star delta

I think it's time to get a qualified electrician on site to fix that motor.

You are talking about this one, right? 3 phase compressor

You never explained how it got wired backwards, but I think this should be fixed by an electrician.

Do you know how to use Wikipedia? That may shed some light on this question, if you're just curious.

What the load (you) calls star is called Y in the power industry. The motor should be wired to match the supply and / or voltage supplied.

You can see more about this at

Y-Δ transform

Sir,

Use of a purely star or purely delta connection for a motor is determined by the motor's design and internal connections of the windings. You can have three leads coming out and the motor could be internally wired either star or delta. If you have 6 or 9 or 12 or 15 leads coming out, then the proper connection needs to be determined by the data plate or data sheet from the manufacturer. In the USA, motors have to have this data on a plate or permanent decal attached to the motor (a few exceptions exist). I don't know about elsewhere. "Which capacity is used in star...delta?" Entirely determined by the manufacturer's design.

Various methods exist to reduce the inrush current when a motor is started. Until it gets up to speed, the current into the motor will be much higher than its running current. At "locked rotor" conditions (not turning yet), this can be as much as 6x or even 8x the running current for many motor designs. Therefore, different connections are designed--one for the starting portion of the motor's operation and one for the running portion. All of these are described with the same generic term "reduced-voltage starting".

Choice of which method of reduced-voltage starting depends on the type of load, the energy supplier's rules, and available budget. A star-delta (also called wye-delta) method is often used, if you have a motor with both ends of each winding brought out to the terminal box, and if it is designed to be run in the delta connection.

Your second question points to a detail that is important. During the time that the starter is changing from star to delta, the motor is still connected to the equipment it drives. Therefore it will slow down (even if only a little bit). The longer the transition time, the more reduction in speed. This then results in a greater jump in current when the delta connection is made. For this reason, you want the transition to be quick. However, if it is too quick, then the "shorting contactor" that made the star connection will not have opened completely before the "run contactor" applies line voltage to the same leads. BOOM! Again, "How much time delay?" As little as possible, but enough so the contactors can change state safely. I know of no written rule that sets a specific time, because of the variables present (load inertia, type of load, contactor size, contactor design, voltage being used, etc.).

--JMM