I got a call late today, The customer just had a commercial appliance tech tell him that he has a "voltage problem". The complaint was low voltage at the oven @ 180 volts 2-Pole single phase. I stopped in to take a quick look, I measured the voltage to the panel supplying the oven and found 114/109/115 to phases AB&C to ground respectively and about 196 volts between phases which is higher than the appliance tech measured 4 hours earlier. The oven is name plated for 208 single phase and is connected to legs B&C. There is a 45KVA 480D to 120/208Y transformer supplying the tenant space and the primary side of the txfrmr reflected the same voltage difference on "B" phase. Could this be a simple primary tap change on B phase? I am going back early tomorrow morning to dump the panel and test the transformer voltages with no loads, then turn the loads on 1 at a time while metering the load on "B" phase.
Transformer Primary taps
- Thread starter dken31
- Start date
- Status
beanland
Senior Member
- Location
- Vancouver, WA
Voltage Drop Issue
Voltage Drop Issue
If I understand, there is a 45kVA 480D-208Y/120V transformer feeding load. Part of the load is a 208V single-phase oven. When the oven is operating, the voltage drops to 180-190V on the 208V.
(1) if the voltage is low when there is no load, the problem is upstream of the transformer. Check for voltage levels at the feeder breaker feeding the transformer. If the voltage is low all the time, retapping the transformer will help. Retap all the windings the same.
(2) if the voltage is good when there is no load, the problem is impedance. If you change the transformer taps, the voltage will go high when the oven is off. How large is the oven load relative to the transformer? What is the impedance of the transformer? You may need a larger transformer, or one with lower impedance. The problem may even be with the 480Y/277V service being overloaded.
Voltage Drop Issue
If I understand, there is a 45kVA 480D-208Y/120V transformer feeding load. Part of the load is a 208V single-phase oven. When the oven is operating, the voltage drops to 180-190V on the 208V.
(1) if the voltage is low when there is no load, the problem is upstream of the transformer. Check for voltage levels at the feeder breaker feeding the transformer. If the voltage is low all the time, retapping the transformer will help. Retap all the windings the same.
(2) if the voltage is good when there is no load, the problem is impedance. If you change the transformer taps, the voltage will go high when the oven is off. How large is the oven load relative to the transformer? What is the impedance of the transformer? You may need a larger transformer, or one with lower impedance. The problem may even be with the 480Y/277V service being overloaded.
zog
Senior Member
- Location
- Charlotte, NC
Could also be open or shorted windings on the suppling transformer, a TTR test will verify this.
Well, heres what I found this morning. The stores manager told me that a few years back they had an electrician come in to "fix" the transformer. I curiously opened it up to find that primary tap#3 (the 480v tap) on the "B" phase of the transformer is broken off and the previous electrician just twisted (spliced) the windings together and used a higher tap (the 502v tap) on the transformer to reconnect the primary. The supply to the transformer is only 459 volts anyway so the entire txfrmr was tapped wrong from day 1. So as it sits right now The "B'" phase is tapped at 502v at the primary and its only really getting 459 volts to it so the secondary side of "B" phase is only producing 109 volts to grd. Needless to say I want to sell him a new transformer.
- Status