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Hoist motor not slowing
3

Hoist motor not slowing

Hoist motor not slowing

(OP)
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.
 

RE: Hoist motor not slowing

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

RE: Hoist motor not slowing

"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.

RE: Hoist motor not slowing

Are the brakes part of the motor?

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

RE: Hoist motor not slowing

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

Keith Cress
kcress - http://www.flaminsystems.com

RE: Hoist motor not slowing

(OP)
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.

RE: Hoist motor not slowing

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

RE: Hoist motor not slowing

(OP)
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.

RE: Hoist motor not slowing

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

RE: Hoist motor not slowing

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?

RE: Hoist motor not slowing

(OP)
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.

RE: Hoist motor not slowing

(OP)
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.

RE: Hoist motor not slowing

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.

RE: Hoist motor not slowing

(OP)
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.

RE: Hoist motor not slowing

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.  

RE: Hoist motor not slowing

(OP)
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.

RE: Hoist motor not slowing

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 ?

RE: Hoist motor not slowing

(OP)
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.

RE: Hoist motor not slowing

We need exactly the uncoupled current only.

Good luck with this.

RE: Hoist motor not slowing

2
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

RE: Hoist motor not slowing

So it regens and a mechanical brake is applied?  Maybe too much inductance in the new motors (can't change speeds fast enough)?  Or, cheaper lamination material (saturates with high regen currents)?

RE: Hoist motor not slowing

(OP)
Yes, the brake sets to get it from low speed to stop.

 I do not have any good info on the rotor. Only generic description of the squirrel cage parts. No, I can't get one of the old motors to tear apart.

RE: Hoist motor not slowing

I take it you've checked the gearbox ratios, cable pulley diameters the platform mass and anything else like that which could have changed?

If the systems are the same except for the motor manufacturer then I think you already know the answer. The motors can not be the same despite claims by the manufacturer. However, to prove it you will either need to test the new motors against a known torque specification or you will have to test an old motor vs a new motor. Hopefully, being a military specification there is a requirement for the torque the motor will produce when changing speed which can be tested against.
 

RE: Hoist motor not slowing

(OP)
Gear boxes etc have all been verified as correct. I get the right amount of travel for given motor revolutions.

 We did get factory data from the old motors and there is really no discernable differences, including amperage loaded and unloaded.

 But, what we have found, is the conductivity of the phosphor bronze rotors bars is 18%. The design (and supplied certifications) call for 11%. Now I think we will look for some 11% material and build a rotor to but in the newer stators.
 

RE: Hoist motor not slowing

I would check that the low speed winding is connected for constant torque rather than variable torque. You need constant torque for a hoisting application. If you have been supplied with a low speed winding designed for variable torque that would certainly give problems.
If the rotor performs well on the high speed winding, it may be alright despite the different bars. How does the cross sectional area of the bars compare?

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

RE: Hoist motor not slowing

(OP)
cross section is identical.

RE: Hoist motor not slowing

The resistance of the rotor bars will have a major impact on the high slip torque. If the bar resistance is different, then the torque will be different.
Lower resistance results in lower torque.

Best regards,
Mark.

Mark Empson
L M Photonics Ltd

RE: Hoist motor not slowing

I agree with what Mark posted. Lower rotor resistance will typically lower the torque when the slip is high.

 

RE: Hoist motor not slowing

(OP)
I will post the results of a rotor swap once we can get some spec material. I hope that is the only cause of the problem.

RE: Hoist motor not slowing

(OP)
Higher resistance rotors bars (the material that was spececified but not delivered & used originally) were installed in a new rotor, inserted in a previously failed stator, and that test motor installed. It performed perfectly. Stars to those that zeroed in on the rotor conductivity.

Now I can get down to the business of swapping out rotors and motors.

RE: Hoist motor not slowing

Could it be the slow speed current is too low allowing extra slip?
Is there any way the windings could have been changed from delta to Star?

RE: Hoist motor not slowing

Thanks for the feed back, rovineye. That's one you don't see every day!

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

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