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| Induction Motors and Inrush Currents Questions |
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| C:
As many have stated, a VFD is best. Going back to old school. Extreme was a multi-stage starter. Each stage increased power to motor increasing the RPM. You had a relay for each stage. When relay is DE-engerized the contacts are bypassed with a resistor. The series resistors of each stage limited the Current. Each stage starts with max current allowed and as motor RPM increases the current drops some before switching to next stage. Wire was often used as resistors, steel or something not a good conductor. Huge extreme was using a starter motor to get main motor started. C |
| oldway:
I described in my previous post the different ways to start a three phase asynchronous motor following the old school methods. Inserting resistors in the stator circuit was not used, it was preferred to use a three-phase autotransformer generally with 3 or 4 taps. But to start with a low starting current and a maximum torque at the time when the VFDs did not exist, we used a three-phase asynchronous motor with wound rotor and we connected resistors in the rotor circuit ... We gradually decreased these resistances when the motor accelerated ... When the starting sequence was finished, one short circuited the rotor. https://electricalbaba.com/difference-between-squirrel-cage-and-slip-ring-induction-motor/ NB: For indication, in Europe, the maximum 230V single phase connection authorized by the electrical energy distributors is 40A. Exceptionally and with additional costs, this value can be increased up to 100A. Above this, the connection is three-phase. |
| C:
--- Quote from: SeanB on May 27, 2018, 04:16:25 pm ---However most common in the USA is split phase supply, with the single phase distribution transformer having a grounded centre tap and 2 115VAC line phases. --- End quote --- Think you will find that it is commonly stated as "2 115VAC line legs" as both are of the same phase. C |
| tautech:
--- Quote from: Mukrakiish on May 25, 2018, 01:39:42 pm ---In my industry I generally work with motors (3/4 hp to 7.5 hp) and we've come to an application where inrush is a problem. Most of the time it's not a big deal but we wanted to experiment with using an inverter coupled to our motors (cheaper than using DC motors). The catch is our motors are either starting under full torque or < 1 sec it is full torque which pushes inrush current + FLA very quickly. Any of you guys have experience that could fill in a few gaps for me? ;D --- End quote --- The solution is very easy; fit D curve motor rated breakers on the mains supply. They're rated for substantial overloads for a specified period to overcome the current surges on startup. |
| HackedFridgeMagnet:
--- Quote from: tautech on May 27, 2018, 11:14:44 pm --- --- Quote from: Mukrakiish on May 25, 2018, 01:39:42 pm ---In my industry I generally work with motors (3/4 hp to 7.5 hp) and we've come to an application where inrush is a problem. Most of the time it's not a big deal but we wanted to experiment with using an inverter coupled to our motors (cheaper than using DC motors). The catch is our motors are either starting under full torque or < 1 sec it is full torque which pushes inrush current + FLA very quickly. Any of you guys have experience that could fill in a few gaps for me? ;D --- End quote --- The solution is very easy; fit D curve motor rated breakers on the mains supply. They're rated for substantial overloads for a specified period to overcome the current surges on startup. --- End quote --- Yes that was my first thought. A VSD would be nice though. |
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