Hoist motor not slowing 3

[rovineye]

I have a speed problem with a hoist motor. In the down direction, it shifts to slow speed at the correct point, but when the platform reaches the stop proximity switch it has not fully decelerated, causing it to be going too fast when the brake sets and resulting in an over travel.

The background is that the proximity switches are placed at very specific points, which I can not change. I have had 2 dozen or so motors installed without issue. Then the place of manufacture changed, and since then all 7 of their motors have failed to slow down rapidly enough. The manufacturer is using the same specs as the previous manufacturer, with only one exception: the wire diameter is slightly larger due to a shift to AWG. Other than that, they claim the motors are identical.

Before I learned of the shift of manufacture I was concentrating on the ship's power, raising voltage trying to see if torque might be limited in regeneration. No help. I can't think of any other issue with my power plant that could be a limiting factor. The hoist motor reacts the same way on utility power as well.

These are simple 2 speed, dual winding motors, across the line controllers.

The motor manufacturer is on the way to run some tests, but if what they say is true about using the same manufacturing details, I am stumped.

 

[Skogsgurra]

If it doesn't behave identical, there is a difference and that difference is in the motor.

How "slight" is the wire gauge change? Percent area change?

Braking from high to low speed will put you on the "wrong side" of the peak torque and that's where torque production is very dependent on rotor characteristics. Are the motors NEMA types or European types? There are different rotor classifications for the two types. You need to have identical rotor types to have identical braking.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[edison123]

"with only one exception: the wire diameter is slightly larger due to a shift to AWG"

Which wire diameter ? Stator or rotor ? If it is rotor, then its resistance will change along with the torque.

 

[waross]

Are the brakes part of the motor?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[itsmoked]

You can also have an inertia difference on the rotor with different wire size.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[rovineye]

Thanks for the replies.

The wire change decreased winding resistance by 9% (stator).

The motors are MIL-SPEC, not NEMA

I am told the rotors are identical, but I dot have any information on the rotor yet. I have asked that the air gap be verified as well.

The brakes are mounted on the end of the motor. Machinists spent a lot of time tearing apart brakes thinking that was the problem, because at one time all these hoists tested good under load. As soon as I heard the motor brake set I knew the motor was running way too fast (1000 RPM vice 300 RPM) and the brake might have held when very new, but won't last.

 

[waross]

If the motor is running 1000 RPM instead of 300 RPM, it is probably wound for the wrong speed.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[rovineye]

No, it is slowing from high speed to slow speed, loaded in the down direction. If I don't let the brake set it will slow to 300 RPM. It just doesn't do it quick enough.

 

[edison123]

How do change from 1000 to 300 RPM ? Timer or manual ?

 

[BobM3]

Have you disassembled and inspected the motors (old and new)? You might spot something that is adding significant inertia. Are the two windings installed in the same order on the stator? Maybe the pole shape of the iron is a bit different?

 

[rovineye]

The platform starts down at high speed (1800 RPM) and when it reaches a proximity switch it changes to the slow speed windings (300 RPM) via a simple across the line contactor controller.

It continues down for a set, predetermined distance, and reaches the stop proximity switch, at which point the contactor drops out as the brake sets. At that time the motor should have slowed to the 300 RPM when the brake sets. But it isn't, it is still at 1000 RPM. We don't use braking resistors as this is not a VFD.

The motor manufacturer is floating a theory that there may not be enough load on the island grid to accept the regenerated power, but I don't buy it. These are only 150/25 HP motors. They can regenerate all the way back to the very large generators in the unlikely event that we had a small load on the bus.

 

[rovineye]

BobM3,
That is problematic. All the good motors are on delivered ships, I don't expect them to be receptive to giving me a motor to tear apart and leave them without one. But it certainly has been discussed!

Right now we are trying to find someone from the closed factory in Europe to see if they can come and see a difference at the new facility. Long shot.

 

[edison123]

Have you counter-checked the starting speed is 1800 RPM ? If the speed checks ok, then I see only shifting first prox switch to a higher point would help.

 

[rovineye]

Yes, high speed is correct.

Yes, I can raise the flag or switch a foot or so and all is well. All I need is a few more seconds. But, since the spacing of the switches and flags is a military specification, it would be easier for me to rewind the motors using copper I personally mine from passing meteors than get the spec changed. I will have better luck getting that change once I can explain why it is needed, which I can't do for certain at this point.

The manufacturer is is doing some dyno testing at factory and on site testing on 3 ships next week. I hope at that point they come around and conclude that THEY have a problem.

 

[edison123]

The lower stator resistance is not the problem here. On the other hand, it is good for the motor due to lower stator copper loss.

The question is how did they achieve the higher cooper area if the frames are identical to the old motors. If they cut no. of turns to achieve higher copper area, then the flux density increases resulting in a different torque when compared to old motors.

 

[rovineye]

I don't know how they cram in the extra copper. I know rewind shops do it from time to time when the wire they have on hand is not exactly the same diameter. A quote I have from the manufacturer is "New windings increased the diameter of the wire when converting to AWG, but the we use the same length of copper".

The winding diagram has not changed, still showing the same number of turns.

 

[edison123]

Have you compared no-load (open shaft) currents of the old and the new motors ?

And does the winding diagram really show the number of turns ?

 

[rovineye]

The winding diagram lists the number of turns.

I haven't been able to get factory test data on the old motors yet. That would be the only uncoupled data available. I have no way to get it shipboard. At least not yet.

 

[edison123]

We need exactly the uncoupled current only.

Good luck with this.

 

[Marke]

I would be looking very hard at the rotor.
If the motors are identical, is it possible to swap a rotor from an old motor to a new one and see what the difference in performance is?
The rotor is the primary influence in high slip torque assuming that the flux density is the same.

If there is a difference in the supply voltage, or the stator turns, or the air gap, or the stator lamination steel, there will also be a variation in the high slip torque.

Best regards,
Mark.

Mark Empson
L M Photonics Ltd