Single-phase Induction Motors
A three phase motor may be inductoon from a single phase power source. Figure below However, it will not self-start. It may be hand started in either direction, coming up to speed a few seconds. The single coil of a single phase phasw motor does not produce a rotating magnetic field, but a pulsating field ;hase maximum intensity at 0 o and o electrical.
Single phase stator produces a nonrotating, pulsating magnetic field. Another view is that motkr single coil excited by a single phase current produces two counter rotating magnetic field phasors, coinciding twice per revolution at 0 o Figure above -a and o figure e. When the phasors rotate to 90 o and o they cancel in figure b.
An analogous situation exists in figure d. The sum of these two phasors is a phasor stationary in space, but alternating polarity in time. Thus, no starting torque is developed. Little torque see torque vs slip curve other than a double freqency ripple is developed from the counter rotating phasor. Thus, the single phase coil will develop torque, once the rotor is started.
If the rotor is started in the reverse direction, it will develop a similar large torque as it nears the speed of the backward rotating phasor. Single phase induction motors have inductikn copper or aluminum squirrel cage embedded in a cylinder of steel laminations, typical of poly-phase induction motors. One way to solve the single phase problem is to build a 2-phase motor, deriving 2-phase power from single phase.
This requires a motor with two windings spaced apart 90 o design of single phase capacitor start induction motor, fed with two phases of current displaced 90 o in time. This is called a permanent-split capacitor motor in Figure below. Permanent-split capacitor induction motor. This type of motor suffers increased current magnitude and backward time shift as the motor comes up to speed, with torque pulsations at full speed.
The solution is to keep the capacitor impedance small to minimize losses. The partnervermittlung bahnhofstrasse zurich are less than for a shaded pole motor. The direction of the motor is easily reversed by stzrt the capacitor in series with the other winding. This type of motor can be adapted for use as a servo motor, described elsewhere is this chapter. Single phase induction motor with embedded stator coils.
Single phase induction motors may have coils embedded into the design of single phase capacitor start induction motor as shown in Figure above for larger size motors. Though, the smaller sizes use less complex to onduction concentrated windings with salient poles. In Figure below a larger capacitor may be used to start a single phase induction motor via the auxiliary winding if it is switched out by a centrifugal switch once the motor is up hpase speed.
Moreover, staft auxiliary winding may be many more turns of heavier wire than used in a resistance split-phase motor to mitigate excessive temperature rise. The result is that more starting torque is available for heavy loads like air conditioning compressors. This motor configuration works so well that it is available in multi-horsepower multi-kilowatt sizes. A variation of the capacitor-start motor Figure below is to start the ihduction with a relatively large capacitor for high starting torque, but leave a smaller value capacitor in place after starting to improve running characteristics while not drawing excessive current.
The additional complexity of the capacitor-run motor is justified for larger size motors. Capacitor-run motor induction motor. Such AC rated electrolytic capacitors have such high losses that they can only be used for intermittent duty 1 second on, 60 seconds off like motor starting. A capacitor for motor running must not be of electrolytic construction, but a lower loss polymer type. If an auxiliary winding of much fewer turns of smaller wire is placed at 90 o electrical to cesign main winding, it can start a single phase induction motor.
Figure below With lower mootor and higher resistance, the current will experience less phase shift than the main inruction. About 30 o of phase difference may be obtained. Resistance split-phase motor induction motor. This motor has more starting torque than a shaded pole motor next sectionbut not as much as a two phase motor built from the same parts. The current density in the auxiliary winding is so high during starting that the consequent rapid temperature rise precludes frequent restarting or slow starting loads.
Singlespeed trier is based on the premise that induction motors capaciyor inefficient at less than full load. This inefficiency correlates with a low power factor. The less than unity power factor is due to magnetizing current required by the stator. This fixed current is a larger proportion of total motor current as motor load is decreased. At light load, the full magnetizing resign is not required.
It could be reduced by decreasing the applied voltage, improving the power factor and efficiency. The power factor corrector stwrt power factor, and decreases motor voltage, thus restoring a higher power factor and decreasing losses. There is no savings for a fully loaded motor since all the stator magnetizing current is required. The voltage cannot be reduced.
Eesign there is potential savings from a less than fully loaded motor. A nominal VAC motor is designed to work at as high as VAC, as low as VAC. That means that it is not fully loaded when operated at greater than VAC, for example, a VAC refrigerator. It is safe for the power factor starrt to lower the line voltage to VAC. The higher the initial line voltage, the greater the design of single phase capacitor start induction motor savings. Of course, if the power company delivers closer to VAC, the motor will operate more efficiently without any add-on device.
Though, it needs to operate a large number of hours per year. And the more time it idles, as in a lumber saw, punch press, or conveyor, the greater the possibility of paying for the controller in sstart few years operation. The cost of a PFC designn be recovered for a motor operating only a few hours per day. Published under the terms and conditions of the Design Science License. ARTICLES LATEST NEWS PROJECTS TECHNICAL ARTICLES INDUSTRY ARTICLES.