| Electronics > Beginners |
| Induction Motors and Inrush Currents Questions |
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| tautech:
--- Quote from: HackedFridgeMagnet on May 28, 2018, 12:11:15 am --- --- 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. --- End quote --- Sure, the the mention of inverters/VFD was only in some effort to manage the startup overload condition when neither are necessary with properly rated motor rated breakers. They're the standard industry solution for motor start overload management. However like any MCB their long term performance is influenced by the # of overload trips they endure. I also find them useful for older simple transformer type welder mains supplies. |
| drussell:
--- Quote from: Benta on May 27, 2018, 04:49:41 pm ---Ain't no such thing as 230 V 3-phase mains, and it would never give 115 V phase-neutral. --- End quote --- I think he meant 208 volt. The vast majority of 230 volt equipment will obviously run just fine on 208 (wired delta for 3-phase motors, or across two phases for a single phase 230, the motor will just draw a bit more current than if it sees 230), an electric range will just have elements that put out slightly less heat, etc. but it basically just works. |
| Gregg:
--- Quote from: drussell on May 29, 2018, 01:33:46 pm --- --- Quote from: Benta on May 27, 2018, 04:49:41 pm ---Ain't no such thing as 230 V 3-phase mains, and it would never give 115 V phase-neutral. --- End quote --- I think he meant 208 volt. The vast majority of 230 volt equipment will obviously run just fine on 208 (wired delta for 3-phase motors, or across two phases for a single phase 230, the motor will just draw a bit more current than if it sees 230), an electric range will just have elements that put out slightly less heat, etc. but it basically just works. --- End quote --- 230 volt delta 3 phase is quite common in the US in older industrial areas. It is 230 volts between any two legs and one phase has a center tap to give the standard pair of 115 volt legs to the center tap which is the grounded neutral. The other leg is 208 volts to neutral and seldom is anything wired for 208 volt single phase. For instance, Bridgeport milling machines run on 230 volt 3 phase only as they are two speed motors. |
| SeanB:
Scary motor you get here in south Africa is a 22kW single phase capacitor start capacitor run motor. Very popular to drive irrigation pumps, as all the farmer has to supply is a single line of poles to the pump house, with a single 11kV line running at the top, in aluminium wire, with ground return at the pump station using a reasonably big earth mat. The 11kV line runs a small 30kVA transformer on the pole, and this then gives a 100A feed using aluminium wire to the motor. All lowish cost, and more importantly, the wiring is less valuble to the ever present thieves, as the Eskom cabling they use in rural areas most commonly has 7 strands in it, 5 aluminium cores, one steel centre core and a single one of the outer 6 strands is an aluminium coated high carbon steel wire, which is roughly Rockwell 40C hard. Can be cut with difficulty in the field using a hacksaw or angle grinder, but the scrap yards do not want this cable unless it has the high carbon steel core stripped out, as this destroys the yard power cutters that make the cable an easy to handle length. As this is easy to identify yards do not want to have it on premises when inspected, it makes them become the focus of investigation for dealing. |
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