Page 39 - Industrial Power Engineering and Applications Handbook

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Under normal conditions, the starting current is too 4 To meet the load demand for more frequent starts and
high, whereas the corresponding starting torque is not so reversals, as for cranes and other hoisting applications.
high. If we can shift the Zst point to the left, or in other S To achieve the required speed variation through
words increase the slip, the torque will increase and ISt variation in rotor circuit impedance.
will decrease. This is used, in a slip-ring motor to control
both Tst and Ist with suitable external resistances. For Nore The latest trend, however, is to select only squirrel cage
instance, if the starting torque is required to be equal to motors as far as practicable and yet fulfil most of the above load
Tpo, Le. C3C4, then the start-up current should be controlled requirements. Fluid couplings and static (IGBT or thyristor) drives
can meet the above requirements by starting at no load or light load
to OC3. The starting resistance, R,, therefore should be and controlling the speed as desired, besides undertaking energy
such as to achieve this current during start-up: conservation. See also Chapters 6 and 8.
Le. R,, = - 1.11.1 Choice of voltage
ss e2
AC3
and external resistance Re = RSt - R2 Because of heavy start-up inrush currents, the use of LT
motors should be preferred up to a medium sized ratings,
say, up to 160 kW, in squirrel cage motors and up to
Corollary 750 kW in slip-ring motors. For still higher ratings, HT
The higher the full load slip, the higher will be the rotor motors should be used.
losses and rotor heat. This is clear from the circle diagram
and also from equation (1.9). An attempt to limit the
start-up current by increasing the slip and the rotor 1.12 Mounting of motors
resistance in a squirrel cage motor may thus jeopardize
the motor’s performance. The selection of starting current The common types of mountings are illustrated in IEC
and rotor resistance is thus a compromise to achieve 60034-7, and the most frequently used are shown in Figure
optimum performance. 1.17. The legends to represent these mountings are given
below:
1 Floor mounting B3
1.1 1 Types of induction motors 2 Wall mounting B6fB7
3 Ceiling mounting B8
These are of two types: 4 Wall mounting v5
5 Wall mounting V6
I Squirrel cage rotor 6 Flange mounting B5
2 Slip-ring or wound rotor 7 Flange mounting v1
8 Flange mounting v3
In a squirrel cage motor the rotor winding is made of 9 Foot-cum-flange mounting B3fB5
solid metallic rods short-circuited at both ends of the 10 Foot-cum-flanges on both sides B17
rotor. Short-circuiting of rotor bars leads to fixed rotor
parameters. In slip-ring motors the rotor is also wound Nore For brevity, the prefix IM, representing ‘International
like the stator and the six winding terminals are connected Mounting’, has been omitted from thcsc legends. In common practice
to a slip-ring assembly. This gives an opportunity to vary the different types of mountings are represented as noted above.
the rotor circuit impedance by adding external resistance While designation B represents a horizontal mounting, designation
and thus vary the rotor circuit parameters to achieve the V represents a vertical mounting. For more details and numerical
required performance. designation of motors refer to IEC 60034-7.
Although it may seem easy to alter the speed-torque Care should be taken in selecting the mounting for applications
and speed-current characteristics of such a motor through where the motor weight would apparently fall on its flange or the
foot, as in mountings 55, B6, B8, V3, V5 and V6. In such mountings,
its rotor circuit, the use of such motors is recommended the foot or the flange is subject to a shearing force due to a cantilever
only for specific applications where the use of a squirrel effect and is vuinerable to breakage. For small sizes, where thc
cage motor may not be suitable. The reason is its slip- weight of the motor may not matter so much, these types of mountings
rings and the brushes which are a source of constant can be employed, otherwise reinforcement may be necessary either
maintenance due to arcing between the rings and the to the foot or to the flange, if possible. For larger motors it is
brushes, besides a much higher initial cost and equally advisable to select an alternative mounting.
expensive control gears.
Specific applications for such motors are rolling mills, 1.1 3 Enclosures
rice mills, paper mills and cranes etc. for one or more of
the following reasons: A motor can be constructed in different enclosures to
suit a particular location as follows.
1 To contain the start-up inrush current, as a result of
low start-up impedance and to control the same as 1.13.1 Protected motors (degree of protection
needed through external resistance in the rotor circuit.
2 To provide a smoother start. IP 12, 21 or 23)
3 To meet the requirement of a high start-up torque and 1 Screen protected (SP): Not a common enclosure.
yet contain the start-up inrush current. For locations clear of dust and water.