AC Motor Speed

This not a PLC direct related question, but I know there are plenty out there with a lot of theoretical knowledge.

I have 2 drums that I need to match surface speed on, they do not have to be exact, but within tolerance.
My approach has been to work backwards to get the desired RPM to the motor. If I take the surface speed required, and divide by the drum size, I get the drum RPM req, then multiply by the gearing ratio between drum & motor gives me the motor RPM req. If I then calculate the % of the motors rated speed, I can work out the required frequency, from this I can calculate the value of the signal I need to send from the PLC.

Surface Speed = 10m/min #multiply by 1000 to give mm/min
Drum Size = 640mm
Gearing Ration = 6.25:1
Motor RPM = 1455

Drum RPM = (10 x 1000) / 640 = 15.63
Motor RPM = 15.63 x 6.25 = 97.66
Speed % = (97.66 / 1455) x 100 = 6.71

My maximum value on Analogue OP is 4095, so the value I need to give to VSD is (4095 x (6.71 / 100 ) = 275.

One of the drums is within acceptable tolerance, but the 2nd does not seem to follow the same formula.

Both motors are running in open loop control, the accuracy is not that critical they both need to be exact, I would just like them to match each other.

Am I on the correct track with this theory, what else do I need to take into consideration, motor slip, quality, model VSD type, etc?

Are both motors identical?
Is one drive setup different from another?

No, unfortunately the motors, gearing ratios & drum size are different.
I appreciate that this is possibly a major factor, but based the calcs, I thought I would be close?

If you are running open loop, the loads on the drums will determine their speeds. Will the drums be subject to transient loads? e.g. do they need to match as they are starting up. Please give some more info about the application.

The application is a printing machine.

Yes they do need to match as they are starting up, if anything, the lead needs to be slightly faster, so there is not a droop. This is not a closed loop tension control system, however, tension needs to be at a certain, non-critcal level.

scottmurphy said:

The application is a printing machine.

Yes they do need to match as they are starting up, if anything, the lead needs to be slightly faster, so there is not a droop. This is not a closed loop tension control system, however, tension needs to be at a certain, non-critcal level.

Click to expand...

To insure that there is always web tension, especially during startup, I believe you are going to have to do some sort of closed loop on one of the axes. At least it will make your life easier and the stability of the applicaiton much more solid and repeatable.

You can drive the master open loop with a regular V/Hz VFD. The other motor should be slaved to that one perhaps with a vector VFD. Experts here will have to chime in on whether to try to gear off of the speed of the master or close a loop on the torque of the slave but unless you do this, the whole scenario is up the physics of the system including all frictions, temperatures, humidities etc. These things will vary over time and will make your open loop system varry along with them.

I'd say that you have the math right but, since there are two drums of differing diameters, the math you've given us must only apply to one of the drums. The other drum will result in a different motor speed and there for a differently proportioned speed signal. Or, simply said, the smaller drum clearly must turn faster than the larger drum to acheive the same surface speed.

As for the comment above about the loading determining the drum speed, that is only true to a limited extent. If you are running both drives open loop, V/Hz, or scalar (lots of terms to describe the same thing--no speed feedback), then the load will cause speed variations up to the slip speed range of the driving motors. Modern high efficiency motors rarely have more than 2% slip these days so you could expect the speed difference to be within 2% worst case from the calculated values.

It sounds like you are using these drums to somehow effect tensioning in the sheet. As expressed above, I doubt that running both of these drives open loop will give you the tensioning control that you need. More than likely, you will need to tighten up the speed control (probably run it as a sensorless vector speed regulator) on the following drum and put some kind of tensioning detector in the speed trim to the leading drum. Especially at really low speeds, I doubt that the tension can be properly regulated with open loop drives.

Good luck. Hope we've helped

Dick DV, you are right, I only gave one of the equations to see if I was on the right track. The other motor is 1430 RPM, drum size 740, with a ratio of 5.2. This gives me a motor RPM req of 70.27, which equates to 4.91%, & a PLC value of 201.In the overall system, there is a total of 3 nips. Nip 1 runs in torque control, Nip 2 open loop speed control & Nip 3 also open loop speed control. Tension is maintained between Nip 1 & 2 by means of a load cell, this part of the system works fine, but at Nip 3, which is the outfeed, the tension control is not that great, there is no feedback device to indicate what the tension is. It is not critical that a certain tension is maintained, but would like it to be consistent, I though I was on the correct track doing the math beforehand, and then tuning as it is implemented.

Scott,

It appears that you will have to run both drives at very low speeds. This may not be good over long time periods, because the motors will not receive very much cooling from the built-in fans. You need to change the gear ratios or use lower RPM motors, if possible. This is in addition to adding the sensorless vector and tensioning detector as outlined by DickDV.

Best Regards,
Lancie

Thanks for all your replies chaps.

I have unfortunatly have inherited this machine from someone else, it was upgraded from DC technology. I have been back and forth from other projects, and just as I get one problem sorted, another pops up.
I realised yesterday that needed different speed motor and gear ratios to get the level of control that I need, but then I may as well go to a servo motor. All the DC motors should of been replaced with servos to get the same level of control that the DC motors provided.
I thought that the calcs would be a good way to start, but at the lower speed end is where the motor speed and gearing ratio are haunting me.

I am going to plug away and try a few different things, not the best way, to put a band aid on a broken leg, but it may work.

Actually, servos may be significant overkill for the application you describe. If conventional DC drives and motors did the job, then modern AC drives can too and for a lot less money than servos usually. You will need to choose drives capable of sensorless vector operation which, today, is common and software-selectable in the drive. Encoders on the motors are not required with sensorless vector control.

Lancie1 makes a good point about motor speed and cooling. If the power train can be inexpensively changed to shift the motor speeds up to at least half of nameplate speed, you will get better control and can use common TEFC motors. If you must stay with the same power train, choose TENV motors with the slowest available base speed. These are available from about 30hp and smaller (I like Marathon Black Max but there are others)and will operate down to zero speed without extra cooling.

If the numbers in your calculations are accurate, by changing the power train to increase the motor speed you can reduce the hp of the motor and the drive by the same ratio as the speed is increased. So, for example, if the existing system used 20hp motors running really slow, you could double the reduction in the power train so the motor operating speed doubles and you could half the hp to 10hp to preserve the same torque. If you could further double the reduction again thus increasing the motor speed to double again (a total increase of 4), you could drop the motor hp to 5hp. I am assuming that the speeds you gave us are at or near maximum desired operating speeds.

I too am a HUGH fan of Marathon Black Max motors. They have excellent low speed capabilities.

I suggest you use the speed output of the primary drive as the speed input to the follower. Then put a TRIM pot on the last drive to allow the operator to make a visual adjustment of speed.

You also might be able to scale the drive input to more closely follow the primary and let a manual trim finalize it. You did say speed matching was not super critical.