SINGLE-PHASE OPERATION OF THREE-PHASE INDUCTION MOTORS
A line wire feeds the stator windings of a three-phase induction motor. If this wire is opened, the motor will operate as a single-phase induction motor. It will not have enough torque to start when energized from a single-phase source. However, if the three-phase motor is running when the break in the line wire occurs, it will continue to operate with a greatly reduced capacity. If the rated load is applied to the motor when it is operating as a single-phase motor, it will overheat. The insulation of the windings may be damaged as a result.
The three-phase motor will not start on a single phase because the induced voltage and currents in the rotor set up a magnetic field in the rotor. This field opposes the stator field. (This situation is an application of Lenz’s law.) The rotor current produces a rotor field in which the rotor poles are centered with the stator field poles, as shown in Figure 16–18. As a result, there is no torque in either a clockwise or counterclockwise direction.
If the three-phase motor is operating at the rated speed when the single-phase condition develops, the rotor continues to turn. The moving rotor cuts the stator field flux and causes induced voltages and currents in the rotor bars. The rotor currents create a rotor field with poles midway between the stator poles. The rotor has high-reactance and low- resistance components. Therefore, the rotor current will lag behind the induced voltage in the rotor by nearly 90%. As a result, the rotor and stator fields are practically 90% out of phase with each other. The rotor current produces magnetic polarities that are 90º out-of- phase with those produced in the stator. The motor continues to operate due to attraction and repulsion of magnetism in the same manner as a single-phase induction motor. The three-phase motor will continue to run, but at reduced capacity. Once the motor has been stopped, it cannot restart because a rotating magnetic field cannot be produced with a single phase.
THE WOUND-ROTOR INDUCTION MOTOR
Many industrial applications require three-phase motors with variable-speed control. The basic squirrel-cage induction motor is a constant speed motor. Thus, another type of induction motor is required for variable-speed applications. The wound-rotor induction motor meets these needs.
The wound-rotor induction motor has nearly the same stator construction and winding arrangement as the squirrel-cage induction motor. Figure 16–19 shows a typical stator for a wound-rotor induction motor.
A wound rotor is shown in Figure 16–20. The cylindrical core of the rotor is made up of steel laminations. Slots are cut into the cylindrical core to hold the formed coils for three single-phase windings. These windings are placed 120 electrical degrees apart. The insulated coils of the rotor winding are grouped to form the same number of poles as in the stator windings. The three single-phase rotor windings are connected in wye.
Three leads from these windings terminate at three slip rings mounted on the rotor shaft. Carbon brushes ride on these slip rings and are held securely by adjustable springs mounted in the brush holders. The brush holders are fixed rigidly, because it is not necessary to vary their position. Leads from the carbon brushes are connected to an external speed controller.
Principle of Operation
When the stator windings of a wound-rotor motor are energized from a three-phase source, the rotating magnetic field formed travels around the inside of the stator core, just as in a squirrel-cage induction motor. The speed of the rotating magnetic field depends on the number of stator poles and the frequency of the source. The formula used to find the synchronous speed for squirrel-cage induction motors can also be used for this type of motor:
As the rotating field travels at the synchronous speed, it cuts the wound-rotor windings and induces voltages in these windings. The induced voltages set up currents that form a closed-circuit path from the rotor windings through the slip rings and brushes to a wye-connected speed controller. Figure 16–21 shows a wound-rotor induction motor connected to a wye-connected speed controller.
At start-up, all of the resistance of the wye-connected speed controller is inserted in the rotor circuit. This additional resistance causes an excellent starting torque and a large percent slip. The added resistance in the rotor circuit increases the impedance. Because the rotor circuit has a large resistance component and a small reactive component, the rotor current is nearly in phase with the stator field flux. Thus, there is a maximum interaction between the two fields, resulting in a strong starting torque.
As the motor accelerates, steps of resistance are cut out of the wye-connected speed controller. When all of the resistance is cut out, the rotor slip rings are short-circuited. The motor then operates at the rated speed like a squirrel-cage induction motor. The speed of the wound-rotor motor can be changed by inserting or removing resistance in the rotor circuit using a wye-connected speed controller.
This motor can be operated at heavy loads by cutting in resistance to the rotor circuit to obtain a below-normal speed. However, the I2 R losses in the rotor circuit are high and cause a large reduction in the motor efficiency. Additional resistance inserted in the rotor circuit leads to poor speed regulation. This effect is due to the large increase in slip that is necessary to obtain the required torque increase with an increase in the load.
The curves in Figure 16–22 show the torque performance of a wound-rotor induc- tion motor. When the proper value of resistance is inserted in the rotor circuit, the starting torque has its maximum value at 100% slip (at start-up). If all of the resistance is cut out of the speed controller, and the motor is started, the starting torque is poor.
In this case, the rotor circuit has a large reactive component and a small resistance component. This means that the motor will have the same starting torque characteristic as a squirrel-cage induction motor.
The direction of rotation of a wound-rotor induction motor can be reversed. To do this, interchange the connections of any two of the three line leads feeding to the stator wind- ings. Note that both the wound-rotor and the squirrel-cage induction motors are reversed using the same procedure. Reversing the phase sequence of the three-phase input to the stator windings changes the direction of rotation of the magnetic field produced by these windings. Therefore, the direction of rotation of the rotor is reversed. Figure 16–23 shows the connection changes that are required to reverse the direction of rotation. There is no reversal in the direction of rotation of the motor when any of the leads feeding from the slip rings of the speed controller are interchanged.
The stator leads of a three-phase, wound-rotor induction motor are marked T , T , and T . This is the same marking system used with three-phase, squirrel-cage induction motors.
The rotor leads are marked M , M , and M . The M lead connects to the rotor ring nearest the bearing housing. The M lead connects to the middle slip ring, and the M lead connects to the slip ring nearest the rotor windings.
The wound-rotor motor and the squirrel-cage motor have the same percent efficiency, power factor, percent slip, speed, and torque characteristics. This statement is true if the wound-rotor motor is operated from no load to full load with all of the resistance cut out of the rotor circuit. If the motor is operated at or near the rated load while there is resistance in the rotor circuit, there will be I2 R losses in the resistance components of the speed controller. These losses cause a decrease in the motor efficiency. When the motor is operated with resistance in the rotor circuit, there is also a sharp increase in percent slip with an increase in the load. The percent slip must increase to obtain the required increase in the rotor current so that there is a larger value of torque to meet the increased load demands.
If the motor is started with all of the resistance of the speed controller inserted in the rotor circuit, the starting torque will be at a maximum with 100% slip. The starting surge of current to the stator is limited to a relatively low value. The current is low because of the high resistance inserted in the rotor.
The wound-rotor induction motor is used when a strong starting torque and a range of speed control are required. Typical applications for this motor include cranes, large compressors, elevators, and pumps. This type of motor is also used in the heavy-steel industry for applications requiring adjustable speed.