3 phase question

[Fritz Maurer]

Just wondering how the single phase spreader motor in my drying bin works on 3 phase.

The output from the transformer is ostensibly 220V at 75K kilowatts.

I am dividing the total voltage by 3 , but the voltage per leg doesn't match the voltage of the motor. Is this the wrong computation?
Thanks, Fritz

 

[Turbo Weasel]

You don’t divide the voltage, the load needs to be rated for the output of the transformer. Plus, a single phase motor doesn’t use all three phases. Only line-to-neutral (single phase 120v) or line-to-line (single phase 208/220/230/240v) Can you post a pic of the motor and transformer nameplate?

 

[used red MN]

The output from the transformer is ostensibly 220V at 75K kilowatts.

Do you run other items that plug into 110v single phase at this bin site? That is line to neutral. If you ran a regular cracker box welder on 220 at the site it would be single phase line to line. For example purposes and disregarding proper circuit protection if you have a cord going to a 3 phase motor at this same site the conductors would be available in that cord to power either of the above examples by choosing the correct ones.

 

[Turbo Weasel]

Do you run other items that plug into 110v single phase at this bin site? That is line to neutral. If you ran a regular cracker box welder on 220 at the site it would be single phase line to line. For example purposes and disregarding proper circuit protection if you have a cord going to a 3 phase motor at this same site the conductors would be available in that cord to power either of the above examples by choosing the correct ones.

A 3-phase motor doesn’t use the neutral, just the three hot legs. If you connect a 120V motor to any two of the circuit conductors feeding a 3-phase motor, it will burn out.

 

[Destroked 450]

3 phase voltage is measured across any 2 legs, not all 3
If you have 240 volt 3 phase each leg will measure 120 volt but total voltage is only measured on two legs

 

[miangus]

It depends on the transformer. When I worked for the silo company, we would run the single-phase motor that powered the hydraulic dig-out tool by connecting to one leg and the neutral/ground in the box. It had to be 220 three-phase.

 

[Fritz Maurer]

Do you run other items that plug into 110v single phase at this bin site? That is line to neutral. If you ran a regular cracker box welder on 220 at the site it would be single phase line to line. For example purposes and disregarding proper circuit protection if you have a cord going to a 3 phase motor at this same site the conductors would be available in that cord to power either of the above examples by choosing the correct ones.

No, this transformer was installed solely for the bin. This is how it was originally wired.

It is my desire to tap the bin at the junction box and have 3 phase in my garage. But I have to to learn more about it.

In an emergency, our local motor repair shop said run any 2 legs into the breaker box ( from that Delco generator I posted a couple months ago) and that would work. Which i did. All the motors worked (compressor, both garage doors) but poofed the 24V operating systems in both door openers and the waste oil furnace. That is why I want to approach this project with a little more caution.... and currently the numbers don't make sense to me.

 

[olddavid]

There are different 3 phase transformers to consider. One type has what is called a wild leg and will measure 200+ volts to neutral ground on that wire. If you run an outlet from that point and plug a 120 volt motor into it, you will likely burn it up. Measure the voltage to neutral ground before hooking an outlet to it. Also, it needs to be properly fused to operate it safely.

 

[Fritz Maurer]

You don’t divide the voltage, the load needs to be rated for the output of the transformer. Plus, a single phase motor doesn’t use all three phases. Only line-to-neutral (single phase 120v) or line-to-line (single phase 208/220/230/240v) Can you post a pic of the motor and transformer nameplate?

I am currently in the middle of a meter setting re-do, an the transformer is currently disconnected so I can obtain no voltages at this time.

If the name plate is on the transformer, it is locked inside the cabinet. There are just some numbers stenciled on the outside.

I can't get to the motor thru the top of the bin, will require scaffolding on the inside. But the wire going to it is through a single blade switch, a single fuse, and a neutral.

 

[used red MN]

A 3-phase motor doesn’t use the neutral, just the three hot legs. If you connect a 120V motor to any two of the circuit conductors feeding a 3-phase motor, it will burn out.

Sorry for the misinformation, you’re right there is only a bonding ground in the 3 phase supply to a motor, bad example.

 

[Turbo Weasel]

I am currently in the middle of a meter setting re-do, an the transformer is currently disconnected so I can obtain no voltages at this time.

If the name plate is on the transformer, it is locked inside the cabinet. There are just some numbers stenciled on the outside.

I can't get to the motor thru the top of the bin, will require scaffolding on the inside. But the wire going to it is through a single blade switch, a single fuse, and a neutral.

Ok, well, from your description, it sounds like it is a 120V single phase motor. Do you have any literature on the spreader that might indicate electrical specs? You really need to be sure on the voltage requirement of the motor and the output of the transformer. Then sizing the wire correctly comes into play. As you are aware, trial and error with electricity can lead to a safety issue not to mention equipment failure.

 

[Janicholson]

Different words, may be helpful: three phase service has three hot conductors they are any two of them have the rated voltage of that system Yours (from your original post) is 220. There should be two additional wires, these are a neutral and an equipment ground. In many cases where the system does not use any 120volt single phase lights or motors, there is no neutral. Because yours has a neutral, the voltage to that is 120 from any one of the hot line wires. Yours is running a motor in the bin with 2 wires, one of the hot wires and a neutral. That would make it a 120volt motor.
If you connect 3 phase from the existing system, put in 4 wires L1,L2,L3, Neutral from the first box, and an equipment ground. (that may have additional ground rod/s at the garage, but must be also connected to the the neutral but only back at the first box. Jim

 

[MarkB_MI]

There's really not enough information in your post to give a definitive answer, but I can take a stab. If I understand correctly, you have a bin dryer than is powered by 220 vac, three phase power. This setup also powers a 120 vac single phase spreader motor. You would like to run 220V three phase power into your garage.

First off, there are two different flavors of three phase: wye and delta. In the wye configration, there are four conductors: Three hot and one neutral. In a typical wye circuit, the voltage between each hot leg and neutral is 120V. The phase-to-phase voltage between any two hot legs is 208V. (120 times the square root of 3.)

In a delta configuration, there is no neutral, only three hot legs. The phase-to-phase voltage might be 240V between any two legs.

What you might have is a "high leg delta" setup. In a high leg delta, one of the transformer windings is center tapped and a neutral conductor is provided. If the phase-to-phase voltage is 240V, you will get 120V between either of the legs connected to the center-tapped transform and neutral. My guess is this is what you have.

As for what happened when you tried to draw power off your three-phase to power your home, it sounds like the neutral was left floating. This caused the voltages on either leg to neutral to rise and fall; most of the devices in your house could tolerate the voltage swings but the 24V transformers could not.

 

[Jim Wendt]

Sounds to me you need a qualified electrician before you fire yourself, someone else or equipment. It only hurts for a little while.

 

[used red MN]

it sounds like the neutral was left floating. This caused the voltages on either leg to neutral to rise and fall; most of the devices in your house could tolerate the voltage swings but the 24V transformers could not.

Do you have any rough idea of what amount these voltage swings could be?

 

[MarkB_MI]

Do you have any rough idea of what amount these voltage swings could be?

It depends entirely on how well the loads in the house are balanced, but the voltage on each leg could swing as much 240 volts. Let me explain.

If the loads between the two legs are exactly equal (same current), there will be 120 volts on each leg, for a total of 240 volts. So it will be indistinguishable from when the neutral is properly grounded.

Now let's consider the extreme condition: One leg has a dead short on it, and the other leg has no load (an open circuit). The shorted leg will have zero volts, and the open leg will have 240 volts.

In the real world, of course, neither of these conditions are likely. You're going to have an imbalance between the two legs, and the voltage is going to swing high on one leg and low on the other. The more load there is on a leg the lower its voltage will be and the higher the voltage on the other leg. But if you turn off everything on one leg and turn everything on on the other, you're going to have the second scenario where there's 240 volts on the leg where everything is off. And if you were to turn on, say, a light, on the leg with 240, it's going to pop.

Note that the 24 volt transformers in your home are normally on 24/7, which makes them vulnerable to voltage swings.

 

[olddavid]

You need someone knowledgeable to check and see what you have. This is the "high leg delta" that Mark mentioned and I was alluding to. The center of one of the windings is grounded. Between the transformer connection points you have 240 Vac. The connection points of the center tap grounded coil are at 120 Vac to ground but the other connection point is at 208 Vac to ground. It's not uncommon. Our old church building was wired that way.

 

[RJKeith]

Just wondering how the single phase spreader motor in my drying bin works on 3 phase.

The output from the transformer is ostensibly 220V at 75K kilowatts.

I am dividing the total voltage by 3 , but the voltage per leg doesn't match the voltage of the motor. Is this the wrong computation?
Thanks, Fritz

The single phase motor uses two of the three hot legs of the transformer. A few cautions. First, the load on the transformer will be unbalanced. This is OK, but it means you will not get the full 75 KVA out of the transformer. So if it is a large motor it would be best to replace it with a three phase motor. Second, your voltage out of the transformer is 208 volts phase to phase, some motors that say 220 volts will not tolerate 208 volts, though most will. If it will tolerate 208 volts it usually says so on the motor nameplate. If needed, a buckbost transformer can be used to get the 220, but at that point it would be just as cheap to replace the motor with three phase.

 

[RJKeith]

3 phase voltage is measured across any 2 legs, not all 3
If you have 240 volt 3 phase each leg will measure 120 volt but total voltage is only measured on two legs

No, if he is getting 240 hot to hot he will get 240 between each hot, but he will get 120 to neutral between two hots and neutral but 208 volts to neutral between the third hot to neutral. This is a 'high-leg' or center tapped delta system, and is not used much in new work anymore. The normal way is a wye system with 208 volts phase to phase and 120 volts phase to neutral. But it is important to remember is you are getting 240 volts phase to phase then one phase to neutral will be 208 volts.

 

[Fritz Maurer]

Im still gathering info to present here, but just want to quickly clarify that the 24V transformers failed at another location, where I had a 3 phase generator inside the garage at the time I made the emergency hookup and cooked the transformers.

I was not hooked to the transformer in question.

Here is the crusty remains of the schematic of the generator I used to fry the transformers. As I recall, the doors worked one cycle, so the transformers didn't fry right away... but they didn't work in the morning when the power came back on