Weir Warman 650 M200-MCR-G, Руководство пользователя, Страница: 119 / 287

TOROMOCHO PROJECT - WEG MOTORS
TAG´s :
210-PP-001-M 210-PP-003-M 210-PP-101-M
210-PP-002-M 210-PP-004-M
The advance of the electronics has allowed creation of the solid state starting switch which is composed of a set of pairs
of tiristors (SCR) (or combination of tiristors/diodes), one on each motor output borne.
The trigger angle of each pair of tiristors is controlled electronically to apply a variable voltage to the motor terminals
during the acceleration. At the final moment of the starting, typically adjusted between 2 and 30 seconds, voltage
reaches its full load value after a smooth acceleration or an increasing ramp, instead of being submitted to increasing or
sudden jumps. Due to that it is possible to keep the starting current (in the power supply) close to the rated current and
with slight variation. Besides the advantage of controlling the voltage (current) during the starting, the electronic switch
has also the advantage of not having moving parts or those that generate arc, as it happens with mechanical switches.
This is a strong point of the electronic switches as their useful life is extended.
3.2.3 - MOTOR PROTECTION
Motors in continuous use should be protected from overloads by means of a device incorporated into the motor, or by
an independent device, usually a fixed or adjustable thermal relay equal or less than to the value originated from the
multiplication of the rated feed current at full load by:
- 1.25 for motors with a service factor equal or superior to 1.15; or
- 1.15 for motors with service factor equal to 1.0 (IEC 34)
Some motors are optionally fitted with overheating protective devices such as thermoresistances, thermistors,
thermostats or thermal protectors.
The type of temperature detector to be used are selected taking into consideration the motor insulation temperature,
type of motor and customer requirement.
THERMOSTAT (THERMAL PROBE)
They are bimetallic thermal detectors with normally closed silver contacts. They open as the temperature increases and
then return to the original position as soon as the temperature acting on the bimetallic decreases, allowing new closing
of the contacts. Thermostats can be used for alarm, tripping systems or both (alarm and tripping) of three phase electric
motors when requested by the customer. Thermostats are series connected directly to the contactor coil circuit.
Depending on the safety level and customer requirement, three thermostats (one per phase) or six thermostats (two per
phase) can be installed. In order to operate as alarm and tripping (two thermostats per phase), the alarm thermostats
must be suitable to act at the motor predetermined temperature, while the tripping thermostats must act at the maximum
temperature of the insulating material.
Thermostats are also used on special applications of single phase motors. On these applications, the thermostat can be
series connected with the motor power supply as long as the motor current does not exceed the maximum acceptable
current of the thermostat. If this occurs, connect the thermostat in series with the contactor coil.
Thermostats are installed in the coil heads of different phases.
THERMISTORS (PTC and NTC)
These are semi-conductor heat detectors which sharply change their resistance upon reading a set temperature.
PTC - Positive temperature coeficient.
NTC - Negative temperature coeficient.
The PTC type is a thermistor whose resistance increases sharply to a temperature defined value specified for each
type. This sudden variation of the resistance interrupts the current in the PTC by acting an outlet relay which switches
off the main circuit. It can also be used for alarm and tripping systems (two per phase). NTC thermistors, which act
adversily of PTC’s, are not normally used on WEG motors as the control electronic circuits available commonly apply to
PTC’s. Thermistors have reduced size, do not suffer mechanical wear and act quicker in relation to other temperature
detectors. Fitted with control electronic circuits, thermistors give complete protection for overheating, overload, sub or
overvoltages or frequent reversing or on - off operations. It is a low cost device, similar to a PT-100, but it requires a
commanding relay for alarm or operation.
RESISTANCE TEMPERATURE DETECTORS (RTD) PT-100
The RTD operates on the principle that the electrical resistance of a metallic conductor varies linearly with the
temperature. It is an element usually made of copper, platinum or nickel which allows a continuous follow up of the
motor heating process through a control panel of high precision and acting sensibility. Highly used in the industry in
general where temperature measuring and automation techniques are required. Also widely used on applications that
require irregular intermittent duty.
A single detector can be used for alarm and tripping purposes.
THERMAL PROTECTORS
These are bimetallic thermal detectors with normally closed silver contacts. Mainly used as protection of single phase
motors against overheating caused by overloads, locked rotor, voltage drop, etc. They are normally fitted in the motors
when requested by the customer. The basic components are a bimetallic disc, two flexible contacts, a resistance and a
pair of fixed contacts. It is series connected with the supply voltage and, due to a thermal dissipation caused by the
current pass through its internal resistance, the disc is deformed enough to open the contacts, and then motor feeding is
interrupted. As soon as the temperature comes down, the protector should react. Based on the resetting, there are two
types of thermal protectors:
a) Automatic overload protector where the resetting is done automatically.
b) Manual overload protector when the resetting is done through a manual release.
Table 6 shows a comparison between motor protection systems.