Synchronous Motors Under Short Circuits

[pwreng1]

Hi all, new to this forum scene but here goes.

A new project will have some large synchronous motors with speed control by 4-quadrant 12 pulse thyristor drive (VSD). These motors also have a large number of poles on both the stator and rotor.

In the event of a mains power failure, the motor effectively becomes a generator. The UPS backed high speed controller for the VSD can keep firing the thyristors for some time (milliseconds) resulting in a short circuit across the motor stator windings. The rotor excitation controller output takes some seconds to decay to zero.

The short circuit period can exist for a period of the order of 100 milliseconds until the current waveform passes through a current zero and the thyristors start to commutate (switch-off on reverse current).

I am looking for information relating to the current and mechanical/ magnetic forces that will occur within the motor during such "Short Circuits" for both two phase and three phase "Short Circuits". In particular:
- the possible magnitude of the currents and forces,
- the direction of the forces (i.e. radial components versus tangential components with respect to the rotor poles)
- any available information on the typical current waveforms
- any information on the change in the magnetic field/forces that occur from motoring to generating including distortion of the fields during the short circuit period.

Thanks in advance

 

[jbartos]

Suggestions marked ///\\pwreng1 (Electrical) Oct 13, 2003
Hi all, new to this forum scene but here goes.

A new project will have some large synchronous motors with speed control by 4-quadrant 12 pulse thyristor drive (VSD). These motors also have a large number of poles on both the stator and rotor.

In the event of a mains power failure, the motor effectively becomes a generator. The UPS backed high speed controller for the VSD can keep firing the thyristors for some time (milliseconds) resulting in a short circuit across the motor stator windings.
///Please, would you explain how the four quadrant 12 pulse thyristor drive works in view of double ended VFDs that have IGBT DC-AC inverter and IGBT AC-DC converter/rectifier?\\
The rotor excitation controller output takes some seconds to decay to zero.
///O.K.\\The short circuit period can exist for a period of the order of 100 milliseconds until the current waveform passes through a current zero and the thyristors start to commutate (switch-off on reverse current).
///The IGBT reverse power flow is very fast, almost instant. This is the strength of 4-quadrant drives with their ability to supply electrical energy back to the power supply.\\I am looking for information relating to the current and mechanical/ magnetic forces that will occur within the motor during such "Short Circuits" for both two phase and three phase "Short Circuits". In particular:
- the possible magnitude of the currents and forces,
- the direction of the forces (i.e. radial components versus tangential components with respect to the rotor poles)
///The motor short circuit currents are usually calculated over the motor subtransient reactance Xd".\\
- any available information on the typical current waveforms
///It depends on a type of the fault, whether it is symmetrical or asymmetrical.\\- any information on the change in the magnetic field/forces that occur from motoring to generating including distortion of the fields during the short circuit period.
///The subtransient reactance Xd" is actually linked to the motor amortisseur winding.\\

 

[pwreng1]

Hi JBartos,

Not sure how to attach a sketch - drew one up quickly in Excel so anyone can see it but cannot work out how to attach it. I will try and email the sketch directly to you.

Your response is based around use of IGBT transistors as used in conventional VVVF type VSD's with three stage operation (i.e. rectifier, DC link, and inverter).

These thyristor based units are single stage conversion (cycloconvter) type VSD's. With the thyristors, turn-off does not occur until the current passes through a zero. (believe they do not have gate turn-off type thyristors).

Like larger 12 pulse conventional VSD's there is a 3-winding transformer except we will have 3 off three winding transformers, each transformer providing 6 phases to power one phase of the thyristor VSD.

The thyristors are configured as a 3-phase full wave bridge across each transformer winding (i.e. 12 thyristors per bridge). the second winding of each transformer has an identical thyristor bridge and one side of the two bridges is connected together. Of the other two "ends", one goes to the motor for a single power phase and the other is connected to the next group of thyristors as a neutral/start-point.