Inverter drive oscilates holding a load

We have 3 identical inverters each lowering a load.

When we raise the speed, two of the loads are unstable. They jitter going down. There is an oscilation as if we where rapidly changing the speeds. Those two loads are the heaviest ones.

The drives are equiped with external braking resistors and without these we would get DC Buss Over Voltage failures.

Raising the lowering speed increases this effect even more.

Raising the loads, the drive have ample power. Max Amps is 6 but they use only 4.5 at 60HZ rising. Lowering, they cannot do it with stability above 15 HZ.

Any suggestions?

They are running at fix speed with accel and decels of 2 seconds.

Say we lower the loads at 20Hz, they jump all over, then we reduce while running, at 15Hz... they become stable again.

Hey Pierre

Nice to hear from you again. What type of drives do you have? What type of feedback if any do you have? Are you running them in an open loop or a closed loop?

Assuming your running either open or closed loop vector, it sounds as though maybe you gains need adjusting. I'd start by lowering the proportional.

TWControls said:

Hey Pierre

Nice to hear from you again. What type of drives do you have? What type of feedback if any do you have? Are you running them in an open loop or a closed loop?

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Its not an ideal setup. We where first asked to be on a fast-track limited budget project and the poo it the fan.

At first the load was to be insignificant... but its not.

Its not a vector drive, its an LS model SV015iG5A-4.

Open loop. No feedback... only mechanical rotary limit switches for positions.

Motor is SEW 2HP 480VAC 3 phases.

Everything looked good. Its a VERY tight space system with multiple slip rings feeding one into the other.

We can see that the drive could not hold the load going down... so we installed resistor to flush out the DC buss. It worked.

Now the load lowers itself smooothly until we raise the speed. I'm curious at why when we increase the speed we get that jittery oscillation. Could it be the IGBT triggering to flush the DC Buss that is causing this. I was convince that when I would increase the speed it would only be less intense on this poor little drive but its not!

The load is max 200 pounds, the motor is a 2HP at 1800 RPM with a 5:1 gearbox... the cable is on a 6 inch circumference drum.

We want to lower that thing 40 feet and it needs to go to 42 HZ to do so in the time required...

Now with the 15 HX we get it down in about 30 seconds.

I have very limited access to the machine so next time I'm thincking of raising the torq boost to its maximum...

I have a 10 minutes access to it per day and its with a manlift... 85 feet in the sky... fun stuff.

There are limited possibilities with this kind of inverter but it looked OK in the shop with weights...

As for the process, its very simple. We now start from ZERO, raise the lowering speed to 15 HZ in 2 seconds and at the end stop.

Then the same thing coming back up.

Thanks for any suggestions.

Never heard of that brand before.

Based on your description, the thing that jumps out at me is cable length. How long are the motor cables and what kind of cables were used? You could be dealing with harmonics. Might want to check to see if things are grounded and shielded according to the MFGR spec.

I would also be concered that the braking transistor might not be rated for the amount of duty you are using it for, but I don't think it would be related to your "surging".

Being regular old V/Hz, there are not many things it could be.

Maybe try monitoring the bus voltage with a meter as well.

Its not a vector drive, its an LS model SV015iG5A-4.

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When I searched for this drive, it actually is a Sensorless Vector thus the SV at the beginning. According to the manual parameter H40 sets the mode. Check to see which mode it's in. Factory default is 0 which would be V/Hz. It may be worth trying in SV mode for a little tighter control(3).

At 5/1 gear ratio you wouldn't normally get binding but just doublecheck that your gearbox and powertrain do not bind up when driven backward.

DickDV said:

At 5/1 gear ratio you wouldn't normally get binding but just doublecheck that your gearbox and powertrain do not bind up when driven backward.

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Please, being french ... I do not understand ''binding''.

Can you elaborate?

flowdam said:

When I searched for this drive, it actually is a Sensorless Vector thus the SV at the beginning. According to the manual parameter H40 sets the mode. Check to see which mode it's in. Factory default is 0 which would be V/Hz. It may be worth trying in SV mode for a little tighter control(3).

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Good thoughts, we will.

We have not taken that approach yet because we beleive the cheaper the drive the worst there 'extras' operate. But we will certainly give it a try.

DamianInRochester said:

Never heard of that brand before.

Based on your description, the thing that jumps out at me is cable length. How long are the motor cables and what kind of cables were used? You could be dealing with harmonics. Might want to check to see if things are grounded and shielded according to the MFGR spec.

I would also be concered that the braking transistor might not be rated for the amount of duty you are using it for, but I don't think it would be related to your "surging".

Being regular old V/Hz, there are not many things it could be.

Maybe try monitoring the bus voltage with a meter as well.

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Cables are 40 feet long.

Jumping happen in the first foot... going down.

I am concerned about the IGBT... do they trigger with an hysteresys... then it could be it!

The smaller load does not do this. For discussion, lets say the smaller load is 50 pounds, the mid load is 100 and the larger one is 200.

The 50 pound load... we can do anything we want with it and it will not fail... we have not even needed to install resistors on it...

This system does that:

Starting in the UP position.

Strat lowering... ACCEL in 2 seconds to 15 hertz
Reach Limit switch... STOP in 2 seconds.

All 3 load do this...

When we raise the speed above 15... Mid and big load... jump. It even bends the supports... we have made then stronger... they barely bend... but the loads still jump...

DB Resistors installed are 300 OHM with 300W capacity

I remember having a problem with a cheap drive and a scrap winder. It was funny feeling the motor shake it's little tits off with the rotor only turning 1/10 of what the field was trying to turn it. Turned out the slip compensation was set too high.

Pierre said:

Cables are 40 feet long.

DB Resistors installed are 300 OHM with 300W capacity

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Hi,

I find 300 Ohms is high. You may have to check in the drive specs for the lowest DB resistance that is permissible. Calculate the Wattage using the following formula.

W = V * V * 2 * duty cycle.
R
where
V = Max DC Bus (usually 760Vdc)
R = Permissible DB resistance


Ron

2rlp said:

Hi,

I find 300 Ohms is high. You may have to check in the drive specs for the lowest DB resistance that is permissible. Calculate the Wattage using the following formula.

W = V * V * 2 * duty cycle.
R
where
V = Max DC Bus (usually 760Vdc)
R = Permissible DB resistance


Ron

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Resistor was selected according to specs manual of drive.

Pierre said:

Resistor was selected according to specs manual of drive.

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Dear Pierre,

Yes I have gone thru the specs. This wattage is for 5% operating duty & 15 seconds duration. You will have to size it taking into account the time taken to lower the load & as well as the duty cycle (which for a lowering system can be as high as 60%. I hope this helps.

Ron

Pierre, I've been watching this thread with interest and would like to help you find a solution to your problem. I suspect that the drive is not defective but maybe programmed less than optimal.

My first question about the data: Are you sure the winding drum is 6 inch circumference? Is it possibly 6 inch diameter?

Second question: You have 200 lbs you need to lower in a fixed period of time. Can you give us the distance traveled and the amount of time to make that move?

Third: Do you have a means of displaying or monitoring the DC bus voltage while lowering the load? If yes, can you tell us what the voltage is doing when the motor starts to oscillate?

Finally, just an observation. I suspect that you are bumping up against some kind of braking capacity limit. I say this because, as you increase descending speed, you also increase the braking hp. It seems that a limit is reached around 15hz and the oscillations begin.

Please advise above requested data.