HT motors

These figures are for general reference only. For actual values the reader should refer to the motor manufacturer. Motors can, however, be designed to suit a particular application. Large LT and all HT motors are generally custom built. 

Note According to the. stipulations of lEC 60034-1, any factor noted above which may influence the performance and cause an excessive current than rated, or because of an excessive load itself, motors rated up to 315 kW and rated voltage up to 1.0 kV should be capable of withstanding a current equal to 150% of the rated current for a minimum of two minutes. No such tolerance is, however, recommended for motors beyond 315 kW and all HT motors. This stipulation would not apply to motors that arc designed and manufactured to specific requiremenl.s. 

Uh = per unit summated value of all the harmonic voltages in terms of the rated voltage n = harmonic order not divisible by 3 (presuming that the star-connected motors (normally HT motors) have only isolated neutrals) in 3-0 motors, i.e. 5, 7, 11 and 13, etc. Beyond 13, the content of harmonic quantity may be too insignificant to be considered. 

Because of heavy start-up inrush eurrents, the use of LT motors should be preferred up to a medium sized ratings, say, up to 160 kW, in squirrel eage motors and up to 750 kW in slip-ring motors. For still higher ratings, HT motors should be used. 

Manufacturers, depending upon market needs, may adopt all or a few such designs or even have their own designs, still conforming to such stipulations. Special applications may, however, call for a custom-built motor as noted later. As a standard practice all HT motors are custom-built for each application and no rotor designs are prescribed for these. 

Spci d Ns r.p.m. at 50 Hz Vibration Icvcd, peak to peak (d(nd)le antpli/iule l.T motors (microns) HT motors (microns)... 

Note In higher speed ranges and for HT motors, Ihese levels of shaft vibration are generally of the same order or slightly belter than prescribed in lEC 60034-14. corresponding to Table 11,3. 

In HT motors, however, these tenninal boxes are always separate because two or more voltages (main and auxiliary). For main terminals there are normally two terminal boxes - one on one side of the stator to house the main three-phase stator teiminals and the second on the other side to form the star point. These boxes are generally interchangeable to facilitate cable routing. 

Voltage surges, caused by system disturbances or switching operations (for HT motors). 

HT motors must be provided with phase-segregated terminal boxes as illustrated in Figure 7.17. 

In HT motors the terminal box for space heaters and embedded temperature detectors must be separate from the main terminal box. 

HT motors must be generally wound with class F insulation and the temperature rise should not exceed the prescribed limits of class B insulation. LT motors up to 250 kW, however, can be wound with class B insulation. 

Frequency of start The HT motors must be suitable for two starts in quick succession when the motor is hot or three equally spread starts in an hour (not to be repealed in the second successive hour). 

HT motors must have a minimum six numbers of resistance temperature detectors (RTDs) (See section 12.8). 

Procedure for vacuum pressure impregnation (with particuiar reference to HT motors)... 

Analysis of insulation failures of an HT motor at a thermal power station 10/241... 

In HT motors even electrostatic effects between the windings of the stator and the rotor due to stator voltage... 

Cognisable shaft currents may exist in large LT and HT motors of 2000 kW and above, using circular laminations and all motors with segmented laminations due to the magnetic field cau.sed by asymmetries. 

Field experience has revealed that one of the major causes of failure of an HT motor is weak insulation, caused by environmental pollution and ageing. 

In LT motors, a standard test finger as shown in Figure 11.7 is used, connected by an incandescent lamp to one pole of a supply of at least 40 V, the other pole of the supply being connected to the parts intended to be live in normal service. All parts must be connected electrically. The lamp should not glow when an attempt is made to touch the bare live parts or insufficiently insulated parts. Insufficiently insulated parts may be covered with a metal foil connected to tho.se parts that are live in normal service. Conducting parts covered with varnish or enamel only or protected by oxidation or by a similar process may be considered as insufficiently insulated. In HT motors, the clearance is verified with the minimum clearance required to withstand the dielectric test as in Section 11.4.8,... 

In all these conditions protection against 2, 3, 4 and 5 is generally applicable to large LT motors, say 100 h.p. and above, and all HT motors. This protection is normally provided by a single-device motor protection relay, discussed in Section 12.5. 

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