در متن زیر اطلاعات پایه مناسبی در خصوص تغییر دور الکتروموتورها، شرایط کاربردی آنها و مدار فرمان مرتبط ارائه شده است.
The first type of multispeed motor that will be considered is the two-speed squirrel-cage induction motor in which the speed is changed by changing the number of poles in the stator winding. Increasing the number of poles decreases the speed at which the flux wave moves around the air gap between the rotor and stator and therefore the angular speed of the rotor is decreased. For a number of years after the development of the induction motor, speed changing with one winding was performed with a consequent pole connection, which always produces an approximate 2:1 speed ratio. This connection can be designed to have one of the following characteristics: constant horsepower, constant torque, variable torque.
Figure below illustrates how the consequent-pole connection works. For operation at high speed, the windings are connected so that the current produces four poles. The currents in adjacent coil sides of the two windings are in phase. When the phase is reversed in one of the windings, the number of phase reversals doubles, forcing a pole between adjacent coil sides of the two separate windings. Thus, the rotor will spin at low speed, approximately one-half of the original speed. It is the forced pole, in this case the S pole, that is the consequent pole, and so this winding connection is called a consequent-pole connection.
While the winding configuration shown in above Fig. is common to all consequent-pole motors, the terminal connections are different for the constant-horsepower, constant-torque, and variable-torque characteristics. The constant-torque consequent- pole induction motor, as the name implies, is designed to develop the same torque no matter what steady-state speed is. Typical applications are compressors, positive-displacement loads such as hoists and elevators, and friction loads such as conveyors, grinders, and stokers.
Power is a function of both torque and speed. The constant torque consequent-pole induction motor will be rated for twice the horsepower at the higher speed. The high-speed connection will also have a much greater locked-rotor current.
A circuit that will start a constant-torque consequent-pole motor can be found in reference books. The circuit works as illustrated in below Figure. This circuit is known as a compelling starter because the motor will not accelerate to high speed from rest, even if the fast pushbutton is depressed. The motor must be started at slow speed first.
To switch to high speed, the fast pushbutton is depressed. The S coil drops out, which permits the 1F coil to pick up. This connects T1, T2, and T3 together in the motor circuit after T1, T2, and T3 have been separated from the feeder circuit. An “a” contact from the 1F coil energizes the 2F coil which, in turn, connects T4, T5, and T6 to the feeder circuit. Now the currents in both windings of each circuit are 180 degrees out of phase and the motor operates at high speed. Constant-horsepower loads have decreasing torque requirements at higher speeds. Typical applications are machine tools such as lathes, boring mills, winches, and mixers. Not only is the motor horsepower constant, but locked-rotor current tends to be about the same for each speed connection.
The T1, T2, and T3 motor terminals are connected to the feeder circuit at low speed as was the case for the constant torque induction motor. However, the T4, T5, and T6 motor terminals are shorted together at low speed rather than open circuited. At high speed, motor terminals T4, T5, and T6 are connected to the feeder circuit and motor terminals T1, T2, and T3 are open-circuited. Variable-torque loads have higher torque requirements at higher speeds. Accordingly, they have horsepower ratings that are the third power or more as a function of speed. Variable torque consequent-pole induction motors have locked-rotor currents that are substantially greater at higher speeds. Typical applications include fans, blowers, and centrifugal pumps.
The starting circuit external to the motor is identical to the constant-torque connections described above. The terminal connections inside the motor are quite different, as illustrated in below Figure. At low speed the windings in each phase are connected in series, while at high speed they are effectively in parallel. This is an obvious switching connection to make since the low-speed current and power are much lower than at high speed.