Running a Delta-Connected Motor with a Star Connection

Why ?

Star connected motors typically provides you with lesser torque - saving of course in starting current - but is used for starting purpose only. When it reaches near the rated speed, normally it is switched over to delta.

Usually your motors should have all the six turminals brought out in the terminal box, and you can connect it as star or delta as you want.

Why do so many people ask this question? Is there someone out there falsely promoting the re-connection of motors to save energy or something? We get this question a lot in here, makes no sense.

In some IEC motors, you have the ability to connect it in Star or Delta. But that is for the purpose of adapting the motor to different incoming line voltages. For example, if you have 240V, you connect in Delta. If you have 415V, you connect the motor in Star. If you have 240V and you connect it in Delta, the effective violtage across the windings will become 240/1.732 or only 138V. That means the motor output torque will be 33% of normal. So unless your load is drastically reduced, your motor will stall. It may also be true that technically, the kW rating of the motor is reduced as well, because of the relationship of kW to speed and torque, and since the base speed will remain the same, the kW rating will reduce to 33% of normal as well. But a motor only draws as much power as the load demands, so even if your motor were properly connected, it would not draw much more.

The only difference is a slight reduction in the magnetic losses, because they are based on the applied voltage. So a reduction in the voltage across the windings by connecting in Star instead of Delta will reduce the losses. But they don't amount to much when the risk of stall is so great. If your motor is so unloaded as to make this a possibility, why do you have that large of a motor connected to it in the first place?

"If you have 240V and you connect it in Delta, the effective violtage across the windings will become 240/1.732 or only 138V."

I think the bolted delta must be Star.

Correct me if i'm wrong.

Hi

u can convert delta connection in to star but provided that all 6 termainals of motor are accessible. By removing delta link and providing link to common terminal of individual winding, you will have a 3 terminal and 1 common or neutral terminal

Best Regards

Divyesh patel

Each motor, in the calculation/design process, is designed and built in such a way that the rated values ( output power, torque, speed) are achieved in case of a certain current-number of turns combination. This is valid only if the voltage applied to its terminals is the one the motor has been designed for. Under normal circumstances that number of turns is constant, so, in order to achieve the rated parameters, the voltage applied should be the one the motor is designed for. Starting processes involving a Y-delta switch are only to lower the start-up current. Even if the motor has 6 terminals and, in theory, the windings can be connected in Y or delta, the voltage applied to each winding is a ratio of sqrt(3). From the same system (208/120; 480/277; 400/230; 415/240, etc) there is no possibility of obtaining as phase-to-phase (line-to-line) the same voltage, without a transformer.

If your motor runs on 230 V in delta, if you switch it to star, you will need 400 V.

Dear abhimishra.

I would like to inform u that I am new to the website and I hope that i can help all the friends as soon as I can ,so please write the reason why u whant to change the connection since all the the motors are designed for special connection process in order to save the rated values for output power,torque ,and the speed.

best regard .

Noori