Ice NP900 Series, Инструкция по применению

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Application Guide - NP900 Series 235 (504
)
A996A
When the motor is run in nominal load for the time
enough the temperatures to stabilize (5 x time constant)
and the final temperatures are reached is said that the
motor is running in its nominal temperature. Now the
heat transfer is stabilized so that the heat generated in
the motor is conducted to the surrounding air and the
inside parts temperatures are not increasing anymore.
If the motor is overloaded the stator winding starts to
heat up according its heating time constant and if the
overload is not released in time it can lead to melting of
the stator winding insulations which leads to short
circuit and motor is said to be “burn”. For the rotor this
situation does not differ much from its nominal
conditions. When load increases also the slip increases
slightly thus causing small increase to the induced
voltage and current but when it is rotating the heating is
not significant compared to nominal loading.
When the hot motor is de energized the temperatures
inside the motor start to stabilize. Hottest part in the
motor is the rotor which temperature conducts to the
motor body through the stator windings and from the
body to the surrounding air. When surface cooled motor
is stopped this is the reason why the motor body
temperature seems to keep still rising after the motor is
stopped. Stopped motor cooling follows same equation
than heating with difference that when the surrounding
air is not moving (due to the fan is stopped) and air
temperature is also increasing the cooling in the
beginning is faster and slows in time since the temperature difference decreases and the heat conducts
slower than in the beginning of the cooling. When the motor is run in cyclic load start and stop applications
the cooling of the motor has to be modeled in thermal replica accurately in order to avoid situation where
the calculated used thermal capacity “runs” from actual used thermal capacity causing the protection to be
overshooting significantly.
Figure 3.3.2-110 Motor heating during overloading and motor cooling.
In the previous figures were presented the motors thermal behavior in principal level. In
reality the temperature of motor inside the motor windings can also be measured with
RTD elements. Motor temperature is highest on the drive end due to the cooling being the
weakest there as can be noted in the figure.