motor turns slowly with rpc power

jamie1107 · Aug 9, 2008

-hi
im having some problems getting three motors i have running from my rpc

the rpc is 10hp and the motors which wont run are 1.5hp 3hp and .5 hp

the rpc voltages are good and it runs my other motors fine

the motors are all brook and compton multi voltage 240 380 415 v

i have tried them in star and delta and none of them run at the propper speed the 2 larger ones bearly turn over and the small one just about makes half speed
all of them work fine on my friends 3 phase mains supply
im just lost as to why they ont work with my rpc

any ideas anyone

johnoder · Aug 9, 2008

If they are muti voltage, they HAVE TO BE reconnected in the motor terminal box for the specific voltages.

Isn't UK "real" three phase 415v?

If they run on that, it is because they are connected for that in the motor terminal box and are being fed that.

John Oder

JST · Aug 9, 2008

1) if they are turning, ESPECIALLY turning slowly, your RPC IS in fact giving them 3 phase. Otherwise they would just hum.

2) running slow is very typical of good bearings and a very low voltage. Induction motors run on "slip" , which is the difference between the synchronous speed and actual motor speed. That difference is how power gets into the motor.

Anyway, at very low voltage, not much power can be gotten into the motor, and the slip has to be extreme to make it turn at all. That means slow speed.

So, presumably your motors are NOT wired for the voltage your RPC is actually putting out.

jamie1107 · Aug 10, 2008

hi
thanks for the advise although im still not understanding whats going on
as i said i tried wiring them in star (low voltage mode 240v 3 phase which i guess would run with two phases of 240 )
and also delta (high voltage 380v -415v)
the voltages i get out out of the rpc are 230v and 209v not very well ballanced i know but it seems to be ok with all my other motors some of which are of the same type by brook (dual voltage)
any ideas what i could try next

johnoder · Aug 10, 2008

any ideas what i could try next

What is voltage at:

friends 3 phase mains supply

?

I.E., when motors run fine?

You could also explain in detail what you do when you reconnect for the low or high voltages.
Here, with our common 9 wire three phase motors we, for example, connect them thus for low voltage:

4,5 & 6 together, hooked to nothing
1 & 7 together, hooked to L1
2 & 8 together, hooked to L2
3 & 9 together, hooked to L3

The above in the motor terminal box.

which i guess would run with two phases of 240

This is an incorrect guess. Since they are three phase motors, they will not start on "two phases"

John Oder

jamie1107 · Aug 10, 2008

its a 6 wire motor wires u1 u2 u3 to input uu1 uu2 uu3 together low voltage
u1 and uu1 u2 and uu2 u3 and uu3 inpust high voltage just as the brook diagram shows
they run fine on 3 phase mains eg the compressor they are fitted to works as it should
3even with the compressor disconnected on the rpc the motor just about ticks over

2 phases of 240 plus neutral always present i will use that in future to avoid confusion

Pierce Butler · Aug 10, 2008

jamie1107 said:
-hi
im having some problems getting three motors i have running from my rpc

the rpc voltages are good

Please define what "good" is as measured at the motor terminals.

ToughTool · Aug 10, 2008

jamie1107 said:
hi
... the voltages i get out out of the rpc are 230v and 209v not very well ballanced i know but it seems to be ok...
quote]

jamie,
This and your comment of "2 phases of 240 plus neutral always present", leads me to believe you are working with a single phase circuit, not 3 phase.
Are your RPC voltages 230, 230, 209? (AB, AC, BC) Joe

JST · Aug 10, 2008

If he has only single phase the motors won't start, they would need "help" to start.

Since I don't think he mentioned that, and he has an RPC operating, he must have 3 phase......

ToughTool · Aug 10, 2008

I didn't say he was using single phase motors and it's his "Friend", that has the real 3 phase power. I was trying to understand what he is working with.
So why did he mention "two phases and a neutral", and only lists two voltages instead of three, in all of his posts. Also he said"(low voltage mode 240v 3 phase which i guess would run with two phases of 240 )", makes no sense to me. Do they connect neutral to a 3 phase motor as a phase in the UK? I think not. Maybe RPC circuits are different over there than here in the states. Joe

johnoder · Aug 10, 2008

I wonder if six wire multi voltage motors can be three phase. :rolleyes5:

John Oder

JST · Aug 11, 2008

im having some problems getting three motors i have running from my rpc

the rpc is 10hp and the motors which wont run are 1.5hp 3hp and .5 hp

the rpc voltages are good and it runs my other motors fine

Apparently the RPC is operating to some extent, unless we are using the same words and descriptions in totally different ways.

ToughTool · Aug 11, 2008

JST,
OK, from the information in your (jamie1107's) quote, the only motor that is apparently running, is the RPC's motor. I just wanted to know the three, 3 phase output voltages of his RPC, to help jamie with his problem. He only listed two voltages, and neutral. Joe

Pierce Butler · Aug 11, 2008

ToughTool said:
JST,
I just wanted to know the three, 3 phase output voltages of his RPC, to help jamie with his problem. He only listed two voltages, and neutral. Joe

I did too and without those voltage loaded and unloaded it is almost impossible to troubleshoot.

jamie1107 · Aug 11, 2008

thanks for the advise
they are loaded rpc voltages
uk three phase can be odd 415 v over 3 terminals (which is what i have )generally two phases of 207 and neutral which is a potential earth
they are definatly 3 phase motors hence the 415 volt tag they are also multi voltage as stated on the tag and confirmed with brook uk
the rpc was simply rigged from a spare motor to get these compressors running whilst i carry out maintanance hence no balancing was deemed to be needed
415 volts is hard to produce from our uk 240v mains
and i have found multi voltage motors tricky to run before without a vfd drive

ToughTool · Aug 11, 2008

jamie,
Here is a schematic of a typical US power system. I have always thought that European power systems were pretty much the same, except that the frequency is 50 Hz and the voltage was double. i.e. 240 volts between L1 and neutral and 480 volts between L1 and L2. If this is the case then you would not be using the Neutral unless you were using a low voltage motor wiring as the RPC. This would also produce a low voltage 3 phase output of 240/240/240 with the motor producing a manufactured leg (B) and two manufactured phases (AB and CB), with or without the run capacitors. Please advise if my assumptions of your UK power's nomenclature and power systems are incorrect. It would be nice to get the phase to phase voltage reading of your RPC (Not phase to neutral). Referencing the schematic I have posted, what is the voltage between A and B, A and C, and B and C? I have added a BASIC schematic of a 415 volt single phase input RPC. Please notice the Neutral is not used. However I am sure a (Green) safety "Grounding Wire" must be used. The run capacitors C1 and C2 are voltage and current balancing capacitors, if you use a small motor (i.e. 1.5 to 3 times the size of the load motor).
Joe

toolnut · Aug 11, 2008

Guys,

I am not specialist on European electrical but I do remember Peterh5322 posting a reply, to a posting from a member in South Africa, on this very subject. Peter posted a schematic of a rotary converter for use on the type system in question here. The converter has to be connected sightly different than the American systems normally posted here. If you will post a question directly to Peterh5322 I suspect he will answer it.

ToughTool · Aug 11, 2008

jamie1107 said:
hi
...i tried wiring them in star (low voltage mode 240v 3 phase which i guess would run with two phases of 240 )
and also delta (high voltage 380v -415v)...

Toolnut,
I tried to find your reference using the search but I think peter has too many posts and not having the right keywords, makes it almost impossible to find. Anyway I think jamie will end up wiring the motor 415 volt Delta and most likely wire the RPC idler motor Delta with 415 volts to L1 and L2 (A and C) as indicated in the basic schematic. Here is a link to some information about Delta and Wye systems and until peter or some other engineer confirms the correct required connections, then it would be good reading. Here is an excerpt from phaseconverterinfo.com.

"If a piece of three-phase equipment operates only on phase-to-phase voltages, it does not matter whether the power is configured as wye or as delta. If none of the circuits in the machine or the motor being operated derive a voltage by referencing neutral/ground, the machine does not know or care where ground/neutral is in reference to any of the phases. It only “sees” the relationship between the phases. It is evident that if you reference the three voltages of wye power and “erase” the connection to ground/neutral, you have a triangle, the same as in delta configured power. Only when equipment has single-phase 120V circuits that are derived from phase-to-neutral connections does it matter if the power is configured as wye or delta."

So, I think neutral is not needed and you will need to connect the RPC as Delta with your 415 Volt single phase power to one of the three 415 volt motor windings and label them L1 and L2, which will also be A and C. When started, the motor will provide B and phases AB and CB. I guess it is possible to connect 415 volts to a Wye wired idler motor winding but I do not know for sure how to connect it that way. Joe

http://www.phaseconverterinfo.com/phaseconverter_deltawye.htm

JST · Aug 11, 2008

OK....

Agreed on the line-to-line voltage is what matters.

The building I work in has 230V "corner-grounded" power..... meaning that one line is grounded..... but it is NOT "neutral".

All motors work as normal, even 3 phase rectifiers work fine, so long as the load is floating. But some voltages vs a neutral could be pretty wild.

And the RPC should certainly not reference neutral in a US type system. Mine goes across the (US) power line which is as ToughTool posted. No neutral needed, not used. Contactors are 230V coils.

But in a UK system, if you have 230 (or other voltage) from hot to neutral, you will end up with a system after the RPC, which is similar to our "corner-grounded" system. One RPC output will be at neutral voltage, but it will also be a regular circuit conductor, and can be treated as one.

ToughTool · Aug 12, 2008

JST,
Cool. I recall a post about an Australian on a farm, that had three phase wired with one phase at ground potential. I suspect jamie has his 220 volt RPC wired as phase to neutral, which would produce a three phase system similarly, with one phase at ground potential, assuming of course, they ground the neutral at the service entrance, in the UK. I believe also jamie was just not measuring phase to phase and presenting the values to us in a way we could understand. Referencing the schematic I have posted, is this how you would connect a Wye wired motor? Joe

motor turns slowly with rpc power

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