3-phase motor slowing down

[cerdeira]

I've powered an old 3phase planer/thicknesser through an inverter, one of those fancy units which convert 230V to 400V. It fires up ok, except for a little chatter in the starter, even the independent dual speed feed motor works. Takes about 10 sec to reach full speed but then runs smoothly, no noticeable strange sounds or vibration.

My problem is that I can easily slow down the motor under load. I've installed brand new knives and I'm taking about 1mm per pass on 100mm wide ash, yet I can almost grind it to an halt. The motor (british Crompton make) is rated 2.2kW (3HP), so I think it should be powerful enough for this task. I've done visual inspection of it but found nothing suspicious. It also doesn't get very hot, in fact the pulley was hotter than the motor.

I'm kind of lost here, is this a problem with the motor or the inverter? I reckon that if the inverter was faulty, I'd be unable to start the motor in the first place but I don't know.

Any input is appreciated.
thanks.

 

[WhyDi]

Good morning,

Unfortunately I am not electricity inclined but set up the variable frequency drive on my combination machine. So I endeavoured to go a bit further and wrote some papers about that tricky subject. You may read about vfd and motor here. That document describes what kind of motors could be driven.
I also was told that motors manufactured before the eighties do probably not correctly work.

Best Regards

 

[gardenshed]

Is it a static inverter, rotary version or the electronic version ?.

Is the motor dual voltage ?.

Need a bit more info to help you.

I also was told that motors manufactured before the eighties do probably not correctly work.

Sorry but Not true.



.

 

[PeterSk]

Interesting that you say the pulley is getting hot, it shouldn't really be getting too hot in general use. Is the pulley slipping on the motor shaft or the belt slipping on the pulley maybe?

 

[dickm]

Not much help, but I had a similar problem with the single phase 3hp motor on my Lurem. Consulted the oracle on all these things (Bob, aka 9fingers), and he made lots of useful suggestions. Never did discover what was wrong, but it seemed to sort itself out!
But the moral is - for anything electric motor wise, ask 9fingers!

 

[9fingers]

I suspect that the problem lies within the inverter.

If there was 2.2kW going into the motor and virtually no mechanical power going in, something would have to be getting very hot.

Check the inverter programming carefully for preset limits etc.
Check to see if it has measured the motor windings correctly on start up. In fact I don't think it can of as you say the starter chatters a bit implying that the motor is not connected when the inverter fires up.

Inverter manuals are often quite difficult to follow being written by someone who knows the peculiarities of the design rather than by someone who knows how to explain things. You might have to talk to the manufacturer or agent.

Sorry i can't be more helpful but I have never played with a voltage converting inverter.

Bob

 

[miles_hot]

is it worth marking the pulley and belt in some way to see if the belt is slipping on the pulley (would explain the pulley being hotter than the motor maybe)?

Miles

 

[cerdeira]

Thanks for the responses.

I think I've found the problem, at least part of it. The pulley is indeed slipping on the shaft, not the motor one but the cutterblock's. That shaft is not keyed, and so the pulley is held with a grub screw instead. I found it was broken. I think it was replaced before because the grub screw seems to be made of mild steel and its head is slotted I was expecting one with an hex socket.

What's the best way to remove off a broken screw? I also need a replacement one, and to make things worst i believe it's an old whitworth screw. I could try to tap a metric thread but I'm not going to risk that for now. I don't know if the cutterblock steel is easy to tap manually.

Does anyone know an online source of machine parts such as pulleys, screws, etc that stocks imperial and whitworth sizes?

For the electrically inclined my inverter was reading 400V and 1.8A with the motor running under no load. The motor rated current is 4.55A (for 400V Y-wound). Does this no-load current seem acceptable?

thanks in advance

 

[9fingers]

In my experience you are never going to successfully transmit 3hp without a keyed pulley and shaft. You "might" just get away with a taperlock pulley.
Forget grubscrews it will end in tears.

Bob

 

[cerdeira]

Forget grubscrews it will end in tears.

Probably I haven't used the right term, it's a headless screw that goes into a threaded hole perpendicular to the shaft and a part of it stands above thus providing a "key" to slot into a notch cut in the pulley. That screw broke. I hope that made sense, I'm not a native english speaker.

I also think the shaft should be keyed, but ironically that's an 35 year old cast iron british machine so they should know what they were doing.

 

[9fingers]

No - you had the correct term. I'm equally surprised that your motor does not have a keyway cut in the shaft for that power.
I would still investigate taperlock pulleys. You need a bush to suit the shaft diameter - they are made in inch and metric sizes and the pulley with the same taper as the bush. They grip very well on the whole shaft not just at a point like the grubscrew.

You English is very good - don't worry!

Bob

 

[WhyDi]

Good morning,

I also was told that motors manufactured before the eighties do probably not correctly work.

Sorry but Not true.

May you explain a bit more ? is there really a limit in term of age ? not from a theoretic perspective but in the real world and for men like me that are not specialists.
Best Regards

 

[9fingers]

Inverters as well as producing 3phase supplies from single phase allow variable speed operation.

When motors are fun a significantly below design speed the cooling is poorer. More modern motors have a slightly better tolerance to high temperature running and so this just might have been the reason you were told this.

Also when inverters became popular some motor manufacturers claimed their motors were inverter tolerant as they specified for running at greater than 50 Hz.

I have run all sorts of motors old and new from between 10Hz and 100Hz ie between 20% and 200% of plated speeds with no ill effects. Most inverters measure winding resistance and therefore can tell when motors are overheating and they then shut down.
I have had this when doing lots of low speed screw cutting on my engineering lathe. All I need to do is brew a cuppa whilst it cools down or change the drive ratio. Inverters let you be lazy!!

hth

Bob

 

[dickm]

When motors are fun

Only you could possibly say that

 

[9fingers]

When motors are fun

Only you could possibly say that

Doh! Another tryping error but of course motors are fun!!!

Bob

 

[cerdeira]

Hello,

I've managed to disassemble the cutterblock. I took some pics along the way.

Here's the whole assembly. Notice that the bearing housing for all the rollers is machined from one single piece of steel. Also no standard keyways in any shaft. The feed rollers shafts have an half female thread (kind of a threaded cove) that aligns with its counterpart in the sprockets providing a regular hole for a grub screw.

The cutterblock pulley which is unusually long. Those notches slot into the grub screw.

now the main offender. that cutterblock alone weighs more than 15kg. notice the broken grubscrew bit at the bottom right.

I've managed to redrill the shaft with a 5mm bit, then off to ebay to get a whitworth 1/4" tap to revive the original threads.
I'm not so lucky sourcing a BSW grubscrew so I'll try an UNC one and retap it. I think the difference between them is that the thread angle in UNC is bigger by 5deg, so converting to BSW with a die should be possible,

Anyway, something tells me I'm still going to scr*w this before it gets finished.

 

[9fingers]

I would have expected the shaft to have a plain steel pin right through from one side to the other and no thread.
The pin would be equal in length to the diameter of the slotted end of the pulley.

The purpose of the pin is to transmit the drive but also to shear off in the even of a serious mechanical problem.

I have seen people (incorrectly) replace this type of pin with a bolt. This may or may not be what has happened to your machine.

I am not familiar with this machine at all but the use of shear pins is a standard engineering practise.

Another standard engineering method or holding something onto a shaft is to use a tapped hole half in the shaft and half in the pulley or wheel. I suspect this is what has been done one the feed roller shafts.
The two parts are keyed together by the screw which is locked in by the torque of the drive.

Bob

 

[WhyDi]

Good morning,

Thanks for the reply based on your own experiment, 9fingers. I would have liked to go further on inverter issues but Cerdeira needs to find out a solution and I don't want to sidetrack. On the other hand photos larger than 640x480 pixels embedded into content make me unable to read the messages . May be later... and good luck Ceirdera.

Best Regards