reversing motor
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A DOL starter with a reverse function.
It has two 3 pole contactors, the outputs of which are commoned up, before they hit the overload. The supply to both contactors is also commoned up, except on one contactor the L1 and L2 are reversed. Two green start buttons, one forward, one reverse.
There is also a 'fourth auxiliary' pole which opens when the contactor energises. This is in series with the start button of the opposite contactor. This prevents the second contactor from energising.
Swapping ANY two phases of a 3 phase motor will reverse the direction.
It has two 3 pole contactors, the outputs of which are commoned up, before they hit the overload. The supply to both contactors is also commoned up, except on one contactor the L1 and L2 are reversed. Two green start buttons, one forward, one reverse.
There is also a 'fourth auxiliary' pole which opens when the contactor energises. This is in series with the start button of the opposite contactor. This prevents the second contactor from energising.
Swapping ANY two phases of a 3 phase motor will reverse the direction.
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A Motor should generally be protected by a DOL starter.
A DOL starter (dirct on line) is basically a contactor with an overload incorporated. The overload is selected depending on the FLC of the motor (full load current). The overload is adjustable to half an amp usually.
If the motor is mechanically overloaded, the current rises, and the overload will 'trip' the contactor and turn the supply off.
Some motors also have thermistors burried in the windings, and these can be incorporated into the control circuit to trip the contactor if the windings overheat.
A DOL starter has a green and red button on the front of the enclosure, and the contactor and overload inside.
An MCB will offer little protection against mechanical overload of a motor.
If the motor is mechanically overloaded, the current rises, and the overload will 'trip' the contactor and turn the supply off.
Some motors also have thermistors burried in the windings, and these can be incorporated into the control circuit to trip the contactor if the windings overheat.
A DOL starter has a green and red button on the front of the enclosure, and the contactor and overload inside.
An MCB will offer little protection against mechanical overload of a motor.
Total. But beware that the current in any one winding may not be as you expect, depending if it is delta or star wound internally.
Of course, once the currents have combined, the current in any one phase will be 1/3 of the total, but this may split into 2 wires each to 415, which each appear to carry about 2/3 of the total (but not in phase - thats the point).. or down one wire to a central pseudo neutral (which on some machines is used to power 240v indicators, if the lamp load is 'small' compared with the winding loads -avoids the need for a neutral core in the supply)
Of course, once the currents have combined, the current in any one phase will be 1/3 of the total, but this may split into 2 wires each to 415, which each appear to carry about 2/3 of the total (but not in phase - thats the point).. or down one wire to a central pseudo neutral (which on some machines is used to power 240v indicators, if the lamp load is 'small' compared with the winding loads -avoids the need for a neutral core in the supply)
B
Big_Spark
Map, what are you drivelling about? (Ignore comment..posted when underthe influence)
The current in a winding of a 3 Phase motor is square root of 3, or 1.732, the full rating of the motor.
In this case 5.5kW = 5500/400 = 13.75A / sq3(1.732) = 7.938A per phase This is the current in the supply NOT the winding, did not make this clear, apologies.
Where is this "pseudo neutral" you speak of on a 3 phase supply? You either have a Neutral, if you have a TP&N supply, or you do not. Indicators on pure 3 phase systems are often 400V rated lamps etc that operate across phases. If there are electronics in the machine then it usually has a true neutral, but this is not always the case as they have a power Tx's that operate at 400V across phases and the electronics use the Earthed casing to provide the ground for the contols etc just as a computer does.
The current in a winding of a 3 Phase motor is square root of 3, or 1.732, the full rating of the motor.
In this case 5.5kW = 5500/400 = 13.75A / sq3(1.732) = 7.938A per phase This is the current in the supply NOT the winding, did not make this clear, apologies.
Where is this "pseudo neutral" you speak of on a 3 phase supply? You either have a Neutral, if you have a TP&N supply, or you do not. Indicators on pure 3 phase systems are often 400V rated lamps etc that operate across phases. If there are electronics in the machine then it usually has a true neutral, but this is not always the case as they have a power Tx's that operate at 400V across phases and the electronics use the Earthed casing to provide the ground for the contols etc just as a computer does.
FWL, what are you drivelling about?
The current in a winding of a 3 Phase delta connected motor is 1/3 watts/volts, assuming 100% PF and efficiency.
In this case 5.5kW = 5500/400 = 13.75A / 3 = 4.58A per winding, plus PF plus losses. Note that the current in a winding is NOT the same as the current in the phase conductor of the supply.
Most three phase motors are not connected internally but, as suggested by mapj1 have all 6 terminations available. If this were not so, you would not have the option of connecting a star-delta starter. Connected in delta, each of the windings carries 1/3 the total current and the ends of two windings are connected to each phase. But that does not mean that the supply current per phase is 2/3 the total because the phase of the current in the two windings is different. Indeed, the supply current in the example is 7.94 A (plus PF & losses).
The correct full load current, including losses and PF, is normally included on the nameplate.
The current in a winding of a 3 Phase delta connected motor is 1/3 watts/volts, assuming 100% PF and efficiency.
In this case 5.5kW = 5500/400 = 13.75A / 3 = 4.58A per winding, plus PF plus losses. Note that the current in a winding is NOT the same as the current in the phase conductor of the supply.
Most three phase motors are not connected internally but, as suggested by mapj1 have all 6 terminations available. If this were not so, you would not have the option of connecting a star-delta starter. Connected in delta, each of the windings carries 1/3 the total current and the ends of two windings are connected to each phase. But that does not mean that the supply current per phase is 2/3 the total because the phase of the current in the two windings is different. Indeed, the supply current in the example is 7.94 A (plus PF & losses).
The correct full load current, including losses and PF, is normally included on the nameplate.
B
Big_Spark
Many apologies, should not post when ****ed....misread map1j's post and then cocked up my reply... 
As regards the use of the star point as a pseudo-neutral, for a small load, when no neutral is supplied, I have certainly seen this done on American 3 phase equipment, actully, now I remember it better I have feeling the whole thing was fed by a 3 phase auto- transformer primary, rather than a true motor load, and it was the non-existant neutral at the star point of this transformer that was used. I do remember being a bit surprised not to find a phase-phase double wound transformer.
It worked as the off balance load was only a few % of the full 3 phase load.
I'm not saying its a good thing - just that you may find things like this, particularly in imported apparatus designed for the north american market, and not to be too surprised.
It worked as the off balance load was only a few % of the full 3 phase load.
I'm not saying its a good thing - just that you may find things like this, particularly in imported apparatus designed for the north american market, and not to be too surprised.
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