The thermistors are built into the motor windings and protect the motor against locked-rotor conditions, continuous overload and high ambient temperature. Thermal protection is then achieved by monitoring the temperature of the motor windings with PTC sensors.
Why use a thermistor for a motor?
The main advantages of thermistors are:
Their low thermal inertia gives rapid and accurate response to winding temperature changes. They measure temperature directly irrespective of how these temperatures are initiated. They can be used to detect overload conditions in motors driven by frequency converters.
How do thermistors prevent overheating?
Sirius 3RN2 thermistor motor protection relays reliably protect motors against overheating by measuring the temperatures at points at risk of overheating within the motor. PTC (positive temperature coefficient) temperature sensors are integrated into the motor windings to monitor critical overheating.
What is the main function of a thermistor?
Thermistors are thermally sensitive resistors whose prime function is to exhibit a large, predictable and precise change in electrical resistance when subjected to a corresponding change in body temperature.
How does a thermistor protect a circuit?
In this application the thermistor is used to protect a circuit by limiting the amount of current that can flow into it. If too much current starts to flow into a circuit through the thermistor this causes the thermistor to warm up.
In what type of device are thermistors used for overheating protection?
NTC thermistors are heat-sensitive resistor elements of which resistance values rapidly decrease with rise of temperature. With this property, they are used as temperature protection devices for protecting circuits from overheating as well as temperature sensors.
What are the advantages of thermistor?
They are more sensitive than other temperature sensors. High sensitivity allows them to work well over a small temperature range. They’re low cost and therefore cheap to replace. They provide a fast response.
How are thermistors used in fire alarms?
The thermistor method of detecting fire requires heat to be the only force necessary for activation. Thermistors are set to detect temperatures, and when temperatures are higher than the designated threshold the alarm will sound. There is no need for any smoke to be present, which lessens the amount of false alarms.
What is thermistor motor protection relay?
The thermistor motor protection relay CM-MSS monitors the winding temperature and thus protects the motor from overheating, overload and insufficient cooling in accordance to the product standard IEC 60947-8, control units for built-in thermal protection (PTC) for rotating electrical machines.
How do you protect a single phase motor?
You use a triple-ganged circuit breaker instead of three individual breakers or fuses. You also use a three-phase disconnect. For more extreme protection, you can install a three-phase latching magnetic starter which drops out (unlatches itself) if any of the three phases fails.
Do all motors need overload protection?
The NEC electrical code requires thermal overload protection on all motors 1/3 HP and above because circuit breakers and fuses have to be oversized so much to allow for the motor startup current, which is about 6 times the full-load running current.
How does motor protection work?
The basic working principle is similar to all other circuit breakers. Thermal protection is used to guard the electric motor against overload. It is based on an expanding and contracting contact that disconnects the motor if excessive current is detected.
What is a motor protector?
A Motor Protection Circuit Breaker is a simple electromechanical device that protects an individual electric motor against overload, fluctuations in input current or unscheduled interruptions to the main circuit. This includes line faults and phase loss or imbalance in three phase motors.