Agitator motor

Hey everyone, great threads as always! I have a motor/VFD question, if anyone is up for it...maybe DickDV is around! At any rate, I have a 230V, 5HP, AC motor, on a VFD. The motor turns an agitator blade in a silo. We have 14 silos, all configured with the same silo/motor/agitator/VFD. All of the Drives also share the same line source. On one of the agitators we are continually breaking off blades. We changed out the VFD, and broke another blade with the new drive. So I put my data logger on the drive output for about 3 weeks, measuring VAC out to the motor, logged every second (got about 1.3 million data points) Anyway, I noticed that MOST of the time it was the nominal 230VAC, but everyonce in a while the voltage would drop to about 130VAC, and then go back up to 230VAC. It is a 2 speed agitator (30 and 60hz). Can someone tell me why the voltage intermittently varies by 100 volts or so??? This would be indicative of an intermittantly open motor winding wouldn't it???

Thanks for any input.

Greg

Did the setpoint of the VFD Change in conjunction with the Voltage Drop? If so, the fault could be the VFD, diodes, SCRs etc..

Does your VFD have a torque monitor of some kind? If so, the VFD should be tripping on that.

Did your Current spike when the Voltage Dropped? If so, Then I would say there could be a problem with the motor.

I'm really leaning towards it being the motor. The drive has been replaced, and still breaking blades.....unfortuneately the motor has no torque monitor.....I would assume the current spiked when the voltage dropped, but it wasn't enough to trip a breaker or trip the drive.....

And no, the setpoint wasn't changed at the time of the drop in voltage.....

The voltage drops? for how long?

Looking in the right place?

On one of the agitators we are continually breaking off blades.

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of 14 installations I would suspect a mechanical problem in this one, and not an electrical problem. What makes agitator blades brake? Is it mechanical force or some electrical quantity?

It's only in the one silo the blade breaks......nothing is in the silo, except milk! It's odd. There's quite a bit of torque at the shaft, after the motor it's gear reduced 13:1, so it's slow, but powerful. Any spike I would think would have a lot of sudden torque....The voltage dropped for a period of about 10minutes....then went back to 230V, then a few days later dropped again....(not at the same time), and it's very sporatic. Nothing mechanical is in the silo, other than the agitator itself.....We've broken 4 stainless blades now....the only thing wrong I can see is that the voltage out of the VFD for some reason cuts in half for a period of time, and comes back to 230V...It comes back to 230V in a matter of about 2 seconds also......and the drop to 120V, is about 2-3 seconds also.

Greg

Randy, again, to your point. I am suspecting that a momentary spike could very quickly, and for a short duration suddenly torque the shaft? And being that it's gear reduced by 13:1, any sort of spike may cause a jaring? I don't know, I don't know what MOMENTARILY happens to the frequency as there are severe voltage drops. I I guess there's really 2 questions:

1.) why is the voltage output from the VFD very sporatically and intermittently dropping so severely?

2.) Would a large voltage drop across the output of the drive momentarily cause a sharp VERY momentary increase/decrease in frequency?

Thanks, Greg

I'm inclined to agree with Randy on this. The VFD can't break the agitator blades, only a mechanical jam will do this. The VFD only supplies current in response to the motor torque load. If you had the agitator spinning in air, the current would be very low.

In most VFDs the nominal output voltage is proportional to output frequency, regardless of torque. At 30 Hz the output voltage should be around 115 VAC.

This leads me to a suggest some possibilities for you to check.

First, it may be that the agitator is jammed and the high momentary current draw is causing a momentary voltage drop.

Second, it is possible that the output frequency setting is momentarily dropping from 60 Hz to 30 Hz, which would cause the output speed and voltge to drop. It is possible that this is caused by a loose speed command wire or such. It is further possible that the sudden deceleration/acceleration is causing a higher torque and a higher load on the agitator blades.

Third, this VFD may have a different max current limit setting than the the other VFDs. This would allow a jam or other load to put a higher torque at the motor than is alllowed by the other VFDs, breaking the blades.

Tom, why would the voltage vary? current will vary based on motor frequency, but why would the voltage vary?

Greg

GregPLC said:

Tom, why would the voltage vary? current will vary based on motor frequency, but why would the voltage vary?

Greg

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Current does NOT vary with frequency. Current varies with the torque required to rotate the load at the motor up to the point where the current limit of the VFD output is reached. This current may be limited by the VFD settings (the max current setting in the VFD during normal operation or starting) or by the thermal capacity of the VFD (the VFD max rated output current) or by the thermal capacity of the motor (the FLA is the point where the motor can't dissipate the heat generated by I-squared R losses).

Motors operate best at a constant Volts/Hz ratio. The typical VFD's inverter portion sets the operating voltage provided to the motor as a function of output frequency up to 60 Hz (in the US anyway). The normal Volts/Hz ratio is 7.6 Volts/Hz for a 460 Volt motor.

Motor speed is a function of frequency: synchronous rpm = (120 x Hz) / NumberOfPoles. Actual rpm = synchronous rpm - slip

The required torque to rotate the load is function of the load. Your agitator is probably a variable torque load. That means that the torque varies proportionally with the square of the speed, just like a centrifugal pump. The constant of proportionality is a function of the agitator characteristics, the fluid viscosity, the gearing, and so on.

Lets talk in general terms and for illustrative purposes only - I'd need a lot of data and a consulting contract to do the analysis for your specific system.

If your agitator is rotating in air: at 50% speed it may require 1% torque because of friction, say 10% current (because of motor magnetizing characteristics zero percent load isn't zero current) 50% voltage, and 50% frequency. At 100% speed it may require 2% torque because of friction, say 11% current, 100% voltage, and 100% frequency.

If your agitator is rotating in milk: at 50% speed it may require 25% torque because of friction and the power needed to induce fluid movement, 25% current, 50% voltage, and 50% frequency. At 100% speed it may require 100% torque, 100% current, 100% voltage, and 100% frequency.

If your agitator is rotating in ice cream: at 50% speed it may require 50% torque because of friction and the power needed to induce fluid movement, 50% current, 50% voltage, and 50% frequency. At 100% speed it may require 200% torque, 200% current, 100% voltage, and 100% frequency. Since the drive won't be rated to provide 200% current, either it will trip out or it will limit the speed to the point where motor torque and current are at 100% - in my example that would be 71% speed, 71% voltage, and 71% frequency!

If your agitator is "rotating" in solid ice: at a 50% speed command it would require infinite torque and infinite current. The VFD would put out 50% voltage at 50% frequency. Since the drive can't provide infinite current, it would provide as much as it could to try to accelerate the agitator, say 150%. If 150% torque is enough to break blades, your agitator will be trashed. If not, the VFD or motor will eventually over heat and trip out.

Always follow the cause and effect chain in your system. If your agitator blades are breaking, it is because something is physically inducing a load that exceeds their structural capacity, and the VFD is simply supplying the electricity that will provide the rotational torque that the motor must have to overcome that load and/or break the blade. The VFD can't break the blade all by itself - just imagine the agitator rotating in a vacuum!

Greg

I cant help alot with this but thought I would post this website.
It has many useful formulas.

http://www.reliance.com/mtr/flaclcmn.htm

Greg, if you have a VFD of any capabilities at all, you can use it to do your troubleshooting for you.

Most VFD's have an analog output that can be mapped to various internal drive operating elements. Usually this includes speed, frequency, current, and torque.

For your application's problem, I would suggest you place your recorder on the analog output of the drive and map the analog output to torque. Since torque is the direct variable that normally would be associated with agitator blade breakage, this would give you a direct insight into what is happening with your motor and the machinery that it is driving.

Let your recorder run thru a period where you would normally expect to get several of these voltage drop episodes. Then study the data for corresponding abnormalities in the torque.

At this point I am guessing but I suspect that you will find that the machinery is drawing unexpectedly high torque at the times that the blade breaks and also during the periods of low voltage. The low voltage and corresponding rapid reduction in frequency is the drive's attempt to get away from a severe overtorque condition. This is proper drive operation. The problem will be found in the machinery. Remember that a rapid reduction in agitator speed is just as likely to break blades as a rapid increase.

If you still somehow suspect the drive of operating the motor in some abusive fashion, go into the drive parameters and set the torque limit down to 100% of the motor torque. If a torque limit is not available in the software, set the output current limit to 100% of the motor nameplate full load current. Either of these will limit the drive's ability to excite the motor beyond it's full load rating. Having done this, a broken blade can pretty well be assigned to the machinery leaving the motor and drive out of it. (The obvious exception here is if the motor and drive are so large that they can break the agitator blades within their normal rating--unlikely).

Hope this helps. Keep us advised on your progress and we will keep trying to help.

put maximum current on low value. this will take care of slow starting or slower revs up.
If voltage is half motor will stop and when voltage is back it will start violently.
so at that time the blade will break.
put the magnetic trip very very low, this way it will prevent breaking.normally use only 20% of rating.

How does a VFD work?
most of them have a feedback to see how fast the motor is actually going. so check that.
if need more just call

Milk shakes anyone?

originally posted by shooter

If voltage is half motor will stop and when voltage is back it will start violently.

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It looks like if this guys milk tanks are getting jarred around like this he would end up with whipped cream or butter. I know milk is a little thick but enough to break a mixer blade off? Wow, that must be some torque surge.