Many transformers contain more than one primary winding, more than one secondary winding, or both:
- Each winding on the high-voltage side has a maximum voltage of the lower of the two voltages.
Each winding on the low-voltage side has a maximum voltage of the lower of the two secondary voltages.
- Any voltage higher than these ratings could damage the insulation.
Each winding can handle half the rated kVA that the transformer is rated for.
We can connect either side in series or parallel to get the voltage we want.
Multi-Coil Distribution Transformer
The transformer below is rated 50 kVA, 2400/4800 V – 120/240 V. This means that each winding of the high-voltage side is rated for a maximum voltage of 2400 V (always the lower of the two voltages). Each winding of the low-voltage side is rated for a maximum voltage of 120 V. Remember that any voltage higher than these ratings could damage the insulation.
To connect the high-voltage side of this transformer to a 4800 V bus, the two windings are connected in series so that the bus voltage is divided equally (2400 V and 2400 V) across each of the two windings.
To connect the high-voltage side of this transformer to a 2400 V bus, the two windings are connected in parallel so that each winding sees 2400 V.
To connect the low-voltage side of this transformer to a 240 V bus, the two windings must be connected in series so that the bus voltage is divided equally (120 V and 120 V) across each of the two windings.
To connect the low-voltage side of the transformer to a 120 V bus, the two windings must be connected in parallel so that each winding sees 120 V.
Series and Parallel Low-Voltage Connections
Each coil of this transformer can handle only half of the total kVA. So each of the high-voltage windings and each of the low-voltage windings are rated at 25 kVA.
To find the maximum current rating of each winding in the above drawing, simply divide the volt-amperes by the rated voltage:
25 KVA/2400 = 10.4 Amps (Primary)
25 KVA/120 = 208.3 Amps (Secondary)
Notice that we get the same value for current whether we use a single coil and half the VA or both coils and the full VA:
50 KVA/4800 = 10.4 Amps (Primary)
50 KVA/ 240 = 208.3 Amps (Secondary)
Observing Proper Polarity on the Supply Side
The video below goes into detail as to how and why it is important to connect your multi-winding transformers properly. At best, if a transformer is connected incorrectly you will see zero volts on the secondary. At worse, you will experience kablazalflam (Dutch for melt your face off) or a dead short.
The video below discusses how to properly connect a multi-winding transformer without blowing yourself up.
By tapping off the center connection, we can get 120 V, or by tapping off both lines we can get the full 240 V.
With three-wire secondary connections (120/240 V), the transformer is capable of supplying the full kVA only if the load is perfectly balanced.
If the full kVA is supplied with an unbalanced load, one of the windings will be overloaded. (That is, its current rating will be exceeded.)
Each winding would only be able to handle half the rated kVA.
This video describes the danger of overloading one of the windings and how to avoid it. Transformers are practically not going to be fully balanced so care needs to be taken on how they are loaded.
This video explains how a transformer can still work if one of the primary windings has burned out. This is the concept of flux linkage. The main point to take away is that if you lose a winding then you have only half of the KVA (Power) available.