Star Delta Contactor Sizing Calculator

Now that’s where working with a star delta starter panel presents its own problem, it has three contactors, one timer and one motor nameplate. If you’ve worked with one before, wiring is simple enough but it’s easy to get tripped up in sizing. You might size everything correctly to handle the full load current but size components for winding current, thus damaging main contactor on first start. It’s an engineer mistake as they thinks all three contactors are designed for the same amperage. Not true. The calculator above use your motor data to help you avoid this costly mistak.

So what’s the big idea with star delta? Well, reduce the voltage. When you crank up a big old induction motor directly on line, it pulls six to seven times rated current. This stresses mechanical couplings, dims lights and trips circuit breakers. By connecting the windings in star, you reduce the voltage on each winding by the square root of three, meaning the starting current decreases about two thirds.

How to Size Star Delta Starter Parts

It is a simple solution and good enough for many applications but there is a downside: you lose torque. Because torque is proportional to voltage squared, you’ll recieve only one third the starting torque. It is not a great way to try to start something like a conveyor belt carrying heavy load from a standstill. Compressors, pumps, and fans starts easily, running light then slowly ramping up, star delta is the ticket for these device.

To properly rate the contactors you need to know direction of flow. When running, current from the motor travels through the delta contactor connecting just two corner points of triangle. In a well-balanced three-phase system, the current through each leg of the delta winding are reduced by the square root of three compared to line current. Therefore, a delta contactor should of been rated at approximately 58 percent of the motor’s full load current. Because the star contactor only “sees” current for very short time when starting, it gets an even smaller rating; about one third of the full load current. Out of habit, engineers frequently oversize this equipment which wastes both money and precious panel space.

Balancing physics and intuition, you set the timer. You cannot switch to delta immediately, or motor will stall because it has not built up enough speed, but if you wait too long then you’ll run it at low torque and windings will get too hot. The calculator find this period for you given the load type and its inertia. If it’s a big industrial fan with lots of inertia, it may take ten or even fifteen seconds to spin up to point where the transfer shock won’t damage drive train. If it’s a centrifugal pump, which spins up quickly, you only give it brief start time (say four to six seconds).

Another item that trips folks up is where the overload relay is located. When positioned on line, set the overload at full load amps for motor. When the relay is within the delta circuit (common with compact starters), set the overload for the smaller current winding. This factor of root three can cause the overload protection to fail or trip nuisance faults and is something not to be forgotten. Always verify whether the overload is on the motor side or contactor side before adjusting setting.

Sizing is a matter of honoring the direction of flow rather than complicated math. The full load goes to main. The winding share goes to delta contactor, while star contactor performs momentarily and then gives way. Visualize it as three separate circuits with their own electrical demands, and sizing tables appear logical more than strange. Yes you should always check surrounding temps and thermal limitations for packed panels but for all practical applications, the proportional relationships remains the same. Respect the math, set timers conservatively, and position overloads based off direction of current.