More motor problems / queries

PLucas

Member
Join Date
Apr 2002
Location
Gillingham Kent
Posts
1,742
Hi All, especially DickDV

As I have seen a number of motor related questions posted recently, so I thought that I would have a go.

Here is the problem, we have 1 Siemens VFD powering 4 motors, speed feedback to the drive is taken from a pulse encoder mounted on the shaft of 1 motor. The motors run at 1485rpm full load. The motors via a 19.8 – 1 ratio gearbox drive the wheels of the crane. A new wheel size is 630mm diameter and the crane is designed to run at 150M/min. Now then by my calculations each wheel will run at 75 (ish) rpm.
So, the motor with the speed feedback signal has a full size wheel and is happily running at 1485 rpm, 1 of the wheels has worn by, say, 2mm diameter so the wheel size is now 628mm, now with the motor running at 1485 rpm then this wheel will still turn at 75 rpm. But over the distance of 150M the wheel will need to turn 76 times, (1 more revolution than the other 3 wheels). I assume that this smaller wheel will be dragged by the other wheels thus forcing it to run that little bit faster, then, via the gearbox the motor will also run that little bit faster and bring the motor speed up to practically sync speed (76*19.8=1504rpm), at which time that motor will not have any torque, so it will be quite happy being pulled along by the other wheels (I think).

Now then, can someone shed some light on to what detrimental effect this will have on the following (if any):

1. Current of this particular motor
2. Current of the other 3 motors
3. DC link voltage in the drive
4. Anything else you can think of

Unfortunately when I served my apprentiship, I worked for British Rail where they used only DC motors for traction etc so I did not take too much notice of AC machines. Now I have to work with AC motors the old grey matter gets a bit clogged up. Any help would be gratefully appreciated.


Paul
 
Hi PLucas

One plant that I worked at used 3 or 4 motors to drive a continuous chain using only one VFD unit. For speed monitoring we measured the speed from a non-driven wheel.

We used standard thermal overloads on each motor. If one motor wasn't pulling its weight so to speak, the other motors would have to take up the slack & eventually would trip out on o/load. If for example, a drive pinion in a gearbox was slipping, the current for that motor would be down, with higher current on the other motors.

Another crane-type application I saw would use encoders to count the position of each wheel relative to other wheels - If the count difference was too high, the crane would stop on "software skew" fault. We also had limit switches as a backup system to check for the crane going out of skew (this was known as hardware skew).

I would be more worried about your crane going out of skew, rather than current drawn by the motors that could be detected by your overloads.
 
The motors in question only drive one side of the crane, we do monitor for skew faults between sides. This is mainly a theorectical question rather than an actual 'live' situation. Each gantry on the crane has 4 driven wheels and 12 non driven, the position feedback via an incremental encoder is driven by a non-driving wheel. The speed feedback encoder mounted on the shaft was put there to allow the crane to final position (within 8mm of it's target position). Just for the info our crane are fully automated rail mounted 1 over 4 container stacking gantry cranes. 24M cross travel and 520M long travel.
 
PLucas

I will try to shed some light on your query.

By my calculations, the small wheel will be turning 75.24 rpm (630/628 x 75) and the motor will be turning at 1490 rpm. This speed is probably lower than the synchronous speed of the motor. In this situation the motor will be drawing low current (at synchronous speed the motor will be drawing no load current) and the load will be shared by the other 3 motors. The synchronous speed of the motor is determined by the output frequency of the VFD and will be constantly changing slightly as the drive maintains a constant 1485 rpm (or whatever speed the VFD is set at) at the motor with the encoder.

We can take this situation one step further and consider what will happen if the wheel is worn more than 2 mm and the motor is turning faster than synchronous speed. Now the motor with the small wheel will be producing braking torque in an attempt to slow down the wheel. The amount of braking torque and the motor current will increase the more the motor speed increases above synchronous speed. The load on the other motors will increase as they have to overcome the braking effect produced by the motor with the small wheel. The braking motor puts power back into the DC link of the drive but the other motors are consuming power from the same DC source so the DC link voltage will not change. If all the motors connected to the drive are braking, then the DC voltage will increase.

I hope this answers your question.
 
multi-motors on VFD

Paul, what you describe here is a good example of what is often called "load sharing" and also a good example of a poor way to deal with it (sorry, no insult intended here, Paul).

In my experience, this arrangement of multiple motors on a single VFD never works with any real precision. However, precision may not be required.

To optimize the operation of this type of load sharing, you should choose HIGH slip motors, preferably 4-5% slip minimum. Avoid premium efficient 1 or 2% slip motors. Then, you must limit the difference in drive train ratios (including wheel wear) to not more than one half the percent slip of the motors. The drive should be configured with open loop scalar or V/Hz control with no slip compensation added. Do not use sensorless vector or flux vector with this arrangement.

Using these rules, you at least get a system that will not drive you crazy with OL trips or excessive wheel wear. It also is important to oversize the drive output amps to cover the full load amps on the motors plus 10% for each extra motor on the system--in your case with four motors it would 130% of the total nameplate amps. This will allow the drive to start all four motors from stall with good torque (avoiding drive overcurrent tripping) and also gives you a little headroom for motors fighting with each other due to differences in the drive train. Speed regulation and torque control will be a rather sloppy.

The preferred method, in my view, is to use lead-follower software (master-slave is no longer politically correct!) in each drive, one drive per motor. You configure the systems to have one lead drive with the remaining drives following at sync torque or sync speed with torque trim. Precision with this configuration will be very good. Of course, it costs more but that shouldn't be any surprise!
 
Dickdv & Vic

Thanks for your replies, they are imformative.

Dick, unfortunately this system has been on these cranes since commissioning, almost 4 years. I am sure the crane manufacturer and the Siemens drive commisioners took into account the points you brought up when they designed the cranes. This question arose because fairly recently a wheel was changed on a crane because it had bad flats and after the wheel was changed the crane 'ran like a pig'. As I was asked to look into this, the first question I asked was 'when you changed the wheel did you match it for size to the other wheels?' to which the reply was no. The wheel was changed again for one near enough the same size as the rest and lo and behold the crane ran like a dream. I was asked by the mechanical engineer to explain to his staff why this occurred, but as my AC motor theory is not up to scratch I had to bull**** my way through it, they knew no better so they accepted my explaination. I knew the wheel sizes would cause problems and I knew roughly why, which is why I was hopeing someone would be able to tell me exactly the reasons why.

Many thanks

Paul
 
Paul, personally I think your Bull**** probably got thru to them as well as anything. I think you had the problem figured out..ie mismatched wheels creates a situation that overworks the drives but doesnt necessarily make them overload/fault.

Look at it this way...you have a car/truck and one tire is low or flat..the vehicle doesnt motivate correctly. You probably explained it better to them then I just did.

What I am saying is that you "hit the bullseye" with your diagnostics, what you described has to be an OJT type learning experience to figure out. MY hat is off to YOU. For you this may be a normal occurence (I doubt that because others didnt figure it out) and for me would be a twice in a lifetime thing, maybe). BY the time it came around the second time I would have forgotten the first.

Kudos to you my friend and an excellent description of the why was posted to me.

I will state this tho in conclusion... a motor is a motor..ac or dc it has to follow the path...ie the theory behind them is proven many times over. Motors/generators can be more confusing than PLC's overall..so many variables but so much is fixed, certain things are fixed...current in relationship to voltage/hp/efficiency/load etc.

Do you realize that 2mm is approximately .078 inch, that is small. Take your thumb and index finger and point them straight in parallel to each other and its approximately an inch..a mm is approximately .039 inches.
 
rsdoran said:
Take your thumb and index finger and point them straight in parallel to each other and its approximately an inch

Say what?... :confused:

And all this time I thought I had skinny fingers... :p

-Eric
 

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