Running a VFD controlled motor with lower than rated Voltage setting

[KallH]

Hello I have a question about a VFD controlled motor.

I have an AC motor controlling a relatively constant torque load that has a rated torque far above the requirements for the system. Once in a while the mechanical system will jam causing physical damage to the system before any over protection is able to kick in. I would like it to be able to stall and trip before this happens. So for my question, am I able to simply reduce the motor rated voltage setting in the drive to below the rated nameplate to reduce output torque or will I also need to adjust the frequency to keep the same V/Hz profile?

If I do change these settings and the motor stalls, will there be any implications for the overload protection of the drive assuming I set the rated full load amps to the value on the motor nameplate?

It is using a fairly inexpensive drive, GS2 from automation direct which is fairly limited in overload protection settings from other VFDs I have used.

 

[controlsdude]

There is a parameter on most drives that will stop based on a current setting. Its called by a few names but here are two drives I know what its called.
Shear pin or software current trip, this setting is set to just above steady state run current to trip when you see a current level slightly above running current. This is usually no cost on your side except changing parameters on the vfd.

Otherwise, you would get something like a torque sensor on the shaft to do the same thing and use that to trip the drive, but I am no expert on that setup.

Allen Bradley Powerflex 525
Example from 525 2 HP motor
Motor OL current
Parameter 33 = 1.9
Motor NP FLA = 2.8
a486 Shear Pin1 Level = 1.7 which = para A098 above

Allen Bradley Powerflex 40

control method v/hz
parameter 125 torque perf mode = v/hz
slip compensation
parameter 114 set to 0 hz
SW Currrent Trip
Parameter A098

 

[itsmoked]

Try the over-torque settings. The torque is settable in 1% increments which should be fine enough for your application.
It should also be possible to mess with the OT settings without harming your machine. Drop the limit until something happens with the machine running. When you find the tripping setting raise it a bit.

P6.07
Over-Torque Detection Mode
Default Setting: 00
Settings: 00 Disabled
01 Enabled during constant speed operation
02 Enabled during acceleration

P6.08
Over-Torque Detection Level
Range: 30 to 200% Default Setting: 150
• A setting of 100% is proportional to the Rated Output Current of the drive.
• This parameter sets the Over-Torque Detection level in 1% increments. (The AC
drive rated current is equal to 100%.)

P6.09
Over-Torque Detection Time
Range: 0.1 to 10.0 Default Setting: 0.1
This parameter sets the Over-Torque Detection Time in units of 0.1 seconds

Keith Cress
kcress -

http://www.flaminsystems.com

 

[KallH]

Thanks itsmoked.

Yes I am using this but it only allows over torque in either acceleration or constant speed, not both. I have it set to constant speed and checked it worked fine, if they happen to actuate the motor as it snags during acceleration I believe this is when it does the damage.

I want to avoid moving to a different drive with better overload protection if possible and reducing output torque of the motor with what I have is my preference.

 

[jraef]

Use P6.10 and P6.11, Over Current Stall Prevention (in Accel and Constant Speed, respectively). Those will reduce the output automatically rather than trip the drive off. I think that may be more of what you want.

In a VFD, Current and Torque are basically synonymous.


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

 

[flexoprinting]

Hello Jeff

Use P6.10 and P6.11, Over Current Stall Prevention (in Accel and Constant Speed, respectively). Those will reduce the output automatically rather than trip the drive off. I think that may be more of what you want.

But Jeff is doing this something that when the machine jams it just stops while drive remains energized casing an unsafe condition for operators?

Chuck

 

[itsmoked]

Jeff: Stall protection can be used to reduce current?? I always thought that was for increasing it (stall prevention).

Chuck; The intent is to prevent breaking the machine - not operator protection. I would expect other means to do that, light curtains, barriers, doors, etc.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

I think that you are confusing torque limiting it with the boost function, Keith.
Starting torque may be 150% of full load torque.
As the motor accelerates the torque drops to the pull up torque which may be 120% of full load torque.
As the motor continues to accelerate the torque then increases and rises to the breakdown torque which may be 200% of full load torque.
A VFD boost function may allow a motor to develop breakdown torque or 200% of full load torque over the entire starting speed range, starting from zero RPM up to rated RPM, to allow difficult loads to be started.
Torque limit may be used to reduce the maximum torque to avoid breaking machinery.

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

 

[itsmoked]

Bill; Aren't you making my point? Jeff is suggesting torque boost while the OP is trying to prevent excessive torque from breaking the machine for the umpteenth time. I'm wondering how torque "boosting" helps not breaking the machine faster yet.

However if the torques boost parameter actually allows a reduction of normal torque I guess it would work.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

Aw, what's in a word. A rose by any other name will stick you with its thorns.
I think we are getting our words confused.
Jeff may have slipped the wrong word in.
I hope that he does an edit, and then we can lose our posts.

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

 

[Bill West]

I took the OP's problem as being that as the motor is oversized, any product or mechanism jam is being overpowered by the motor and thus damages the mechanism. If he can get the VFD to limit the maximum torque and just stall for a jam then he will avoid suffering damage for these incidents. Chuck's point is that if a worker walks up and sees the mechanism at a standstill he may think it has stopped and feel free to reach in and remove the jam, the problem then being that the system will instantly resume moving and possibly hurt him. The torque limiting in the VFD needs to actually push the stop button as the OP suggested. The sensing of torque would presumably be by current measurement so either a torque or current setting in the VFD will do the job, it's only a matter of exactly what adjustments and actions the maker has offered in his program settings.

For the OP's concerns over motor protection, limiting torque to figures below maximum inherently means limiting current, right down to stall. I think that eliminates any possibility of the stall exceeding the motor current limits. Setting the torque limit to cause a shutdown also stops overcurrents.

Bill

 

[itsmoked]

Well summed up Bill W.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[LionelHutz]

Can you set the drive up to change a relay or DO when the current reaches a certain level and then feed that output back to a digital input set to trip the drive?

 

[Compositepro]

The first question to answer: is the problem due to mainly motor torque or motor inertia. If the inertia alone is enough to break things then a torque limit will not fix things. If that is the case, then a clutch or spring is required to fix the problem.

"the mechanical system will jam causing physical damage to the system before any over protection is able to kick in." This implies that electronic torque limiting is not working.

 

[jraef]

Don't go by the name the manual assigns to things, that can be misleading (especially through translation). I go by the described FUNCTIONALITY.

P6.10:
Over-Current Stall Prevention During Acceleration
Range: 20 to 200% Default Setting: 150
A setting of 100% is equal to the Rated Output Current of the drive.
• Under certain conditions, the AC drive output current may increase abruptly,
and exceed the value specified by P6.10 This is commonly caused by rapid
acceleration or excessive load on the motor. When this function is enabled, the
AC drive will stop accelerating and maintain a constant output frequency. The
AC drive will only resume acceleration when the current drops below the
maximum value.

P6.11
Over-Current Stall Prevention During Operation
Range: 20 to 200% Default Setting: 150
• During steady-state operation with motor load rapidly increasing, the AC drive
output current may exceed the limit specified in P6.11. When this occurs, the
output frequency will decrease to maintain a constant motor speed. The drive
will accelerate to the steady-state output frequency only when the output
current drops below the level specified by P6.11.

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

 

[itsmoked]

Jeff; You sly dog, reading the descriptions instead of the headings! Nice trick. I'll do that next time.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[KallH]

Thanks all for your input. Just a few notes.

There is little risk to personal safety if the machine jams and the functions of P6.10 and P6.11 prevent movement. The machine is guarded and controlled by a remote pendant away from the machine so the operator would need to stop pressing the button in any case to figure out why it stopped moving. I want to program an automation feature in the PLC but I can do this if the machine will break itself. At least with an operator they can catch the event and stop before it breaks.

I did consider these functions but I wasn't sure if it would allow for an output to tell the PLC that it had stalled. At the moment It is doing exactly as Lionel suggested, using a DO from the drive to stop the system in case of a trip however this is only available in accel or constant speed not both. If I can use the stall prevention and get it to set a DO when it does then I can use it. Which is why I just wanted to control the actual torque output of the drive. I was thinking I may just be able to set a lower rated voltage in the drive settings and that would do the trick?

Composite, the load is no inertial, it stops pretty abruptly on it own on free run. The motor moves a linear boom via a chain and sprocket and moves quite slow as well. Ideally I would like to prevent the cause of the mechanical jams altogether but unfortunately, at least without a great deal of pain, I cant do much about the mechanical system. So I need to solve it with other means. It does not help that the motor/gearbox output torque far exceeds that required to break itself when it does.

I am happy to change VFDs (much to distress of my back pocket) as I just need the machine working as it is much more costly replacing parts all the time. The GS2s do their job but they are cheap drives.

 

[KallH]

Ok I think I have found a potential solution?

There is a DO I can use that will tell the PLC when the motor increases passed a preset current value (in P3.17) up to the rated current of the motor. I can essentially use to set my maximum torque, if the motor slips and increases current due to the jam then the output will tell the control system to stop the drive. I can use this in conjunction with the stall prevention as a backup possibly.

The question is will this act fast enough to prevent physical damage to the system?

 

[itsmoked]

Indeed. CompositePro has made a good point of that.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Compositepro]

KallH, you seem to have missed my point. I was referring mainly to motor inertia. The motor keeps coasting after torque drive torque is removed. You say " it stops pretty abruptly on it own on free run". But then you ask "will this act fast enough to prevent physical damage to the system?"