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.

 

[BobM3]

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)?

 

[rovineye]

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.

 

[LionelHutz]

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.

 

[rovineye]

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.

 

[waross]

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

 

[rovineye]

cross section is identical.

 

[Marke]

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

 

[LionelHutz]

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

 

[rovineye]

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.

 

[rovineye]

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.

 

[roydm]

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?

 

[waross]

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

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