OT: V/F controled motor problem

[Krcedinac]

Hello everybody,

I know it may not be a question for this forum but I have to post it. Inverter is SSD (Eurotherm) 650V, controls 1.10kW 5.04A motor which is connected to the gearbox. When it gets stop command from PLC (rising edge of the cam), stops in the right position but if the machine is stopped for some period of time it can not hold it`s possition. Eventualy after some time (15-20m) it moves for 25-30 degrees and cam together with it causing the alarm. I must say that there is some load that the motor has to hold in that position. There is one more motor with the same inverter working in the practicaly same conditions but it holds its position while stoped.
I am 99% sure that it is not mechanical (gear box) problem.
Is there some suggestions what to do or what to check?

Regards

 

[rsdoran]

IF both units are identical i.e. stop at the same position with power from the inverter OFF then it sounds to me like it must be in the gear box or the cam itself is moving.

 

[Krcedinac]

I do not think so. The movement is on the shaft that comes out of the gear box. Does it seams reasonable that the gear box shaft can alow 25-30 degrees without moving the motor!?!
I wrote that I am 99% sure that it is not gear box... tomorrow I hope I will be 100% sure. The motor has a brake that is ON when the unit is OFF. I am waiting for the right conditions(highest loads) to test. If the shaft moves with the motor brake on than it is that 1% - gear box problem. If not then it is motor and/or inverter problem.
If that is the case what can be done?

 

[rsdoran]

Since there is an electromagnetic brake involved I would look closely at it, they can wear out but they also use springs for the brake pressure, on some the spring tension can be adjusted.

 

[brucechase]

My bet would be the brake on the motor. Since the inverter has a stop command, it is basically out of the equation. If you think that the gearbox can rotate 35 degrees without moving the motor (and an identical one does not), then I would consider that excessive slop and you would probably be hearing the banging as the motor changes speed. If the brake is on and the motor fan turns, then the brake is bad. If the brake is on and the motor does not turn but the gearbox shaft turns, then I would start looking at the keyways (but I am not a mechanical type of person).

 

[Krcedinac]

Let me explain situation a little bit better. Motor break is on when the unit power is off(unit main switch is off). Electricians asured me that they checked the brake and that it is OK and I trust them. While the unit is running - motor has start-stop commands without brake involved. Brake is used for security reasons when the unit is not working - when it is powered down. When the motor has STOP command on the shaft (that is coming out of motor+gear box unit) there is significant torque and the brake is OFF (100% it is not controled by start-stop commands from PLC). Anyway after some period of time shaft moves for 25-30 degrees. Since I am Mechanical Engineer I asure you that I checked mechanical aspekts of the problem.
Is there some possibility that something is wrong with the drive or the motor. I checked everything from the manual, comparing it with the other drive+motor that is working OK. What I plan to do next is to change control mode from V/F to Sensorless Vec but I need to discconect the motor from gearbox to do the Autotune. So I have to wait for opportunity (production stop) in order to do that. In the mean time I would like to hear secon oppinions and expiriences.

 

[rsdoran]

SO what you are saying is that when in production the STOP command takes the motor to zero speed with full torque and the brake is not applied in this situation?

If that is the case then you probably have an issue involving the inverter.

I am wondering if it is possible the inverter could be temporarily dropping out or faulting then resetting itself. A heat issue may fault it but it reset itself. Take a look at the inverters fault log.

 

[milldrone]

Motor break is on when the unit power is off(unit main switch is off)

Ok so there is a brake but it used only when the disconnect is locked out. Is this correct? If it is then we can assume that the machine is supposed to control it's shaft position with the drive when powered up.

Anyway after some period of time shaft moves for 25-30 degrees.

Is this when the machine is powered up?

If it is then these questions
1. Is there a significant number of total operating hour differences between the two "identical" machines? The reason I'm asking this is because I once had two identical machines that used worm gear drives. The machine that was new had very good "self braking" action in the gearbox. The machine that had lots of run time had almost no "self braking" action from the gearbox.
2. Is the drive capable of full vector control with a encoder? There are many hoist type applications where the hoist motor is controlled by a full vector drive using the encoder. In these apps the drive holds the motor at zero speed when commanded to do so. In these applications there is a brake but it only grips the shaft when the drive no longer has a "health" output. So when the power is removed or when the drive is in a fault condition is the only time the brake grips the shaft. Of course the motor needs some sort of cooling under these conditions.

 

[leitmotif]

What is the motor gearbox load ie what do they make go up or down etc??
Is this load constant thus imposing constant torque on gearbox and motor?
What is reduction ratio of gearbox??

Milldrone says one of two identical worm gear setups will not hold load I have seen this also.

OK so here is what I understand
Takes 15 or more minutes for load to cause gearbox to drift.
When you have STOP command to VFD and gearbox motor you are expecting motor gearbox to hold the load.
When you deenergize the VFD and motor the brake is set.

I do not believe your problem is in the output side of gearbox - with approx 30 degree movement you should not have that much slippage on shafting HOWEVER you must check this otherwise you are going to strip a keyway, grind up a shaft etc etc. I for sure would check all shafting on gearbox output for properly set keys, shearpins etc etc.
With approx 30 degree movement of gearbox output shaft it's input (motor) shaft will turn several revolutions. This should be easy to mark and measure the total revs of the motor shaft. IF motor turns where number motor turns = gearbox out shaft degree movement X reduction then you know you are back driving the gearbox.

IF it were mine I would set the brake even during STOP command to VFD.
ASSUMING the motor is not restarted for several minutes even though this is frequent restarting of motor it should cool enough given several minutes before restart.

I do not think I would rely on motor gearbox (using STOP command) to hold the load - UNLESS you have independent cooling to motor.

Dan Bentler

 

[kamenges]

Originally posted by Krcedinac:

What I plan to do next is to change control mode from V/F to Sensorless Vec but I need to discconect the motor from gearbox to do the Autotune.

This should help, at least some. A drive in V/F mode will not put out anything at zero speed so there is nothing to stop the shaft from turning. In SV mode, the motor should stay fluxed up so it will at least have a chance to respond to forced rotation.

I think milldroneis correct. One of the drivetrains turns easier than the other or one machine/location has a little more vibration than the other. That is all it takes to spin a gearbox output shaft the amount you are talking about.

It is also possible that, if the drive is getting it's speed command by analog interface, that you have a small offset on the drive speed command.

Keith

 

[milldrone]

It is also possible that, if the drive is getting it's speed command by analog interface, that you have a small offset on the drive speed command.

Keith

I'll bet Keith has been bitten by this one before. The wealth of experence and knowledge on this forum is unbelievable.

 

[allscott]

This should help, at least some. A drive in V/F mode will not put out anything at zero speed so there is nothing to stop the shaft from turning. In SV mode, the motor should stay fluxed up so it will at least have a chance to respond to forced rotation.

Keith

Yes, in order for the motor to actually hold the load it would need to be in closed - loop vector mode with an encoder on the motor shaft. Unfortunately a 650 drive does not offer that as an option, you need a 690+. You would also need a motor designed to operate at 0 speed without overheating ie, a blown motor, or a TENV motor.

I agree with both Keith and Mildrones conclusions.

Edit:
Just to add, it is very common to use the drives at 0 speed signal to operate the brake on the motor, in fact this is almost always how I have seen it done.

 

[bguinn]

You did not mention which drive you were using, but most drives have a DC injection option for holding a shaft still when the motor does not have a run command. Many drives use different variations of this and some have more options than others. I have found that by using this DC injection it also helps cut down on regen/overvoltage faults, because it takes the power off the drives DC bus and sends it back to the motor for braking. Many times the current can be adjusted on this DC brake. I have found that 10-25% motor FLA is usually plenty and have only damaged the cheapest motors. A quality motor seems to handle it just fine. Infact, Marathon claims FLC at a stall indefinetly on one of its motor models.

 

[milldrone]

bguinn,

I'm under the impression that DC injection will only slow down a shaft and will not stop it all together. And there has to be some shaft movement for DC injection to work. The original poster's problem seems to occur over a long period and I thinking the shaft creeping at this speed will not be held by DC injection

Eventualy after some time (15-20m) it moves for 25-30 degrees
Regards

If I'm wrong then this would be the easiest solution.

 

[bguinn]

DC injection will actually hold a shaft in place depending on the load. If the load is too heavy it can usually be overcome by going up on the injection current.