1. Overview

The shaded pole motor is one of the simplest type of miniature single-phase induction motor. Because it has the advantages of simple structure, convenient manufacture, low cost, reliable operation, strong overload capacity, convenient maintenance and so on, it is widely used in various low-power driving devices. The disadvantage is that the running performance and starting performance are poor, and the efficiency and power factor are reduced. It is usually used for small-capacity occasions with no-load or light-load starting, such as electric fans.

2. Working principle

A motor without a shaded pole ring and only a main winding has no starting torque and cannot be used in practice. In order to obtain starting torque, the measures of additional auxiliary winding are used. This winding is not powered by an external power supply, but by maintaining an off angle of θ <90 between it and the main winding axis, see Figure 1. After the main winding power-on, a part of the main magnetic flux Φm ' will pass through this shaded coil, and the induced potential generates a current. The shaded coil, like the secondary winding of the transformer produces demagnetizing flux Φk, and after synthesis with Φm ', it will be Φs in the shaded pole section, and finally determines the potential Ek on the shaded pole ring. So that Φm and Φs with different time phases are pulsed at different sections between the main pole and the shaded pole, to form an elliptical magnetic field and generate a starting torque. Under the condition that the rotor is closed, the rotor will start. Since Φm is ahead of Φs, the magnetic field moves from the leading magnetic flux to the lagging, so the rotation direction of the motor is clockwise from the main pole to the shaded pole.

(a）Working principle   (b) Vector Diagram

Figure 1 is the principle and vector diagram of shaded pole motor

3. Technical indicators and Term

3.1 Technical indicators

Rated power
Rated voltage
Rated current
Rated speed

3.2 Term

3.2.1 fficiency: The ratio of motor output power to input power.

3.2.2 Power factor COSØ: The ratio of the effective power input to the apparent power of the motor.

3.2.3 Starting torque Tst: Torque generated by the motor at rated voltage, rated frequency and when the rotor is blocked.

3.2.4 Maximum torque Tmax: The maximum torque generated when the motor does not drop suddenly under the rated voltage, rated frequency and operating temperature.

3.2.5 Noise: A-weighted sound power level dB (A) when the motor is running at no-load steady state.

3.2.6 Vibration: the effective value of vibration acceleration (m / s2) when the motor runs at no-load steady state.

4. Basic structure

The shaded pole motor is a single-phase motor with the simplest structure, and its structure can be divided into two types. One is the hidden pole type. From the appearance, the fixed rotor is uniformly slotted, and the rotor is a squirrel cage type. The self-closed auxiliary winding is called the shaded pole winding. The two windings can be made into equal coils or sinusoidal windings. However, the two windings must not be placed orthogonally, that is, the angle between the winding axes is less than 90 degrees. Its The stator has two sets of main and auxiliary phase windings, but most of its main windings are in the form of concentrated windings, and the auxiliary winding is a shaded coil placed on the local magnetic pole, that is, the shaded pole coil (also called short-circuit ring). This type of motor can be divided into two types, a circular structure as shown in Figure 1 (b), and its stator can clearly see the salient pole type. The main winding is placed on the magnetic pole, and the shaded pole ring is embedded in There is one corner of the magnetic pole, and there is one more. The other is a square structure. The iron core is like a transformer, as shown in Figure 1 (a), the main winding is sleeved on a core post, and the magnetic pole and rotor are on the other side of the iron core. On the pole, put two more shaded rings at the corner of the magnetic pole. In the shaded pole motor, as long as it manages to generate a rotating air gap magnetic field, the motor has a self-starting capability and can operate normally. In the shaded pole motor, the main and auxiliary phase windings of the stator and the axis are arranged non-orthogonally in space, and in order to improve the performance of the shaded pole motor, various measures have been taken, such as stepped air gap, magnetic bridge, etc., magnetic asymmetry has occurred , And because the current in the secondary winding is induced by the main winding, it causes electrical asymmetry, respectively generating magnetic potentials with different time and space phases, which are synthesized into a moving magnetic potential similar to a rotating magnetic potential. The moving magnetic field established in space interacts with the rotor to start and run it.

Its structure is shown in Figure 2:

Figure 2  Three typical structures of shaded pole motor

 

5. Characteristic analysis

5.1 The efficiency of the shaded pole motor is relatively low, only between = (5 ~ 30)%, so it is mostly used in low-power drives.

5.2 The main and secondary phase currents of the shaded pole motor are not changed much, so the loss and temperature rise of the motor are mostly calculated based on the current when the motor is not moving. Therefore, the shaded pole motor will run when the rotor is locked without problems. Reliable operation Is its biggest advantage.

5.3 The starting and maximum torque multiples of shaded pole motors are specified as T * st = 0.3, T * max = 1.3, which are all too small. Therefore, shaded pole motors are mainly used in places where the starting torque is not required highly.

5.4 The shaded pole motor is specially designed to be able to rotate in two directions. The poles of such shaded pole motor are notched at both pole tips to place the shaded pole winding. Close a shaded pole winding as required, and the motor is Rotate in that direction.

5.5 The shaded pole motor can use step-down or tap speed regulation like a single-phase asynchronous motor. The motor with winding tap speed regulation is to add more speed coils to the windings of the motor. Connect these speed coils into the loop to connect to When the power supply goes up, it is as if a reactance is connected in series in the motor circuit to achieve the purpose of speed reduction.

 

6. The impact of structural factors on performance

6.1 Magnetic bridge (magnetic shunt)

The function of the magnetic bridge is to improve the magnetic flux distribution of the air gap and improve the mechanical characteristics of the motor. The introduction of the magnetic bridge is to intentionally increase the magnetic leakage between the poles. Although the excitation reactance is reduced, the excitation current is increased, and the maximum torque is reduced, but Because the magnetic flux Φb of the magnetic bridge does not link with the rotor turn, the mutual magnetic flux of the main and auxiliary windings is increased, and the air gap magnetic flux under one pole is changed from rectangular to trapezoidal, as shown in Figure 3d, thereby reducing the harmonic Wave component.

Figure 3

The influence of the magnetic bridge on the performance of the motor can be explained as follows:

6.1.1 Torque speed characteristic (T-n) curve

If the magnetic bridge is canceled, the magnetic flux leakage of the motor is reduced, the excitation reactance is increased, and the maximum torque Tmax of the motor is increased. However, because the air gap magnetic flux changes from trapezoidal to rectangular wave at this time, the harmonics increase, and thus the harmonics The increase of the torque component (mainly 3 times) reduces the T of the motor in the low- and medium-speed region and produces a significant depression. If the magnetic bridge is too wide, the magnetic leakage is too large, and the excitation reactance is reduced too much, although the harmonic When the wave is small, the Tn curve tends to be smooth, but according to the theorem of magnetic flux continuity, the air gap magnetic flux must be reduced. Not only does Tmax fall too much, but it also causes Tst to decrease, so it is also undesirable.

6.1.2 Tst  Starting torque Tst

When the width of the magnetic bridge increases from 0, Tst increases faster first, and then gradually decreases after exceeding the maximum value (the optimal width at this time). The appropriate width of the magnetic bridge can increase Tst to 1.2 ~ when there is no magnetic bridge. 1.5 times.

It can be seen that the width of the magnetic bridge is important. In order to improve the magnetic potential waveform of the magnetic bridge without causing too much torque drop, the magnetic bridge is always designed to be in the state of super saturation of the magnetic density. Above 2.2T to limit its excessive magnetic flux leakage. For this reason, the initial design can take the following values: In Figure 2a, in order to maintain a certain rigidity, the magnetic shunt piece should not be too thin, so its axial direction can be reduced The length can be taken from 1/2 to 1/3 of the stacking length of the iron core. In Figure 2b, c, the two salient poles are connected by a pole tip to form one body. In order to ensure mechanical strength, it is obvious that the width of the pole tip cannot be too small, so it is used as Magnetic bridge is not feasible. For this reason, two symmetrical half-circle notches should be punched on the outside near the intersection axis, and the remaining width is used as the width of the magnetic bridge. Generally, half of the original width is taken because of the amplitude. It can be simply considered that the magnetic flux of each pole is evenly distributed throughout the pole, that is, the magnetic density in the pole is equal everywhere, and the magnetic density in the salient pole is always about 1.1 ~ 1.5T. If the width of the current magnetic bridge is 1/2 of the width of the pole tip, The magnetic density in the magnetic bridge is always above 2.2T.

Figure 5 shows the effect of the magnetic bridge width of a 8W square motor on mechanical characteristics.

6.2 Stepped air gap

The air gap is locally increased at the tip of the front pole, which is a stepped air gap. The larger the air gap, the larger the magnetic resistance. Since the magnetic field lines always try to shorten their path, the magnetic flux density in the stepped air gap is always smaller than the main air gap. From the electromagnetic ratio, the magnetic resistance of the stepped air gap is parallel to the magnetic resistance of the main air gap, the larger the magnetic resistance (resistance), the smaller the magnetic flux (current). Therefore, the use of the stepped axial clearance makes the air gap under one pole The magnetic flux changes from a rectangular wave to a stepped wave, as shown in Figure 3e, thereby reducing the harmonic component. The effect is better than the stepped air gap is a gradual air gap, because the length of the air gap gradually decreases from the front pole tip, thus Make the air gap pass wave into a slope shape, as shown in Figure 3f. The same function as the stepped air gap is to punch a closed long hole (circular motor) at the tip of the front pole or a long notch (square motor) on the outside, and make the air gap there by increasing the magnetic resistance of the local interval. The magnetic flux is smaller than the main air gap. However, because the gradual air gap is difficult to control and the punching hole is complicated, the mold is rarely used in practice.

The effect of the stepped air gap can not only improve the running performance, but also increase the starting torque. This is also caused by the characteristics of the magnetic field lines. At the junction of the stepped air gap and the main air gap, part of the magnetic flux starts from the stator on the main air gap. The rotor that reaches the stepped air gap, that means the magnetic field lines are bent toward the stepped air gap, makes the path longer and the magnetic resistance increases, and the rotor strives to be oriented with the minimum magnetic resistance of the magnetic circuit, which produces a Torque in the direction of the main air gap. Since the stepped air gap is located at the tip of the front pole, the torque is consistent with the direction of the rotating magnetic field.

The effect of the stepped air gap length δc and width (expressed in arc angle) θc on the Tn curve is as follows: δc does not change θc increases, or θc does not increase δc increases, the effect of the two is roughly the same. When δc (θc) increases , Tst and Tmax will increase, and because of the reduction of harmonic torque, the depression in the middle speed region decreases. However, too large δc (θc) will make Tmax decrease, and the characteristics will become softer, the slip of the operating point will increase, thus the loss will increase and the efficiency will decrease, but Tst will always be larger than that without stepped air gap. It can be seen that the values of δc and θc are very important. According to the data recommendation, the general values are around δc / δ = 2.5 ~ 3.5, θc / θp = 0.15 ~ 0.20.

6.3  Shaded pole ring

The role of the shaded pole ring is to cause the stator to generate a rotating magnetic field pulsating flux Φ. Without the shaded pole ring, only the main winding constitutes a single-winding motor, which generates a pulsating flux Φ in the air gap, as shown in Figure 3a, so The motor not only has no starting ability, but also has a small positive torque during operation. With the shaded pole ring, a part of Φm passes through the main air gap, and the other part Φ1 passes through the shaded pole region, thereby inducing current in the ring. Since the mask pole ring is an inductive component, the magnetic flux generated by the current in the ring is constant to prevent the change of Φ1, which causes the composite magnetic flux Φs of the mask pole region to lag behind the main magnetic flux Φm. In this way, there are two in the air gap Pulse beat flux Φm and Φs, as shown in Figure 3b. Because Φm and Φs have a phase difference in time, the two axes are staggered by another angle in space, thereby synthesizing a rotating magnetic field, generating starting torque, and starting and running the motor. However, since the angle θ between the two axes is less than 90 ° (there will be no Φs induced when θ = 90 °), the phase angle difference is also less than 90 ° (due to the resistance of the ring), and then Φs is less than Φm, so The combined magnetic field of the two is always an ellipse. And because Φs lags behind Φm, the rotation direction of the combined magnetic field always moves from the main pole to the shaded pole, that is, the motor cannot change its direction.

6.3.1 The ratio of shaded pole 

The ratio of shaded pole Ks is defined as the percentage of the width of the magnetic pole in the shaded area to the width of the entire magnetic pole. Its size has a great influence on the performance of the motor. If the strength of the two pulse magnetic fields is the same, the smaller the Ks, the closer the angle θ between the two axes to 90 °, the smaller the ellipticity of the rotating magnetic field. However, since Φs is induced by the main winding, the smaller the Ks, the weaker the Φs, and the greater the ellipticity of the rotating magnetic field. From this, we know that there must be an optimal value of Ks, which weighs the angle between the two magnetic fields and The amplitude minimizes the ellipticity of the resultant magnetic field. Analysis and experiments show that the optimal shaded of the single-shaded ring motor is about 33% (that is, 1/3). At this time, Tmax and Tst are large, and the Tn curve is relatively flat. It is known from the experiment that when Ks is large, The mid-speed region of the Tn curve is very concave, but the Tst is low. When the limit Ks = 100%, the axis of the main and auxiliary windings coincide, which is equivalent to a shaded coil of transformer, the induced current in the auxiliary winding is the largest, and only the pulse magnetic field can be generated at this time, Tst = 0. When Ks is reduced, the change in Tmax is very small , Tst increases first, but the depression in the mid-speed region gradually increases; after less than 33%, Tst decreases, and when the limit Ks = 0, Tst = 0.

In the square iron core, due to the structural permission, two shaded rings are generally used to form a three-winding motor. In a three-phase symmetrical motor, each phase occupies 60 ° in a pole (180 ° electrical angle), that is, 60 ° phase band. Although it is impossible to achieve such a symmetrical distribution in a double-ring motor, the principle is the same, so the size of the ring taking the long and making up the shortcomings, the motor characteristics are obviously much better than the single ring: Tmax and Tst are increased, and the depression in the middle speed area is not serious. The experiment shows that in the double ring motor, when the main pole is about 110 °, the large ring is about 70 °. When the ring occupies about 40 ° in the 70 ° of the large ring, the motor characteristics are better. In other words, generally the large ring Ks1 = 40% and the small ring Ks2 = 22%.

As mentioned earlier, in order to obtain a large output torque, the synthetic magnetic field should be as close to a circle as possible. For this, Φm and Φs must have three conditions: the intensity is equal, the axis differs in space by 90 °, and the phase angle differs by 90 °. But this is contradictory: Since Φs is induced by the main winding flux Φ, the closer the angle between the main and auxiliary winding axes is 90 °, the smaller the mutual induction between the two, that is, the weaker Φs. At the limit position of 90 ° , The magnetic flux generated by the main winding cannot pass through the secondary winding, at this time, no current can be induced in the secondary winding, so that Φs is 0, and no torque can be generated. In order to produce a large secondary phase magnetism in a small ratio of shaded pole. It is completely necessary to use a magnetic bridge through Φs. Due to the existence of the magnetic bridge, a small part of the magnetic flux does not pass from one pole to the other through the air gap-rotor-air gap, but passes through the magnetic bridge, that is to say this small part of the magnetic flux is not linked with the rotor turns and is a leakage flux. In this way, because part of the magnetic flux in the shaded pole ring does not pass through the air gap, the magnetic resistance of the magnetic circuit of the shaded pole ring decreases, so that the magnetic flux Φs in the ring increases. It can also be understood that the magnetic bridge increases the main Mutual inductance between secondary windings, thereby increasing mutual inductance.

6.3.2 Shaded ring impedance

After the position of the shaded ring is determined, its own parameters have a great influence on the performance of the motor. The resistance rs of the cover ring has an optimal value for the starting torque. If rs is too large or too small, it will cause Tst to drop. In the motor, generally speaking, for thick copper wire or flat copper wire with only one turn, the rs is small, and the multi-turn type secondary winding wound with enameled wire is rs.The rs is too large. In this case, try to use a thicker gear. For a motor with a small rs, if it is replaced with an equal diameter wire such as brass, the electrical density will not change, and rs will be improved again, which is the performance of the motor. The increase in the leakage resistance of the cover ring xs will cause Tst to decrease. The end of shaded ring be as close to the lamination as possible, and its slot should be as close as possible to the inner diameter of the stator, in order to reduce its leakage flux.

6.3.3 The loss of shaded ring

Although the number of turns of the shaded ring is small, the induced potential in the ring is very small, but due to its extremely small impedance, the current in the ring is usually very large, resulting in large losses and high temperature, especially when blocked. Above 150 ° C (semiconductor point thermometer). Because the shaded ring is involved in the operation, the efficiency of the motor is very low, and the motor has the advantage of little change in the current of the machine when overloaded or even blocked, and is not prone to failure. If the design of the motor is unreasonable or the welding of the cover ring joint is not good, the heat of the shaded ring will be very serious, and even the welding point may be burned out, so that the motor cannot work normally, so the welding quality of the shaded ring must not be underestimated.

6.4 Main winding impedance (resistance r1 and leakage reactance x1)

The impedance of the main winding increases and the voltage drop on it increases. Because this is a useless consumption, the induced potential in the winding decreases. Since the induced potential is E1 =, when the power frequency f and the number of turns W1 remain unchanged, E1 The decrease means that the magnetic flux Φ of each pole decreases, and the motor output decreases. When the main winding resistance r1 is increased, the loss increases, the output decreases, and the temperature rise increases. However, when r1 increases, it is not a decrease in output It is equal to the increase in loss, but part of the loss increase is compensated by the power input, and part is compensated by the decrease in output. Therefore, when r1 is reduced (for example, the wire diameter is reduced by one gear) without reducing the number of turns, the For a fixed load, the input of the motor increases, the output decreases slightly, the speed decreases, the power factor rises slightly, and the temperature rise rises significantly. For the active load of wind blades, due to its power consumption is basically the same as The cube of the speed is proportional to the speed.When the speed is slightly reduced, the required torque drops a lot, so the motor behaves as the input is reduced, the output is slightly reduced, and the temperature rise is apparently increased. 

For wind-blade active loads, because the power consumption is basically proportional to the cube of the speed, when the speed is slightly reduced, the required torque drops a lot, so the motor behaves as the input decreases, the output slightly decreases, the temperature rise is slightly increased. Therefore, for the fan motor, when the impedance protection is used, if the blocked temperature exceeds the standard, it can be considered to use a thin line, which can often solve the problem, and the impact on the speed and air volume is very small The stator leakage reactance includes slot leakage reactance xs, end leakage reactance xe, harmonic leakage reactance xδ and magnetic bridge leakage reactance xb. Reliable features tend to arrange the rated operating point near the maximum torque (especially the fan motor), where xb increases, the torque in the high-speed section will be significantly reduced, which is more disadvantageous than the rest of the leakage reactance. The thickness of the bridge must never be too thick, resulting in excessive magnetic leakage.

6.5 Rotor resistance r2

When the air gap magnetic field is circular and elliptical, the effect of r2 on torque is different.In a symmetrical motor, the air gap is a circular rotating magnetic field.When r2 increases, the slip rate of Tmax increases, and The value of Tmax is unchanged, the concave in the middle speed region decreases, and Tst increases. In the shaded pole motor, the magnetic potential of each winding cannot meet the three conditions of the circular magnetic field, so there is always a negative sequence component. When r2 changes The positive sequence torque and the negative sequence torque change according to the Tn curve of the symmetrical motor, so that the resultant torque, when r2 increases, not only the slip of Tmax increases, the depression decreases, but the Tmax value decreases, and Tst becomes very small, this situation can be illustrated by Figure 5.

Figure 5  Effect of rotor resistance on mechanical characteristics (resistance increases with serial number)

In fact, there is an optimal value between Tst and r2. When r2 is at a certain value, Tst will get the maximum value. Therefore, it is better for shaded pole motor to reduce r2 as much as possible to obtain a large output. However, when the motor is adjusted If the speed is reduced, the entire Tn curve will decrease when the speed is reduced, which may cause the motor to crawl into a low speed and not reach the stable speed.

In small motors of a few watts, the rotor with fewer slots and shallow slots should be used as much as possible, which is great for the stability of cast aluminum, because the punching plate is originally very small, if there are many slots, the area of each slot is small, and if the slot is deep At one point, in particular, the bottom of the slot is prone to under-playing, or even broken rows, causing fluctuations in the quality of the motor.

6.6 Air gap length δ

For asynchronous motors, from the theoretical analysis of the sine wave, we can see that δ should be as small as possible. Because δ is small, the excitation reactance increases, thus the excitation current decreases, the no-load current is reduced, and the power factor increases and the efficiency increases. However, if δ is small, the precision of precision machining is high, and it is easy to increase the eccentric value (relative value), which makes it difficult to manufacture and operate. In addition, the air gap is not a sine wave, δ is smaller, the harmonic magnetic field and harmonic leakage reactance increase, resulting in the reduction of Tst, Ist and Tmax, and the increase of harmonic torque and additional losses, resulting in higher temperature rise and Loud noise. In shaded pole motors and other single-phase motors, it is beneficial to take δ larger than three-phase motors:

(a)  The δ is larger, the leakage of the fixed rotor harmonic and the rotor chute are reduced. The result is that both Tst and Ist increase. Since the increase rate of Tst is greater than Ist, a suitably large δ can improve the starting performance of the motor . And because Tmax is proportional to the leakage resistance of the motor, the air gap increases and Tmax can increase.

(b) δ is larger, which reduces the harmonic flux amplitude of the stator and rotor.Since the stray loss is proportional to the square of the harmonic flux amplitude, a large δ can reduce the stray loss of the motor and make The efficiency has increased slightly. Of course, if δ is larger, not only does the no-load current increase and the power factor decreases, but also the load slip rate decreases (that is, the speed increases), and the load current also increases.The actual efficiency increase or decrease depends on which factor is leading.

(c) A larger δ can reduce the rotor surface loss, so that the rotor surface heat is greatly reduced, and the heat radiation to the stator is reduced, so the temperature rise should be reduced. However, if δ is increased too much, it will make the air gap required As the magnetic potential increases, this is bound to be supplemented by increasing the input current, thereby increasing the saturation of the motor, the copper consumption increases, and the temperature rises instead.

(d) δ is larger, even if the actual eccentricity value of the air gap is unchanged, its relative value (air gap unevenness) is smaller, which is especially important in single-phase motors. Because the air gap unevenness will cause single-phase motor There are different starting torques at different rotor positions, and reducing the unevenness of the air gap will reduce the fluctuation of Tst. And because the increase of δ weakens the higher harmonics, the additional torque is reduced, so that the Tmin of the motor increases , That is, the depression in the velocity region in the Tn curve decreases.

(e) The radial force generated by the interaction of any two harmonic magnetic fields is approximately proportional to δ2, so a larger δ can reduce noise and vibration. Theoretical analysis shows that when the air gap changes from δ1 to δ2, The corresponding bending of the electromagnetic noise level is approximately L1-L2 = 10 lg (δ2 / δ1) 4dB. Of course, due to the increase of no-load current and other reasons, it is actually less than the calculated value of the above formula. 

Based on the above reasons, in single-phase motors, the air gap value is generally 0.05 ~ 0.1mm larger than that of three-phase motors. In general, when using ball bearings, 0.25 ~ 0.35mm is used. Due to the increase of δ by 10% ~ 20%, for the shaded pole motor, due to the more serious harmonic component in the air gap, δ should be larger, especially for square motors, the use of brackets is more likely to cause eccentricity δ is often taken to be 0.35 ~ 0.45 mm.