Protection of the electric motor at start-up. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Power Supplies
Comments on the article
There is a flaw in a good article [1]: the protection circuit (Fig. 15) is inactive at the time of launch. After all, the "Start" button acts bypassing the block contacts of the relay that control the presence of voltage in phases A and B, so the magnetic starter is attracted when there is voltage in phase C alone (and remains attracted while holding the "Start" button). If thermal relays were included in the circuit, responding to the overload current, then the overload from starting on two phases would last only until the moment of their operation.
But even in the absence of thermal protection, "vain" and useless engine starts can be avoided when one of the phases fails (see figure).
The normally open contacts of the voltage monitoring relay K1 and K2 allow you to supply phase C voltage to the winding K3 (magnetic starter) only if there is voltage in the phase wires A and B.
We connected the control button chains between the second terminal of the K3 winding and the ground. This is due to the following reasons:
1. Auxiliary contact K3, connected parallel to the "Start" button, must have a common wire with the output of the coil K3 (to save wires and simplify the circuit).
2. The keypad must be connected to the rest of the circuit with three conductors so that our circuit is not more complicated than [1].
We also supplemented the circuit with an automatic switch S., which de-energizes the motor during short circuits and overload modes.
Circuit operation
Turning on S. three-phase voltage is supplied to the control circuits and contacts K3.
The presence of phase A voltage leads to the operation of K1.
The presence of phase B voltage - to the operation of K2.
The voltage of phase C (if it is connected) gets to the upper terminal of the coil K3 only if the voltage of phases A and B is simultaneously present.
Normally closed contacts of the "Stop" button bring "zero" to the contact of the "Start" button.
In the presence of three-phase voltage, pressing the "Start" button leads to the operation of the relay K3, which with its block contacts (connected in parallel with the contacts of the "Start" button) becomes "caught".
The engine is switched on by the contacts of the magnetic starter K3. If the voltage in each of the three phases is maintained, then the motor continues to run.
Loss of voltage in phases A or B leads to a break in the contact in the power supply circuit of coil K3. The same loss of power to the coil will be in the event of a break in phase C.
A short press of the "Stop" button leads to a power interruption of the K3 coil from the "zero" side.
After a short disconnection of K3 for any of the above reasons, its auxiliary contacts are broken, K3 is de-energized.
Restarting K3 (and the engine) is possible only after restoring the normal power circuit and pressing the "Start" button.
References:
- Bearded Yu. Notes on the operation and repair of electric motors//Electrician. - 2002. - No. 12. - P.14-15.
Author: N.P. Goreiko
See other articles Section Power Supplies.
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