Permanent magnet d.c. motors are same as that of ordinary d.c. shunt motor with the difference that there is permanent magnet instead of stationary field winding for producing the required magnetic flux. These stationary electromagnets are fixed to the outer shell of the motor.
1.1 Construction
The constructional features of permanent magnet d.c. motor is shown in the Fig. 1. As seen from the figure the cylindrical steel stator supporting the electromagnets also provides return path for the magnetic flux as it is made up of magnetic material.
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| Fig. 1 Cross section of a typical permanent-magnet motor |
The armature (or rotor) consists of slots for windings, commutator segments and brushes same as those in conventional d.c. motors. The stator is having some structure having cylindrical shell of uniform thickness which is magnetized in radial direction. The material used for permanent magnet is having high residual flux density and high coercivity. For the motors having the rating upto 150 kW. The material like Alnico may be used. The ferrite magnets are are used in the fractional kilowatt motors while rate earth magnets even though costly are economical in small and large motors. The materials used in rate earth magnets are newly developed materials like somarium cobalt and neodymium-iron-cobalt which gives high residual flux density, high coercivity with maximum energy product.
The latest trend is to use neodymium-iron-cobalt material which gives large flux density, coercivity and maximum energy product than somarium cobalt. It is also having good mechanical properties and comparatively less expensive.
1.2 Working and Performance Characteristics
These motor normally run on 6 V, 12 V or 24 V d.c. supply. This d.c. supply can be made available from batteries or from rectification (a.c. to d.c.). Because of the interaction between flux produced by permanent magnets and current carried by the armature, the torque is produced.
The equivalent circuit of a permanent magnet d.c. motor is as shown in the Fig. 2.
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| Fig. 2 |
Ra is representing the resistance of the armature winding. The field winding connections are absent because of permanent magnets.
In a conventional d.c. motor, the relation between speed and voltage can be written as,
Also the relation between torque and current in conventional d.c. motor is given as,
But in case of permanent magnet dc motors the resulting flux Φ is constant. Hence the above relationships can be expressed as,
Back emf, Eb = Km . ω , where Km = Ka . Φ = constant
... Km = Eb/ω
The supply voltage V can be given as,
V = Eb + Ia . Ra = Km . ω + Ia . Ra
1.3 Performance Characteristics
The set of typical performance characteristics of permanent magnet d.c. motors are shown in the Fig. 3.
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| Fig. 3 |
The speed-torque characteristics are almost linear which makes it suitable in servomotors. With increase in torque the current increases. The efficiency of this motors is better than conventional motors owing to absence of field losses. As the field flux is remaining constant, the speed control is not possible with flux control method. Thus the method used for speed control is armature control using a rheostat or using electronic circuits. The speed obtained are below the normal speed.
1.4 Advantages
1. These motors do not require external excitation for producing magnetic fields. Thus there is saving in energy required for creating magnetic fields.
2. As the windings on the field are absent, the size of such motor is small as compared to equal rating conventional motor.
3. The cost of these machines is low.
4. The efficiency of these motors is high compared to conventional motors as the field losses are absent.
5. The motors designed upto 12 V or less produce less TV and radio interference.6. These motors produce less air noise.
1.5 Disadvantages
1. The excessive currents in the armature windings of these motors may demagnetize the permanent magnets because of armature reaction m.m.f. The other sources of demagnetization are improper design, or brush shift or temperature effects.
2. The flux density produced in the air gap by the permanent magnets is limited.3. The speeds above normal speeds are not possible flux per pole can not be controlled.
4. As the magnets are totally enclosed to prevent them from magnetic junk, as compared to conventional motors their temperature is higher which is limitation in applications where the motor is used for short period.
1.5 Disadvantages
These motors are extensively used in automobiles for windshield wipers and washers. They are also used in blowers used in heaters and air conditioners. They are also used to raise and lower windows and in slot cars. For disc drives in personal computers these motors are used. The rating available for these motors is upto 150 kW. These motors may also be used in applications such as fans and radio antennas, electric fuel pumps, marine engine starters, wheel chairs and cordless power tools. In toy industry, tooth brush, food mixer, ice crusher, vacuum cleaner and in portable electric tools these motors are extensively used.
A permanent magnet dc motor has armature resistance of 1 Ω. The speed of the motor is 2000 r.p.m. when fed from 50 V d.c. source while taking 1.2 A
Determine :i) No load rotational losses
ii) The motor output when running at 1800 r.p.m. when source voltage is 48 V.
iii) Stall torque when fed from 20 V source.
Solution :
The given values are :
Ra = 1 Ω, Ia = 1.2 A , V = 50 V
i) Eb = V - Ia Ra = 50 - 1.2 = 48.8 V
At no load, all the power developed is used to supply rotational losses.
... No load rotational loss = Eb .Ia = (48.8) (1.2) = 58.56 W
ii) Now, Eb = Km . ω
... Km = Eb/ω = (48.4x60)/(2x2000) = 0.2330 V-s/rad
For a speed of 1800 r.p.m.,
ω = (2πN)/60 = (2πx1800)/60 = 188.49 r/s
Back e.m.f., Eb = Km . ω = (0.2330) (188.49) = 43.91 V
Armature current Ia = (V-Eb )/Ra = (48-43.91) /1 = 4.09 A
Power developed = Eb . Ia = (43.91) (4.09) = 179.59 W
Motor output = Eb . Ia - Rotational losses = 179.59 - 58.56 = 121.03 W
iii) When motor stalls, Eb = 0, V = Ia Ra
... Ia = V/Ra = 20/1 = 20 A
... Stall torque = Km . Ia = (0.2330) (20) = 4.66 N-m
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