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Motor/VFD Question

Motor/VFD Question

(OP)
Greetings all,

I would like some opinions/suggestions on an issue being experienced with a VFD and motor.

Motor Specs:
Volts 460
F.L. Amps 239
S. F Amps 275
RPM 1200
Hertz 60
HP 200
Duty CONTINUOUS
TYPE TD
Frame 447T
Serv. F. 1.15
Phase 3
Design B
Code G
Insul Class F
Eff 100% 96.2
Connection 6 Lead Star-Delta
Link http://www.leeson.com/CnxDocRequest/PublishedPDF/d...

Drive Specs:
Product: Allen Bradley 23C-D460A103NNMANN
Description: PowerFlex 400, Fan & Pump Packaged Drive

Version Voltage Code 480 VAC, 3 PH
Output Current 460 Amps
Human Interface Module Fixed Keypad
Enclosure IP20 / Type 1
Version RS485
Frame Size H

Setup:
Motor is driving a P type fan via direct coupling
Motor connection = DELTA (connection has been verified twice)
Drive tuned and configured for 200HP motor, all settings default with exception to motor nameplate data, voltage input and auto-tune data.
Motor Data = (matches above info)
Voltage Input = 480 Volt
Auto-tune = calculated by VFD based on above info
Motor Control Mode = V/Hz
Application Mode = None, no PID inputs, being controlled via HMI

ISSUE: (Note that I am not performing the install/setup, I got involved because someone asked me to look into this).

The motor will not come up to full speed before O/C limit is reached. With frequency set-point at 60Hz: as the motor ramps up to speed (ramp time = 30 seconds), at +18HZ an over current alarm is indicated on HMI. Longer ramp times have no effect. The VFD will ramp up to ~18Hz and go into 'current trim mode' as this is highest frequency achievable before current output peaks at ~275 amps(SF amps). Adjusting the frequency beyond 18 Hz triggers O/C alarm.

Motor manufacturer recommends leaving the motor connected in delta and starting ATL to determine if this is a drive or motor issue. This was thought I had as well but Size 5 starters are not laying around like they used to be 20 years ago so, for the time being, that option is off the table. My thoughts are centered on drive parameter settings but the settings are as basic as they need to be for nominal operation. Any thoughts?

RE: Motor/VFD Question

Doesn't look like you did anything wrong. It's a single voltage motor according to that drawing (assuming that's the correct drawing for that motor), so delta is the only connection choice you have.

Did you megger the motor and leads? Note that you must disconnect the leads from the VFD first.

What's the distance from drive to motor? Could (remote possibility) be cable capacitance charging current is exceeding the capacity of the transistors.

If anything were wrong with the drive itself, you would be getting a Fault immediately. It will not Current Limit into a bad transistor.


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

RE: Motor/VFD Question

Too low torque.
Try to correct V/Hz curve at low frequency (0-15Hz) by increasing this ratio to 8.5-9.

RE: Motor/VFD Question

One of your windings may be reversed.

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

RE: Motor/VFD Question

If the delta is externally linked, you can temporarily connect the winding in wye and see if the motor speed picks up without any current issues.

Muthu
www.edison.co.in

RE: Motor/VFD Question

(OP)
Thanks all for responding, all good advice.

Answers / clarifications to your responses:

@jraef: Connection diagram is correct. Yes, motor was not only checked but replaced (had new spare on hand). Distance between drive and motor = ~250', line and load reactors installed within 6' of the drive. VFD was replaced (with new one) after new motor was installed and did not remedy the problem.

@iop95: I am looking into this. The V/Hz curve for a default Fan/Pump setup (current configuration, none adjustable parameters) is drastically different from the "default custom" (adjustable via parameters) curve. You may be on to something.

@waross: Do you mean a flipped coil or external connect? Motor(s) are new, not rewound. External connections have been confirmed to match nameplate and attached connection diagram.

@edison123: If the motor, while connected STAR, was to run at base speed drawing nominal current, what would that suggest as the problem?

RE: Motor/VFD Question

Well I once saw a motor with a bad bearing on the brake. The race had failed and the bearing balls bunched up and jammed the shaft.
You could see the problem by looking at the bearing closely.
The millwrights went around in circles for hours trying to release the brake. Direct current to the control solenoid, remove the solenoid and put air pressure directly to the brake.
For hours, the answer to the suggestion that the bearing had failed was:
"There can't be anything wrong with the bearing. It's brand new."
I would connect a 12 Volt battery to T1 and T2 and go around the end coils with a compass.
Then connect the battery to T2 and T3 and repeat.
Then connect the battery to T3 and T1 and repeat.
You should see a repeating pattern of magnetic polarity.
But then why waste the time? It's brand new.
Another option is to disconnect the fan and run the motor unloaded. See if it comes up to speed with balanced fairly low currents.
The fan may have a mechanical problem or it may be intended for a belt drive.

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

RE: Motor/VFD Question

About a year ago, I was called by a ship building company that had two brand new motors that were tripping on overcurrent during starting. I found the delta leads were not properly connected. Instead of A1B2, B1C2 and C1A2, both motors had A1A2, B1C2 and C1B2. A proper re-connection on-board solved the problem in 15 minutes. The shipbuilder was fuming over the german OEM's mistake. Something for you to think about.

If you connect in wye (for test purposes), then better to decouple the load since the torque is not going to be great.

Muthu
www.edison.co.in

RE: Motor/VFD Question

(OP)
@waross: Bro, I would NEVER consider someones advice as a waste of time while trying to resolve a problem bigears. Regardless of the motors condition (new vs rewound), I have all ready suggested that it be uncoupled and ran. That was done, results; motor ran up to base speed, current about 25% of name plate (or at least that whats I am being told). In regard to the power transmission, I agree with your thinking that it may be undersized for direct coupling. There are setups currently on-site that are utilizing 200HP motors to drive fans that are half the size AND at higher speeds (1800RPM motor driving fan at ~2000RPM). I mentioned this while on-site and got the 'deer in the head lights' stare. I then recommend that the fan manufacturer review and verify the HP requirements; no feed back yet. If I provide specs on the fan, could you approximate the needed HP?

@edison123: The motor will run on DELTA when uncoupled from the load. We considered running it on STAR while coupled to load but our concerns then focused on HP/Torque. This being a fan (load increasing with RPM) I don't think it would come up to speed.

I am going to suggest that the low end torque parameters be adjusted per iop95's post. If that fails, I will follow up with reconnecting the motor in STAR and attempt to run under load. I will advise on the results.

RE: Motor/VFD Question

If you can run it up to speed, unloaded, in delta, make sure that the currents are well balanced.
I suggest that you post what information you can about the fan here.
Manufacturer and model number may allow us to locate specs for the fan.
I would also suggest posting in the "HVAC/R engineering" forum under a heading such as "I need help checking fan sizing."
Them instead of posting the info, post a request that the fan gurus look in on this forum. I believe that you can post a link to this thread.
Sorry about the attitude, it was uncalled for.
My rancor was not directed at you, but at those who will not follow trouble shooting steps because of an assumption.
Not always but too many times, the assumption has turned out to be false.

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

RE: Motor/VFD Question

First, set the VFD panel to display current or load.

Manually start the motor with the VFD and then MANUALLY, slowly, over a minute or two speed up the motor with the VFD panel. If the fan is too big you'll see it probably loading-up or rather overloading somewhere before running speed. Because the fan load goes up non-linearly it will be easy to see the problem if it's fan/load.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Motor/VFD Question

Another assumption that we are all making is that the voltage is correct. Has anyone verified that the VFD is getting a solid 480 Volts?

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

RE: Motor/VFD Question

Something that occurred to me was what you meant by "P type fan", so I googled it. Is this a variable pitch vaneaxial fan then? If so, are you starting it with the vanes turned in to where there is no flow, or turned all the way out for maximum flow? If turned out, the motor may be unable to accelerate it, because in that type of fan the load may be excessive from the outset. You may need to accelerate it with the vanes closed until you have overcome the inertia of the fan itself, then open the vanes up and trim the speed after it is running.


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

RE: Motor/VFD Question

(OP)
I have some time scheduled this weekend to visit the site and attempt to resolve this problem.

@waross: I will monitor the phase currents and advise. I have asked multiple times for the fan manufacture name and model; no response. I will try and locate this information myself this weekend. The incoming voltage to the drive is steady and constant @ 480VAC both loaded and unloaded.

@itsmoked: I suggested this as well but was unable to make a determination at the time. I believe we will see noticeable results after changing the torque curve.

@jraef: 'P Type Fan' = Centrifugal fan type P-wheel - Straight backward blades. High efficiency, self cleaning, changing in pressure have little influence on air volume. Ref. http://www.engineeringtoolbox.com/fan-types-d_142....

@iop95: The VFD is configured operate in 'Fan/Pump Mode'. Having read your comments, I believe we should see some improvement in changing this to 'Custom V/Hz' with a value of 9 or 10 set for P60. I am posting a link to the manual(s) if anyone would like to review the default settings and make any suggestions.
Ref manual: http://literature.rockwellautomation.com/idc/group...
Prog manual: http://literature.rockwellautomation.com/idc/group...

The limited information on the fan that I have so far is:
1) Fan wheel diameter = ~96", weight = 1,200 lbs
2) I hope to have more for you all on Monday

Note: This fan is part of a baghouse ( https://en.wikipedia.org/wiki/Baghouse ) type filtering system, the system is new. The manufacturer has been unable to get this fan up to speed.

RE: Motor/VFD Question

Quote:

he system is new. The manufacturer has been unable to get this fan up to speed.

Yikes that smacks of a 'problem' rather than an adjustment. Good luck. Most of us are around on weekends so please keep us informed.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Motor/VFD Question

P60, P61 and P62 must be set in voltage not V/Hz ratio.
I suggest to try these settings: P63=25Hz, P60=25V, P61=10V and P62=210V
I can't find a function to mask current alarm for few seconds (or at stat-up) to avoid current limit alarm.

This motor have nominal torque about 1200Nm; if supose fan load at nominal speed is 90% of motor torque, mean about 1100Nm; I think is correct to supose fan torque as Mf ~ k(speed)^2, so at 18Hz, fan need about 100Nm torque only.
If motor trip: or inertial load is too much (reduce accelerating time) or sometingh is wrong in V/Hz relation; may you check output voltage when motor trip?
With high aproxiamtion, J fan is about 200kgm^2, so need to keep acclerating time (0 - 60Hz, if is a single ramp) above 30 seconds.

RE: Motor/VFD Question

(OP)
FAN DATA:


Fan Diameter: 72 inches
Fan Width: 19 inches
P Type, Forward Pitch

Measurements are the actual fan

MOTOR DATA:



DRIVE DATA:

RE: Motor/VFD Question

Why would someone hook a 150HP motor up to a fan with a 200HP requirement?

Keith Cress
kcress - http://www.flaminsystems.com

RE: Motor/VFD Question

Keith,

That's the 380V / 50Hz data on the right side of the plate.

It's good for 200HP / 150KW at 460V / 60Hz

RE: Motor/VFD Question

AHh. Thanks for that Scotty. I've never seen that on a motor plate before! Lots of info, unlike the blower plate that doesn't even state the RPM. LOL

Keith Cress
kcress - http://www.flaminsystems.com

RE: Motor/VFD Question

2
Uncouple the fan and see if the drive still does this when connected to the motor, but no load on it at all. If it does still go into Current Limit, you have a bad transistor in the drive. If not and the drive goes ahead and ramps the unloaded motor up, it might be that you will need to have that fan (blower) system choked off for no flow until after the motor gets to speed.

Also, double check the programming on A179 (Current Limit) to make sure someone has not accidentally entered some unrealistic value in there. Then also check A182, which tells the drive what to do about an over current situation. From the sounds of it, A182 is probably programmed to a value of 2 which automatically forces it to lower the speed, but if set to a value of 1, it will automatically lower the current limit. If set to a value of 3 it will do both but it will lower the speed first, then lower the current limit setting. See what that is set for.

Lastly, look at the setting of A168, which is the Carrier Frequency (called the PWM frequency for this drive). If someone set the CF to a really high value, like 10kHz, in order to try to make the motor quiet, they did so without paying attention to the fact that you must ALSO de-rate the drive when you do that. So for example if you were to set it to 10kHz, the drive must be de-rated by 50%, so the drive may be trying to let you know it can't handle what you are asking it to do.


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

RE: Motor/VFD Question

jraef,
Wow, that's a heck of de-rating for carrier frequency. Is that fairly typical of all VFD drives? Definitely something to remember.

RE: Motor/VFD Question

Yes, it's a function of increasing the switching losses in the transistors because you are switching them more often in the same time frame. All VFDs must be de-rated, some cheap ones just don't make that easily to discover until AFTER you burn them up.


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

RE: Motor/VFD Question

Thanks jeff. Learnt something new today.

Muthu
www.edison.co.in

RE: Motor/VFD Question

Thanks for your contribution to my continuing education Jeff.
Yours
Bill

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

RE: Motor/VFD Question

(OP)
I wish I had good news to report...

@iop95: We input your parameter recommendations which yielded marginal results. Was unable to get past 18Hz before reaching FLC.

@jreaf: Motor uncoupled from load will run up to base speed (1160RPM) connected delta, current draw ~53 amps (2nd hand information, can not verify as I was not on-site). With load coupled, using the damper to reduce air flow (closed position) did not work as far as getting past 18Hz before reaching FLC. The parameters you suggested be reviewed do not match up to the description you provided. I Included a link to parameters in an above post for review. From the description you provided, I can confirm the following:
420
Drive OL Mode = Disabled
421
Current Limit Sel = Port 0 : 422
422
Current Limit 1 = 390.00
38
PWM Frequency = 2.0 kHz

I have more accurate information on the fan:
Fan Diameter: 72 inches
Fan Width: 19 inches
P Type, Forward Pitch
Design CFM: 51000

We re-connected the motor Y to see if we got any improved results. We did see improvement; was able to get to 48Hz with current at ~260amps. We reduced frequency to 40Hz bring the current down to ~240amps (name plate current). However the motor leads were not sized for Y connection, so we reduced to 30Hz which brought current down to ~120amps. Although the Y connection yielded improved results, it was not designed to run on the Y (high slip, reduced HP, etc..). I contacted AB tech support on Monday and shared our results. Was informed their drive would not function correctly on a delta connected motor which is why we saw some improvement when the motor was connected Y. I questioned this statement and was not given a solid answer. We have a AB PowerFLEX 400 VFD driving a 200HP delta connected motor on site with no issues. Parameters are identical (2nd hand information). The nameplate on the Leeson motors states that it is VFD Rated (see above photo). Can anyone explain the voltage and current ratios shown on the nameplate and how those correlate to a VFD?

RE: Motor/VFD Question

Quote (OP)

We have a AB PowerFLEX 400 VFD driving a 200HP delta connected motor on site with no issues.
If this is driving a similar fan, or if you have any similar fans on site, can you compare the fan dimensions?
I wonder if the fan has been designed for a much lower speed?

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

RE: Motor/VFD Question

(OP)
@waross: The difference in fans are as follows:

EXISTING FANS:
~36 inch diameter, ~24 inch width, belt driven, running ~1800+ RPM

NEW FAN:
~72 inche diameter, ~19 inch width, direct coupled, 1160 RPM

RE: Motor/VFD Question

Looks to me comparing apples to potatos?

Muthu
www.edison.co.in

RE: Motor/VFD Question

(OP)
@edison123: Please explain...

RE: Motor/VFD Question

Neither of those fans is a substitute for the other.

RE: Motor/VFD Question

With no load, 53Amp?! That mean 42kVA... about 30kW considering 0.7 PF. I think is too high value... motor have a so powerfull internal fan? May you clarify.
Fan models are too different for any comaparison.
If do some calculation for Y connection with values showed above, result motor torque values above nominal... up to 1800Nm, that is not possible and make me to think about driver current accuracy reading.
2kHz PWM may lead to high current spikes, worse in delta and drive go in current limit... but is far away of limit; a trueRMS clamp meter may help to clarify.
May you increase PWM to 4-5kHz and/or add a reactor?

RE: Motor/VFD Question

Quote (OP First post)

The motor will not come up to full speed before O/C limit is reached. With frequency set-point at 60Hz: as the motor ramps up to speed (ramp time = 30 seconds), at +18HZ an over current alarm is indicated on HMI. Longer ramp times have no effect. The VFD will ramp up to ~18Hz and go into 'current trim mode' as this is highest frequency achievable before current output peaks at ~275 amps(SF amps). Adjusting the frequency beyond 18 Hz triggers O/C alarm.

Quote (From the drive nameplate:)


250 kW (200 kW)
Continuous Amps 456 (367)
60 Sec Amps 501.6 (550.5)
Looking at this information there should be no way that this drive will reach a current limit of 275 Amps in 30 seconds.
Possible causes.
1 The drive is configured for less than 250 kW or 200 kW
2 The drive has other configuration issues
3 The drive is faulty
It seems strange that the 60 seconds Amps is lower for 250 kW than for 200 kW.

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

RE: Motor/VFD Question

53A is quite typical for the no-load motor current, possibly even lower than typical. You certainly don't get a 0.7 power factor on the motor with no load. Maybe 0.1.

Still, if you want to make sure the no load idle current is reasonable then monitor the input current to the VFD. Without the load coupled, the input current should be very low, likely under 10A.

Assuming the fan is 80% efficient, it could move 51,000 cfm if the fan outlet pressure is under 10psi. So, what is the fan outlet pressure?

The VFD will be fine with a delta connected motor. The motor connection doesn't matter as long as the motor and the VFD V/Hz pattern match.

I find it odd that the manufacturer plated the fan with the HP but without the rpm. That's a really odd thing to do for a fan since the HP is useless without the rpm.



RE: Motor/VFD Question

Yes, my mistake with 0.7PF... maybe 0.1-0.2 at no load.
Regarding fan.. we don't know torque curve or at least rpm for 200HP power requsted.

RE: Motor/VFD Question

(OP)
@LionelHutz: At ~18Hz, we are drawing less then 1 inch WG on inlet side of duct. It seems to me that there is hardly any load at all.
I agree that the VFD should work fine on a delta connected motor. Can you elaborate on motor and VFD/Hz pattern needing to match?
Again I agree, the fan manufacture indicating the HP without indicating RPM for the equipment is odd. I can only speculate from the motor they installed that the operating speed of the fan is 1160RPM.

RE: Motor/VFD Question

I don't know who you got connected to at tech support, or how the question was framed, but that issue of the drive not working on delta connected motors is just bunk. That has no bearing on how the drive functions. All that's important is that the motor is connected in the pattern commensurate with the voltage level. Your motor is designed for delta connection at 460V, you must use it that way.

So what I'm curious about is the "old vs new" fan issue. Going from 36" diameter to 72" diameter represents a HUGE difference in inertia of the fan blades themselves. So was the 36" fan using a smaller motor so that when you changed to 72", the motor size was increased too?

Also I agree with LH, stating the fan HP requirement without stating the RPM is very odd. For all we know, that 72" fan was intended to run at 10RPM and the 200HP assumed a 6:1 belt reduction, meaning the torque requirement if directly driven would be the equivalent of 1200HP! I just really think there is some sort of mechanical mistake taking place here and some mechanically inclined person is thinking the VFD performs magic. I've had that happen in more than one occasion.


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

RE: Motor/VFD Question

The motor is rated 460V @ 60Hz. this means it has a V/Hz ratio of 7.67 and the VFD needs to be programmed to match this ratio. There are various ways this is done and a typical way is programming motor voltage to 360 and a base frequency setting to 60Hz. However, I can't say how to do this exactly with that VFD.

Did you take any rpm measurements when connected in wye? It'd be interesting to know what rpm the motor and fan achieved.

jraef - wasn't there a poster here recently thinking the motor hp was the same at any speed? The basic theory was along the lines of taking a 200hp @ 1200rpm motor and directly connecting it to a load that required 200hp @ 300rpm. Then, use a VFD to run the motor and load at 300rpm. Unfortunately, the theory was wrong because the motor would only be 50hp @ 300rpm.

RE: Motor/VFD Question

(OP)
b]@jreaf:[/b] One of the AB tech support persons made that statement, I was very clear in my question, "Will the PowerFLEX 753 VFD operate a delta connected motor?", long answer was no. I agree with you and questioned it, and again, was not given a clear answer. I contacted AB support again today and got a different specialist who had no reservations about the PowerFLEX 753 drive working on a Y or delta connected motor, said it makes no difference. We went over the motor data and drive parameters. He made some suggestions which we will try this weekend.
There is no "old vs new" fan, this is a completely new baghouse. The reference was to EXISTING system (3 actually) currently in operation vs the NEW system setup.

@LionelHutz: In regards to programming the drive to match the motor V/Hz ratio, AB tech specialist says that is taken care of (calculated) by the V/Hz algorithm.
Yes, at 30Hz we drew ~2 inches WG on inlet side of duct. the RPM was ~550 (can not verify, did not witness the measurement). I will be able to document accurate info this weekend.

Quote (LionelHutz)

wasn't there a poster here recently thinking the motor hp was the same at any speed? The basic theory was along the lines of taking a 200hp @ 1200rpm motor and directly connecting it to a load that required 200hp @ 300rpm. Then, use a VFD to run the motor and load at 300rpm. Unfortunately, the theory was wrong because the motor would only be 50hp @ 300rpm.
That is very interesting, I wonder the poster is somehow involved with this particular fan manufacturer!




RE: Motor/VFD Question

The photo you posted is a PowerFlex 400, you asked about a PowerFlex 753, they are different animals.

Still, the issue of Y vs Delta connection is no different, still irrelevant. The first person may have been somehow conflating this with the LINE SUPPLY configuration. All PowerFlex drives can be configured to allow them to be used on a Delta power system, but some other VFDs are only allowed to be connected to a Wye system (typically Asian mfrs). But even then, the PF400 can also be reconfigured to be used on a Delta system so I don't know why he would have said that... maybe he was a noobie.

I once had an ME from a pump mfr sell a customer a new 100HP pump to which I applied a VFD to maintain a tank level against a variable inflow. When it failed to keep up with the inflow even at full speed, I investigated the application and found that they had abandoned an old pump in place, and it had a 400HP motor on it! When I questioned the validity of replacing it with a 100HP pump, the ME told me to just turn up the speed...


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

RE: Motor/VFD Question

Oh, duh...

I guess I saw what I was predisposed to see:

Quote:

Drive Specs:
Product: Allen Bradley 23C-D460A103NNMANN
Description: PowerFlex 400, Fan & Pump Packaged Drive

OK then. Did you perform an AutoTune on that 753 drive? Even if you are using it in V/Hz control, it still needs to do an AutoTune. I don't know why, but I know it does.


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

RE: Motor/VFD Question

(OP)
@jreaf: Initially, yes, a Static auto-tune was performed. However, the drive has be reset to factory defaults in preparation for this weekends trial setup per AB Tech Support recommendations.

RE: Motor/VFD Question

I sent you an email, but now please edit your post to remove the address, otherwise it attracts spam-bots to the site.


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

RE: Motor/VFD Question

From the spec plate it appears as though the drive may be either a 250 kW drive or a 200 kW drive.
Is this something that is set in the parameters?
If the drive was inadvertently configured to 100 kW, that may explain the low current limit.
Is this possible Jeff, or am I lost in lala land.

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

RE: Motor/VFD Question

(OP)
@waross: The drive is a setup for 480 Class (right data column of nameplate)

RE: Motor/VFD Question

I'm pretty sure the problem is in drive current measurement.
Y motor connection is working and for 48Hz output, motor torque result about 2000Nm... much above maxim motor torque.
In delta, FLA is reached at lower speed but real current is much smaller than what drive see; this is the reason drive stop at 18Hz.

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