High leg voltage on delta-delta 3 phase

[dt38k]

I have read discussions regarding a 3 phase delta with the wild leg that measures 208 to ground
In a search, I came across thread237-95429
For the most part, I understand the voltage and learned shortly after this system was installed into my new pole building years ago about the wild leg
This wild leg sits in the center of the main panel, and in two additional panels marked with orange tape
I have two lathes wired directly after the disconnect into 5 position,6 pole drum switches and the wild leg position is burning the contacts
The remaining contacts also wear at a lesser rate except for the 6th pole that is for a 90 volt dc brake
I wondered if a resistor could be used on the wild leg after the disconnect to lower the voltage
I feel that it might be best to design these machines with an on/off switch and then use magnetic starters, using the drum switch for the coils only
Any suggestions would be appreciated, there has to be a better way
Thanks, Randy

 

[itsmoked]

Hi Randy.

I dislike drum switches. I hate being called out to deal with those evil things.

What's likely happening is that with the accursed drum switches you can reverse the motor while it's still turning which puts enormous electrical stress on the switching elements. That is called 'plugging'. All you need to do is plug it a couple of times and the contacts are toast. With a stinger leg that set of contacts gets it just a little worse.

I would go with a reversing contactor and delay-on switching to prevent any-and-all plugging. If you want to retain the ability to plug then you need to size the contactor and the reversing contactor to handle it. Typically something like three sizes bigger.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

You haven't mentioned if the motors in question are single phase or three phase.
If the motors are correctly connected to three phase, the wild leg is immaterial.
The motor sees 240 volts three phase and the motor windings can't tell which leg is the wild leg.
If the motors are single phase the wild leg should never be used and so should not be an issue.
The wild leg is called the wild leg to identify it as the leg which should never be used for single phase loads.
The only time when there may be problems is when a piece of equipment needs three phases and a neutral. Motors do not use a neutral connection, so no problem.
In some instances a voltage unbalance may cause one phase to have a higher voltage and as a result a higher curent.
That may lead to accelerated contact burning one that phase.
That is because the voltage, phase to phase is higher, not because it is a wild leg.
As for plugging to reverse or to stop, that was common years ago with the old "U" frame motors.
It is not a good idea with "T" frame motors. It is abuse of a "T" frame motor and can lead to motor burn-out and/or contact burning.
Note: The wild leg has a higher voltage to ground and to the neutral but the phase to phase voltages are equal and the motor windings see only the phase to phase voltages.

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

 

[Compositepro]

You say 5 position drum switch. Does this mean that the positions are FWD-BRAKE-OFF-BRAKE-REV? Switching DC is much harder on switch contacts than switching AC. Perhaps this is the cause of the contacts burning. A solution would be to put a capacitor across the problem contacts as done for breaker points on old ignition systems.

 

[dt38k]

Keith
I’m no big fan of the drums switches either
Using the switch to power switches would eliminate the contact burning as you have mentioned
Bill
The motors are 5 hp 3 phase
Composite Pro
The drum switch is wired where when in the lever is in center position, pole number 6 simply makes a connection of the wire that is fed through a rectifier to the dc brake
Those contacts actually show no arc wear
It is the other five wires associated with the 3 phase wires to the motor that are taking the hit
Positions are
Center is brake
One up and down has no power which allows the spindle to free wheel coming off of two up and down which are the forward and reverse positions
When I first got the machine, the wild leg was being switched and is was cooking two sets of contacts
After rebuilding the contacts, I positioned the wild leg so that the switching was done one the other two
This caused the wild leg to only burn one set of contacts
Another downfall of this system is that unless I shut off the disconnect, the brake is actually hit all the time
As to quick changing from forward to reverse, I have never done that on this machine

 

[waross]

Are you fed from an open delta transformer bank?
Often a large single phase transformer feeds numerous loads. Possibly 50 KVA.
To generate the wild leg, a small transformer may be added in open delta.
If this is for only a few three phase motors, it may be lightly loaded and the voltage may be higher than the larger transformer.
It doesn't take much extra voltage to cause a lot of extra current.
Whatever, try rewiring the switch so that the coasting positions connect the wild leg.
When the switch is placed in the run position the wild leg will be already connected.
Both Murphy's law and the law of unintended consequences may come into play.
The other two poles may wear faster than before but there is a good chance of overall improvement.

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

 

[waross]

By the way. I have seen quick reversing on a lathe unscrew a three jaw chuck and drop it onto the ways, still spinning.

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

 

[dt38k]

Ive always understood that my service was wye delta 3 phase, and it seems like the main panel is 600 amps
Two power wires were ran underground and tapped into two transformers sitting alongside my shop
They buzz 24-7 and it costs me around 70 per month by themselves, before I even hit the light switch inside the door
If I would have known then what I know now,I probably wouldn’t have chose this option
There was only a 1,500 difference in the initial cost to go this route but I don’t recall anyone mentioning the stand by minimum fee
Not too long after this was installed, I was at an auction over in Indiana and there were close to 30 transformers for sale on the list
Most of these were step down from 480 volt and by luck, a person that I had only met a few weeks before, spotted the one and only delta unit and it was a 75 kva
Of course on average, most of the others sold around 100 each and when they came to the one and only, they added a small one for farm use and it cost me around 500 for the pair
I have it wired for a buck boost 240-480, but I haven’t hook anything up as of yet
I have heard stories about lathe chucks spinning off, most of the big lathes are either L series taper,cam lock, or A series which don’t have that issue
I tried several times to post a picture of the wiring diagram from this particular machine, and it just won’t recognize my photo library
I just unloaded another machine exactly like mine but 5 years newer
Someone has modified the wiring on this one, with four extra boxes hanging off a 3/8” plate
The 110 volt trigger coil wires from 4 separate magnetic switches, are fed into the drum switch and the contacts look like brand new pennies
This could be the ticket to replicate
I have a 5 ton bridge crane that is 3 phase and all the motors are driven through the 24 volt pendent into two good size boxes with a but load of electronics inside
I use this almost every day and have had zero issues with it,,maybe I should take a look at that wiring diagram?

 

[waross]

There are not many 460 volt motors around.
The standard is 230/460 Volts.
It is easier by far to reconnect a motor for 230/240 Volt operation than it is to transform up to 460/480 Volts.
I don't know the current practice but years ago, when a 480 Volt plant sent a motor in for re-wind it may have come back wound for 460/480 Volts only.
Motors below 10 HP were scrapped and replaced.
Bt the way:

I would go with a reversing contactor and delay-on switching to prevent any-and-all plugging. If you want to retain the ability to plug then you need to size the contactor and the reversing contactor to handle it. Typically something like three sizes bigger.

The crane contactors will be sized for severe duty.

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