Motor current

As answered by Sparkstation the effective out put will reduced. It will draw more current from the supply in comparison to Delta, if it continuous runnings under star

You don't know what you're talking about, do ya?

Thats not what sparkstation said. In star mode the coils of the motor are connected between phase and earth and that is less than between two phases. When the windings of the motor are in delta then they actually start to draw more current. It is this very reason why it starts up in soft mode to reduce the effects of the motor cause power jerks and surges during start up to the supply and to near by consumers (as sparkstation it is a mechanical soft start before the days of solid state tech come about).

Do not preach bull...if you do not know what you're on about!!! This blog's suppose to be helpful instead of confusing or destructive. It's no point waisting peoples time long before they get the right kind of answer.

This is why I sometimes cannot trust this blog for proper answers because some jack a...s, like you, are trying to be smarter than they really are.

This applies to the guy directly after you who also gave his opinion rather then his expert advise.

motor will heat up. Also may burn out if full 25 hp load on it.

Dear Friend

Motor can run continuously on STAR point, but the torque output would be 1/3 of the rated torque. The internal connection of of the winding of 25 Hp motor are terminated in Delta connection to get the max. output of the torque.

Star point is an option for the user to start the motor with safety of the system. As the current drawn from the system is almost 1/3 of that would be drawn if the motor is started with delta connection.

Regards

Ramesh Kumar

Star or Delta ?If a motor is designed for running in Delta it can be started in star with a reduced voltage.A motor can also be designed to run in star.All HT motors have their stator in star connection.When a run a motor designed to run in delta to continuously in star two things can happen.motor will be developing less torque and cannot be loaded to full extent.The rpm will be lower.This can result lower cooling and if temp rise goes beyond limits motor can burn out.

Every thing said above relating to torque and output power is correct. One more addition. If you look at the current, the motor is taking from the supply grid, and you take from this current a fourrier transform, then you get the harmonic content of this signal. The fundamental will be 50Hz (in Europe), but depending on the way the windings in the motor are build up, you see higher harmonics (150Hz, 250Hz, 350 Hz etc.) These higher harmonics 3th, 5th, 7th etc, cause a pollution of the supply grid. In a motor the biggest polluting harmonic is the 3th (150Hz) this harmonic carries the highest current. If you connect your motor in delta, by nature the 3th harmonic will go away. You can prove this mathematical.

So for the supply grid, it is better to connect the motors in delta.

When the motor is designed, it is designed for a specific voltage across its winding, i.e. a specific number of turns per winding per phase. If the motor is designed for delta connection, (i.e. either 208V or 480V in the US and 380V in EU), there is a number of turns per phase which is based on certain magnetic characteristics of the laminations, higher that if the motor would have been designed for Y (i.e. 120, 277 or 220V). When the motor is run at a lower voltage (corresponding to the Y connection: 120V/277V respectively 220V), the real number of turns is higher than required for that voltage, so the current drawn is lower.

The reason of usually designing a motor for delta is that at the same output power (which is basically proportional to the product of current x voltage), at a higher voltage the required current is lower, requiring smaller wires, etc.

Continuing to run the motor at a reduced voltage will not be able to develop the rated output power (torque x speed).

By using the Y/delta starter the motor is basically fed at a reduced voltage ( 1/sqrt(3) ) in Y,to lower the starting current, when reaching a specific speed, it is switched to the initially designed voltage to be able to develop the rated parameters.

Good answer, very useful.

I'll only add that, AC motor torque in a fixed frequency supply is proportional to the square of the applied (or effective in this case) voltage. So since the effective voltage is 1/sqrt(3) or 57.7% of normal, the torque is reduced to .577 x .577 or .333, the 1/3 mentioned earlier. If your LOAD is reduced to (or otherwise at) 1/3 or less, the motor current will remain low and the motor will NOT overload. But if your load remains the same as what the motor was designed to perform against, the loss of torque will cause the motor to slow down, falling into severe slip, the motor will pull more current and it will overload very quickly.

Excellent. I would only add that a motor can be designed for the rated out put to run in Delta or star.As said in my earlier post All ht motors designed for star as it is difficult to have more connections as required in delta.Very small motors below 1 hp also are designed to run in star.Its a matter of Convenience.The problem we are discussing arises only when a motor designed to run in delta is run for a prolonged period on star.Further starting is decided by the ability of the power system to withstand the in rush as well as the load inertia .High inertia loads will need longer acceleration time which should be below the thermal withstand time of the motor.Even with a assisted start what we actually achieve is to reduce the duration of starting current.At the instant of changing over a kick up will be noticed.Soft starters have solved the problems to a certain extent.

You are perfectly right. One additional comment: The duration of the starting period is usually taken into account when designing the motor, assuming a certain maximal duration to avoid overheating the windings. Therefore it is recommended to start the motor initially at no load, only when it reaches a speed close to its rated speed load shall be applied (brake torque). In this case the mechanical inertia is the lowest possible, and the starting process is the fastest, limiting the heat quantity developed in the motor. But this is not always possible, depending on the nature of the load.

But basically the reason of a start/delta switch is to lower the impact on the supplying power grid, less than for motor protection.

Further, since the normal operation point of a machine is at the intersection of 2 curves ( torque vs. speed) one for the motor itself (at rated voltage) and one for the load, and the machine curve for a lower voltage (line-neutral) is under that for higher voltage (line-line), a new operating point may form on the portion between speed=0 and speed for max torque, which is an unstable point and leads to overloading of the machine. So the consequences of running a delta designed motor in star connection depend on the load nature.