Motor Help

Thread Starter

benha

Joined Jan 4, 2011
58
Howdy!

Hoping this is a reasonable/appropriate forum for this question.

I'm trying to get a new sander working in my shop. It runs on 480V 3ø. It's fed from two 230V phase converters coupled together (one 20HP the other 30HP), which then goes through a 45KVA, reverse-fed transformer to get to 480V for the tool. That part's all working.

The sander has two sanding heads. One's got a 30HP motor, the other has a 17HP motor. The smaller one works fine. The larger one notsomuch. When I hit the starter switch (DOL wired) the contactor stutters. I assume what's happening here is that the inrush current is dropping the voltage sufficiently that the coil in the contactor releases and the circuit opens, which then bumps the voltage back up allowing the contactor to close, ad nauseum until I release the button. (Which I do quite quickly.)

There's plenty of amperage to run the tool at sanding load as long as I don't push it really hard, but I need to figure a way to get that big motor spinning. I'd really like to avoid buying a soft start or something like that since it's both expensive and requires a bunch of rewiring. Is there a way to add some big capacitors or something to get past the inrush?

Thanks,
-Ben
 

MaxHeadRoom

Joined Jul 18, 2013
23,092
You may need to research where the contactor coil is fed from, it sounds as though the motor current is also causing the coil voltage source to drop.
If the coil is 120AC or 240AC, a DC version may prevent the contactor from chatter and dropping out.
AC coils are not the best once engaged, it takes very little volt drop to get them to chatter.
If this is the reason, a DC version with possibly a LV transformer and a Electrolytic Cap across the bridge rectifier would solve it.
Max.
 

LesJones

Joined Jan 8, 2017
3,172
Can you give more details of the phase converters. I assume they are not VFDs as there would be no way to synchronize their outputs. I assume that they use capacitors for phase shift. If this is so then the phases are probably not very well balanced. It could be that the contactor coil is being fed between two phases (Or between a phase and neutral.) whose voltage drops under the starting load. Rotating the phases MAY solve the problem.

Les.
 

Thread Starter

benha

Joined Jan 4, 2011
58
Can you give more details of the phase converters. I assume they are not VFDs as there would be no way to synchronize their outputs. I assume that they use capacitors for phase shift. If this is so then the phases are probably not very well balanced. It could be that the contactor coil is being fed between two phases (Or between a phase and neutral.) whose voltage drops under the starting load. Rotating the phases MAY solve the problem.

Les.
One 30HP one 20HP, rotary converters. They're phase matched. This part of the system has been working correctly for a couple of years. It's only the 460V part that's new.
 

Thread Starter

benha

Joined Jan 4, 2011
58
You may need to research where the contactor coil is fed from, it sounds as though the motor current is also causing the coil voltage source to drop.
If the coil is 120AC or 240AC, a DC version may prevent the contactor from chatter and dropping out.
AC coils are not the best once engaged, it takes very little volt drop to get them to chatter.
If this is the reason, a DC version with possibly a LV transformer and a Electrolytic Cap across the bridge rectifier would solve it.
Max.
This is actually what I'd been exploring this morning. It LOOKS like the control circuit is actually 110V powered by an onboard transformer. If that's the case, I may look at just providing a direct 110V line to the machine and bypassing the little transformer. That would put the source for the contactor upstream of the RPCs and the step up transformer which could (maybe) create enough separation that it doesn't get drawn down and chatter.
 

Thread Starter

benha

Joined Jan 4, 2011
58
This is actually what I'd been exploring this morning. It LOOKS like the control circuit is actually 110V powered by an onboard transformer. If that's the case, I may look at just providing a direct 110V line to the machine and bypassing the little transformer. That would put the source for the contactor upstream of the RPCs and the step up transformer which could (maybe) create enough separation that it doesn't get drawn down and chatter.
Didn't work. I got a bit further down the road, but all that I accomplished was letting the thing draw long enough to trip the 125A breaker that feeds the phase converters. Looks like I'm going to need a soft start. :-(
 

MisterBill2

Joined Jan 23, 2018
8,740
Since the problem sounds like it is with the contactor in the motor starter losing voltage, a simple solution would be a separate power supply for the starter contactor, instead of powering the starter from the rotary converters. Another single phase step-up transformer rated for the starter coild current would not cost that much.
 

Thread Starter

benha

Joined Jan 4, 2011
58
Since the problem sounds like it is with the contactor in the motor starter losing voltage, a simple solution would be a separate power supply for the starter contactor, instead of powering the starter from the rotary converters. Another single phase step-up transformer rated for the starter coild current would not cost that much.
Yeah. Just tried this. Per the above it just exposed the next issue which is that the 1ø feed to the RPCs isn't enough to handle the inrush.

Sigh.
 

MisterBill2

Joined Jan 23, 2018
8,740
Another possible option would be a separate motor, lower power single phase, to spin the sander up to speed, and then connect the 3-phase motor. Possibly a lot more mechanical complexity, but I have seen such schemes used many years ago.
 

MaxHeadRoom

Joined Jul 18, 2013
23,092
Yeah. Just tried this. Per the above it just exposed the next issue which is that the 1ø feed to the RPCs isn't enough to handle the inrush.

Sigh.
So it would seem that even sourcing the coil from a non-interrupted source does produces the same result?
I cannot quite see how a contactor armature can 'chatter' with a known fixed voltage?
But it also sounds as though you have supply problems also.
Another alternative is set up a star/delta starter arrangement.
Max.
 

MisterBill2

Joined Jan 23, 2018
8,740
I have seen some very destructive "chattering" in a multi-speed starter for a 200 HP motor in a machine sold to a third-world country. There is seldom such a thing as "fixed voltage" at these power levels, because the connecting mains are often not heavy enough. Consider that at 100 amps a one ohm resistance will create a 100 volt drop in the voltage.
In the case of the multi-speed starter it resulted in the contacts being burned away and the starter being made unrepairable.
 

Thread Starter

benha

Joined Jan 4, 2011
58
Another possible option would be a separate motor, lower power single phase, to spin the sander up to speed, and then connect the 3-phase motor. Possibly a lot more mechanical complexity, but I have seen such schemes used many years ago.
Heh. Yeah. Certainly possible but seems like by the time I got done with all that engineering I would be into it for more time and money than just getting a soft start and getting on down the road.

Would be fascinating though. The two head motors are right next to each other. Add clutch pulleys to them and tie them together with a belt...
 

MisterBill2

Joined Jan 23, 2018
8,740
Yes Max. And that was not the total solution because the incoming power capacity was not adequate. So while a VS drive or a soft start drive could work, it would need to be an improvement in efficiency to be able to do it with the present mains connection.
And I am trying to imagine what kind of sanding needs 30 HP. Nothing that I am familiar with.
 

Thread Starter

benha

Joined Jan 4, 2011
58
Yes Max. And that was not the total solution because the incoming power capacity was not adequate. So while a VS drive or a soft start drive could work, it would need to be an improvement in efficiency to be able to do it with the present mains connection.
And I am trying to imagine what kind of sanding needs 30 HP. Nothing that I am familiar with.
It's a 37" wide belt sander. Sanding is a notoriously power-hungry application.
 

MisterBill2

Joined Jan 23, 2018
8,740
I don't see anything about ratings suitable for a 30HP motor in that table, nor anything that looks like a current rating. So how they could be used is not clear.
 
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