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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.

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    \$\begingroup\$ Revise the question to list supply voltage, motor rated voltage and current for star and delta connections. Describe the driven equipment. Motor design type, e.g. normal, high slip NEMA class might also help. \$\endgroup\$ – Charles Cowie Sep 14 '16 at 15:35
  • \$\begingroup\$ What fuse rating are you using? What's the rpm when you switch over form Y to delta? \$\endgroup\$ – winny Sep 14 '16 at 16:11
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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.

enter image description here

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First look at the motor name plate, (in star delta starters check the six leads connected properly to their terminals). If the delta formation is improper, the motor will first start and run ok. But after it changes over the motor will stop if the contactors are in closed position in the starter. Place the relay below the delta contactor while after the change is over.

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  • \$\begingroup\$ There is a circuit diagram tool, its wise to use it or post a picture. Avoid run on sentences (put proper punctuation in your posts) \$\endgroup\$ – Voltage Spike Sep 21 '16 at 15:58
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Check the motor connection plate.

If the motor connection plate's U1, V1, W1 side is ok, may be U2, V2, W2 side have some mistake. In that case, the would start normally but after motor goes into delta, it would trip the circuit breaker because of high current.

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Perhaps your motor is designed for example 230/400V delta/star. When you switch the motor on in star connection, it has already the nominal voltage. P=0.4kV*1.73*40A = 27kW, which is already the rated power of described motor. For example a delta/star motor of 400/690V, suitable for star/delta starter should have a power 1/3rd in star connection, compared to nominal power when delta connection is established, so something arround 7kW.

But it hard to say without any nameplate data.

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