 
400% current on an Across the Line start is not comparable to what happens in a VFD. It's all about TORQUE, not current. When starting ATL, the 400% current is NOT all developing torque in the motor, most of it is just reactive current because the motor Power Factor is extremely low until the motor starts to rotate. So even if you have the full 600% Locked Rotor Current, that only translates to about 150-160% of the motor Full Load Torque (Locked Rotor Torque). With a VFD, because the frequency at voltage are controlled together, you can get full torque at any moment from a dead stop, and in the case of a HD (CT) rated drive, you should be able to get 150% torque from the motor for about 60 seconds, and 200% torque for around 3 seconds. So that means the VFD is actually capable of accelerating the motor FASTER than an ATL starter can.

So back to what others have said, this is more likely a programming problem; someone has hobbled the VFD from being able to provide you the peak performance it is capable of delivering. So here are my suggestions:

[ol 1]
[li]Is this a Sensorless Vector Control drive? If so, is that feature enabled? Some SVC drive are defaulted to be in SVC mode, some are set to V/H as the factory default. You cannot assume. If the VFD is not capable of SVC, consider replacing it with one that is, this is the type of application that needs that.
[/li]
[li]If it is SVC and that is enabled, did you perform an "Autotune" (sometimes called "Motor ID") procedure? This is absolutely necessary, even if you just use the basic "static" tuning method (no rotation of the motor). For SVC to function, the VFD must know the motor equivalent circuit parameters and it can learn that from testing. Without that information, it is not capable of getting peak performance from the motor.
[/li]
[li]Has someone programmed a "stall prevention" feature that might be interfering with starting a high inertia / high stiction load? If so, turn that off to let the VFD do it's maximum output.[/li]
[/ol]


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden
 
Go with jraef's suggestions.
jraef has forgotten more about VFDs than many of us know, and he has not forgotten much.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!
 
Thanks Jraef,

I have an ATV930D22S6 drive.
So its actually a 25HP VFD. (I wrongfully said 30HP earlier)
The motor name plate FLA is 19.3Amps
Current limitation is at 40.8(max of vfd)
Speed loop type is 'standard'
My torque ratio is set a 100%
Toque offset 0%
Torque control time out 60s
High torque 300%

I've lived something similar 10 years or so ago on a trommel screen.. I ended up oversizing the VFD from lack of know how.. I hope to figure it out with some help here to avoid a repeat.

I've used VFD to control speeds on conveyors for years, but my knowledge on them is limited.
My applications are usually quite simple and only require 30 to 90hz speed variation.. so thanks for all the tips.
Nat
 
OK, so that does appear to be a 25HP HD rated drive, and it is capable of SVC. So you have the right product to do the job, ergo it must be something in the setup that is getting in your way. I don't have a manual for that drive (and I'm not paid to care about their products...), but have you engaged Schneider Tech Support to help you with getting the drive set up to maximize the starting torque capability? That would be my best suggestion. Again, the drive IS capable of doing what you need, so it's a programming issue for your application.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden
 
Field Oriented Control will give you maximum starting torque while still allowing a gradual ramp up. You'll need to add a shaft encoder to your motor but your drive should otherwise be fully capable.
 
Why don’t you enquire with the VSD OEM? - do they have a support Engineer who can go to site and check config? He will likely know within an hour what the problem is and can test output
As others have suggested, it’s likely a settings issue in config but best sorted actually stood in front of the system

I.Eng MIMMM MIET MIPowerE AIOSH
 
Actually wattyeng
Original Poster said:
Nat_Lortie (Electrical)
(OP)
8 Jan 21 21:20
I left my rep a voicemail.
Thanks,

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!
 
Is that because of improvements in senseless control schemes?
 
Yes correct. Adding a shaft encoder would improve the ACCURACY of the torque and add the ability to attain 100% torque at zero speed, but Sensorless Vector Control is good for something like this. Some high-end drives (I don't know about this one) also now have "Encoderless FOC" available as well if you do need the accuracy but can't use an encoder.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden
 
I only have experience with the PowerFlex 700S and 753 models. There is quite a degradation in low speed performance in sensorless mode versus full vector control. These are in winch applications. I'm not stating anything is fact here. I only troubleshoot VFDs I have never built one.
 
Winches/hoists are one of those apps that absolutely need encoders to do a good job.

For FOC the controller needs to read the voltage on the instantaneously undriven phase. The problem is that for the system to work at high speeds the voltage being read back is high but at low speed it's very low and the measurement front-ends don't have the dynamic range required to measure the high and very low voltages. The end result is that the low readings become buried in the noise floor and that's why trying to control at the low end sensorlessly goes unstable. The controller sees a noise spike and controls to it.

Keith Cress
kcress -
 
Yes, for winches and hoists you absolutely need the encoder feedback (in my humble opinion). Although others think that Encoderless FOC is OK for that, my challenge to their beliefs is to stand under a test weight on a winch or hoist during a test run because when the mechanical brake is released, you are trusting your life to the current sensors and algorithm that makes the drive create full holding torque at zero speed. I would never do it of course, but it makes salespeople rethink their convictions...


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden
 
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