|Forum topic by Chris||posted 04-26-2015 10:22 PM||624 views||0 times favorited||2 replies|
04-26-2015 10:22 PM
I have recently acquired a three phase pneumatic drum sander. The kind that possesses an inflatable drum on each end of the motor’s horizontal shaft. The motor is a dual voltage (240/480) 3ph. motor. It must be a wye start – delta run motor because there are 12 leads in the motor junction box.
My dilemma is, the motor must have been specifically custom wound for some owner of the yesteryears to be able to execute some wanted function. It has a typical -forward, off, reverse- function drum switch mounted upon it. Within that drum switch someone has ‘crossed’ the line and load center connections (B phase). The A and C phase connections are wired in typical fashion.
The dilemma is- I don’t believe that the individual mounted the drum switch to obtain a reverse function on this sander. I think it was a cheaper alternative to purchasing two separate definite purpose contactors, one to start the motor and the other to run in delta, after the first contactor fell out.
I am familiar with drum switches…to switch the line and load of one phase through the switch simply doesn’t make much sense. That is the purpose of the drum switch after all, to reverse the leads for you by its design. Thus, the reason I am leaning towards the wonder if someone used the forward function for the wye start portion of the motor and the reverse functionality of the switch to run the motor in delta.
The fact that the motor doesn’t have much of a load on it…only two lightweight inflatable drums, this arrangement might have been a cheaper alternative to obtain a two speed motor….(hence wye start would be slow and the high rpms would be in the delta run mode) that much I do know.
Just wondering if anyone has ever heard of a drum switch being used in this function? I am thinking that by design, after motor’s locked rotor inertia has been overcome through the wye start, if someone wanted to they could simply switch the drum switch over which would carry the motor into its delta run mode. It seems like a manual way of performing what two contactors linked to timers would normally do.
Does this make sense?
The only concern is, arc flash when the switch makes connection on its secondary side. The switch by design breaks contact and passes through its neutral plane (off)- we know. However, whether or not there would be significant arc flash during the connection of the reverse side of this switch, to switch the motor over to delta run is unknown. The fact there is minimal torque created because of such a small shaft load leads me to think that perhaps this manual switch could indeed be used in this capacity without the arc flash. Well, at least on the lower 240 volt app.
Just not sure.
One thing, I do feel confident in, is that I doubt very seriously the switch was installed to provide a reverse function. Just would not make any sense to do so. Plus, the nuts on shaft would run off. So the ‘installer’ had some other plan in mind. Apparently it worked for years…secondary owner purchased and used on 240 volt system. “Never used the reverse function the entire time”, he said. So this leads me to believe second owner was in essence running in wye mode all the time…reduced speed and did not realize the motor even had a second, much higher speed built into its design
-- Chris Harrell - custom callmaker "Quacky Calls" Eastern NC. http://www.quackycalls.com