induction motormotorthree phase
We are using 3 Phase Six Wire out standard ABB foot mount motor of 25HP.
We are using star-delta starter with delay of about 30 second from star to delta.
We are seeing around 40 Ampere when machine starts in star, but that amperes jumps to 130 ampere when the starter switched to delta, which trips the whole circuit in six to ten seconds.
How do I diagnose such behavior ? As I understand, the ampere should be reduce when it is used in delta configuration.
Thanks.
The whole idea is that a smaller voltage is delivered to the motor windings during start up.
The end game is that the motor (or load) is configured as a delta winding formation. In other words, after the load is up and running, each winding receives line voltage.
However, during start-up the windings are put into star formation and each winding will receive 57.7% of the AC voltage.
When starting the motor (above) KM3 and KM1 activate whilst KM2 is deactivated. You should be able to see that KM1 terminates the motor windings into a star point thus the supply voltage to each winding is only 57.7%.
After a short while KM1 deactivates and KM2 activates. This puts all three motor windings into a delta configuration thus each winding receives 100% line voltage.
Here's a picture that shows (or implies) that math: -
Here's the SE question where I borrowed the final picture from.
Adding a V/F control system could be an approach. However with only V/F control the regulator can not see if the motor is acctually turning. One step further is adding an encoder to the system so the regulator can see that the motor is turning. In both situations the available starting torque will be more or less 150% at low frequencies (3Hz)
A different approach could be OLV (Open loop vector control). Here the motor can realise a torque of about 200% of their rated torque at very low frequencies. Also here an encoder is possible turning the system into an CLV closed loop vector control. Here a torque control mode is possible. Thereby the motor is controlled at torque rather then motor speed.
Try to understand the principles first and then look at your situation.The Closed loop vector control would have my preference for your mixer. For this approach you can look at the SINAMICS G120.
For your specific questions.
CLV can work in a standstill condition and would deliver enough torque during startup. You can compare this with a DC motor.
V/F might be enough but is not sure. For the other possibilties see above.
I do not think that energy savings can be achieved. The motor has to deliver its energy whatever the situation is.
Best Answer
If the rated voltage for the delta connection is applied to a star-connected motor, the resulting torque and current are about one third of the characteristic torque and current for the delta connection. As a result, at the transition from star to delta, the change in current and torque is about a three times increase. Therefore, an increase from 40 to 130 amps is within the normal range.
If the current doesn't drop to a normal running current in six to ten seconds after the transition, the motor is not adequate for the load, there is a problem - possibly an unconnected phase in the delta connection, there is a mechanical problem with the load or some other problem.
Star-Delta Starting Characteristic Curves and Explanation
The torque and current vs. speed curves shown below are for a typical IEC Design N or NEMA Design B motor with star-delta starting. The load torque requirement shown increases as the speed increases as it would for a fan or centrifugal pump load. The substantial torque requirement at zero speed would be a requirement of a pump with a substantial static head. Note that the torque available to accelerate the motor and load is the difference between the steady-state load torque requirement and the available motor torque shown by the motor cur torque vs. speed curves.
In this case, there is very little star-connection acceleration torque available between 40% and 70% of full speed. If the load torque was a little more, or the motor torque a little less, the motor capability torque curve would cross the load torque requirement torque curve and the motor would not accelerate the load to a higher speed until the star-delta transition. The load and motor curves are shown as crossing at a little above 90% of rated speed, a little below the star-delta transition speed.
At the star-delta transition, the torque and current increase to the points at which the vertical transition line crosses the delta torque and current curves.