3 Phase Induction Motors

1. Why was a motor that had been working fine for many years replaced?
2. Why was a motor with a different speed selected?

The motor was corroded by the harsh environment.These motors are from the same manufacturer but different dates.

Of what significance is the difference in dates?

I do? How stupid of me. I've got two different speed motors on the same shaft!

I really need to change one of them quickly before the high current burns one of them out.

Pass the spanners, there's a dear.

You could always put the crane on a circular track!

Please forgive the snarky remarks of my colleagues, we all get a little grumpy sometimes.

Yes, the difference in base speed will make all of the difference. The one with the higher base speed will drag the other motor as an additional load, until the total physical load becomes more than it can contribute and the 2nd motor begins running in slip. But while running, the slip and therefore the load on that first motor will always be more.

In addition on a crane bridge travel situation, matching the speed is critical. Without that, your bridge can skew, meaing one side moves faster than the other and it gets crooked (hence the funny comment on using it on a circular rail). If / when it skews enough, it will either jam on the rails or worst case, a wheel will slip off. Bad news either way.

When you replaced the one, you should have replaced them both with IDENTICAL motors from the SAME mfr bade at the SAME time. They need to match as exactly as is practical.

I concur.

I worked on a dockside crane for unloading containers. They were getting uneven wear on the trolley rails due to poor torque split between the motors, along with issues of other misalignments.

I discovered I don't like being 100 ft above a ship on a crane swaying with the loads and wind.

You either need matched pairs of motors or you need to run each of them from AC Drives. Running them in "open loop vector" mode (you need drives that can do this) will probably suffice, it does depend on how much "crabbing" the layout/rails can accommodate.

Often buying two matched motors however is the easier route.

If you get problems or the application already looks difficult you need to get help from the crane manufacturer or a control system company that has good drive experience.

You can run 2 bridge motors on 1 VFD. We run both on a 75KW drive on"ECL"cranes and have for years with no problems.

It isn't rocket science.

^{"You can run 2 bridge motors on 1 VFD. We run both on a 75KW drive on"ECL"cranes and have for years with no problems.}

^{It isn't rocket science."}

I think you missed the point. His motors do not match. If they are both on the same VFD, the mistmatched speed and the problems with it will just follow, it will not solve the problem. Having separate VFDs allows you to force them to match.

The nominal speed of both motors is 1500

1440 & 1480 means that the motor, respectively, will develop its maximum power or torque at these speeds.

1440/1500 = 0.96 which is the slip while 1480/1500 = 0.9866 giving a difference of 2.7% slip

Now, all depends on the installation and the point of balance:

If the 1480 motor is forced to run at a lower speed because it has to drag the 1440 motor, then it will be overloaded and will heat excessively. Otherwise it will be taking a bigger share of the load but within its capability.

Please check the running rpm (difficult task on a moving crane?!), or the current on each motor to see if they are running above or below their minimum rated speeds (or currents).Also, indicate which motor is running hotter? You might have a different problem(!?).

Ideally, both motors should be identical to minimize the load difference between them, but if not overloaded, then they can operate with a small difference as long as the crane railing etc can accomodate and it is well within the maximum loading allowed.

......... your motors need to be match pair motors. There are no other fixes for your problem. I use to work on the worlds longest suspension crane years ago, we had a set schedule to pm all our motors on this crane to keep from being down. We also had 3 spare motor pairs in-case there was a catastrophic incident as well. I know your feeling about being 100ft plus spanning the docks.

Good luck

Maximo

You've answered your own question - if one motor is drawing a lot more amps then it is working harder. You don't say which, but presumably the new one is drawing more, or there's something strange going on.

The new motor tries to go faster, so there's a potential to crab or go off a rail, as other posters have said, but you haven't mentioned that, so presumably the crane works OK, and both motors run at same speed (as mechanically coupled).

On that basis, if the original motors both gave power P, I calculate the new one gives 1.5*P, and the old one 0.5*P, both running at 1470 rpm. Current ratio also 3:1.

Is the new motor current above FLC? If the motors are generously sized, it may not be, and then you might get away with things as they are, at least for a while. But as other posters have said, safer to fit identical motors.

Codey

I am sure if you look on the maint hand book it will tell you to replace BOTH long travel motors together............not difficult is it to work out!

Thanks, everyone for the good answers.It is pretty clear to me now that matching the speed close as possible plays an important role on the operation of the motor.

The motor characteristics should be identical.In such cases it is better to replace both motors with two new identical motors

You can put VFD on each of the motors and adjust the speed of the motor.

Re-reading all the posts we all made some rather large assumptions.

The following needs to be confirmed:

- both motors are on a common shaft? (or equivalent due to connection to track and gear box arrangement etc?)

- What are the N/P ratings of the motors, rpm, volts, power - are they identical?

- when you state 1440 and 1480 rpm are these measured speeds or motor N/P ratings?

- is the motor at 1480 rpm (N/P or actual rpm ?) the one drawing more current?