What are rotary transformers?
High current transformers are used for example in heating plants, electric furnaces, glass melting plants, test fields, etc.
Finally, “stepless voltage adjustment under load, without any sliding contacts” is the concise characteristic of this type of transformer.
Structure
The construction of rotary transformers is basically the same as that of asynchronous machines.
The fixed stator contains the rotatable rotor, and both contain grooved laminations and windings, as are common in three-phase motors. The windings are two-pole in order to achieve better type utilization. Four-pole windings are only provided if extremely high through currents can only be achieved by fourfold parallel connection. All materials commonly used in electrical engineering with the appropriate insulation classes are used for insulation. The excitation winding is normally located in the rotor, since the additional winding in the stator often carries relatively high currents. The rotor with the excitation winding is rotated 180° for voltage adjustment. The current is supplied via spirally arranged copper strips which are permanently connected to the winding. There are therefore no contacts or slip rings that could cause interference. Amply dimensioned needle bearings, or combined axial-needle bearings, are used to support the rotor.
The rotor is adjusted by a worm gear and held in its respective position relative to the stator. The adjustment is performed continuously under load by a geared motor. A brake on the motor prevents the rotary transformer from overrunning or moving on its own.
A limit switch is provided in each end position to prevent overrunning of the adjustment range. A further switch is located directly in the motor circuit to additionally switch off the motor in the event of a fault in the control system.
Rotary transformers are built in dry type or oil type. In many cases, they are housed with a fixed transformer in the common oil tank.
Mode of operation
The mains voltage U connected to the excitation winding generates a rotating field. In the additional winding, an additional voltage U is generated according to the turns ratio, which has the same magnitude in each position, but rotates 180° in phase with respect to the line voltage. If the start of the additional winding is connected to the mains, the resultant output voltage at the end of the winding can be varied continuously in the range U +- U. The additional winding can also, instead of the winding ratio, be used to generate a voltage of the same magnitude as the mains voltage.
The additional winding can also be connected to any other fixed voltage instead of the mains voltage, e.g. to the secondary of an additional fixed transformer. In this combination, therefore, all desired control ranges can be achieved independently of the mains voltage.
Between input and output, however, there is practically no deterioration of the mains cos phi due to the rotation of the additional voltage, since at the same time as the voltage, the current in the additional winding is also rotated in the opposite direction and the rotary transformer thus compensates itself, so to speak.
Design limits
At present, rotary transformers are built up to a type output of about 3.2 MVA.
The supply voltage is normally less than 1000 volts, but values up to 10 kV are also provided for higher power. However, since the winding is housed in slots, the increased insulation effort considerably reduces the available winding space and thus also the type rating. It must therefore be checked in each case whether it is expedient to feed in high voltage as part of the overall system.
The additional voltage can be selected practically freely. However, since the winding must be symmetrically distributed over a certain useful number, there is a minimum number of conductors which limits the executable voltage for each type.
Function
If the additional winding (stator) is connected to the excitation winding (rotor), the vector of the additional voltage Uz1, Uz2, Uz3 rotates around the fixed excitation voltage U1, U2, U3 with the rotation of the rotor by the angle @. The resulting of both voltages gives the output voltage U1 + Uz1, U2 + Uz2, U3 + Uz3, which changes continuously between U1 + Uz1 to U1 – Uz1. The rotation of the rotor is only from 0 to 180°, because afterwards the output voltage increases again.
In no-load operation
I = I1 = Im
The no-load current of the excitation winding of the rotary transformer is greater than that of conventional transformers. However, a small air gap between the stator and rotor allows this to be set to reasonable values.
Definition
Throughput power: Product of output voltage U1 + Uz1 und output current.
Type power: Product of additional voltage Uz1 and output current.