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Three phase reversing

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John Haine16/01/2022 15:49:47
4622 forum posts
273 photos

On this question of reversing whilst running. As far as the motor is concerned, you could think of it like this. Suppose the motor is running at normal speed, say 1400 rpm for example. The rotor sees a rotating stator magnetic field going 100 rpm faster than it, so the circulating currents in the rotor will be at about100/60 = 1.67 Hz. At this frequency the rotor impedance will be mainly resistive, and low, and so the currents in phase with the rotating field, which they have to be to generate maximum torque.

Now throw the reversing switch. This makes the stator field rotate in the opposite direction at 1500 rpm, so the rotor now sees a field rotating at 2900 rpm. The "slip" frequency is now about 48 Hz, so the inductance of the stator will be much more significant, and the stator current much lower. The reverse torque will also be much lower than normal starting torque as well (since the rotor current is lower and in the wrong phase relationship). So the motor will slow down significantly more slowly than it normally starts until it approaches standstill when it generates normal starting torque in the reverse direction.

So I can see reasons why electrically there should not be so much of an issue and the transient current at the point of reverse could be significantly lower than normal starting torque. Also the braking torque at the point of switchover will be much lower than normal running torque so shouldn't load the machine more than normal startup.

Where does the energy go? Well, an induction motor which is driven faster than synchronous speed generates power and feeds it back into the supply - this can be exploited in small-scale wind and water power plants.

Nigel McBurney 116/01/2022 17:13:42
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999 forum posts
3 photos

in these eastern area videos,every handle or wrench has a longer handle welded on,and every thing hammered true with a hard hammer,no one has told the operator that he should lubricate the tap,perhaps the big lathe is being reversed by clutches rather than electrics, Its a long time ago but I think that the smaller Ward capstans were quick reversed by clutches.

old mart16/01/2022 17:18:32
3717 forum posts
233 photos

Thanks for all the help, but having looked at that video, it would be alright for experienced people to see the many undesirable work practices, but the novice might think they were ok and end up having an accident.

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