Motor Overspeeding

[plugsandsparks]

Hi guys, i have been asked to look at a machine as there appeared to be a fault with the plc / inverter as the motor controlling a tension roller did not speed up when it hit the top limit.

Anyway, to cut a long story short - plc fine, inverter fine,

Issue is that the motor , a small 90W 3T 220V jobbie driving a set of rollers through a 15/1 gearbox is already running at 1780rpm normal speed and 1820 at full speed (when the tension roller hits the top limit)

The rating plate at 60hz is 1630 rpm.

I can reprogramme the inverter to up the frequency but have no experience overspeeding small motors and dont really know how far to go without materially compromising reliability.

any thoughts ?

 

[AndyOz]

First question I would ask is has it ever worked? If it has then mechanical slippage between drive rollers and product would be where I would start.

I have known rollers being changed for ones with different diameters. This will give these symptoms.

 

[Marvo]

I would also establish the root cause of the speed issue before treating the symptoms by altering the motor speed. Maybe look for a safety clutch or slipping drive, if the motor speed is to spec (or >15% over in this case) it's pretty certain it's a mechanical issue. It sounds like someone might already have tried to compensate for the problem with a higher motor speed so I'd be very hesitant to increase it even more.

 

[Knobhead]

As Andy has said has it worked right before. If yes then I would be looking for mechanical problems. Unless someone has being playing about the PLC and VSD parameters are unlikely to change.
What interface is there between to two?

 

[Darkwood]

When you say the tension roller hits the top limit how does the drive ref' the speed does it work in torque mode monitoring current demand or is it watching an input like a 0-10v signal from a position sensor or pot.

 

[plugsandsparks]

Thanks guy, history is that the machine was built many years ago, manufacturer does not support anymore, the client has been using different products through it and as such the mechanical engineers are looking at the machine from a point of view of getting it to work reliably under all scenarios. The mech engineers thought the top limit did nothing but when i forced the machine to think it was on the top limit there was a very slight speed increase when measured with a mechanical tacho held against the shaft.
The motor control is very simple, there is a tension roller that rises and falls in a guide, the guide has two proximities, top and bottom.
When the tension roller drops to the bottom, the motor stops, when it is between the two proximities it runs at "normal" speed and when it hits the top the motor runs at "full" speed.
The inverter has a number of fixed frequency settings programmed into it. These are control by the digital inputs on the inverter. By using the plc to operate a relay the 20V common voltage on the inverter is applied to one or more of the three digital inputs on the inverter. Depending on which combination you get different speeds applied.
Have ruled out slippage and the machine mainly runs OK. The other machines they have produce a noticably higher speed at the top limit but not this one. I understand that i might be being asked to overcome a mech issue but have watched the machine and cannot see anything to indicate a slippage or any mech issue. Drive from motor is though a gearbox , then toothed belts to the drive rollers.
One idea was to slow the front end rollers down a bit (basically slow the whole machine down) and this would give more scope for control at the back end but dont think clients wants a less productive machine )

 

[Darkwood]

If the drive is capable can you change it to run in torque mode... set the desired speed and the top limit become obsolete but still keep the lower to shut of drive in case of jam, also are you able to swap motor and gear for similar one on a identical machine ... its possible the gearing is getting stiff, is it maintenance free or does it require routine top up.... this would possible show a slow creep of reduced speed over its usage ... check if gearing is running hot..... ive had this a few times it lasted hours then slowed down as the gear cogs expnded with the heat generated through lack of gear oil..... like others have said it is pointing to a mech issue

 

[plugsandsparks]

Will check data sheet on inverter for torque option, fair point about change over time, i will ask the mech guys to give it a full days running and see if it changes its behaviour. The motor and gearbox are a combined Lenze product, no oil or grease maintenance points (sealed for life)
thx

 

[plugsandsparks]

Inverter has torque mode option but the torque setpoint is apprently lost when mains is removed, not really much use as this machine is on a plug top and is switched off regularly. Hey ho

 

[Darkwood]

Rig a pot up as an external speed control the torque point is then referenced from the pot on power up a very simple task the pot can be left in the panel if you dont want it adjusting.

P.S. ... im suprised it loses memory on power down what make is it?

Some drives have a seperate save command to log new setting in while others log it in auto.

Can i stress here this option may be an alternative set-up but you still need to know why the underspeed issues existed in the first place or you'll be setting the drive up to a system which may have running problems... you need to correct the initial problems first.

 

[plugsandsparks]

Lenze 8200 Vector inverter. Looked at all other parameters and this appears to be the only one that is lost on power cycle. I can rig up a pot, no problem, have several that i use to just get them fired up and to test the motors so i could hide one on the control panel. Agree i wont do a wholesale change of mode until we can reliably replicate the problem. The inverter manual is indicating a max of 87Hz for a 4 pole motor in delta.

In manual it says:

Torque setpoint ortorque limit value​

(MCTRL1 MSET)
[FONT=HelveticaNeueCondensed,Bold][FONT=HelveticaNeueCondensed,Bold]

The value set will be lost when​

switching the mains!


[/FONT][/FONT]

 

[Darkwood]

I work on a few Lenze drive 'old buggers i work on' and they have a seperate save function once you change values, im not sure about your drive but it can get a little complicated but its probably possible to tell setpoint to reference another function i.e. a pot input or a set speed point, its sounds like its just an option for configuring the initial motor speed when commisoning machine and not a true torque/run option but i figure you should be able to get it to reference another input address, but the idea of doing this only wants entertaining after you have sorted out the original issue of it running slow.

 

[Knobhead]

Is this for web speed and tension? If it is leave the torque side of it alone.

Let me know if it is. I'm not writing chapter and verse for the fun of it.

 

[plugsandsparks]

Hi Tony, this application is not constant tension stuff, the machine simply measures out a preprogrammed lenght of material and cuts to length. The tension bar at the back end is just to help control the material and stop it getting messed up by touching the floor.
Thx for thoughts anyway

 

[malcoa]

Hi, So you have proved that the original fault of the motor not speeding up does not exist as you say it does increase in speed when the top limit is flagged. I assume that production just want the speed tweeking a touch more? If the current limit is correct on the inverter then i guess there isnt much of a mechanical issue as you would perhaps see an overcurrent alarm (you could always monitor the current via the parameters) if you increase max frequency i doubt it will increase the speed further as i'd imagine that there is a preset frequency for a given input to the inverter, as an example input to R1=0Hz, input to R2=40Hz, input to R3=60Hz, these frequencies can be adjusted and it will alter the speed but not above the Max Freq setting