Electric motors - Philips

1 Philips tech. Rev. 33, 215-234,1973, No. 8/9 215. Electric motors E. M. H. Kamerbeek types of Electric motor The function of an Electric motor is to convert elec- able interest recently in developing electronic methods trical energy into mechanical work. The electrical ener- of commutation; motors with an 'electronic commuta- gy supplied is usually obtained from mechanical work tor' are also called brushless motors . applied simultaneously somewhere else in a conversion The motor is the obvious choice when battery in the other direction by means of generators. Electric su pplies have to be used. A characteristic feature of the motors and generators are both referred to as electrical motor is that its speed of rotation can be varied machines and are essentially the same in construction; over a wide range.

2 When this is a requirement, in principle the sarne electrical machine can perform motors are sometimes used even though power has to both functions. be taken from mains [31. The current then has to be In this article a description will be given of the fun- rectified; modern circuits for rectification and control damental principles of the operation of the electrical include semiconductor diodes, transistors and thyris- machine. Various types of machine will be considered, tors. all of them types that can be applied as small Electric motors . Here small motors are taken to be motors with a rating of I kilowatt or less. Most Electric motors carry out the energy conversion by way of a rotating movement - exceptions are the linear Electric motor , in which the movement is a linear one, and the vibrator rn The rotating part of the machine is called the rotor, and the stationary part the stator.]

3 At least one of these is provided with one or Fig. 1. The Lorentz force. A conductor carrying a current i in more windings that are connected to the source of a magnetic field of flux density B is subject to a force F, the Lorentz force. In the diagram the conductor is assumed to be Electric power. Depending on the type of source, the straight. F is at right angles to the plane of the vectors i and B;. machine will be a motor or an motor . the magnitude F of the force is proportional to the sine of the angle a between i and B, the length I of the conductor, the In both cases the operation of the motor can often strength of the current and the magnetic flux density.

4 In vector be described by starting with the Lorentz force - the notation F = li In motors i and B are usually perpendicular so that the magnitude of the force is given by F = liB. force experienced by a conductor carrying a current in a magnetic field (see jig. J). If we imagine that the magnetic field in a motor is stationary, then to ensure that the Lorentz force on a cond uctor at the circumference of the rotor has the correct direction during the whole of each revolution, the current must be regularly reversed. This is the function of the com- mutator, an essential feature of every motor [21. The commutator is usually a mechanical device and generally consists of a cylinder formed by insulated Fig.]

5 2. Commutator. Each segment is connected to a part of the rotor winding. The current is supplied via two stationary brushes, conducting segments mounted on the shaft; the wind- which make a sliding contact with the segments. ings are connected to the segments and the current is fed to the segments by brushes (jig. 2). [1] J. Timmerman, Two electromagnetic vibrators; this issue, A commutator of this type is subject to wear, par- [2] An exception is the unipolar machine, but this is rarely used. ticularly when there is sparking between brushes and A special form of this machine, which made use of supercon- ductivity, has been described by J. Volger in: A dynamo for segments.

6 The sparking also causes radio-frequency generating a persistent current in a superconducting circuit, interference. For these reasons there has been consider- Philips tech. Rev. 25, 16-19, 1963/64. [3] See the article by R. Raes and J. Schellekens about a Dr Ir E. M. H. Kamerbeek is with Philips Research Laboratories, motor with speed control for a washing machine, to appear Eindhoven. in the second issue on Electric motors . 216 Eo M. H. KAMERBEEK Philips tech. Rev: 33, No. 8/9. In machines there is no need for a commutator; load on the motor . Because of this difference in speed use can be made here of the periodic change in>the currents areinduced in closed circuits in the rotor; this direction of the current in the mains.

7 This means induction is essential to the operation of this type, that a relation must exist between the frequency of the which is therefore usually referred to as the induction current and the speed of revolution of the machine. motor . The induction motor is the most common type In synchronous machines the number of revolutions per of motor . second is proportional to the frequency of the current. In some applications the relation between the speed Synchronous machines are therefore widely applied and the mains frequency is a nuisance. One way around where constant speed is a first requirement. Then there the difficulty is to use a commutator, even though the is the group of asynchronous motors ; these motors run supply is This is because a commutator motor in at a speed a little lower than the 'synchronous' value, the correct kind of circuit can operate from a mains and the difference (the slip speed) increases with the supply of any frequency - and not just if that fre- Table J.

8 Survey of the most important types of motor . The torque-speed characteristics show the variation of the electromagnetic torque Te as a function of the rotational speed 11 of the motor . Type Torque-speed characteristic Special features See page I I I. ~----"'I. motor with permanent-magnet rotor ~. ti _n I. Also applied as stepping motor and brushless motor 217. Synchronous motors ~~ @ . Also applied as stepping motor and 223. I I brushless motor o: Reluctance motor ~----"'I Self-starting 224. Hysteresis motor I I. P<\. 0 An external resistance Rext ~----"'I in the rotor circuit can be used for starting motor with slip-ring rotor ~ Rext '". Asynchronous or induction motors ~~ @ 0.

9 Torque-speed characteristic affected by shape of cage (double cage, skin-effect cage). 226. Squirrel-cage motor ~. ~----"'I. motor with CO ~. Skin effect in the rotor is of major importance solid rotor =----"'1. Separately excited motor Q I \. Permanent magnet often used in small motors instead of field winding Commutator motors Ll Q. Shunt motor =. I \ 230. I~~-l ~. I~. Can also be used for (universal motor ). Series motor Philips tech. Rev. 33, No. 8/9 Electric motors 217. , quency is zero, as for a motor . In practice a motor fore the end of the nineteenth century [**l. Neverthe- to be used in this way has to be specially designed, and less, new developments are still under way.]

10 The main is called a universal motor . If such a motor is run from trends are: an supply the currents in the rotor and stator wind- - Improvement of the various characteristic features ings continually change sign simultaneously. This (the efficiency, the variation of torque with speed, means that the interactive force, which is proportional the starting characteristic, etc.). to the product of the two currents, maintains the same - Increasing the power/weight and power/volume sign. ratios. - Improving reliability and safety. The speed of a synchronous or asynchronous motor can also - Improving methods of motor control. be varied over a wide range if it is supplied from a source of variable frequency.