Electrical Starters: Star and Delta

In order to overcome friction and initial shaft momentum on heavy loads - larger 3 phase motors take high values of current, this excessive current can affect overload protection devices and cause the electricity companies a lot of problems.

When a motor is started direct from the main supply this called DOL Direct On Line.

Power companies normally have a rule stipulating the largest size of motor which can be started in this manner.

One of the earliest solutions to reducing the starting current without drastically affecting the torque was the Star Delta starter.

It consists of several contactors in a local control station. Initially the operator held a button or handle in the Star position until the motor run up to speed then upon releasing the contactor would engage in the normal running Delta configuration.

These days everything automatic with the changeover being done by a timer controlling the contactors.

In the normal running Delta mode the Line voltage is applied to one winding load and the motor runs its normal rated load when running at its rated speed.

Motor windings have very little resistance and the impedance which restricts the current flow in the windings is actually a voltage which is induced in the windings by the motor shaft revolving this called the Back EMF. As the motor shaft turns faster this Back EMF increases and opposes the current drawn by the motor windings from the supply. At rated speed this Back EMF is at its maximum and this results in the rated current for the motor.

To reduce the amount of current drawn by the stationary motor shaft at start up the starting contactor connects the windings in Star configuration.

The same line voltage is applied as in the Delta configuration, but in Star connection the Line voltage is applied across two windings in series - one in each branch of the Star leg. The Line voltage is now opposed by the Back EMF in two windings which in series simply doubles in value. As a result of the higher impedance, the current drawn in the Star configuration is much less.

As the motor runs up to speed the Back EMF's in the windings are already present so when the Delta contactor times in, the Line voltage is applied across only one winding which now has a resistance value instead of no resistance when starting from standstill.

The changeover itself is instantaneous and no shaft momentum is lost, the time between the Star to Delta changeover is dependent each individual application and how long the motor takes to reach full shaft speed in the Star. If attempted too quickly the overloads will operate in Delta mode otherwise the motor could stall or the windings burn out.

In simple language:

Input-supply remains same for Start & run.

Motor windings are connected in Star configuration on Start so less voltage is applied to motor windings.

On run the motor-windings are connected in Delta , so applied voltage to each winding is increased to take full load at full speed.

Now Soft-Starts are used which start motor slowly & increases its speed gradually at a set-slope till reaches full speed.

Dear Guest your description of the subject is quite comrehansive. I tried to give a brief note

Regards

All this is well know! What is the question?

Greg

Where reduced starting torque is acceptable, they are used to reduce heating from high inrush currents and stress from high starting torque.

Actually, it's all a lie!

Star-Delta starters serve no real purpose other than to placate the utility into thinking that there is a reduction in starting current, and do so with the cheapest possible component configuration. In fact, when a Star-Delta starter transitions from star to delta, the transition spike is at least as high as the initial DOL starting current would have been, and if the transition happens at the wrong moment in the sine wave, can even be up to 2 times as high! The accompanying torque spike that occurs when this happens has been well documented as being strong enough to break motor shafts, and if the motor slows even a little bit during an open transition, when the motor is reconnected out-of-phase with the line frequency, a voltage spike occurs that can damage other nearby electrical equipment.

But go right ahead and keep using them, I sell replacement SCRs and Diodes for soft starters, VFDs, power supplies and UPS systems and your transition spikes keep me in the black every month!

actually the star/delta you are talking about is the open transition,which in most code of practices is not permitted. you might want to consider closed transition type.

Actually, closed transition requires a 4th contactor, another timer,a large resistor and a way to dissipate it's heat. So by the time you are done with going to closed transition, a solid state soft starter would have been cheaper and more effective.

Do some star/delta applications use speed switches instead of timers? If so, are they common or rare?

Dan

You can initiate a transition based upon motor speed, but that should not preclude the timer. A Star-delta timer serves two functions: controls the transition time AND ensures a transition delay, the gap in time during which all contactors are open (assuming an open transition). You can do it with separate timers, but the special star-delta timers make it simpler. If you did the transition timing with a speed switch alone and just used a transition delay timer to ensure the contactors do not close too soon, but what happens if the motor fails to get to the proper speed because of loading? It will stall and overload, which, although the OL relay should offer protection, is not a good thing.

Or you can just forget all this risky star-delta nonsense and use a solid state starter.