I am going to wire a air compressor that has a tag 10 hp 230/460 volts 3 phase.the service is 120/208 3 phase so i will need to use a 24 volt boost transformer at .75 kva. it will be connected open delta requiring two transformers and i understand how to do that.The motor amps are 28 on table 430.250 and can be increased by 250% if using a circuit breaker.So a 70 amp breaker at the panel can be used to feed the motor. When I install the boost transformer will it still be protected if using a 70 amp circuit breaker?do i need any protection on the secondary besides the motor starter?
buck boost transformer
- Thread starter norsky63
- Start date
- Status
- Location
- Connecticut
- Occupation
- Engineer
No, I don't think you can do this. I think your buck-boost (2-0.75kVA open delta) will have a rating of 12.39kVA. The input current would be 34.4A at 208V-3ph. 450.4 says that the autotransformer shall be protected at not more than 125% of the input current (or the next standard size up.) This would be 45A.
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
norsky63: Is there a typo in your original post? I do not see how even three single phase 24 volt boost transformers or a three phase 208 to 24 volt isolation transformer can convert 208Y/120 to anything but a very strange near-230 delta with possible neutral problems. Which would be within the motors operating range, but still on the low side. (230 volt rated NEMA motors really expect 240 supply, IMHO.) In any case, that would ONLY work with a closed delta on the output side, not an open delta, since the third output phase wire would also have to be offset from the incoming phase voltage.
If you are intending to use a 120 to 24 volt isolation transformer or autotransformer on each line to neutral leg to convert the 120Y to 148Y, in hopes of making use of the 256V delta that could be formed from the L1, L2 and L3 terminals, then it would theoretically work, but be nominally too high a voltage for your motor.
When you say that the motor is rated for 230/460 delta, you mean that it is a six winding motor which has the option to be wired parallel or series for use with either 230 Delta or 460 Delta? Because there is no such standard thing as a 230/460 supply.
Executive summary: It can be made to work, and the transformer rating would only have to handle between ~1/8 and ~1/5 the power rating of the motor, but it will really confuse anyone who has to work with it later, possibly to the point of being dangerous.
so if the motor will start on a 60 amp breaker and i use (2) 1.0 kva transformers hooked up open delta.2 x 10 x .86 =17.2 kva @236 volts=42,12 amps x125% =52.65 amps next higher breaker 60....does that sound right?
- Location
- Connecticut
- Occupation
- Engineer
:?
Two 0.75kva 240-24V transformers would boost the voltage to 229V with sufficient capacity to serve the load.
Or two 1.0kVA 240-32V transformers would boost the voltage to 236V.
- Location
- Connecticut
- Occupation
- Engineer
Yes, I think you could use 1.5kVA transformers with a 70A breaker or 1.0kVA transformers with a 60A breaker.
thanks
- Status