Introduction
A motor with two separate windings will give the motor two speeds, because the windings are independent of each other the possible pole configurations are great. For each winding the number of poles may be any even number 12 to 2. For 50Hz supplies this gives speeds from 500rpm to 3000rpm for each winding. For 60Hz supplies this equates to 600 to 3600rpm.
Power Circuit
Control Circuit (where direct on line starting at any speed is permitted)
Circuit Operation
When the speed 1 start button is pressed it’s contacts a3-a4 close and a1-a2 open. The contact a1-a2 is in the speed 2 contactor coil S2C circuit, thus if the speed 2 contactor is energised it is de-energised and it’s normally closed contact on terminals 21-22 close which allows the speed 1 contactor coil S1C to energise which causes it’s main contacts on terminals 1-2, 3-4 and 5-6 and normally open auxiliary contacts on terminals 13-14 to close and normally closed auxiliary contacts on terminals 21-22 to open. The auxiliary contact on terminals 13-14 maintains power to the speed 1 contactor coil when the start button is released. Power is connected from the power supply to the motor terminals and the motor will start and accelerate to it’s first speed.
Similarly the motor may be started in speed 2 the operation is as speed 1 above with the substitution of the relevant contactors.
To change speed whilst the motor is running it is only necessary to press the appropriate button which will cause the energised contactor to de-energise and the de-energised contactor to close.
To stop the motor the stop button is pressed its contacts a1-a2 open which de-energises the energised contactor, 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 speed 1 S1C and speed 2 S2C contactors are placed into each others coil circuit so that electrically the two contactors cannot be simultaneously energised which would cause an overload in both windings, this can be seen by examination of the above circuit. It is normal to mechanically interlock the speed 1 and speed 2 contactors in addition to the electrical interlock.
Control Circuit (where direct on line starting at low speed only is permitted)
Circuit Operation
If the motor direct on line starting current of the speed 2 winding exceeds the limit for it’s point of common coupling then a variation of the above circuit can be used. If the motor is at standstill and speed 2 is selected the motor will start at low speed and accelerate to full speed in speed 1 then switch over to speed 2. This is achieved by adding a pilot contactor to the speed 2 circuit and a start sequence complete relay.
When the motor is at standstill and the speed 2 start button is pressed it’s contacts a3-a4 close and a1-a2 open. The speed 2 pilot contactor S2PC energises it’s contacts 13-14, 43-44 close and 21-22 open, the contact on terminals 13-14 maintains power to the pilot contactor when the button is released, the closing of the contact on terminals 43-44 energises the speed 1 contactor coil, the normally closed contact on terminals 21-22 opens to prevent the speed 1 contactor maintaining circuit from keeping the speed 1 contactor energised. Power is connected from the power supply to the motor speed 1 terminals and the motor will start and accelerate to it’s first speed. When the speed 1 contactor closes it’s auxiliary contact on terminals 43-44 closes, which starts the timing sequence of speed 1 timer when that timing sequence is completed 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 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 enable the speed 2 contactor to close. When the speed 1 contactor de-energises the normally closed contact on terminals 21-22 of the speed 1 contactor closes which completes the circuit to the speed 2 contactor coil which energises. The motor now accelerates to full speed in speed 2.
To change speed whilst the motor is running it is only necessary to press the appropriate button which will cause the energised speed contactor to de-energise and the de-energised speed contactor to close.
To stop the motor the stop button is pressed its contacts a1-a2 open which de-energises the energised contactors 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 of the speed 1 S1C and speed 2 S2C contactors are placed into each others coil circuit so that electrically the two contactors cannot be simultaneously energised which would cause an overload in both windings, this can be seen by examination of the above circuit. It is normal to mechanically interlock the speed 1 and speed 2 contactors in addition to the electrical interlock.