Thermal protection, motor

Overload Protection Overload relays lor protecting motor insulation against excessive temperature are located either in the motor control or in the motor itsell. The most common method is to use thermal overciirrent relays in the starter. These relays have heating characteristics similar to those ol the motor which they are intended to protect. Either motor current or a current proportional to motorline current passes through the relays so that relay heating is comparable to motor heating. 

Kolfertz, G., Full thermal protection with PTC thermistors of three-phase squirrel cage motors, Siemens Review, 32, No. 12 (1965). 

A form of cooling, and the one of prime interest, concerns ablative cooling. It is essentially a heat and mass transfer process in which mass is expended to achieve thermal dissipation, absorption, and blocking. The process is passive in nature, serves to control the surface temperature, and greatly restricts the flow of heat into the material substrate. As a result of these desirable attributes, ablative cooling (includes use of plastic compositions) has been widely used for thermal protection of solid propellant motors and less extensively in liquid propellant motors. 

Mains circuit protection Fuse, circuit breaker, type (T, F, etc.) Size A Vac/DC Thermal protectors (motors, transformers) type size other info Fuse/therm protector approyals and locations VDE/S or other, location(s) ... 

For this reason NRC states in RG l-lOti that the thermal protections should be bypassed in case of accident or regulated in a way which simultaneously takes into account all the most unfavourable circumstances (which is, as already said, very difficult to implement). The practical answer adopted in the industry has been to completely eliminate all the thermal protections or to bypass them in all the cases where an accident could happen (safety-related conditions). As a consequence, cases of burnt-out motors have occurred. 

Figure 6-21. This 47.2-m- (155-ft.) tall rocket-motor external fuel tank weighs 720 metric tons (1.6 million lbs.) at launch and must withstand the strain of liftoff. The thermal protective system on the tank is composed of a 1-in. layer of a closed-cell polyurethane foam and a plastic charring ablator.

When the motor overheats, the thermostat opens, interrupting motorline current. Pilot thermostats mounted on the windings of larger motors trip the motor starter rather than interrupt line current. This method gives good protection for sustained overloads, but because of the thermal time lag between the copper winding and the thermostat it may not provide adequate protec tion for stalled conditions or severe overloads. 

Adequate single-phase protection is provided on low-voltage ac motor starters by three overload relays, which are now standard. Rotor heating is not particularly a problem on smaller motors which have more thermal capacity, but it is important to protect the stator windings of these machines against burnout. 

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. 

The fuse rating, if used before the starter, may have been inadvertently high, inconsistent with the thermal withstand capacity of the motor. Thus the fuse did not blow which would have protected the motor from such severe and persistent overloading. 

In all the above conditions, the rotor would heatup much more rapidly than the stator due to its low thermal time constant (t), and its smaller volume compared to that of the stator, on the one hand, and high-frequency eddy current losses at high slips, due to the skin effect, on the other. True motor protection will therefore require separate protection of the rotor. Since it is not possible to monitor the rotor s temperature, its protection is provided through the stator only. Separate protection is therefore recommended through the stator against these conditions for large LT and all HT motors. 

Protection against overloading This can be achieved by an overcurrent relay. The basic requirement of this relay is its selectivity and ability to discriminate between normal and abnormal operating conditions. Three types of such relays are in use thermal, electromagnetic and static. Thermal relays are employed for motors of up to medium size and electromagnetic and static relays for large LT and all HT motors, as discussed in Section 12.5. 

Characteristics of a bi-metallic thermal relay The thermal characteristics are almost the same as those of an induction motor. This makes them suitable for protecting a motor by making a judicious choice of the right range for the required duty. (See Figure 12.11 for a typical thermal overload relay and Figure 12.13 for its thermal characteristics.) Ambient temperature com-... 

Figure 12.16 Supplementing a thermal relay with an IDMT relay for complete motor protection...

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