Guide to the Power Circuit and Control Circuit of the Wound Rotor AC Induction Motor

In certain exceedingly enormous type of industrial applications where a tremendous amount of power is required to drive a specific type of load too large for the ordinary type squirrel cage induction motors to carry, the most appropriately preferred type of motor for use with such extensively large demand of power application is another type of large capacity motor which has the capability to run with high starting torque immediately attainable all throughout starting from when the motor is just beginning to run from rest even while it is at loaded condition up to when the motor reaches its peak rated speed while also carrying the same amount of heavy load, which would also necessitate for this type of motor to possess the ability to run continuously at different variation of steps of speed levels while driving a very heavy load. This type of motor is known as the wound rotor induction motor.

This so-called wound rotor motor as the name implies is a type of AC induction motor which does not only have stator windings but is also equipped with rotor windings. The terminal ends of its rotor windings are externally connected to slip rings and brushes as part of the motor assembly where these rotor winding terminates to terminal ends which are wired externally and connected further to an isolated resistor bank which is a separate necessary component to accompany the intended function of the motor to serve its purpose as a one whole complete functioning unit. The separate resistor bank unit connected to the motor's wound rotor can be externally switched on and off at different intervals of resistor branches in the resistor bank, thus providing multiple stages of speed levels and torque capacity with an acceleration rate faster than would an ordinary induction motor could accomplish for similar types of bulky load applications to be driven.

Another purpose of the resistor bank connected to the rotor winding is to facilitate for the reduction of the very high inrush starting current, considering that the type of application that the motor will be subjected to requires for the motor to start running at a very heavy load condition. The resistor bank also serves to absorb the heat developed in the rotor winding which is necessary for the intended application of the motor which is to have the ability to effectively dissipate the heat generated in such extreme load conditions, because rapid heat build-up is an inherent characteristic of very large capacity motors operating at very high load condition from zero speed up to its rated maximum speed or even when running the motor in any continuous variation of speed level falling anywhere within the various stages of speed range of the motor.

Fig-1 below shows an electrical schematic diagram of a three phase power circuit of a wound rotor motor. As shown in the drawing, you will notice that the motor consists of a stator winding which is the stationary part of the motor while another component of the motor is the rotor winding which constitutes the rotating part of the motor. The stator winding is directly connected to the three phase power supply which provides the source voltage to the motor. The main supply voltage is supplied to the primary side of the disconnect fuse on the three phase terminals L1, L2 and L3 where the fuse serves as an instantaneous interrupting means with rapid action that immediately detects line side power trouble and motor circuit problem such as over voltage, over current, short circuit or overloads. The Main Magnetic Contactor (MMC) is a magnetic motor starter which serves as the motor operation switch which is electrically activated and deactivated remotely with an external push button switch in the control circuit. The overload relay is another type of overload protection device which is directly concentrated in instantaneously detecting motor overload currents that once detected would instantly trip-off to open the control circuit in order to deactivate the main magnetic contactor (MMC) to isolate power flow from the main supply line voltage going to the motor in order to prevent the destruction of the motor.

Fig-1: Electrical Schematic Diagram of the Power Circuit of a Wound Rotor Motor

The secondary part of the wound rotor motor contains the rotor winding. Fig-1 above shows the rotor winding connected to a resistor bank which consists of three resistors per phase making up a total of nine resistors in the entire resistor bank network. There are correspondingly three branches of resistors in the resistor bank which consists of the first branch comprising R1-R2-R3 resistors, the second branch being R4-R5-R6, and finally the last branch contains R7-R8-R9 respectively. Each stages of resistor branches are provided with their own individual magnetic contactor switches with MC1 for the first resistor branch, then MC2 for the second resistor branch and finally MC3 for the third resistor branch. Each of these resistor branches are shorted individually by MC1, MC2 and MC3 according to an order of operational sequence relative to the efficient running of the wound rotor motor. The single phase step down transformer is made available in the power circuit as shown above in order to provide low voltage power supply for the control circuit taken from the single phase terminals RC and SC.

Fig 2: Electrical Schematic Diagram of the Control Circuit of a Wound Rotor Motor

Fig-2 above is an electrical schematic diagram of a common control circuit which illustrates how the wound rotor motor is operated to run from standstill until it reaches its rated speed. The power supply source voltage of the control circuit is taken directly from the step down transformer provided in the power circuit in Fig-1 above which shows the reference terminals RC and SC coming from the secondary part of the step down transformer from the power circuit. The proper sequence of operation of the wound rotor motor is directly dependent on the construction of the control circuit based on the intended sequence of operation for this particular wound rotor motor.

The start operation sequence in the control circuit in Fig-2 above begins with the closing of the start push button switch which permits power flow to the main magnetic contactor (MMC) provided that both the stop switch and the overload relay contacts are maintained closed at all times during the operation of the control circuit. The auxiliary open contact of the MMC which is connected in parallel to the start switch serves as the holding contact which maintains the MMC coil energized even after the human operator releases the start switch, this in turn provides continuous power flow to the control circuit which further proceeds in commencing the sequence of steps in the operation of the entire circuit of the wound rotor motor.

During the first stage of the operation of the wound rotor motor when power is connected to the primary stator of the motor, the secondary part or the rotor is firstly connected with maximum resistance with all resistors in the resistor bank network fully active which initially runs the motor at its lowest speed. The flow of electricity in the control circuit would then proceed through the closed contact of Timer 4 going down to the Timer 1 coil which upon reaching its specified time delay period would then activate contactor MC1 to short the first stage resistor branch R1-R2-R3 leaving only the second and third stage of resistor branches to remain active in the resistor network to act on the rotor winding which will eventually increase the speed of the motor to a partial 1/3 of its rated speed.

The activation of contactor MC1 achieves the next step of the operational sequence that closes one of the auxiliary contact of MC1 to energize the Timer 2 coil which upon reaching its specified time delay period would then energize contactor MC2 to short out the second stage resistor branch R4-R5-R6 leaving only the third resistor branch to remain active in the rotor winding which further increases the motor speed to 2/3 of its overall rated speed. Then comes the last and final stage where the previously activated contactor MC2 energizes the Timer 3 which when its specified time delay period expires would then activate contactor MC3 to totally short out the rotor winding to the fullest without any resistor branches remaining active in the network that can connect to the rotor winding, this final stage brings the motor to run to its fullest rated speed.

The activation of contactor MC3 would then provide a holding contact with one of its auxiliary contact connected in parallel across the Timer 3 contact while maintaining the MC3 coil energized during the final stage of this operational sequence. This last stage would also activate and maintain the Timer 4 coil which upon reaching its specified time delay period would then open the Timer 4 normally-closed contact to remove power to the coils of Timer 1, MC1, Timer 2, MC2 and Timer 3 in order to release the first stage contactor MC1 and second stage contactor MC2 from shorting out the first and second stages of resistor branches but while also maintaining only the last contactor MC3 to remain activated to maintain the rotor winding shorted out completely hence maintaining the motor to run at its rated full speed.

To interrupt the operation of the circuit, the only thing necessary to manually stop the motor is by pressing the stop switch which would totally remove power to the circuit at any time during the circuit is active no matter under which stage of speed intervals the motor is actively running in. Another interrupting means in the control circuit is the overload relay which automatically shuts down the circuit to immediately stop the motor in case over current is instantly detected at anytime during the motor is running.