Wye-delta starters (or star-delta starters) operate by initially connecting the motor in star, and then after a predetermined period of time, reconnecting the motor in delta.
The objective of this type of starter, is that it is a reduced voltage starter and will reduce the start current and the start torque.
The start current in star configuration, is one third of the dleta current, and the torque is one third of the delta torque.
If the reduced torque is sufficient to accelerate the machine to full speed, there may be some advantage from the use of this starter provided the transients are eliminated. In reality, very few situations actually reach full speed before the transition to delta and so no benefit can be attained. It is very common for machines to rech about half speed before the motor runs out of torque in star, and at half speed, the delta current is almost LRC so there is no advantage.
Standard starters are open transition, and so there is a large transient current and torque developed at changeover, causing more damage than DOL or Full Voltage starting.
There is a scheme called the Wanchop method, for closing the transition on this type of starter. This entails the connection of an auxiliary delta conatcor accross the main delta contactor with resistors in series with each phase. The sequence is to connect the motor in Wye (star) and then close the aux delta contactor. Then open the Wye (Star) contactor and close the main delta contactor.
The theory is this. When you open the wye contactor, current continues to flow through the motor winding via the aux delta contactor and resistors. This keeps the stator field synchronised to the supply and prevents the motor from acting as a generator. To be effective, the current through the resistors must be signioficant and be of the same order of magnitude as was flowing in wye connection. The resistors must be low value and high wattage.
For a brief moment, the resisors are in circuit while the whe contactor is closed, so the resistors are effectively wye connected in parallel with the wye connected motor. This results in a high current draw from the supply and a high current draw through the resistors. They must be able to sustain this!!
The bottom line is: to be effective, the resistors must be sufficiently low in value that a sgnificant current continues to flow through the motor windings while the resistors are in circuit, they must be able to operate in wye (star) connection across the supply so they must have a high power rating, and the motor must get to full speed in wye connection before the changeover, or you would be better off with a full voltage (DOL) starter.
In practice, I have seen amny installations using a small control relay as the aux delta contactor and 10W radio type resistors. This may serve to get round the regulations, but in reality achieves nothing.
To make it work, you are essentially using the components of a primary resistance starter inside a wye delta starter. You would be better off just to use the primary resistance starter (provided it was set up correctly) or a DOL starter.
A correctly selected and setup soft starter is definitely and better option!!
Best regards, Mark Empson
