Running 230V compressor on 208V system used to power servo motors 1

[rmore]

We have a customer that wants to put our 230V equipment consisting of a compressor and some 3 phase ac motors into his 208V (200V to 220V) system. The system consists of our equipment and some 3 phase and 2 phase servo motor circuits, as well as, some ac motors, all running off of a single power transformer that is used to power the entire system. We have decided to put two boost transformers on the front end of our equipment to boost the 208V. Are their any considerations that we should be aware of for placing a 3 phase boost transformer configiration onto the input of a compressor in a closed system with a variety of servo motors?

Thanks in advance.

 

[itsmoked]

I'd certainly do the boost too. I'd rather use three transformers than two since the site currently has true three phase.

Waross will probably agree with two. I don't like creating open delta if it doesn't exist already.

Other than that there should be no issues. Make sure your boost transformers are adequately sized.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

Hi Keith. The connection that I favor does not create an open delta even though it uses transformers in an open delta configuration. You have a wye connection with a slightly offset neutral. The line to neutral voltage on the common phase remains the same. The line to neutral voltage on the other two lines changes slightly. I installed a lot of these and never had a problem. I saw a unit service (Service equipment, a two transformer auto-transformer and two MCCs in one enclosure) in an area where the utility had upgraded to 600 Volts and would no longer supply 480 Volts. The manufacturer used the open delta auto-transformer circuit. The customer had a lot of legacy motors and a lot of 600 Volt motors. So the unit service had a 600 Volt MCC and a 480 Volt MCC fed from the two transformers.
Then they started to move equipment around and we ended up with pairs of transformers all over the plant adjusting the voltages. When we had a 600 Volt motor fed from the 480 volt MCC we had two sets of two transformers, The first set in the unit equipment going from 600 Volts to 480 Volts and the second set at the motor going from 480 Volts back up to 600 Volts.
The two transformer setup is usually cheaper especially for small motors. Another advantage that is important in some installations is that the two transformer connection does not require a neutral connection back to the supply. The three transformer connection should use five wires; three phase wires, a neutral wire and an equipment grounding wire.
Try it. You may come to like it.

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

 

[itsmoked]

I knew you'd have a plan..

Got a picture or drawing handy?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

Hi Keith. Sorry for the delay. Taking it one day at a time when I was jumped by several days at once.
A word picture first.
For this application I would use a 240:24 Volt transformer.
That will boost 208 Volts up to 28.8 Volts. Close enough to 230 Volts.
Connect H2 to A phase and H1-X2 to B phase. That will give you 228.8 volts from A phase to X1 (B' on the sketch).
As you will see from the sketch the Voltage to neutral from B' and C' is a little more than 120 Volts and there is a little phase angle error, but this connection is only used for three phase, phase to phase loads.
Sizing. Easy. The low voltage winding must carry rated full load current of the motor.
Example: Motor full load current = 18 Amps, The transformer must be 18 Amps x 24 Volts = 432 VA You would use two 500VA transformers.
You could use 240:36 Volt transformers. This will develop 239.2 Volts. The transformer sizing will now be 36 V x 18 A = 648 VA. Use two 750 VA transformers.
For the same motor with a wye connected boost, you would use three 120:12 Volt connected transformers @ 216 VA. You would probably use three 250 VA transformers.
For a 239.2 Volt wye connection, it will be 18 V x 18 A = 324 VA for three 500 VA transformers.
The wye connection wants a neutral connection. It will work if the wye point is grounded with the equipment grounding conductor but that is poor practice and against Canadian and US codes. Also, using the equipment ground as a neutral point may cause interesting interactions with ground fault protection schemes.

Where I used this connection the most, running an extra neutral conductor was not a consideration. We had a number of 480V to 120V dry type transformers. About 2 KVA or 3 KVA as I remember. They were adequate for the all the squirrel cage motors in the plant. It was quick and easy to change out a 460 Volt motor for a 575 Volt motor or vice-versa. (460V > 480V, 575V > 600V)
Here's a link from Federal Pacific:

http://www.federalpacific.com/literature/drytrans/IN-7900.pdf

Scroll down to pages 4 and 5, figures G, L, H, H1, M, M1, N and N1.

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