Complete Start Relay Basics Updated in 2021

The start relay is a special component for starting the compressor using a single-phase asynchronous motor. As a separate component, its function is to connect and disconnect the attracting coil to control circuits. In small home equipment such as refrigerators and air conditioners, their refrigeration compressors mostly use single-phase split-phase asynchronous motors, requiring the starter relay activation. After the start is completed, the auxiliary winding is automatically disconnected to avoid burning itself.

Start relays belong to the switch category. They use electromagnetic and electromechanical principles to close or open contacts to drive or control related circuits. Relay is a control device, which has two parts: controlled system (input loop) and control system (output loop). When the input quantity (electricity, magnetism, light heat and other physical quantities) reaches a certain value, the output quantity changes abruptly from zero to a certain value (or from a certain value to zero). So as to realize the control and protection of the circuit, regulation and transmission of information.

The starting relay is a control element that realizes automatic starting of a single-phase split-phase induction motor. For example, when the refrigerator compressor is started, the motor winding is energized; when the start-up ends and the normal operation is on, the starting relay will cut off the power supply of the winding. There are several types of starting relays commonly used in refrigerators, such as integral starting relays, heavy hammer starting relays and VIE semiconductor starting relays.

The function of the automobile starting relay is mainly to control the current of the high-voltage line by controlling the current of the low-voltage line, playing the role of a current switch. The starter relay is one of the components of the starter system. The main function of it in car is to start the motor rotation in a small current control high current mode, thereby driving the magneto rotor inside the generator to rotate, inputting an ignition pulse signal to the igniter, and causing the igniter to generate an electric spark, finally ignite the mixed gas in the cylinder and perform work.

The coil of the current-type starting relay is connected in series with the operating winding of the compressor motor. When the motor is running, the operating current passes through the coil of the relay, and the control contact is connected in series with the starting winding to control it. The heavy hammer type, PTC(positive temperature coefficient) type and mercury start relays are very common type.

(1) Heavy Hammer Start Relay

It is mainly composed of a coil, an armature, a moving contact, a static contact and a wiring terminal.

Heavy Hammer Start Relay Structure

The control contact of the relay is normally open. When the power turned on, only the relay coil and the running winding have current. Because the rotor is stationary, the starting current is very large, so that the relay coil attracts the armature to move upwards, moving and static contacts close, turn on the start winding, and the compressor starts to rotate. As the speed increases, the current decreases, the armature falls by its own weight, the contact is disconnected, and the starting action is completed.

(2) PTC Start Relay

The PTC element is a positive temperature coefficient thermistor, which is a barium titanate type semiconductor that is doped with trace rare earth elements and made by a special process. The resistance value of the PTC heat-sensitive element used in the air conditioner is only a dozen ohms in the cold state, that is, it begins to be in a path state in the compressor starting circuit. When the compressor starts, the current is very large, so that the temperature of the PTC thermistor quickly rises to the Curie point (usually 100°C to 140°C), and its resistance rises sharply and becomes an open-circuit state. This characteristic of PTC thermistor makes it widely used as a contactless start relay in small air conditioner compressors.

PTC Start Relay Wiring

In practical applications, the PTC starting relay is connected in parallel with the starting capacitor and then connected in series with the compressor starting winding. At the moment when the compressor is connected to the power source, the resistance of the PTC start relay is very small, and it is in a state of passage in the circuit, and the current passing through the starting winding is very large, which makes the compressor generate a large starting torque. At the same time, because the resistance of the PTC element increases sharply, the start winding is disconnected, so that the compressor enters a normal operating state.

The PTC start relay has a simple structure, no contacts and moving parts, so its performance is reliable. Due to the thermal inertia of the PTC components, it takes 4 to 5 minutes after each startup to cool down.

(3) Mercury Start Relay

It consists of a coil, a plunger core, mercury and two electrodes. Because the coil of the mercury starting relay is connected in series with the compressor winding, and there is current passing through when running. 

Mercury Start Relay Structure

A current coil is wound on the outside of the starter cylinder, and the inside of the cylinder is equipped with a plunger iron core and mercury, and is filled with anti-oxidation argon gas. The current coil of the starter is connected in series with the compressor winding. The plunger core is at the lower part of the starter due to its own weight before power on, and the mercury does not contact the two electrodes, so the terminals ① and ② are not conductive. When the compressor is powered on, that is, the current coils ③ and ④ are energized, the plunger iron core lifts the mercury simultaneously under the action of electromagnetic force, so that the two electrodes are in contact with the mercury to conduct, and the start winding is connected to the power supply, finally the machine starts. At the same time, the lifted mercury flows out from the hole at the bottom of the plunger core, the mercury level drops, the two electrodes are disconnected again, and the compressor enters normal operation.

Potential start relay is the most commonly used starting relay in household air conditioners. It is composed of coil, iron core, movable iron sheet, movable contact, fixed contacts and lead terminals.

Potential Start Relay Structure

The coil of it is connected in parallel with the compressor starting winding, and the normally closed contact is connected in series with the starting capacitor. After the compressor is started, the voltage across the relay coil changes with the increase of the compressor motor speed. When the motor speed reaches 70% to 80% of the rated speed, the voltage on the coil causes it to generate electromagnetic force to attract the armature, so that the normally closed contact is disconnected, and the starting capacitor is useless; when the compressor is powered off, the normally closed contact is automatically closed under the action of the spring to prepare for the restart of the compressor.

When the start relay fails, the compressor will run out of adjustment, the starting capacitor cannot be cut off from the circuit after starting, or the compressor cannot be started after the normally closed contact is disconnected before starting. When the overload protection relay fails, the air conditioner cannot operate normally. The cause is that the relay wire ends are loose or fall off, the contact points are worn or welded, dirty, uneven, and the coil is short-circuited or disconnected.

For compressors that use starter relays, when the compressor can't start after hearing the noise, check whether the starter relay contacts are in good condition or not. The main fault of the relay is that the elastic force of the return spring is weakened, and the contact is closed when it is started and it is difficult to switch off. The starting current adjustment screw can be adjusted. If the adjustment screw is not easy to turn, the spring can be pulled out with flat pliers.

How Can I Judge the Car Start Relay is Broken?

A. Starter Errors

1. Failure Phenomenon and Causes

When starting, the starter does not rotate, the possible failures are as follows:

(1)Power Failure

The battery is seriously deficient in power, or the plate is vulcanized or short-circuited, etc. The battery pole post and the wire clamp are in poor contact, the starting circuit wire connection is loose and the contact is poor, etc.

(2) Starter Failure 

The commutator is in poor contact with the brush, the excitation winding or armature winding is open or short-circuited, the insulating brush is grounded, the solenoid switch coil is open, short-circuited, grounded or its contacts are ablated, etc. 

(3) Starter Relay Failure 

The starter relay coil is open, short-circuited, or grounded. The contacts are poorly contacted.

(4) Ignition Switch Failure 

The wiring of the ignition switch is loose or the internal contact is poor.

(5) The starting system circuit is faulty. 

There is an open circuit, poor contact or loose wire in the starting circuit.

2. Detecting Methods

(1) Check the power supply.

Press the horn or turn on the headlights. If the sound of the horn is small or hoarse, and the light is dimmer than usual, it indicates that there is a problem with the power supply.

(2) Check the starter.

Use a screwdriver to short the terminal connecting the battery and the conductive sheet of the motor on the starter electromagnetic switch. If the starter does not rotate, it means that there is a fault in the motor, therefore the starter should be disassembled and inspected.

(3) Check the electromagnetic switch. 

Use a screwdriver to short-circuit the terminal connecting the starter relay with the terminal connecting the battery. If the starter does not turn, it means that the starter electromagnetic switch is faulty. The electromagnetic switch should be disassembled and inspected.

(4) Check the starting relay.

Use a screwdriver to short-circuit the "power" and "starter" terminals on the starting relay. If the starter rotates, it means that there is a relay fault. Otherwise, the next step should be checked.

(5) Check the ignition switch and wiring. 

Connect the “battery” of the starter relay with the ignition switch directly. If the starter can operate normally, it means that the fault lies in the line from the starter relay to the ignition switch.

B. Poor Starter 

1. Failure phenomenon and failure reason

When starting, the speed of the starter is obviously low or even stops. The possible faults are:

(1) Power Failure

Battery loss or short circuit due to vulcanization of the plate, or poor contact at the starting power wire connection, etc. 

(2) Starter Failure

The commutator is in poor contact with the brush, the electromagnetic switch contact plate and the contact are in poor contact, and the motor excitation winding or armature winding has a local short circuit, etc.

2. Detecting Methods

If the starter is running weak, check the starter power supply at first. The starter should be disassembled and inspected if there is no problem with the starter power supply. Firstly, check the contact between the electromagnetic switch contact plate, commutator and brush, and then check the excitation winding and armature.

C. Starter Idling

1. Failures and Reasons

After the start switch is turned on, only the starter rotates quickly but the engine crankshaft does not rotate, which indicating the starter circuit is unblocked. So the fault may lies in the starter's transmission and flywheel ring gear.

2. Detecting Methods

(1) If the starter is idling while accompanied by the impact of gears, it indicates that the teeth of the flywheel ring gear or the starter pinion teeth are severely worn or damaged, resulting in incorrect meshing.

(2) The failures of the starter transmission device include: the one-way mesher spring is damaged; the one-way mesher rollers are seriously worn; the spline groove of the one-way mesher sleeve is rusted. These faults will hinder the normal movement of the pinion and cause it to fail to interact with the flywheel ring gear accurately.

(3) Some starter transmission devices use a one-stage planetary gear reducer, which has compact structure, large transmission ratio and high efficiency. But in use, the load is often too large to burn the device. Some use friction plate clutches. If the compression spring is damaged, the spline is rusted and stuck, resulting in the friction clutch slips, which will cause damage to the starter also.

The start relay is actually a low voltage DC relay. When we want to start the power equipment, we connect the coil of the starter relay to the DC voltage, and generate magnetic force to attract the main contact of the electromagnetic switch, and apply the DC supplied by the battery to the starter motor, which used to drive the power machinery and equipment to start. In order to clearly understand the cooperation of the starting relay and the motor, a physical wiring diagram of them is drawn below.

It can be seen from the figure that the electromagnetic starting relay has three wiring bolts, two of which are thick wiring bolts, and one is thin wiring bolts. The thin wiring bolt is a lead wire (positive +) of the relay pull-in coil. At this time, it is connected to the negative (negative ground) through a start button switch (or a knob switch). Press the start button switch, and the electromagnetic relay coil is energized, the main contact is closed, the DC current of the battery and the starter motor coil form a complete closed state, and finally the motor starts to rotate, thus driving the mechanical power equipment to operate.

Note: For the starter relay with three pins, the bolt marked with red is generally the positive input terminal of the battery, and the other thick wiring bolt is connected to the starter motor. If the wiring is reversed, when the fine terminal bolt is connected to the ground, the starting motor does not work. This is because the internal terminal of the starting relay coil is shared with the thick wiring terminal. At this time, the thick wiring terminal should be swapped.

The current starting relay has two thick wiring bolts, a soft wire comes out in the middle, and there are two leads out of the wire. In fact, the working principle of it is the same as that of the three wiring posts. Among the two wires coming out here, one is already inside the starting relay grounds the iron, you only need to connect the two leads out of it to a start button switch to start the motor.

After know the start relay basic, if you want to get to know how to test and replace it, you can check Common Start Relay Guide with Diagrams.