Split-phase motors utilize a second armature winding, connected in such a way that its current is temporarily out of phase with the current in the main winding, at least when the rotor is stalled. This phase shift may be produced by a series capacitor or, if large starting torque is not required, by a resistor. Figure 4.37 shows a capacitor-start motor, one of the most common of split-phase types. In such motors there are two windings, typically arranged to be spatially in quadrature, but not ecessarily identical. The strategy in starting a motor is to try to get the current phasors to appear as they do in Fig. 4.38. During starting, the current in the mainwinding lags terminal voltage because the winding is inductive. If a capacitor is inserted in series with the auxiliary winding, and if its
impedance dominates, its current will tend to lead terminal voltage. If verything is sized correctly, the current in the auxiliary winding can be made to lead that of the main winding by approximately 90°.Note that since the terminal impedance of the induction motor will increase and become more resistive as its speed increases, a capacitor sized to produce optimal currents at start will not produce optimal currents when the machine is running. Further, the large capacitance (economical capacitors often used for starting duty) are relatively lossy and would overheat if left connected to the motor. For this reason, in most capacitor-start motors a switch is used to disconnect the starting capacitor once the machine is turning.
The starting switch is usually operated by a speed-sensitive mechanism (a variation of flyballs) which holds the switch closed when the motor is stationary, but which withdraws and allows the switch to open as the rotor turns at a high enough speed. Such a mechanism is simple and cheap, but has moving parts and always employs a pair of contacts. Such
switches are therefore subject to wearout failures and environmental hazards.
An alternative to the mechanical, centrifugal switch is an electronic
equivalent, usually employing a simple timer and a Triac, which connects
the auxiliary winding and capacitor for a fixed period of time after
energization.
It is not necessary to employ a capacitor in a split-phase motor. If the
auxiliary winding is made to have relatively high resistance, compared
with its inductance at start, the current in that winding will lead the current in the main winding by some amount (although it is not possible to get the ideal 90° phase shift). This will tend to produce a rotating flux wave.
Such motors have smaller starting torque than capacitive splitphase motors, and cannot be used in applications such as pumps where the motor must start against a substantial “head”. For low starting duty applications (for example driving most fans) however, resistive splitphase windings are satisfactory for starting
“Permanent” split-phase motors leave the auxiliary winding connected all of the time. For example, permanent capacitive split-phase motors employ a capacitor selected to be able to operate all of the time. The sizing of that capacitor is a compromise between running and starting performance. Where running efficiency and noise are important and starting duty is light, a permanent split-phase capacitor motor may be the appropriate choice. Permanent split-phase motors do not have the reliability problems associated with the starting switch. In some applications, it is appropriate to employ both a starting and running capacitor, as shown in Fig. 4.39. While this type of motor employs a starting switch, it can achieve both good starting and running performance
Shaded poles.
There is yet another starting scheme that is widely used, particularly for low-power motors with small starting requirements such as fan motors. This is often referred to as the “shaded-pole” motor. An illustration of this motor type is in Fig. 4.42.
In the shaded-pole motor, part of each pole is surrounded by a
“shading coil,” usually a single short-circuited turn of copper. This shorted turn links the main flux, but has some inductance itself. Therefore, it tends to reduce the flux through that part of the pole and to retard it in phase. Thus the flux pattern across the pole has a component which tends to move from the unshaded part of the pole to the shaded part of the pole.
Shaded-pole motors tend to have low efficiency and low power density because part of the active pole is permanently short-circuited. They are used primarily for small rating applications in which starting torque is
not important, such as blowers. A major application for this motor type is now largely obsolete—the synchronous motors used to drive electric clocks. These always started as shaded-pole induction motors, but ran as (weakly) salient-pole synchronous machines.
impedance dominates, its current will tend to lead terminal voltage. If verything is sized correctly, the current in the auxiliary winding can be made to lead that of the main winding by approximately 90°.Note that since the terminal impedance of the induction motor will increase and become more resistive as its speed increases, a capacitor sized to produce optimal currents at start will not produce optimal currents when the machine is running. Further, the large capacitance (economical capacitors often used for starting duty) are relatively lossy and would overheat if left connected to the motor. For this reason, in most capacitor-start motors a switch is used to disconnect the starting capacitor once the machine is turning.
The starting switch is usually operated by a speed-sensitive mechanism (a variation of flyballs) which holds the switch closed when the motor is stationary, but which withdraws and allows the switch to open as the rotor turns at a high enough speed. Such a mechanism is simple and cheap, but has moving parts and always employs a pair of contacts. Such
switches are therefore subject to wearout failures and environmental hazards.
An alternative to the mechanical, centrifugal switch is an electronic
equivalent, usually employing a simple timer and a Triac, which connects
the auxiliary winding and capacitor for a fixed period of time after
energization.
It is not necessary to employ a capacitor in a split-phase motor. If the
auxiliary winding is made to have relatively high resistance, compared
with its inductance at start, the current in that winding will lead the current in the main winding by some amount (although it is not possible to get the ideal 90° phase shift). This will tend to produce a rotating flux wave.
“Permanent” split-phase motors leave the auxiliary winding connected all of the time. For example, permanent capacitive split-phase motors employ a capacitor selected to be able to operate all of the time. The sizing of that capacitor is a compromise between running and starting performance. Where running efficiency and noise are important and starting duty is light, a permanent split-phase capacitor motor may be the appropriate choice. Permanent split-phase motors do not have the reliability problems associated with the starting switch. In some applications, it is appropriate to employ both a starting and running capacitor, as shown in Fig. 4.39. While this type of motor employs a starting switch, it can achieve both good starting and running performance
Shaded poles.
There is yet another starting scheme that is widely used, particularly for low-power motors with small starting requirements such as fan motors. This is often referred to as the “shaded-pole” motor. An illustration of this motor type is in Fig. 4.42.
In the shaded-pole motor, part of each pole is surrounded by a
“shading coil,” usually a single short-circuited turn of copper. This shorted turn links the main flux, but has some inductance itself. Therefore, it tends to reduce the flux through that part of the pole and to retard it in phase. Thus the flux pattern across the pole has a component which tends to move from the unshaded part of the pole to the shaded part of the pole.
Shaded-pole motors tend to have low efficiency and low power density because part of the active pole is permanently short-circuited. They are used primarily for small rating applications in which starting torque is
not important, such as blowers. A major application for this motor type is now largely obsolete—the synchronous motors used to drive electric clocks. These always started as shaded-pole induction motors, but ran as (weakly) salient-pole synchronous machines.
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