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Why do we use star delta connection?

If you connect motor direct on line; at start there will be almost 5-7 times more current drawn. This current will damage the equipments on line like circuit breaker etc. So to overcome this huge current issue we use star-delta connection.

When to use star delta connection?

Generally it is suggested to use star delta connection when the motor power is greater than 4kW.

What happens if we don’t pass to delta and stuck at star connection?

If the motor reaches its rated operating speed (which is in rpm unit) and does not pass to delta then it will work with 1/3 of its normal working moment.

How much time duration do we need to set for conversion from star to delta?

Generally speaking 5-10 seconds but you can get the actual time you need your motor to be connected as star by using an ampermeter and a chronometer. By using these you will try to get the point where current change occurs. That point is where you need to convert to delta.

Which connection to be used? Star or delta?

It is actually related to the motor producer. If you see Δ/ λ 220/380 at the motor nameplate then this means you can use this motor in delta connection with 220V and in star connection with 380V.

What if we run the delta motor in star connection?

It can be done but you need to know that it will be working with less power and moment. You will not get rated values. So this is not good it is better you choose a motor with less power in delta connection then you will be losing less power.

What if we run star motor in delta connection?

If you mistakenly connect star motor (from nameplate you can get info about what connection your motor need for your voltage) then you will apply 3 times more voltage to the windings which will end up with increase in current and your motor windings will be burned.

Star connection

star connection

If entrance side of the stator is names as U, V and W and X, Y and Z are the output side then by shorting U, W and V and applying voltage to the other sides of the stator we construct a start connection. The other way meaning shorting the other side and applying voltage from the reverse side is also possible.

Resistance analogy

At the end since this construction seems like star one ends are connected and other ends are open to apply voltage we say it star connection.

Delta connection

At motor terminals, if you connect X-V, Y-W and Z-U you connect your motor in delta connection.

Resistance analogy

It s more like connecting your resistances in serial.

Trick about motor terminal boxes winding terminal orders

Here we note something that motor companies don’t put the same winding’s terminals opposite. Meaning of you connect U and X that means you shorted a winding. So to prevent this, they put them in U-V-W and opposite in ZXY. So off you put brass jumpers you can put to connect delta or Wye only. It is not possible to short a winding with a standard brass jumper plate.

Star and delta connection voltage and current status

Connection VoltageCurrent
StarV_L = \sqrt{3} V_P I_L = I_P
DeltaV_L= V_P I_L = \sqrt{3} V_P

Star delta power circuit drawing

Star delta control circuit drawing

Selection of thermic overload relay and contactor

Overload protection of the motor will be 1.732xFLC

Overload protection of the line or winding will be 0.58xFLC

Why is it important?

Because if thermic overload relay is not suitable for the amperage then it will not function properly and if contactors are big it is not economic.

Selection of a contactor

Selecting contactor is like selecting circuit breaker where you need to select one bigger size. So if on nameplate of motor current is I then you rated contactor current I_e must be bigger then I . If you choose one more bigger size you will reduce the possibility of contactor damage due to electric arcs.

Closed transition star delta starters

While changing from star to delta there is a very short time when there is no power supplied to the windings and that causas small switching transients. Actually there happens some arc and this is not good for the circuit elements. In closed transition bu using resistors a smooth transition is applied. In closed transition, windings are always supplied with power.

There is also another contactor introduced in the circuit to connect and short the resistors at the system. This contactor removes resistors when delta connection is started.

Resistors should not be selected so big since it will not allow the current to pass and there will be no use for these resistances then.

Torque problem of the star delta starters

Although star delta starters seem to be a perfect solution with its being cheap on the other hand it may not be suitable for some of the motors since motor may need more torque at the beginning. Star delta connection may only give 33 percent of the starting torque oif the motor. If this is not enough starting torque for your motor then you will experience mechanical and electrical problems.

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