Introduction
A motor with a Dahlander or tapped windings will give the motor two speeds, possible pole configurations are 12/6, 8/4 or 4/2 poles. For 50Hz supplies this would give speeds of 500/1000rpm, 750/1500rpm or 1500/3000rpm respectively. For 60Hz supplies this equates to 600/1200rpm, 900/1800rpm or 1800/3600rpm respectively.
Power Circuit
Control Circuit
Circuit Operation
As Korndorffer starting has been determined for speed 1 of this motor it follows that if the motor is at standstill and speed 2 is selected the motor will start at speed 1 and via the Korndorffer starting sequence run up to full speed in speed 1 then switch over to speed 2.
This is achieved by adding a pilot contactor to the speed 2 contactor circuits and a start sequence complete relay.
The contact on terminals a3-a4 of each start push button energises the required direction contactor, either the forward contactor FC or the reverse contactor RC, the relevant main contacts 1-2, 3-4 and 5-6 close which puts power; with either clockwise or anticlockwise phase rotation; to the speed control contactors, normally open auxiliary contacts of the relevant direction contactor on terminals 13-14 close to maintain the power to that direction contactor when the button is released, a second normally open contact of the relevant direction contactor on terminals 43-44 closes which enables the speed contactors control circuit.
When the motor is at standstill and the speed 1 start button is pressed it’s contacts a3-a4, b3-b4 close and a1-a2 open.
The speed 1 push button contact on terminals a3-a4 closes to energise the required direction contactor as described above, the contact on terminals b3-b4 closes to energise the speed 1 contactor S1C which causes it’s main contacts on terminals 1-2, 3-4 and 5-6 to close, it’s normally open auxiliary contacts on terminals 13-14 close and normally closed auxiliary contacts on terminals 21-22 open, the normally open contact on terminals 13-14 closes to maintain power to the speed 1 contactor when the button is released, at the same time the speed 1 star contactor S1SC closes which causes it’s main contacts on terminals 1-2, 3-4 and 5-6 to close and it’s auxiliary contact on terminals 21-22 to open, the star contactor timer SCT also energises. Power at a reduced voltage; that voltage is dependant on the auto-transformer tapping; is connected from the power supply to the motor terminals 1U, 1V and 1W. The motor starts with a reduced voltage and accelerates to a speed that is load dependant. After a preset time the star contactor timer SCT times out and it’s change-over contact on terminals 15-16/18 operates which de-energises the speed 1 star contactor coil S1SC which causes its main contacts on terminals 1-2, 3-4 and 5-6 to open and its auxiliary contact on terminals 21-22 to close; the motor is still connected to the power supply but with the auto transformer acting as an in line current limiting choke. The star contactor timer SCT contacts on terminals 15-18 close when that timer timed out, this energises the run contactor timer RCT after a preset time it’s contacts on terminals 17-18 close which energises the speed 1 run contactor S1RC which causes the main contacts on terminals 1-2, 3-4 and 5-6 to close. The motor is now connected to the full line voltage and accelerates to full speed in speed 1.
When the speed 1 run contactor S1RC energises it’s auxiliary normally open contact on terminals 43-44 closes, which starts the timing sequence of speed 1 timer S1T when that timer times out it’s contact on terminals 15-18 close which energises the start sequence complete relay SSCR.
When the start sequence complete relay energises the normally open contact on terminals 13-14 closes to keep the relay energised, contacts on terminals 75-76 open and 87-88 close; these contacts are a special type they are both late break early make with the contact on terminals 75-76 being normally closed and on terminals 87-88 being normally open. When the start sequence complete relay is energised as part of the starting sequence it takes control of the speed 1 run contactor S1RC from the run contactor timer RCT without an interruption of power to that contactor, this is achieved by utilising the early make late break contacts (there is an overlap with these contacts which means that before the normally closed contact opens the normally open contact closes).
The purpose of the start sequence complete relay SSCR is to facilitate switching between the speeds without having to go through the speed 1 Korndorfer starting sequence; it is de-energised when the stop button is pressed.
When the motor is at standstill and the speed 2 start button is pressed it’s contacts a3-a4, b3-b4 close and a1-a2 open.
The speed 2 push button contact on terminals a3-a4 closes to energise the required direction contactor as described above, the contact on terminals b3-b4 closes to energise the speed 2 pilot contactor S2PC it’s contacts on terminals 13-14, 43-44 close and 21-22 open, the normally open contact on terminals 13-14 closes to maintain power to the speed 2 pilot contactor S2PC when the button is released, the closing of the normally open contact on terminals 43-44 initiates the speed 1 starting circuit which operates as detailed above, whilst the normally closed contact on terminals 21-22 opens to prevent the speed 1 contactor S1C maintaining circuit from keeping the speed 1 contactor energised.
At the completion of the speed 1 starting sequence the starting sequence complete relay SSCR energises the normally open contact on terminals 13-14 closes to keep the relay energised a normally closed contact on terminals 21-22 opens which de-energises the speed 1 contactor and a normally open contact on terminals 33-34 closes to complete the circuit to the speed 2 star contactor coil S2SC which energises causing it’s main contacts on terminals 1-2, 3-4 and 5-6 to close and it’s normally open auxiliary contact on terminals 13-14 to close Terminals 1U, 1V and 1W are short circuited which changes the low speed winding’s number of poles, when the normally open auxiliary contact on terminals 13-14 of the speed 2 star contactor closes it energises the speed 2 contactor S2C which causes it’s main contacts on terminals 1-2, 3-4 and 5-6 to close, it’s normally closed auxiliary contacts 21-22 open. Power is connected from the power supply to the motor terminals 2U1, 2V1 and 2W1. The motor now accelerates to full speed in speed 2.
The motor’s direction of rotation cannot be changed whilst the motor is running. To change the direction of rotation it is necessary to stop the motor by pressing the stop button which de-energises the contactors, relays and timers to reset the control circuits ready to start the motor in the required direction.
To change speed whilst the motor is running it is only necessary to press the appropriate button which will cause the energised contactor or contactors to de-energise and the selected speed contactor or contactors to energise. Where that speed change is from speed 2 to speed 1 the motor is connected in ‘run’ without going through the starting sequence.
To stop the motor the stop button is pressed its contacts a1-a2 open which de-energises all the energised contactors, relays and timers thus stopping the motor by removing it’s power supply and resetting the control circuits.
The auxiliary normally closed contacts on terminals 21-22 of the forward FC and reverse RC contactors are placed into each others coil circuit so that electrically the two contactors cannot be simultaneously energised which would cause a short circuit, this can be seen by examination of the above circuit. It is normal to mechanically interlock the forward and reverse contactors in addition to the electrical interlock.
The auxiliary normally closed contacts on terminals 21-22 of the speed 1 S1C and speed 2 S2C contactors are placed into each others coil circuit so that electrically only one contactor can be energised at a time, if both were simultaneously energised it would cause an overload or short circuit in the windings, this can be seen by examination of the above power circuit. It is normal to mechanically interlock the speed 1 and speed 2 contactors in addition to the electrical interlock.
The auxiliary normally closed contacts on terminals 21-22 of the speed 1 star S1SC and speed 1 run contactors S1RC are placed into each others coil circuit so that electrically the two contactors cannot be simultaneously energised which would cause up to only half the transformer winding to be connected to the full line voltage which will cause an overload within the auto transformer, this can be seen by examination of the above circuit. It is also normal to mechanically interlock the star and run contactors in addition to the electrical interlock.
The auxiliary normally closed contacts on terminals 21-22 of the speed 1 S1C and speed 2 star S2SC contactors are placed into each others coil circuit so that electrically the two contactors cannot be simultaneously energised which would cause a short circuit, this can be seen by examination of the above circuit. However it is normal to mechanically interlock the speed 1 and speed 2 star contactors in addition to the electrical interlock.