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.
When the motor is at standstill and the speed 1 start button is pressed it’s contacts a3-a4 close and a1-a2, b1-b2 open.
The speed 1 push button contacts on terminals a3-a4 energise the speed 1 contactor coil S1C is energised which causes the main contacts on terminals 1-2, 3-4 and 5-6 and its normally open auxiliary contact on terminals 13-14 to close and normally closed contacts on terminals 21-22, 31-32 open. The auxiliary contact maintains power to the contactor coil when the start 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 normally closed 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 its 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 speed 1 star contactor to open and its disconnecting arc to extinguish, it also allows the reset of any mechanical interlock between the speed 1 star and speed 1 delta contactors before the speed 1 delta contactor S1DC coil is energised. Star delta timer SDT delay to close contact on terminals 27-28 closing energises the delta contactor S1DC which causes its main contacts 1-2, 3-4 and 5-6 to close and its normally closed auxiliary contact on terminals 21-22 to open. The power supply is connected to motor terminals 1U2, 1V2 and 1W2. Thus each winding has the full voltage applied to it, the motor accelerates to full speed in speed 1.
When the speed 1 delta contactor S1DC energises it’s auxiliary 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 completed relay SSCR. When the start sequence complete relay energises the contact on terminals 13-14 close 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 75-76 being normally closed and 87-88 normally open. When the start sequence complete relay is energised as part of the starting sequence it takes control of the speed 1 delta contactor S1DC from the star delta timer SDT 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 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 close and a1-a2, b1-b2 open.
The speed 2 push button contact on terminals a3-a4 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 contactor coil S2C energises 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, 31-32 open. 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 close and a1-a2, b1-b2 open.
The speed 3 push button contact on terminals a3-a4 closes to energise the speed 3 pilot contactor S3PC 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 3 pilot contactor S3PC 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 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 to close and it’s normally open auxiliary contact on terminals 13-14 to close Terminals 2U, 2V and 2W are short circuited which changes the Dahlander winding’s number of poles, when the normally open auxiliary contact on terminals 13-14 of the speed 3 star contactor closes it energises the speed 3 contactor S3C 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, 31-32 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.
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 ‘run’ 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, relays and timers thus stopping the motor by removing its power supply and resetting the control circuits.
The auxiliary normally closed contacts on terminals 21-22 and 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 1 star S1SC and speed 1 delta S1DC 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 star and speed 1 delta contactors in addition to the electrical interlock.
The auxiliary normally closed contacts on terminals 21-22 of the speed 1 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.