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What determines the output voltage of a 3 ph idler motor?

lin842

Aluminum
Joined
Sep 27, 2008
Location
Richmond Va
I have a Cincinnati Bickford 21" Super Service and a Baldor grinder that I want to run in my shop. Both are 3 ph and run on 308v. I am also wanting to also get a lathe which will be 3 ph in the 3 hp range.

I have a couple 10 hp and a 20 hp 3ph motors I can use as idlers. I have already started putting together the frame to hold one of the 10hp idler's and have mounted my control box. I have also fabricated a bracket that can hold a pony motor if needed to help start the unit.

I have already ordered a kit from WNY supply that is supposed to have all the parts needed to start and run my new 10hp rpc. That is if they ever get off their butts and ship the kit out...I ordered it the beginning of last week and they still haven't shipped it out yet.

Anyway, what I am wondering is what voltage motor will I be able to run off my rpc, or what determines the voltage you can expect to get out of one of these units? Is the input voltage you have supplying the idler or is it the way the idler motor is wired? I hope that makes sense. Thanks!!
 
First issue, are you sure its 308 volts. I've never heard of that. Sure its not 208?

Next, the output voltage will be that same as the input voltage on the two legs coming from the distribution panel. The third, or generated, leg is balanced by the use of capacitors that probably are included in your kit.

A last word, do not connect any magnetic starters between the generated leg and the line leg. When starting a motor, the heavy demand for current will drop the generated leg voltage too low. Manual starters and drum switches are ok, but not magnetics.

Tom
 
The basic voltage from the idler will be a bit less than the incoming voltage, because it is the "back EMF" of the motor, which MUST BE less than the incoming voltage, by an amount dependent on the loading.

The "balance" capacitors are used to produce a boost by canceling some of the idler inductance (a "PFC-like" action, getting close to resonating the cap and inductance at the line frequency).. That action is also dependent on loading.

Net result is that the generated leg can be at or above the incoming voltage at no load, but drops under load.
 
The basic voltage from the idler will be a bit less than the incoming voltage, because it is the "back EMF" of the motor, which MUST BE less than the incoming voltage, by an amount dependent on the loading.

The "balance" capacitors are used to produce a boost by canceling some of the idler inductance (a "PFC-like" action, getting close to resonating the cap and inductance at the line frequency).. That action is also dependent on loading.

Net result is that the generated leg can be at or above the incoming voltage at no load, but drops under load.

I disagree. I have a solid piece of copper coming from the distribution panel, to the idler and hence to the load. Other than an IR drop, how can there be a voltage gradient along this wire?

Tom
 
You are not disagreeing.....

The only drop is IR drop on the INPUT to the RPC, which is what the wires yon are talking about are.....

The OUTPUT FROM the RPC, which is the GENERATED LEG...... THAT is what is at the voltage of the back EMF.... and is lower in voltage.

The input MUST BE greater than the back EMF, or the motor will have no current and cannot draw power, so the wires YOU are talking about will indeed be at a higher voltage than the basic generated leg... Remember , they are NOT "output" from the RPC, they are actually the "input" to it.

The only "output" that the RPC has is the generated leg. The other two are a "pass though" that only power the RPC (and usually 2 phase wires of the load)
 
Besides back EMF you are also dealing with a series tuned circuit. If you look at the generated leg with an oscilloscope, it typically will be higher than line voltage and too small a phase shift at no load, then progresses through the correct voltage and phase to a lower voltage and too much shift as the load increases. This isn't theory, done it lots of times. Obviously the curve can be moved around by changing the capacitor. What the idler motor thinks of that is another question. Usually it responds by overheating.

Bill
 
Tom, Sorry you are right I meant 208-230/460, Brain dead as usual. I checked yesterday and the big drill is wired for high voltage or 460. I cant figure out what the grinder is wired for because there is no diagram on the tag that shows what is what. I'll have to do some more looking to try and figure that one out. That grinder has a light on it that operates off 110. There is some kind of transformer mounted to the stand it is mounted to but I can't tell from the wiring just what it is for. I was thinking it was for the light and maybe it is but there are a bunch of wires in the transformer box that are not marked so I don't know what I have. All of this equipment was running in place at the plant before they decided to scrap it and I haven't been able to get up with the guy at the plant to ask him what voltage they were using.

I'm thinking I'll have to rewire the big drill to low voltage for it to work for me with the RPC. I guess I could get some kind of transformer to up the voltage to 460 but I'm not sure there would be a big advantage to doing that, or am I wrong again? Or if there is such a transformer option like that.

I checked the line voltage coming into my building and I have 243 on one one leg to ground and 241 on the other. Is that going to cause any problems with running those motors that are wired for 208-230v?
 
That sounds like 240 three phase with the B phase grounded. Check the voltage between the two hot legs. If it is also 243, you may be home free.

Bill
 
I'm with Bill here. With the voltage readings your giving, if they are correct, you may have 3Φ 240V corner grounded delta already in place. This type of service is pretty rare today, and is usually only found on older commercial buildings.

As Bill suggested check the voltage across the two hot legs. Corner grounded delta was a way to supply 3Φ equipment, using 1Φ electrical gear. By using ground as one of the phase conductors. But it can't be used to supply 120V loads, unless a transformer is used.

Can you show a photo of your electrical service?

SAF Ω
 
Sorry guys I haven't been around for a couple days we has a shut down at a plant and it's been 24 hr days for the past week.

With regard to the voltages I posted my numbers were all wrong. My 25 year old Simpson 260 let me down. When I posted those numbers in the back of my mind I knew they were wrong but I was in a hurry when I checked it and wrote down what I saw or thought I saw. I've had that old meter for so long and it is a great meter for some things but has always acted funny when the batteries get low. I packed it up along with a bunch of other tools to use at the plant and when I went to use it at work I realized it was acting funny. I changed the batteries and wow, what a difference.

Anyway these are the real readings In my service panel. 122.2v to ground one leg and 121.1v on the other. Then 243.1 when leads placed between the two. I'm sorry for the mistake......WOW two times in a single post that has to be some kind of record.:D
 
260's use batteries only for the resistance ranges, not voltage. Mine is mid 1970's.

Tom



Yes.... Dunno how the batteries can affect the voltage ranges. I never noticed any effect, and three should not be any, circuit-wise. Maybe your meter actually has another different problem?
 
I have never noticed this problem with a 260, but I had a funny one with an RCA pocket meter that had very similar circuitry. When I had my three phase installed, I checked it with the aforesaid meter and got readings around 500 volts. Thinking they might have put me on a 480V line by mistake, I called Union Electric and was informer, somewhat rudely, that I had a 240 volt service. I went to dinner and came back later. I checked again and got circa 240 readings. The next morning the readings were back up. Turned out that there was an AM radio station no more than 1/2 mile away, on the corner of Manchester and Hanley roads for you locals, and the power line was picking up a strong signal from it. Because the reading of the meter fell off a bit between 60 and 400 cycles, they had put a small capacitor across the resistors that coupled a little more energy around them at the higher frequency. Since the station frequency was over a million cycles, it coupled a lot of energy in that case. The station had a daylight only license, it went off at sundown and my readings returned to normal. I checked with a Fluke DVM and got good readings day and night.

Bill
 
Had exactly the same experience, but it was measuring a PFC output voltage in an SMPS. ONE specific version of the 260 would read double voltage.

It was funny in a way, because I knew there was a chance of odd readings from the high frequency "ripple", and picked an analog meter for that measurement because it would NOT be bothered by the high frequency part.... Three other versions of the 260 had no trouble.
 
That grinder has a light on it that operates off 110. There is some kind of transformer mounted to the stand it is mounted to but I can't tell from the wiring just what it is for. I was thinking it was for the light and maybe it is but there are a bunch of wires in the transformer box that are not marked so I don't know what I have. ...

Put 110V to the light side of the transformer and check the voltage at the other side. That reading will be in the ballpark of your machine voltage.
 
Put 110V to the light side of the transformer and check the voltage at the other side. That reading will be in the ballpark of your machine voltage.

I'll take a look at that when I get back on this project. They used to be pretty good at the plant that I got these machines from about tagging wires when hooking up motors and transformers. They used to have real electricians that would hook this stuff up but every now and again somebody will install something and not tag or leave a wiring diagram in the box.

I'm sure somebody that knows more than I do about motor and transformer wiring could figure this mess out. If your idea doesn't work out I'll try to get someone else to look at it. Thanks!
 
Is there any real advantage to a shop like mine that is running 3ph equipment off of a RPC to run something like this 3hp Super Service drill on the high voltage setting? I mean; I know to do this I would have to add a step up transformer to make it work along with possibly something else also. I have a transformer I could use but is there any real time advantage of doing so?
 
It means you wouldn't have to shift wiring and change thermals. And if there are single voltage motors involved, a transformer would be preferable.

Just depends on which way is the least amount of work, IMO.
 








 
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