Motors protection

FIG. 29-4 Examples of open dripproof, totallv enclosed fan-cooled, and weadier-protected motor enclosures, (Pliolo cowiesij of R/ linnco Eloclnc Coinynmj, Clovolmuf Ohio.)... 

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. 

Figure 1.18(d) Cross-sectional view of a large screen protected motor showing the cooling circuit (Cooling system ICOA1) (Courtesy NGEF Ltd)... 

In protected motors failure may be due to accumulation of fly ash at the overhangs (modern installations use only enelosed motors). Fly ash beeomes an extremely good conductor when damp. The failure will generally occur when a motor is switched on after a prolonged shutdown. 

Purpose Unfavourable operating conditions Fault conditions Protection Single-device motor protection relays Summary of total motor protection Motor protection by thermistors Monitoring of a motor s actual operating conditions Switchgears for motor protection Selection of main components Fuse-free system... 

Most standard motors are manufactured using non-hygroscopic NEMA Class B insulation. For added protection in an offshore environment, open drip-proof or weather protected motors should be specified with a sealed insulation system. NEMA Class F insulation is also available in most motor sizes and is advisable to provide an improved service factor. 

Motor enclosures for outdoor use are NEMA weather-protected Type II, totally enclosed, fan-cooled, or drip-proof with weather protection. Motors of the latter type are widely used. Not only are they less expensive than the other types, but they also have a service factor of 1,15. The above enclosure types are all suitable for the Class I, Group D, Division 2 classifications usually encountered. 

Drachman D. B. and Rothstein J. D. (2000). Inhibition of cyclooxygenase-2 protects motor neurons in an organotypic model of amyotrophic lateral sclerosis. Ann. Neurol. 48 792-795. 

As shown by Leonard (1988), purge air can be used to protect motor (or rotor) bearings from corrosive fumes in the contactor stage. This approach can work well as... 

Table 6.22 Comparison of a non-explosion protected motor with (E)Ex eh design (rated synchronous speed 1500 min-1, frame number 132,50cps, totally enclosed/fan cooled) (by courtesy of Loher GmbH, Ruhstorf/Germany)...

Dracliman DB, Erank K, Dykes-Hoberg M, Teismann P, Aimer G, Przedborski S, Rothstein JD (2002) Cyclooxygenase 2 inhibitor protects motor neurons and prolongs sur vival in transgenic mouse model of AES. Ann Neurol 52(6) 771—778. 

Effective motor protection is crucial and achieved by using overload protection devices, temperature-sensing devices, and current-limiting devices. The protection devices shall protect motors against ... 

Overload relays are intended to protect motors by tripping if heat generated in the motor windings is approaching damaging levels. This interrupts the control circuit and causes the contactor to open, disconnecting current to the motor. 

Reversed-Phase Protection. Phase-failure and phase-reversal relays are used to prevent the operation of elevators and industrial machinery to protect motors, machines, and personnel from potential hazards when phase reversal or loss occurs. Interchanging two phases of the supply of a three-phase induction motor will reverse its direction of rotation. 

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