Across The Line Starters (1945?)

the electric motor which drive this hand milling machine is an induction motor controlled by a manually operated thermal overload switch when an induction motor is connected directly to the power lines the initial electron flow to the motor is approximately six times the normal full load flow this initial overload is useful in quick starting and causes no damage to a modern motor a thermal overload switch provides automatic protection against sustained overloads which would harm the motor this switch is similar in principle to the one used on the milling machine pushing the handle in one direction closes the circuit pushing it in the opposite direction opens the switch with the cover remove the working parts of the switch are seen in motion as the contacts of force tnmn released automatic protection against overloads is provided by thermostats built into the switch when the switch handle is pushed up triggers are set against the thermostat so movement will release the trigger and open the switch to understand how this thermal overload switch operates a clear understanding of the action of a thermostat is necessary this bar consists of two strips of different metals joined together they operate on the same principle as the thermostat in the switch when heat is applied to the strip's they expand since they are made of different metals one stiff expands faster than the other this difference in expansion produces a bending action of the bi-metal bars the force produced by the bending action can be applied to such devices as the trigger in a thermal overload switch in this thermal overload switch thermostat the bi-metal strips are heated within themselves by a sustained excess flow of electrons they are designed to overheat at the same rate as the motor will overheat when subject to an overload there are two bi-metal strips here shaped in horseshoe form as the metal strips are heated they Bend and press against the trigger release the strips will Bend far enough to release the trigger and open the switch before the overload continues long enough to damage the motor wear automatic or remote control is desirable the magnetically operated across the line starter is used a push-button station is one of the most common methods of controlling this starter pressing the start button operates the starter mechanism when the start button is pressed the control circuit is closed electrons flow in from line one across the starting switch and through coil M the alternating flow in the control circuit is here shown as unidirectional flow coil M becomes magnetized as the electrons flow through and it draws up the contacts the contacts mx now short-circuit the start button the button can now be released contacts MX maintain the control circuits independent of the start button as long as coil M is energized now the stop button is pressed operating the stop button breaks the electron flow through the entire circuit when coil em is no longer energized all the contacts open when excess electrons flow through the thermostatic element and the thermal overload relay heats to a certain predetermined temperature it will operate a contact opens and D energizes the control circuit and coil em is de-energized all main contacts and contacts MX open exactly as they do when the stop button is pressed coil M is de-energized whenever the line voltage falls too low thus providing the machine with complete low voltage protection another across the line starter in common use is the drum reversing switch for a three-phase motor here shown in operation on a lathe by moving the controller handle to certain positions the operator is able to start the lathe running forward to stop the lave altogether and to reverse the direction this panel wiring will show the theory of operation of the drum reversing controller the lines mo 3 phase 3 wire servers are connected to terminals at the right side of the panel the 3 wires at the left go to the motor to use this panel first check to see that the service switch is open then connect the service and motor wires straight across the board close the switch the motor turns the spool note the direction open the switch again reverse the connections of any two wires close the switch and the motor now turns the screw in the opposite direction this process of shifting connections is accomplished in the drum controller mechanically when the handle is turned to different positions in this position the center or stop position all contacts are disconnected moving the handle to the left or forward position connects all three sets of contact straight across moving the handle to the right or reversed position connects the two top right terminals together and connects the two top left terminals together the bottom terminals are again connected straight across electrical aid when the handle is in the off position there is no contact between any terminals and no electrons are flowing when the handle is moved to the forward position the contact bars make a connection between L 2 and T 2 and L 1 and T 1 and L 3 and T 3 at a given instant electrons flow through the motor coils in the direction shown by the arrows since the rapid change of direction that is characteristic of alternating electron flow cannot be shown here we are showing the electron flow as if observed during the split second while it was traveling in one direction and before it reverses and starts in the other direction this is called instantaneous polarity the motor is now turning to the right when the controller handle has moved from forward to reverse the motor will turn in the opposite direction contact bars T 1 and L 2 and T 2 and L 1 are connected there is no change between contacts L 3 and T 3 now note that the flow through two of the coils is reversed the motor turns in the opposite direction because it's rotating magnetic field has been reversed another of course the line starter is this magnetic reversing switch it is really to magnetic across the line starters built into one unit one starter is connected for a forward motion the other for reverse it starts stops and reverses the motor when the forward button is pushed the coil is energized and closes the forward contacts at the same time a mechanical interlock prevents the reversing contacts from closing and causing a short-circuit pushing the forward button makes contact between C 1 F and F 2 and at the same time breaks the contact between a R and F for making it impossible for both coils to be energized at the same time this is an electrical interlock electrons flowing through coil FM produce an electromagnetic field which closes all forward contacts and auxilary contact FX after these contacts closed the forward button returns to close the circuit between a R and F for pressing the reverse button stops the forward motion of the motor and causes it to turn in the other direction this is what happens inside the starter when the motor is reversed the interlock now prevents the forward contacts from closing pushing the reverse button breaks contact at AF and R for interrupting the flow and the energizing coil F em all forward contacts are allowed to open as the electromagnetic field disappears as pressure on the reverse button continues contact is made between c1 R and R 2 as coil RM is energized all reverse contacts and auxilary contact RX closed simultaneously the motor can be stopped at will by pressing the stop button which opens the circuit this action takes place inside the starter pressing the stop button opens contact L 1 and s 2 stopping the flow of electrons through coil RM as the electromagnetic field of coil RM disappears all reverse contacts and auxilary contact Rx open simultaneously and all electron flow stops in the magnetic reversing switch