A special form of transformer having one winding, a portion of which is common to both the primary and the secondary circuits. The current in the high-voltage circuit flows through the series and common winding (see illustration).
The current in the low-voltage circuit flows through the common winding and adds vectorially to the current in the high-voltage circuit to give the common winding current. Thus, an electrical connection exists between high-voltage and low-voltage winding. Because of this sharing of parts of the winding, an auto-transformer having the same kilovolt-ampere (kVA) output rating is generally smaller in weight and dimensions than a two-winding transformer. One possible disadvantage of auto-transformers is that the winding are not isolated from each other and that the auto-transformer provides no isolation of the primary and secondary circuits.
Auto transformers are used in applications where electrical isolation is not a critical requirement. When the ratio V2 : V1 is 0.3 or more they are used with advantage. The normal applications are motor starters & booster.
Output VA range
For higher rating contact us for more details. Dimension may change at the time of final design.
PRIMA motor starting auto transformers have been an industry standard for many years. Auto transformers are a significant component of a motor controller. In its simplest form, motor starters start and stop a motor and protect against over voltage and under voltage. In most applications, a simple full-voltage magnetic starter places full voltage across the motor’s winding on startup and maintains full voltage at the motor during the run mode. For larger motors, or motors where the load is connected, the effect of motor-starting inrush can stress the electrical system with significantly lower currents in the distribution network. This can cause all manner of disruption to the system with subsequent interruption in the facility. In these cases, reduced-voltage starting must be applied. The addition of a motor starting auto transformer during the startup sequence limits the starting inrush current. The PRIMA 3P product line is designed for Medium Duty service including applications for drive loads such as pumps, compressors, conveners and fans.
PRIMA 3P Motor Starting Auto transformer: Three phase, 3 coil, suitable for motor starters with 3 contacts, connected in a three phase ‘Wye’ configuration. This provides balanced voltages across each phase.
Special Motor Starting Auto transformers. For applications requiring heavy duty service with frequent starts or jogging, high inertia loads such as paper or steel mill drives, medium voltage or other unusual demands,
PRIMA can offer specially designed units to meet your requirements. Please consult our sales offices.
Output VA range
Starts per Hour