Shaded pole motor

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The shaded pole motor is an electric motor and belongs to the group of asynchronous motors . In contrast to the three-phase asynchronous motor, the shaded pole motor runs with single-phase alternating current . The engine is mechanically very simple.

Typical small shaded pole motor (power: a few watts)

construction

Stator with short-circuit windings

In the shaded pole motor, the stator consists of a laminated dynamo stack with laminated, pronounced poles, which are divided into a main pole and a shaded pole. The stator windings are arranged concentrically. The network winding, the so-called main strand, lies around the stator yoke or around the pole shafts. The shaded pole consists of a smaller groove that is split off from the main pole. There is a short-circuit winding around the shaded pole, which usually only has 1–3 turns. This short-circuit winding, also called a short-circuit ring, forms a short-circuited transformer together with the mains winding during operation. In order to achieve a favorable field distribution, the pole piece tips are allowed to converge or even overlap. In order to achieve the same effect, the pole pieces of some motors are connected with so-called diffuser sheets.

The shaded pole motor has a rotor which is constructed as a squirrel cage and usually consists of crossed rods. A torque is generated in the rotor by an unequal rotating field generated by the stator . The properties of the shaded pole motor are significantly influenced by the design of the spreading sheets, spreading bars and spreading gaps.

Synchronous operation

Multi-pole shaded pole synchronous motor with claw poles

If the rotor of shaded pole motors is made of a magnetically hard material, these motors start up as asynchronous motors and are drawn into synchronism after they have started up. They then continue to run as a synchronous motor . These runners are also called hysteresis runners. The reluctance motor also has a similar operating behavior .

Slow-running shaded-pole motors also work as single-phase synchronous motors. In these, the stator rotating field induces eddy currents in the rotor ring, which cause asynchronous start-up. After run-up, the stator rotating field creates pronounced poles in the rotor's magnetic material . As a result, the rotor takes on the speed of the stator rotating field.

Slow running shaded pole motor

Low-speed shaded pole motors are usually built as external rotors. So that they have a correspondingly low speed, they are provided with 10 or 16 poles . The stator in these motors consists of a ring-shaped excitation coil and two stator halves made of sheet steel. These stand halves have sheet metal tabs acting as claw poles on the circumference. The polarity of the claw poles is determined by the magnetic field of the excitation coil. For this reason, the polarity of the claw poles of one half of the stator is the same.

With this structure, every second pole claw acts as a shaded pole. A common short-circuit ring is built around the split poles of one stator half. This short-circuit ring causes the phase shift of the magnetic fluxes in the gap poles compared to the main poles.

The pot-shaped runner is put over the claw poles. On the inside of the rotor there is a ring made of hard magnetic material , the so-called rotor ring. Due to the hard magnetic material of the rotor ring, the slow-running shaded pole motor shows the typical speed behavior of shaded pole motors with rotors made of hard magnetic material.

With shaded pole motors, a change in speed is implemented with the so-called cross-pole connection.

function

Working principle

A current flow in the stator winding creates a magnetic flux Φ in the stator . This is divided into the main pole field Φ H and the gap pole field Φ S , which goes through the short-circuit winding attached to the stator. The gap pole field induces a voltage in the short-circuit winding , which creates the short-circuit current I s . This builds up the flux Φ ' S through self-induction , which lags behind the main field Φ H. Together, the two flows create an unequal, so-called elliptical rotating field in the rotor, which takes the rotor with it. This elliptical rotating field is a poor quality rotating field.

The magnetic poles of the magnetic field caused by the two phase-shifted currents move one after the other to the following stator poles: main pole 1 to shaded pole 1, main pole 2 to shaded pole 2. Thus, the direction of rotation is always from the main pole to the shaded pole. In order to make the rotating field more rounded, the split poles are sometimes split again, with a second short-circuit winding generating an additional phase shift. The phase angle of the magnetic fluxes can be increased by a precise selection of the scatter bars. A rotating field generated in this way is sufficient to move the rotor. However, it is also heavily load-dependent and leads to a lower starting torque than with three-phase motors with the same power.

The starting torque depends on the shaded pole width and the spread width and increases with the width of the two factors. The starting torque is around 50% of the nominal torque. The shaded pole motor's special stator shape creates large stray fields with corresponding stray field losses. The power density is in shaded pole motors by design much smaller than capacitor motors . The efficiency of the shaded pole motor is also significantly lower than that of an equally powerful three-phase asynchronous motor or a capacitor motor. This is due to the ohmic losses in the short-circuit windings. This disadvantage can be avoided by switching off the short-circuit winding after starting. The motor then runs with good efficiency without a rotating field. The auxiliary winding short-circuit is switched on and off manually (e.g. with older lawnmowers) or automatically based on the power consumption with a magnetic switch that is in series with the main winding, implemented in refrigerator compressors.

Direction of rotation reversal

Circuit for changing the direction of rotation

The direction of rotation of shaded pole motors cannot normally be changed electrically, as it is determined by the pole arrangement and only the main strand is accessible from the outside. However, various modifications to the motor are possible, by means of which the direction of rotation of the shaded pole motor can be changed. So there is the possibility that the direction of rotation is made switchable by a special circuit arrangement. For this, the motor is designed in such a way that an auxiliary winding is attached to both pole sides. A second short-circuit winding is thus built on. There are also motors with four shaded poles, the windings of which can be short-circuited in pairs. The respective winding is short-circuited via a special switch depending on the required direction of rotation. The switches used must be designed for high currents and are also very complex. One possibility for reversible drives is to assemble two shaded pole motors of the same type in pairs. These two motors are assembled in mirror image. However, these are relatively complex special constructions which contradict the purpose of simplicity of the shaded pole motor and are therefore practically not found in practice.

To change the direction of rotation of a simple shaded-pole motor, the motor would have to be dismantled and the stator installed rotated by 180 ° axially. This is very cumbersome and not always possible due to the design (stator not symmetrical, end shields with rivet instead of screw connection, problems during assembly due to manufacturing tolerances).

Advantages and disadvantages

advantages

  • very easy to build
  • inexpensive
  • high running smoothness
  • maintenance free
  • long life span
  • robust
  • automatic start

disadvantage

  • low efficiency
  • bad power factor
  • Can only be used for small outputs (approx. 2 ⁄ 3 of the output of a normal three-phase motor)
  • low power density
  • in the "normal version" no direction of rotation reversal

Areas of application

Shaded pole motor on a radial fan

Due to their very simple structure, shaded pole motors are manufactured in large series. They are used as inexpensive drive machines with low outputs of up to around 300 watts whenever no high starting torque is required. Shaded pole synchronous motors are built for outputs of up to 3 watts. The mentioned advantages and disadvantages of the shaded pole motor compared to other asynchronous motors determine its areas of application: Its low efficiency (approx. 30%) prevents its use in the area of ​​high outputs. Its main advantage, the possibility of being able to operate it on AC voltage without an additional capacitor, has, among other things. a. achieved a certain spread in the household sector. The motor is particularly suitable for drives with short-term operation. Its smooth running, maintenance-free and long service life, typical of asynchronous motors, have made it the standard drive for small axial and radial fans and fans .

Application examples for shaded-pole asynchronous motors

  • Fan motor for electric heaters or hair dryers
  • Drain pumps in washing machines, especially for older devices
  • Pump in tumble dryers
  • Drive motor for turntables
  • Spin-dryer direct drive
  • Motor for fan
  • occasionally used as a vacuum cleaner motor

Application examples for shaded pole synchronous motors

  • Drive motor for program switchgear
  • electric clocks
  • electrical timing relays
  • writing measuring devices
  • in microwaves to cool the magnetron

Web links

Commons : Shaded pole motors  - Collection of images, videos and audio files

Individual evidence

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  14. ↑ R. Gottkehaskamp: Shaded Pole Motors , accessed on April 2, 2012 (PDF; 184 kB).
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