New Efficiency Standards and Efficiencies

Insulation is rated according to maximum allowable

operating temperatures

Insulation Class

Example - a motor operating at 180 Degrees C will have an estimated

life of:

• 300 hours with a Class A insulation

• 1,800 hours with Class B insulation

• 8,500 hours with Class F insulation

• 10.000 Hours with Class H insulation

Insulation Class F is the most common for international use

Motor Protections

• Based on operating temperature

Continuous duty motors must be protected from overload either by a

device integrated into the motor or via an independent protection

system, usually a thermal relay.

• PT-100

There are temperature detectors with operating principle base on the

properties that some materials vary the electric resistance with the

variation in temperature. They are also allowing continuous reading of

motor operating temperature through a monitoring display.

• PTC-NTC

These are thermal protectors consisting of semiconductor detectors

with sudden variation of the resistance when reaching a certain

temperature. PTC are consider a thermistor. This sudden resistance

variation blocks the PTC current, causing the output relay to operate.

PTC does not allow continuous reading of motor operating

temperature.

• Thermal Relay

These are silver-contacts thermal sensors normally closed, that

operate at certain temperature

• Based on operating current

Overloads are processes that usually make the temperature increase

gradually. To solve this problem, the thermal protectors are quite

suitable. However, the only way to protect motors against short-circuit

currents is the application of fuses or breakers.

FORMS OF ENCLOSURE

The most common form of enclosure is "drip-proof" which by definition ensures protection of the windings, commutator and terminals not only from falling liquids but also any other particles. Drip-proof protection as applied to motors in ship requires a slightly different interpretation from that applied to motors ashore, owing to the motion of the ship. It is therefore for ship's motors extended to provide protection from falling water or dirt when the machine is tilted 22 ½ degrees in any direction. Protection must also be provided against liquids striking a surface and running into the machine or being drawn in by the ventilating air.

Another type of enclosure commonly used is "watertight", the standard requirement for which is that they will withstand complete immersion in water to a depth of not less than three feet for a period of one hour. "Watertight" must not be con­fused with "submersible". The essential difference is that a watertight motor is only intended to be suitable for immersion under a comparatively low head of water and for a limited period, whereas a "submersible" motor may be submerged un­der a considerable head for an indefinite period. Submersible motors are necessary for statuary bilge pumps.

Totally-enclosed motors may also be used, but for D.C. they suffer from the disadvantage that carbon dust from the com­mutator brushes is deposited on the windings, core-ducts, and brush gear insulation. Pipe-ventilation or duct-ventilation is sometimes used for large motors to enable clean cool air to be drawn from suitable places outside the engine room.

Water cooling is seldom used for motors, except propulsion motors, though it is sometimes used for generators.

Axis of Rotation. Horizontal-shaft motors should be placed with the axis of rotation in a fore-and-aft direction. If it is unavoidable for it to be athwart ships, provision should be made to take end thrust and to reduce end- play which may arise

due to rolling of the ship.

TASK 3Motor Protection (fill in the missing words)

Protection of motors _____ required mainly to _______ overheating which can ________deterioration of winding insulation and burnout, if severe. Overheating as the result of overloading, stalling, single-phasing or prolonged starting period can be _______ by a rise in line current and by temperature change. Overheating as the result of high ambient temperature or poor cooling due to blocked air passages can only be detected by temperature rise within the windings.

Overload protection is _______ for all motors of more than 0.5 kW although different rules _______ to steering gear motors and others essential to safety or propulsion.

We ______ overload relays, thermistors (a thermal device which can be used in conjunction with an electromagnetic overload trip).

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