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#98795 - 06/07/06 10:41 AM sizing transformers for motors  
bigpapa  Offline
Member
Joined: Mar 2006
Posts: 81
canada
Does this make sense?
to run a 460V motor on a 600V system. All figures are approximate:

3 PH 40HP 460V motor FLA = 52A wire size is min #6 CU

600 to 480V autotransformer would be 13.5 KVA
(600V - 480V) x 1.73 x 52A = approx 11KVA

Use three 600V-120V 5 kva transformer in a buck configuration for a 15KVA bank, you will be wiring a motor that requires a #6
copper wire to feed it, to a transformer that has # 12 GA wires coming out of it. It doesnt seem right.

I bought a 30 KVA autotransformer that has #12 wires for connecton. The #6 wires that feed the motor get a bit warm during normal
use, I can imagine how warm the #12 GA wiring will be.
Am I missing something here?


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#98796 - 06/07/06 11:45 AM Re: sizing transformers for motors  
feather  Offline
Member
Joined: Feb 2006
Posts: 25
Santa Maria CA USA
My notes show a 43.2 kVA xfmr requirement for 40HP with an additional 20% for motors started more than once per hour.


#98797 - 06/07/06 09:36 PM Re: sizing transformers for motors  
bigpapa  Offline
Member
Joined: Mar 2006
Posts: 81
canada
Would this calculation be for an autotransformer or a standard isolation transformer. I get similar numbers as you when I calculate for the isolation type.


#98798 - 06/08/06 12:27 PM Re: sizing transformers for motors  
feather  Offline
Member
Joined: Feb 2006
Posts: 25
Santa Maria CA USA
I have never used a auto xfmr to feed a 3 ph induction motor, however I have installed many triple ganged Variacs in wound rotor secondaries. They have to be sized for the FLA as they are in series with the load. Your correct on the size of transfomer 3 x 15 Kva = 45 Kva, but the winding wire size seems too small to me.


#98799 - 06/08/06 02:30 PM Re: sizing transformers for motors  
winnie  Offline
Member
Joined: Sep 2003
Posts: 649
boston, ma
Several points. I've not figured out this entire issue, but several things jump out at me:

1) The _primary_ current of a 5KVA 600V to 120V transformer is only about 8A. The load current will not be flowing through the primary coil, and I would expect small conductors on the primary side. The _secondary_ current of one of these transformers should be about 42A, and I'd expect larger conductors.

2) While you did calculations for an autotransformer, the transformers that you described using are standard dual coil transformers which you will be connecting in a buck configuration, requiring slightly higher KVA ratings. The _full_ load current must flow through the 120V coil, which means that the transformer KVA must be 120V * full load current.

3) I don't see how you are connecting _three_ 600V to 120V transformers in a buck configuration. You can use an 'open delta' connection with _two_ 600V to 120V transformers (with suitable ratings), and I can see symmetric connections that would work with three transformers that have _dual_ 40V secondaries (the secondaries would be connected in something of a 'zig-zag' fashion). You could also use these three transformers with the primaries connected _wye_ to buck the voltage down, but this uses the primaries at only 347V, which reduces the effective KVA of the transformers (you are using 600V to 120V transformers as 347V to 69V transformers, so the KVA available is reduced to 57% of the nominal rating).

4) I don't know the _code_ for any of the possible connections that I am dreaming up; just scheming with the physics [Linked Image]

-Jon


#98800 - 06/17/06 07:11 AM Re: sizing transformers for motors  
Scott35  Offline

Broom Pusher and
Member
Joined: Oct 2000
Posts: 2,708
Anaheim, CA. USA
***This Message Is Cross-Posted With A Duplicate Thread Found At The Canadian Electrical Code Area. Attached is A Pasted Copy Of That Message, Plus A Link To That Thread ***

P.S.: Schematics for polyphase "Buck/Boost" Autotransformer connections are available in the Technical Reference area. Check the "Menu" located at the Tech. Reference area's main page for links to these drawings, or just search the forum's pages.
-------------------------------------------------------------
-------------------------------------------------------------

bigpapa

Maybe I can offer a bit of assistance with your Motor / Transformer questions.

1:
Quote

3 PH 40HP 460V motor FLA = 52A wire size is min #6 CU
600 to 480V autotransformer would be 13.5 KVA
(600V - 480V) x 1.73 x 52A = approx 11KVA


For the Motor vs. Transformer KVA capacity in this example, the Transformer would require a "Per-Winding" Capacity of at least 23.920 KVA for an Open Delta connection scheme, or 14.404 KVA per winding for a "3 Transformer / Closed" Wye configuration.

2:
Quote

Use three 600V-120V 5 kva transformer in a buck configuration for a 15KVA bank, you will be wiring a motor that requires a #6
copper wire to feed it, to a transformer that has # 12 GA wires coming out of it. It doesnt seem right.


I calculate the individual Transformers for a Voltage Bucking (Output Voltage Reduced) setup - using 600V X 120V Isolated Transformers - to be minimum of 7.5 KVA each.

A 5 KVA Isolated Transformer with a Secondary Voltage of 120 VAC is only good for 41.67 Amps, which falls below the FLA of the Motor.

With an Isolated Transformer of 7.5 KVA Capacity - and a Secondary Voltage of 120 VAC, the maximum load Amperes of which the Secondary Winding is able to _Safely_ handle continuously is 62.5 Amps - which exceeds the FLA rating of 52 Amps for the Motor.

The only part of the Transformer that will be carrying the Motor's Load Current (for the "Buck-Boost" Transformer arrangement), will be the Secondary Winding - or Windings (if "Split-Coil" type is used).
Therefore, the "Primary" Windings' terminal leads may be of small conductors - such as the # 12's you mentioned, without any threats of Barbecued Wires!
The "Primary" side will only draw Magnetizing Currents, which will likely not exceed a few Amperes during normal operation (may be quite high during Motor's Starting).

The rated load current for the Primary Windings of these Isolated Transformers are:
* 5 KVA - 600 VAC Primary = 8.334 Amps,
* 7.5 KVA - 600 VAC Primary = 12.5 Amps.

3:
Quote

I bought a 30 KVA autotransformer that has #12 wires for connecton. The #6 wires that feed the motor get a bit warm during normal
use, I can imagine how warm the #12 GA wiring will be.
Am I missing something here?


This Autotransformer may be too small in KVA size for this Motor's FLA rating.

My figures show:
* MINIMUM 45 KVA total capacity - for a "Full/Closed" Wye Autotransformer configuration (Motor Load should be no less than 14.404 KVA "Per Line" for a continuous load, across 3 separate 1 Phase 2 Wire Transformers),

* MINIMUM 50 KVA total capacity - for an Open Delta Autotransformer configuration (Motor load should be no less than 23.920 KVA per Winding set, with a total capacity of no less than 47.840 KVA across 2 separate 1 Phase 2 Wire Transformers).

4:
Quote

So, my question is... how is it that we can calculate what the curent carrying capability of the windings are? My guess is that the primary windings would need the ampacity of the load. Our codes do not address this issue and there is very little information out there on it.


Applying the expected Load to find the MINIMUM CAPACITY of an Isolated Transformer - when creating an Autotransformer arrangement, is done as follows:

1: Determine the Maximum Continuous Full Load Current of the Load to be driven,

2: Figure what the required OUTPUT Volatge should be to the Load, and compare this to the "Existing" System Voltage available,

3: Find a Transformer with a Secondary Voltage which will bring the Available System Voltage to the desired level of the Load's rated Voltage (by either increasing the Voltage or decreasing the Voltage),

4: Multiply the Voltage rated for the Secondary of the Transformer, by the Load's designed FLA, to find the minimal KVA rating for the Secondary Winding (and overall - the entire Transformer)

*Example:

We have a Motor with nameplate ratings of 500 VAC, and Full Load Amperes (FLA) of 10 Amps.
The existing Power System is only 400 VAC, so we need to increase the Voltage another 100 Volts.

The MINIMUM KVA Capacity for the Isolated Transformer to be used for this scenario will be 1.0 KVA - since the Secondary will be wound for 100 Volts, and we need to carry 10 Amps through it.

This is true for Single Phase Motors and 3 Phase Motors / Loads.
You need to figure the _Highest_ expected and continuous Full Load Amperes for the Load(s), then how much the Voltage needs to be adjusted - and apply the diiference in Voltage X the FLA to calculate minimum Transformer Secondary Capacity.

Again, the "Primary Windings" will not be carrying the Load Current, only Magnetizing Currents.

Good luck - and be sure to respond back if you have more questions (or if things are unclear).

Scott35

p.s. I am "Cross-Posting" this message to the other one in the "Canadian Electrical Code" area.

Sizing Autotransformers For Motors

S.E.T.


Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!


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