Differences between Autotransformer and Reactor started motors 3

[rockman7892]

Can someone please explain to me the differences, as well as pros/cons between Autotransformer vs Reactor starting of a 4.16kV motor?

I am familiar with an autotransformer starter arrangement having dealt with them in the past and understand that the transformer simply reduces the voltage at the motor terminals in order to achieve a reduced starting current. The autotransformer acts simply aa a transformer stepping down the voltage.

Does a reactor work in principle by inserting a higher impedance into the circuit thus limiting the staring current to the motor during starting?

 

[dpc]

The goal of both approaches is to lower the voltage at the motor terminals during starting. This reduces starting current and starting torque.

If the goal of the reduced voltage starting is to limit the voltage dip seen by the rest of the power system, the autotransformer method is highly preferred over a reactor type RV starter. This is because the incoming line current will be less due to the transformer turns ratio. With a reactor-type RV starter, the line current is still equal to the motor current. If the only purpose of the RV starter is to limit the motor starting torque, both approaches have the same effect on torque for the same motor voltage.

Solid-state reduced voltage starters are a better option than either of these older method.

 

[jraef]

Excellent response, I'll only add a few more points worth considering.

An autotransformer starter typically has 3 taps on the transformer that allows for a degree of adaptability to starting torque requirements, and although some reactors do also have taps many do not, especially those used by OEMs. If the current reduction from an un-tapped reactor is insufficient or the torque reduction results in a stall, the only recourse is to change out the reactor. For that reason, you typically see reactor starters used by OEMs who will know the specifics of a starting process and know in advance whether or not it will work. RVATs are often a better choice for field work and/or retrofits.

RVAT starters are typically heavier and larger (physically) than RVPR starters. That can make a difference in some tight space situations. I've seen a lot of RVPR starters used for fire pumps only for that reason. The Architect considers the space to house the fire protection system to be "wasted" and therefore keeps it to a bare minimum.

Also on the down side of Korndorfer type RVAT starters for MV motors is a known possible issue with very high voltage transients occurring inside of the transformer during transition causing transformer insulation breakdown. It is of very short duration and happens when the Star contactor on the transformer opens because of the flux linkage multiplication through the first stage of the transformer (up to the first tap) and the high dv/dt at that instant. It happens at LV too, but the transients are less likely to exceed the insulation rating of the windings. In MV applications this isn't always the case. That problem increases with an increase in the tap point, i.e. it's worse at 80% taps because that first stage is larger than at 65% or 50%. That problem doesn't exist on older Open Transition RVAT starters, but they come with a host of other problems that the Korndorfer method had solved.

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[rockman7892]

Thanks guys!

So if I understand correctly with an autotransformer tapped at 50% the voltage at the motor terminals will be 50% of rated and the current seen upstream on the system will be 25% of the rated starting current (while current on load side of autotransformer is 50% motor starting current). Starting torque on the motor will be 25% or normal starting torque.

The only reactor starter I have seen was one with adjustable taps. So lets stick with the same values as above and say the reactor is sized to provide 50% voltage at the motor terminal during starting. Would this mean that the reactor would be sized to have the same exact impdeance as the motor during starting so that 50% voltage would drop across the reactor and the other 50% would drop across the motor. (I'm assuming the reactor acts more like a volage divider than the auototransformer which transforms the voltage.) The motor current on both sides of the reactor therefore would be 50% of normal motor starting current. Starting torque on the motor will be 25% of normal starting current.

Am I understanding this correctly?

 

[electrolitic]

Yes. You are correct.
I have some points to comment:
When starting torque and/or grid limitations make it possible, I prefer reactor starting.
Differently of AT starter, the voltage applied to the motor is a function of the passing current.
If an air core reactor type is used, a constant and linear acceleration can be achieved;
Air core reactor is really more expensive but when possible must be used ( no risk of saturation, harmonics filtering, etc)
AT starter needs more contactors/circuit breakers (3)if compared with reactor starter (2).
Last, do not forget a time-out control circuit to protect against long starting caused by timer circuit failure or reactor re-insertion into the circuit caused by reactor short circuit device failure.
Regards