Thermal curve

With the help of this equation the thermal curve of a machine can be drawn on a log-log graph for a known r, I versus /j//, for different conditions of motor heating prior to a trip (Figure 3.12). The relay can be set for the most appropriate thermal curve, after assessing the motor s actual operating conditions and hence achieving a true thermal replica protection. 

Figure 3.13 Thermal curves to set the relay for over-current protection corresponding to different operating temperatures...

J)- Thermal curves for intermittent loads, the average heating not exceeding the permissible temperature rise Figure 3.16 Equivalent output of short-time duties... 

Operating conditions that may overload a machine and raise its temperature beyond permissible limits may be called unfavourable. This overheating, however, will be gradual (exponential), unlike rapid (adiabatic) heating as caused during a locked rotor condition. The machine now follows its own thermal curve and therefore a conventional thermal protection device can be used to protect it from such conditions. These conditions may arise due to one or more of the following ... 

These are conditions in which overheating of the machine may not trace back to its own thermal curves as in the first case. The temperature rise may now be adiabatic (linear) and not exponential and hence rapid. Now a normal thermal protection device may not be able to respond as in the previous case. Some conditions causing overheating may not necessarily be fault conditions. Nevertheless, they may require fast tripping, and hence are classilled in this category for more clarity. Such conditions may be one or more of the following ... 

Nevertheless, whenever the rotor is more critical, despite a higher rotor operating temperature, rotor thermal curves are provided by the manufacturer for facilitating protection for the rotor also through the stator. 

Overcurrent protection. To provide a thermal replica protection, the relay is set according to motor s heating and cooling (/ - 1) curves supplied by the motor manufacturer. If these curves are not available, they can be established with the help of motor heating and cooling time constants, as in equations (3.2) and (3.4). A brief procedure to establish the motor thermal curves when they are not available is explained in Section 3.6. 

Plot the starting current and thermal curve of the motor as in Figure 12.33. 

Figure 12.33 Plotting of starting current and thermal curves...

The thermal curve of the motor does not show any significant overload capacity and therefore the relay must be set as close to the full-load current as possible, say at a setting of 110%. Then... 

This setting is only a calculation. The exact thermal curves of the motor and the relay should be available to closely match their characteristics at every point. An ideal relay in this case would be one which, without tripping, will permit two consecutive hot starts, i.e. its characteristic should lie above... 

For small motors with a number of brands and varying thermal characteristics the above may not be practical. Moreover, to arrange the thermal curves for each relay and motor and then match them individually for closer protection may also not be practical. The practice adopted... 

Duty cycles Continuous duty (CMR) (S ) Periodic duties Factor of inertia (FI) Pleating and cooling characteristic curves Drawing the thermal curves Rating of short motors Equivalent output of short time duties Shock loading and use of a flywheel... 

Thermogravimetric studies were done with derivatograph (17). Thermal curves were recorded at 10°/min. The standard was A1203 heat treated at 1600°C. 

It was verified by X-ray analysis that the second endothermal deflection in the thermal curve is the result of decomposition of the silicate structure (appearance of nickel oxide reflections). 

Similarly, differences in manufacturing substantially affect the DSC melting profile (7). In figure 8 we see thermal curves of nearly identical polyester yarns. Sample one and two are identical in composition--both contain a dyability additive--but were annealed at different temperatures as can readily be seen by the position of the annealing "scars" on the thermograms. The effect of the dyability additive apparently is that it lowers and broadens the melting peak destroying its characteristic first-run, double-peak behavior. 

The identification of wool, cotton and rayon by observing their reductive degradation thermograms may be less reliable because the presence of flame retardants, dyestuffs, etc., may somewhat alter the thermal curves. However, as a comparison technique, samples from a truly identical origin, encapsulated or run under the same conditions should give similar results (10). 

Tire rubber. Substantial differences can be seen in the thermal curves of automotive tire rubbers from formulation to formulation [and, one suspects, from lot to lot, since DSC is used as a quality control monitor in tire manufacturing (15)]. If it can be demonstrated that sufficient rubber can be obtained from a skid mark to make such a determination, this could become a valuable tool in hit-and-run forensic work. Because of its sensitivity, perhaps TMA would hold particular promise. 

Figure 8.4 DSC thermal curves of aqueous suspensions ofwaxy maize starch (a, b, c) and resistant starch (retrograded amylose) (d).

Figure 8.8 The B- to A-type transition of B-type potato starch lintners (DP 15) at 35% water content (d.b.). The arrows on the DSC thermal curve specify the temperatures at which the lintners were heated prior to x-ray analysis.320 The model of Perez et al.320 for the solid state polymorphic transition B to A is also shown, illustrating the progressive removal ofwater molecules (small dots) and the change in packing of chain duplexes the chain duplexes marked with 0 and Vi indicate their relative translation along the c axis.

The second sample (Figure IB) benefited from the surfusion phenomenon, materialized on the diagram by the steady drop in temperature down to approximately -6°C, then by a sudden rise in temperature up to 0°C. The thermal curve then shows a short plateau corresponding to quick water icing in the whole sample, and finally a decrease in temperature. 

Isothermal crystallization of the oil or fat was monitored by a Perkin Elmer DSC 7 differential scanning calorimeter. Sample sizes range from 5 to 10 mg. The oil sample is heated to a temperature of 80°C at a heating rate of 5°C/min from ambient and held at that temperature for at least 10 min in order to totally erase all past crystallization memories. The sample was then cooled at a rate of 5°C/min until the desired crystallization temperature had been reached. The sample temperature was then maintained at this crystallization temperature for 2 h to monitor the complete crystallization behavior of the sample. Partial areas under the thermal curve were determined by means of the Perkin Elmer Pyris partial area analysis software. 

---