Reliable motor protector

This motor protector can cut off the working power of the motor in time when the motor is out of phase or overloaded to prevent the motor from burning. This protector has the characteristics of strong anti-interference ability, reliable working performance, low self-power consumption, etc., and is suitable for small three-phase AC motors.

How the Circuit Works

The motor protector circuit consists of a power supply circuit, a current detection circuit and a protection control circuit, as shown in the figure below.

The power supply circuit is composed of capacitors C1, C2, resistors R1~R3, rectifier diodes VD1~VD4, voltage regulator diode VS1 and power indicator light emitting diode VL.

The current detection circuit is composed of current transformers TA1~TA3, resistors R5~R11, R14~R16, diodes VD5~VD7, voltage zener diodes VS2~VS4, potentiometer RP, capacitors C3~C7, etc.

The protection control circuit is composed of operational amplifier integrated circuit IC (N1~N4), resistors M, R12, R13, diodes VD8~VD14, transistor V, relay K, AC contactor KM, start button S1, stop button S2, etc.

After pressing the start button S1, the AC contactor KM is energized and closed, its three groups of normally open contacts are connected, and the motor M starts to run.

The AC voltage at both ends of L2 and L3 is reduced by Cl, rectified by VD1~VD4, limited by R3, stabilized by VS1 and filtered by C2 to generate +121/ voltage, which is supplied to the protection control circuit. At the same time, VL is lit.

After the +12V voltage is divided by R13 and R12, it provides a reference voltage for the inverting input terminals of N1~N3 and the non-inverting input terminal of N4.

TA1~TA3 are used to detect the working current of the three-phase power supply line of the motor, and generate three detection voltage signals on R14~R16. These three detection voltage signals are rectified by VD5~VD7 and filtered by C3~C5 to become DC voltages, which are added to the positive input terminals of N1~N3 respectively and compared with the reference voltage of the negative input terminal.

When the motor works normally, the voltage of the positive input terminal of N1~N3 is higher than the reference voltage, the voltage of the negative input terminal of N4 is lower than the reference voltage, N1~N4 all output high level, VD8~VD11 are all cut off, V and VD13, VD14 are turned on, K is attracted, and its normally open contact is connected to ensure that KM can still maintain power supply and attraction after S1 is released.

If any phase of the three-phase power supply is disconnected due to some reason, the detection voltage signal of that phase will disappear, and the operational amplifier will output a low level, turning on the diode connected to its output end, cutting off V, releasing K, and disconnecting the normally open contact of K, releasing KM. The three sets of normally open contacts of KM will cut off the working power supply of the motor, thereby protecting the motor from damage due to phase failure.

When the motor is overloaded, the voltage at the inverting input terminal of N4 will be higher than the reference voltage, and N4 will output a low level, turning on VD11, cutting off V, releasing K and KM, and cutting off the power supply of the motor, thereby protecting the motor from damage due to overload.

Component Selection

R1 and R4 are both 1/2W metal film resistors: R3 is a 1W metal film resistor; R2, R5~R16 are 1/4W carbon film resistors or metal film resistors.

RP uses organic solid variable resistor.

C1 uses a CBB capacitor with a withstand voltage of 630V; C2 uses an aluminum electrolytic capacitor with a withstand voltage of 25V; C3~C6 all use monolithic capacitors; C7 uses an aluminum electrolytic capacitor with a withstand voltage of 16V.

VD1~VD14 all use 1N4007 silicon rectifier diodes.

VS1 uses a 1W, 12V silicon Zener diode; VS2~VS4 all use a 1/2W, 10V silicon Zener diode.

V uses a 59013 or 58050 silicon NPN transistor.

IC uses LM324 quad operational amplifier integrated circuit.

K uses JZC-21F 12V DC relay.

TA1~TA3 are all modified from 3W power transformers: the secondary winding of the power transformer is removed, and a long copper sheet with a thickness of 1mm and a width slightly smaller than the width of the power transformer skeleton is passed through the gap of the skeleton (it is bent into a U shape, and the opening is bent 90° to both sides), and then connected in series to the power supply line of the motor; the original primary winding of the power transformer is used as a current detector in parallel with resistors R14 or R15, R16.