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Autotransformer motor starter

A three-phase induction motor draws, due to its own constructive technology, multiple times its rated current while being started. In the moment the motor is requested to operate and it is connected to the power grid, it shows a relatively small impedance which depends from the rotational speed: as a consequence, it calls a large a current, proportional to the supply voltage, until its speed has stabilized to the final regime. This, in turns, has effects on the stability of the power network, since it is being used at a rating that is higher than normal, and on the motor itself; it is all further enhanced in case of frequent starting instances.

Among all possible solutions to mitigate this issue, FDUEG offers autotransformers that are specifically designed to start pumps, compressors and fans, applications that in normal circumstances include an induction motor. If this motor is connected and fed by an autotransformer starter, it can be supplied with a reduced voltage so that both starting current absorbed by it and voltage drop on the stator windings are reduced. Once the motor has reached its final speed, the auxiliary autotransformer can be bypassed without any particular unwanted effect on the operativity of the rotating machine. This brings the starting voltage to the motor back to full voltage, so that it can then work at full speed and load.

Autotransformer motor starter FDUEG

Motor starter autotransformer up to 12 kV voltage, powers up to 10 MVA. Features and performances

Small footprint, long life service and total reliability are the most desirable characteristics for a three-phase motor starting transformer.

The construction of this device as an autotransformer allows to reduce weight and dimensions in a consistent manner, while being able to retain the needed power flow to make it possible to start the motor placed afterwards, even if it exhibits a low power factor. Moreover, the losses generated by the auxiliary conversion device are lowered, because of the small weight of material used for its construction: this also directly impacts on the final purchase costs.

The design procedures of FDUEG grant that their autotransformers can tolerate two consecutive starting instances, and a third one within the following hour, without overheating the device more than what is allowed by the standards. Requests for heavier duty cycles can, however, be satisfied if the needs are more stringent.

The Technical Department carefully considers both the thermal aspect and the mechanical one, aiming at a fundamental purpose: not causing an untimely ageing of the static machine and of its internal dielectric insulation.

A solid and simple construction completes the characteristics of the autotransformer. This way, it is made totally reliable and performing for the whole duration of its service life: this can reach up to twenty or more years without degradation in its functions.

Custom product


Mechanical stability

Auto transformer starter, custom products

The core business of FDUEG is made up by special products, designed to specifically meet the needs of a customer. This also applies to autotransformers for motor starting purposes: they are available for motors needing a line voltage up to 12 kV, with 10 MVA rated power.

The number and position of taps in the autotransformer’s winding is a free parameter: it increases the degrees of freedom to perform a motor start, varying as a consequence the voltage ratio and the needed time to regime of the connected induction motor. In fact, being the inrush current smaller, so is the starting torque and the period to reach the final speed stretches as consequence.

The complete performance is granted for any tap considered; this holds both if the motor is built as star or delta connected.

The constructive technology mainly adopted by FDUEG involves the production of dry type VPI (Vacuum Pressure Impregnated) transformers. Other models, insulated in mineral, silicone or vegetable oil, remain available upon customer’s request.

FDUEG always evaluates the solution that offers the best performance in any case of utilization. In order to do this, the Technical Department assists its customers in evaluating the specific application and its boundary conditions.


Technical data

Motor starting autotransformer
Up to 10 MW

Up to 12 kV



Power Plants
fdueg applicazioni steelworks
Offshore Plants


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