Advantages of autotransformer starter
The benefits of autotransformer starter:
- It limits the inrush current significantly.
- It produces low power loss and less radiated heat.
- It has a good performance when starting large motors.
- It is suitable for long starting periods.
- Adjustment of starting voltage can be done by the selection of a proper tap on the autotransformer.
The figure below shows a graph to compare starting current and starting torque for a transformer start at 80% voltage and direct-on-line start. The blue curve is for the direct online start and the red curve is for the transformer start. Dotted lines represent starting current and filled lines represent starting torque. As noticed, there is a lower starting current and starting torque with transformer start compared to the direct online start. This means that the transformer start can be more flexible compared to the star-delta starter.
Disadvantages of autotransformer starter
The drawbacks of autotransformer starter:
- Starting with an auto-transformer is to be considered quite expensive. In lower power, it is the most expensive design. It is also not cost-friendly when it is applied to medium/high power squirrel-cage motors with high inertia.
- The control and power circuits are quite complex.
- Due to the physical size of the system, it might not be possible to apply an autotransformer starter to an application where space is limited.
- It has a low power factor.
- It has a limited duty cycle.
- Autotransformer starting has a distinct “jump” between the starting torque (lower value) and the normal running torque (high value). The mechanical and electrical transient produced by this switching might not be suitable for some loads.
- It is old technology.