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GOALS
- Identify terminal markings for two-speed, one-winding consequent pole motors.
- Discuss how speed of a consequent pole motor is changed.
- Connect a two-speed, one-winding consequent pole motor.
- Discuss the construction of three-speed consequent pole motors.
- Discuss different types of four-speed consequent pole motors.
FIG. 1 Synchronous speed is determined by the frequency and number
of stator poles per phase.
FIG. 2 The magnetic field will travel through the same number of
poles during each complete cycle.
FIG. 3 The direction of current flow determines the number of poles.
Consequent pole motors have the ability to change speed by changing the number of stator poles. There are two factors that determine the synchronous speed of an AC motor:
1. Frequency of the applied voltage
2. Number of stator poles per phase A chart showing the synchronous speed of 60 and 50 hertz motors with different numbers of poles is shown in FIG. 1. A three-phase two-pole motor contains six actual poles. The magnetic field will make one revolution of a two-pole motor each complete cycle. If the stator of a motor were to be cut and laid out flat, the magnetic field would traverse the entire length in one cycle (FIG. 2A). If the number of stator pole pieces, both will produce the same magnetic polarity and are essentially one pole piece. If the current direction is opposite through each pole piece, they will produce opposite magnetic polarities and are essentially two poles.
Two-speed consequent pole motors contain one reconnectable stator winding. A two-speed motor will contain six T leads in the terminal connection box. The motor can be connected to form a series delta or parallel wye (FIG. 4). If the motor is wound in such a way that the series delta connection gives the high poles is doubled to four per phase (FIG. 2B), the magnetic field will traverse the same number of stator poles during one cycle. Since the number of poles has been doubled, the magnetic field will travel only half as far during one complete cycle. Consequent pole motors have an advantage over some others types of variable speed alternating current motors in that they maintain a high torque when speed is reduced.
FIG. 4 Stator windings can be connected as either parallel wye or
series delta.
The number of stator poles is changed by redirecting the current through pairs of poles (FIG. 3). If the current travels in the same direction through two speed and the parallel wye gives the low speed, the horsepower will be the same for either connection. If the winding is such that the series delta gives the low speed and the parallel wye gives the high speed, the torque will be the same for both speeds.
Two-speed consequent pole motors provide a speed ratio of 2:1. For example, a two-speed consequent pole motor could provide synchronous speeds of 3600 and 1800 RPM, or 1800 and 900 RPM, or 1200 and 600 RPM. The connection diagram for a two-speed consequent pole motor is shown in FIG. 5. A typical controller for a two-speed motor is shown in FIG. 6. Note that the low speed connection re quires six load contacts: three to connect the L1, L2, and L3 to T1, T2, and T3; and three to short leads T4, T5, and T6 together. Although contactors with six load contacts can be obtained, it is common practice to em ploy a separate three-pole contactor to short T4, T5, and T6 together.
In the circuit shown in FIG. 6, the Stop but ton must be pressed before a change of speed can be made. Another control circuit is shown in FIG. 7 that forces the motor to start in low speed before it can be accelerated to high speed. The Stop button does not have to be pressed before the motor can be accelerated to the second speed. A permissive relay (PR) is used to accomplish this logic. The motor can be returned to the low speed by pressing the Low push button after the motor has been accelerated to high speed. The load connections are the same as shown in FIG. 6.
FIG. 5 Connection diagram for a two speed consequent pole motor.
FIG. 6 Two speed control for a consequent pole motor.
FIG. 7 The motor must be started in low speed before it can be accelerated
to high speed.
FIG. 8A Constant horsepower.
FIG. 8B Constant horsepower.
FIG. 8C Constant horsepower.
FIG. 8D Constant torque.
FIG. 8E Constant torque.
FIG. 8F Constant torque.
FIG. 8G Variable torque.
FIG. 8H Variable torque.
FIG. 8I Variable torque.
FIG. 9A Constant horsepower.
Three-Speed Consequent Pole Motors
Consequent pole motors that are intended to operate with three speeds contain two separate stator windings.
One winding is reconnectable like the winding in a two speed motor. The second winding is wound for a certain number of poles and is not reconnectable. If one stator winding were wound with six poles and the second were reconnectable for two or four poles, the motor would develop synchronous speeds of 3600 RPM, 1800 RPM, or 1200 RPM when connected to a 60 hertz line. If the re connectable winding were to be wound for four or eight pole connection, the motor would develop synchro nous speeds of 1800 RPM, 1200 RPM, or 900 RPM.
Three-speed consequent pole motors can be wound to produce constant horsepower, constant torque, or vari able torque. Examples of different connection diagrams for three-speed, two-winding consequent pole motors are shown in Figures 8A through 8I.
FIG. 9B Constant horsepower.
FIG. 9C Constant torque.
FIG. 9D Constant torque.
FIG. 9E Variable torque.
FIG. 9F Variable torque.
FIG. 10 Pushbutton control for a 4 speed consequent pole 3 phase
motor.
FIG. 11 Two-speed, two-winding motor controller mounted in cabinet.
FIG. 12 Two-speed, one-winding motor controller mounted in cabinet.
Four-Speed Consequent Pole Motors
Consequent pole motors intended to operate with four speeds use two reconnectable windings. Like two-speed or three-speed motors, four-speed motors can be wound to operate at constant horsepower, constant torque, or variable torque. Some examples of winding connections for four-speed, two-winding three-phase con sequent pole motors are shown in Figures 41-9A through 41-9F.
A circuit for controlling a four-speed, three-phase consequent pole motor is shown in FIG. 10. The control permits any speed to be selected by pushing the button that initiates that particular speed. In this circuit, stacked push buttons are used to break the circuit to any other speed before the starter that controls the selected speed is energized. Electrical interlocks are also used to ensure that two speeds cannot be energized at the same time. Eleven-pin control relays are used to pro vide interlock protection because they each contain three sets of contacts.
The load contact connection is also shown in FIG. 10. The circuit assumes the connection diagram for the motor is the same as the diagram illustrated in FIG. 9F. The circuit also assumes that the starters and contactors each contain three load contacts. Note that 3RD speed and HIGH speed require the use of two contactors to supply the necessary number of load contacts.
A two-speed, two-winding motor controller and a two-speed, one-winding motor controller are shown in FIG. 11 and FIG. 12.
QUIZ
1. Name two factors that determine the synchronous speed of a motor.
2. How many speeds can be obtained from a consequent pole motor that contains only one stator winding?
3. What is the advantage of consequent pole motors over some other types of variable speed motors?
4. A consequent pole motor has synchronous speeds of 1800, 1200, and 900 RPM. How many stator windings does this motor have?
5. Refer to the circuit shown in FIG. 6. You are to install this control system. How many auxiliary contacts should starter 1L contain? List how many are normally open and how many are normally closed.
6. Refer to the circuit shown in FIG. 6.
What is the function of contactor 2L?
7. Refer to the circuit shown in FIG. 7. When the low speed push button is pressed, the motor begins to run in low speed. When the high push button is pressed the motor stops running. Which of the following could cause this problem?
a. 1L contactor coil is open.
b. H contactor coil is open.
c. PR relay coil is open.
d. 2L contactor coil is open.
8. Refer to the circuit shown in FIG. 10.
Assume that coil 2CR is shorted. Would it be possible to run the motor in third speed?
9. Refer to the circuit shown in FIG. 10. Ex plain the action of the circuit if coil 2CR is shorted and the 2ND speed push button is pressed.
10. Refer to the circuit shown in FIG. 10. You are to construct this circuit on the job. Would it be possible to use an eleven-pin control relay for 4CR?