Selection of fuses for protection of asynchronous motors

Discharge of fuses of fuses from inrush currents of electric motors

The main determining condition for the selection of fuses for the protection of asynchronous motors with a squirrel cage rotor is a detuning from the starting current.

The discharge of fuses from inrush currents is carried out in time: the start of the electric motor must be completed before the insert melts from the inrush current.

Operational experience has established the rule: for reliable operation of the inserts, the starting current should not exceed half of the current that can melt the insert during start-up.

All electric motors are divided into two groups according to the time and frequency of starting

Motors with easy start are considered motors of fans, pumps, metal cutting machines, etc., whose start is completed in 3 ... 5 s, these motors are started rarely, less than 15 times in 1 hour.

For heavy starting motors, include crane motors, centrifuges, ball mill motors, whose start lasts more than 10 s, as well as motors that start very often - more than 15 times in 1 hour. This category also includes engines with easier starting conditions, but especially responsible for which false burning of the insert when starting is completely unacceptable.

The selection of the rated current of the fuse for disconnection from the inrush current is carried out according to the expression: Ivs ≥ Ipd / K (1)

where Ipd is the starting current of the motor, determined from the passport, catalogs or direct measurement; K is the coefficient determined by the starting conditions and equal to 2.5 for soft start engines and 1.6 ... 2 for heavy start engines.

As the insert heats up and oxidizes when the engine is started, the section of the insert decreases, the condition of the contacts deteriorates and it may misfire during normal engine operation. The insert selected in accordance with formula 1 can also burn out if the engine is started or started longer than the calculated time. Therefore, in all cases it is recommended to measure the voltage at the motor inputs at the time of starting and determine the starting time.

To prevent burning of the inserts during start-up, which can lead to the operation of the motor in two phases and its damage, it is recommended in all cases, where permissible in terms of sensitivity to short-circuit currents, to select inserts coarser than under condition (1).

Each motor must be protected by its own separate protective device.A common device is allowed to protect several low-power motors only if the thermal stability of the starters and overload protection devices installed in the circuit of each motor is ensured.

Selection of fuses for the protection of the network supplying multiple asynchronous motors

The protection of the power network supplying several motors must ensure both the starting of the motor with the highest inrush current and the independent starting of the motors, if this is permissible according to the safety rules, the technological process, etc.

When calculating the protection, it is necessary to accurately determine which motors are turned off when the voltage drops or completely disappears, which remain on, which are turned on again when the voltage appears.

To reduce disturbances in the technological process, special circuits are used to turn on the holding electromagnet of the starter, which ensures immediate connection to the motor network when the voltage is restored. Therefore, in the general case, the rated current of the fuse through which several self-starting motors are fed is selected by the expression: Ivs ≥ ∑Ipd / K. (2)

∑Ipd — the sum of the starting currents of self-starting electric motors.

Selection of fuses for line protection in the absence of self-starting electric motors

In this case, fuses are selected according to the following ratio: Inom. vt. ≥ cr / K

where Icr = I'start +'dlit is the maximum short-term line current;

I'start — starting current of an electric motor or a group of simultaneously switched-on electric motors, at the beginning of which the short-term line current reaches the highest value;

Idlit — long-term rated current of the line until the start of the electric motor (or group of electric motors) — this is the total current consumed by all elements connected by a fuse, determined without taking into account the operating currents of the started electric motor (or group of motors) .

Selection of fuses to protect asynchronous motors from overload

Since the starting current is 5 ... 7 times higher than the rated current of the motor, the fuse selected according to expression (1) will have a rated current 2 ... 3 times higher than the rated current of the motor and, withstanding this current for unlimited time, cannot protect the motor from overloading...

To protect motors from overloading, they usually use thermal relaysbuilt into magnetic starters or circuit breakers.

If the motor is used for motor overload protection and control magnetic switch, then when choosing fuses it is also necessary to take into account the condition of preventing damage to the starter contacts.

The fact is that with a short circuit in the engine, the voltage of the holding electromagnet of the starter decreases, it drops and interrupts the short circuit current with its contacts, which, as a rule, break. To prevent this short circuit the motors must be disconnected from the fuse before the starter contacts open.

This condition is ensured if the short-circuit current interruption time from the fuse does not exceed 0.15 ... 0.2 s; for this, the short-circuit current must be 10 … 15 times higher than the rated current of the fuse protecting the electric motor.