MEDIUM VOLTAGE AUTOTRANSFORMER STARTER FAULTS
There are two main flaws in the Korndorfer autotransformer circuit that cause destructive voltage transients. These transients only become apparent when used in medium and high voltage applications and electrical engineers cannot explain what is causing these motor starter faults.
The Max Korndorfer circuit has remained unchanged since he filed for a US patent in 1908; his US patent No 1,096,922 shows the simple circuit which is still in use today.
We know that an induction motors starting current amplitude, at transition from the reduced voltage first stage will step change at transition from say, 6 times to 10 times full load current .
What has not been understood is what happens at contact separation of the star switch, within a 5µs time frame.
1st Korndorfer motor starter defect
From an inspection of the schematic circuit for the prior art Korndorfer reduced voltage starter, it can be seen that at the instant before transition, the motor current is supplied from the secondary winding of the autotransformer, then transferred to the primary supply voltage when the Star contacts separate.
What must be recognized is that the action of separation of the contacts of the “star” contactor forces the motor current to instantaneously change from a “secondary” winding supply to a “primary” winding supply, very fast dv/dt event.
Michel Faraday D.C.L, F.R.S, in 1831 published a paper on his discovery of electromagnetic induction which is “the time rate of change of the magnetic flux with a coil induces a voltage in that coil, the amplitude of the induced voltage is proportional to the velocity of flux linkages with the coil”.
This is Faraday’s Law of Induction, the basic law of electromagnetism. It relates to the operating principles of transformers, inductors and explains the source of the destructive transient voltages in the Korndorfer prior art motor starter.
Examination a graph [Figure 4] of a first 1/2 cycle of the motor current supplied from the secondary winding of the autotransformer, if the opening (separation of contacts) of the star switch is on or about the 90 degree of the wave form.
It can be seen that the motor current supplied from the secondary, is instantaneously transferred, to the primary winding power phase’s polarity.
This change in motor current is not a step up from 6 to 10 units, a change of 4, in less than 5µSeconds.
This is a very fast event and is the root cause of the voltage transients.
2nd Korndörfer motor starter defect
The second defect is a hidden step-up autotransformer circuit that only comes into play in the few microseconds time frame starting from the separation of the star switch contact. [Figure 3]
The separation of the star switch contacts forces the motor current path to be, through the part winding of the autotransformer remaining in circuit. I.e. the current flow is from the supply line through the winding and out the 80% voltage tap, assuming the motor is connected to the 80% tap.
With normal operation a step-up autotransformer would have flux linkages at the supply frequency; in the case of very fast dv/dt the voltage induced into the redundant windings of the first starting stage, will be very large.
The voltages, calculated for various tap connections are a further clue to the transient causes. An 80% voltage tap will have a 5:1 step up winding ratio for the pseudo step-up transformer under transient conditions.
The amplitude of this fast rise-time (dv/dt) voltage surge at transition is of concern because each voltage peak may also cause microscopic “pinhole” failures or partial discharge in oxidizing the winding insulation material. Repeat starting of the motor adds to the risk of partial breakdowns.
The high motor current surge and a step-up voltage transformer connection that generates the destructive winding flash-over or burn-out is an unplanned event that depends on the instantaneous angular “point on wave” at which the star switch contacts separate.
Random switching for the transition from the first starting stage means that with a step-up autotransformer connection of the prior art Korndorfer starter, a transition from the first stage to the second stage will eventually generate the destructive voltage surges. Therefore “controlled switching” is desirable and advantageous compared to random switching.
It is also desirable to prevent the step-up autotransformer connection that increases the amplitude of the voltage transient according to the turn’s ratio in use, at contact separation.
Therefore to reduce the risk of such a destructive voltage transient, it is desirable to replace the Korndorfer motor starter circuit with an autotransformer apparatus and method that including a central switch circuit that disconnects redundant windings of the first starting stage from the circuit at the transition [Figure 5], thereby preventing any step-up transformer connection and subsequent voltage escalation. (See US patent 7633260.)
It is a further advantage if the central switch may be a ‘controlled switch’ as that term is understood by persons skilled in the art, where the separation of the switch contacts may be controlled for a pre-determined angular position of the voltage waveform.
Another improvement is the application of a reduced voltage autotransformer starter with a magnetic core having two or three separate magnetic permeable sections, allowing for DC excitation for variable (Primary) impedance control of the autotransformer current after contact separation, from a first starting stage to a second variable reactor second stage, before switching to the final stage of direct on-line.
A central switching means has other benefits, such as improved thermal dissipation, where the windings of the second stage are not buried into the coil mass of the first starting stage. The absence of intermediate tap connections inside a coil layer also reduces insulation stress between layers in both medium and high voltage applications.
Central switching with a 2-coil/phase set, auto-transformer starter can provide a simpler controlled switching at the 30 electrical degree “point on wave” for those skill in the art, for at this point both phase’s are at 0.5 of the peak and are in the same quadrant.
The addition of 5% number of turns above and below the coil stack allows a “Buck and Boost” coil assembly that provides nine (9) torque setting, each at 5% increments around the NEMA preferred taps of 50%, 65% and 80%.
It is expected that the low voltage Korndorfer autotransformer starter is also subject to similar voltage transients with fast rise-time voltage transients, however with a lower operating voltage and state of the art insulation materials in use is a factor that contributes to its longevity.
- Use a central switching auto-transformer to replace the Korndorfer transformer, existing switchgear can be reused with only slight wiring changes.
These modifications will provide a cost effective, improvement in motor starter reliability and integrity.