Introduction
A motor with one Dahlander or tapped winding and a separate low speed winding will give the motor three speeds, possible pole configurations are for the Dahlander winding 8/4 or 4/2 poles and for the low speed winding 12 or 8 poles. For 50Hz supplies this would give speeds of 750/1500 or 1500/3000rpm for the Dahlander winding and 500 or 750rpm for the high speed winding. For 60Hz supplies this equates to 900/1800 or 1800/3600rpm and 600 or 900rpm respectively.
Power Circuit
Control Circuit
Circuit Operation
As star delta starting has been determined for speed 1 of this motor it follows that if the motor is at standstill and speed 2 or speed 3 is selected the motor will start at low speed in star accelerate to full speed in speed 1 in delta then switch over to the selected speed.
This is achieved by adding a pilot contactor to the speed 2 and speed 3 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, b1-b2 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, 31-32 open, the normally open contact on terminals 13-14 closes to maintain power to the speed 1 contactor when the button is released. Power is connected from the power supply to the motor terminals 1U1, 1V1 and 1W1. At the same time the star delta timer SDT energises which causes its instantaneous close, delay to open contact on terminals 17-18 to close, this in turn energises the speed 1 star contactor S1SC which causes its main contacts on terminals 1-2, 3-4 and 5-6 to close and its auxiliary contact on terminals 21-22 to open. Motor terminals 1U2, 1V2 and 1W2 are connected together thus completing the motor circuit. The motor starts with a reduced voltage on each winding and accelerates to a speed that is load dependant. The voltage reduction is ?3 of the line voltage. The star delta timer SDT is a special type of time delay relay that has two contacts. After a preset time the star delta timer SDT instantaneous close delay to open contact on terminals 17-18 opens 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 it’s normally closed auxiliary contact on terminals 11-12 to close. The motor circuit becomes open circuit. The star delta timer SDT delay to close contact on terminals 27-28 closes 50-60milli-seconds after the preset time which opened the contact on terminals 17-18. The additional delay is to allow the star contactor to open and its disconnecting arc to extinguish, it also allows the reset of any mechanical interlock between the star and delta contactors before the relevant delta contactor either S1FDC or S1RDC coil is energised. Star delta timer SDT delay to close contact on terminals 27-28 closing energises the relevant delta contactor either S1FDC or S1RDC which causes its main contacts 1-2, 3-4 and 5-6 to close, its auxiliary contacts on terminals 11-12, 21-22 and 31-32 to open. The power supply is connected to motor terminals U2, V2 and W2. Thus each winding has the full voltage applied to it, the motor accelerates to full speed in the selected direction.
When the speed 1 delta contactor either S1FDC or S1RDC 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 relevant speed 1 delta contactor either S1FDC or S1RDC from the star delta timer SDT without an interruption of power to that delta 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 star delta 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, b1-b2 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 13-14, 43-44 close and 21-22 open, the contact on terminals 13-14 maintains power to the speed 2 pilot contactor when the button is released, the closing of the contact on terminals 43-44 initiates the speed 1 starting sequence 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 contacts on terminals 13-14 closes to keep the relay energised another contact on terminals 21-22 opens which de-energises the speed 1 contactor and another contact on terminals 33-34 closes to complete the circuit to the speed 2 contactor coil S2C which energises causing it’s main contacts on terminals 1-2, 3-4 and 5-6 and auxiliary contacts on terminals 13-14 to close. Power is connected from the power supply to the motor terminals 2U, 2V and 2W. The motor now accelerates to full speed in speed 2.
When the motor is at standstill and the speed 3 start button is pressed it’s contacts a3-a4, b3-b4 close and a1-a2, b1-b2 open.
The speed 3 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 3 pilot contactor S3PC it’s contacts 13-14, 43-44 close and 21-22 open, the contact on terminals 13-14 maintains power to the speed 3 pilot contactor when the button is released, the closing of the contact on terminals 43-44 initiates the speed 1 starting sequence 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 contacts on terminals 13-14 close to keep the relay energised another contact on terminals 21-22 opens which de-energises the speed 1 contactor and another contact on terminals 43-44 closes to complete the circuit to the speed 3 star contactor coil S3SC which energises causing it’s main contacts on terminals 1-2, 3-4 and 5-6 it’s normally open auxiliary contacts on terminals 13-14 to close and it’s normally closed auxiliary contacts on terminals 21-22. Terminals 2U, 2V and 2W are short circuited which changes the motor’s number of poles, when the auxiliary contact on terminals 13-14 of the speed 3 star contactor close it energises the speed 3 contactor S3C which causes it’s main contacts on terminals 1-2, 3-4 and 5-6 it’s auxiliary contacts on terminals 13-14 to close and it’s normally closed auxiliary contacts on terminals 21-22, 31-32 to open. Power is connected from the power supply to the motor terminals 3U1, 3V1 and 3W1. The motor now accelerates to full speed in speed 3.
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 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 either speed 2 or speed 3 to speed 1 the motor is connected in ‘delta’ without going through the star delta starting sequence.
To stop the motor the stop button is pressed its contact a1-a2 opens which de-energises all the energised contactors thus stopping the motor by removing its 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. It can also be seen from the above circuit that it is not possible to change the direction without first pressing the stop button to de-energise the energised contactor.
The auxiliary normally closed contacts on terminals 21-22, 31-32 of the speed 1 S1C, speed 2 S2C and speed 3 S3C contactors are placed into each others coil circuit so that electrically only one contactor can be energised at a time, if two or more were simultaneously energised it would cause an overload or short circuit in the windings, this can be seen by examination of the above circuit. It is normal to mechanically interlock the speed 2 and speed 3 contactors in addition to the electrical interlock. It is not possible with most contactors to mechanically interlock with two other contactors where this is possible all three speed contactors should be mechanically interlocked.
The auxiliary normally closed contacts on terminals 21-22 of the speed 2 S2C and speed 3 star S3SC 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 2 and speed 3 star contactors in addition to the electrical interlock.
The auxiliary normally closed contacts on terminals 21-22 and 31-32 of the speed 1 star S1SC and both delta contactors S1FDC and S1RDC are placed into each others coil circuit so that electrically only one of the three contactors can be energised at one time, should any two or more close simultaneously a short circuit would result, this can be seen by examination of the above circuit. It is not possible with most contactors to mechanically interlock with two other contactors where this is possible the three contactors should be mechanically interlocked.