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Jul 23 2020

How to Test a Start Capacitor?

Introduction

What is the difference between a three-phase AC motor and a single-phase AC motor? If you pay attention to it, you will find that single-phase AC motors have one more equipment than three-phase AC motors, which is the start capacitor(starting capacitor). The most common one is in various household appliances. Almost all the household appliances with motors are equipped with start capacitors.

This article will start with the explanation of the principle of the motor start capacitor, and then describe in detail the failure phenomena, causes and test methods of the start capacitors in the two common home appliances, air conditioners and fans. In addition, the article will also explain some basic questions about start/run capacitors. If you want to learn the related knowledge of motor start capacitors, this article must be worthy of your reading.

How to test a RUN or START CAPACITOR the CORRECT way

Catalog

Introduction

Catalog

I The Principle and Causes of Damage of Start Capacitor

 1.1 How Does the Motor Work?

 1.2 How Does the Start Capacitor Work?

 1.3 Reasons for the Burning of the Start Capacitor

II Troubleshooting of Start Capacitor in Air Conditioner

 2.1 Functional Characteristics of Start Capacitor

 2.2 How to Quickly Figure out Whether the Start Capacitor is Damaged

 2.3 Why is the Start Capacitor of the Air Conditioner Outdoor Unit Easy to Damage?

 2.4 Symptoms and Test Methods

III How Test the Motor Start Capacitor of a Fan?

IV Relevant Knowledge in the Step of Start Capacitor Test

 4.1 How to Choose Start Capacitor?

 4.2 Precautions for Replacing the Start Capacitor

V How to Test the Motor Start Capacitor without a Multimeter

VI Frequently Asked Questions about Start Capacitors

 6.1 Which Motors are Served by the Start Capacitor?

 6.2 Why Does a Three-phase Motor do not Need a Start Capacitor?

 6.3 The Relationship Between Start Capacitors and Motor

 6.4 What are the Functions of Start Capacitors, Run Capacitors, and Centrifugal Switches in Single-phase Motors?

VII Quiz

I The Principle and Causes of Damage of Start Capacitor

1.1 How Does the Motor Work?

The single-phase current flowing through a single-phase motor cannot generate a rotating magnetic field, and a capacitor is needed to separate the phases. The purpose is to make the current in the two windings produce a phase difference of nearly 90 to generate a rotating magnetic field.

The capacitive induction motor has two windings, namely the starting winding and the running winding. The two windings are 90 degrees apart in space. A large-capacity capacitor is connected in series to the starting winding. When the running winding and the starting winding pass single-phase alternating current, the current in the starting winding is 90 degrees ahead of the current in the running winding due to the action of the capacitor, thus reaching the maximum value first.

Two identical pulse magnetic fields are formed in time and space, so that a rotating magnetic field is generated in the air gap between the stator and the rotor. Under the action of the rotating magnetic field, an induced current is generated in the motor rotor, and the current interacts with the rotating magnetic field. The electromagnetic field torque causes the motor to rotate.

Electric-Motor

Figure1. Electric-Motor

1.2 How Does the Start Capacitor Work?

A single-phase power supply is different from a three-phase power supply in that a three-phase rotating magnetic field is generated in the motor.

The principle of capacitor starting of single-phase motor is: using the principle that the current of the capacitor in the circuit is advanced by 90 degrees, so that a magnetic field of 90 degrees ahead of the main winding is generated in the starting winding, so that there will be an alternating 90 degree angle in the motor. The magnetic field, to put it plainly, uses the phase-shifting principle of the capacitor to transform a single-phase power supply into a two-phase power supply of 90 degrees to each other, and a rotating magnetic field of 90 degrees to each other is generated in the motor.

Maybe it is easier to understand to explain from this aspect. The start capacitor is to give the motor a thrust when the motor is started, so that the motor can turn from moving to rotating. Without it, when a single-phase AC motor starts, it will shake at the origin instead of rotating; the start capacitor is a two-phase AC motor, so the magnetic field cannot exert force on the rotor without it, and of course it is impossible to rotate.

Capacitor Start Run Induction Motor

Figure2. Capacitor Start Run Induction Motor

1.3 Reasons for the Burning of the Start Capacitor

Generally, the start capacitor is not easy to burn, because its working time is very short, and it is thrown off by the centrifugal switch at the moment of starting, with no current flowing through the start capacitor. However, not easy to burn does not mean that it will never burn.

If the start capacitor burns out, the possible reasons are as follows:

① Capacitors have low voltage resistance or poor quality, so it is best to use capacitors with a voltage resistance of 500V.

② The centrifugal switch will often produce an arc when it is turned off. It is possible to burn the switch to the motor. After the switch is started, the switch cannot be turned off. There is always current through the capacitor. It is easy to burn the secondary winding of the motor and the start capacitor within a certain period of time.

③ The capacity of the selected capacitor is too small, and the starting current exceeds the allowable value of the capacitor.

④ The motor is bored or the bearing is damaged. It is difficult for the motor to start the centrifugal switch within a certain period of time and it is difficult to reach the disconnected speed, and the start capacitor is easy to burn.

Deteriorated Capacitor

Figure3. Deteriorated Capacitor

II Troubleshooting of Start Capacitor in Air Conditioner

2.1 Functional Characteristics of Start Capacitor

The start capacitor is an important part of the auxiliary compressor to start. The capacitor is a large-capacity capacitor (1~6uF), which is used to provide starting current for the auxiliary winding of the motor to assist the compressor to start. The start capacitor is generally fixed on the bracket or support plate above the compressor, and the pin is connected to the starting end of the compressor.

2.2 How to Quickly Figure out Whether the Start Capacitor is Damaged

First of all, it depends on what kind of capacitors are used in electrical parts.

● If the compressor start/run capacitor is damaged, the compressor cannot start or run intermittently.

● If the capacitor of the fan motor is damaged, some malfunctions such as excessive exhaust temperature, excessive exhaust pressure, compressor overload, and small air output will occur during the operation of the air conditioner.

2.3 Why is the Start Capacitor of the Air Conditioner Outdoor Unit Easy to Damage?

The capacitor of the air conditioner compressor is installed in the outdoor unit. Due to the high temperature of the outdoor unit (ambient temperature + temperature emitted by the condenser), capacitors that have been used for a long time will easily dry up and fail the electrolyte.

When replacing capacitors, be sure to use high-quality capacitors. The capacity must be the same, and the withstand voltage must not be lower than the original standard.

Air Conditioner Outdoor Unit

Figure4. Air Conditioner Outdoor Unit

2.4 Symptoms and Test Methods

① Smoke.

② Cannot operate normally.

③ The display cannot be displayed normally.

④ The power supply cannot be charged and discharged normally.

⑤ Can not heat normally.

⑥ The power factor compensates the capacitor damage, resulting in a waste of electricity.

Different symptoms of damage to compressor start capacitor and external motor start capacitor:

There are two capacitors in the outdoor electromechanical packaging, the larger one is the compressor capacitor, and the smaller one is the external motor capacitor. Different capacitors have different failure phenomena.

① Compressor capacitor damage

the performance phenomenon is that normal compressor startup will be accompanied by loud noise and jitter. If the capacitor is damaged, you will feel a slight jitter in the compressor by pressing the casing above the compressor, and the sound is like the sound of current passing. The compressor will stop running after a period of time.

② The damage of the external motor capacitor

After the compressor is working normally, the external motor stops working for a period of time. If there is a fault code, there will be high pressure protection and compressor exhaust pipe temperature protection.

Basic Electrical Controls of Air-Conditioning Units

Figure5. Basic Electrical Controls of Air-Conditioning Units

Test methods of air conditioner start capacitor

● Method 1:

The start capacitor of the air-conditioning compressor is a large-capacity electrolytic capacitor. When testing, using the capacitance setting of the digital multimeter to determine whether there is any abnormality.

Under normal circumstances, the capacitance of the capacitor used to detect the capacitance of a multimeter should be the same or very close to the nominal capacitance, otherwise the start capacitor is mostly deteriorating, such as dry electrolyte, leakage, etc., which should be replaced.

● Method 2:

In addition to using a multimeter to test its capacitance, the ohmic setting of a pointer multimeter can also be used to test the charge and discharge performance of the start capacitor.

Steps:

① Connect the red and black test leads to the two poles of the compressor start capacitor.

② The multimeter gear is set in ohm gear.

③ Under normal circumstances, the pointer of the multimeter first swings to the right to a position, then slowly swings to the left, and finally stops at a fixed position.

④If the pointer does not swing or the swing range is small, it indicates that the performance of the compressor start capacitor is poor.

Suggested Reading: 5 Ways to Test Capacitors

How to replace the start capacitor?

If it is ensured that the air conditioner failure is caused by the damage of the compressor start capacitor itself, the damaged compressor start capacitor needs to be replaced.

Replacing the start capacitor can generally be divided into three steps: removing the start capacitor, finding a replaceable start capacitor, and replacing the start capacitor.

①Remove the start capacitor

The compressor start capacitor is located on the circuit support board above the compressor. When disassembling, unplug the connecting wire and use a screwdriver to remove the fixing screw of the snap ring.

② Looking for replaceable capacitors

After removing the damaged compressor start capacitor, then select a suitable new start capacitor to replace it according to the specifications and volume of the damaged start capacitor.

The specific content of how to select the start capacitor will be explained in detail below.

③Replace the start capacitor

After selecting the compressor start capacitor, install the new compressor start capacitor in the outdoor unit, fix the metal fixing ring, reconnect the connecting cable, and then power on and test the machine to complete the replacement.

Motor Start Capacitor

Figure6. Motor Start Capacitor

III How Test the Motor Start Capacitor of a Fan?

①Connect all parts on the circuit board of the air conditioner completely.

②Turn on the power supply.

③Use the remote control to adjust the temperature to make the fan motor rotate.

④Connect the ground terminal of the oscilloscope probe to the ground terminal of the circuit board.

⑤Use an oscilloscope probe to detect the white lead on the Hall element plug.

⑥The oscilloscope shows signal waveform.

When testing the start capacitor of the fan motor, a multimeter should be used to measure the resistance of the capacitor. Due to the large size of the capacitor, it is impossible to use the capacitor input jack for testing. At this time, you can use the multimeter pen to test, and judge the quality of the capacitor by the change of the value displayed by the multimeter.

If the resistance value displayed by the multimeter changes from small to large, and then changes to infinity, it means that this is a good capacitor with charging and discharging functions. Check again after changing the test leads, the displayed value still changes from small to large, and changes to infinity.

After measuring the capacitor, if the test leads are not replaced when the test is performed again, and the resistance value is displayed as infinite, it means that charging and discharging are not performed, but it does not mean that the capacitor is damaged. Therefore, when testing the capacitance, the test leads must be replaced no matter which multimeter is used.

Fan Motor

Figure7. Fan Motor

IV Relevant Knowledge in the Step of Start Capacitor Test

4.1 How to Choose Start Capacitor?

How to calculate the starting and running capacitance of a single motor

run capacitance C=120000*I/2.4*f*U*cosφ

Where: I is the current; f is the frequency; U is the voltage; cosφ is the power factor, taking 0.5 to 0.7.

The run voltage of the run capacitor is greater than or equal to (2~2.3) U.

Start capacitor capacity = (1.5 ~ 2.5) run capacitor capacity.

The run voltage of the start capacitor is greater than or equal to 1.42 U.

(It is best when the voltage across the capacitor is 311V during operation) The working capacitor is 1-4UF per 100W, and the start capacitor is 4-10 times the working capacitor (the motor requires a larger starting torque).

Empirical data, if your motor does not exceed 200W, the start capacitor will not exceed 100uF. If you run the capacitor, you can choose several values for power-on test, and see which capacitor has the smallest current in the whole machine, then the capacity of the capacitor is the most Good value.)

The capacity of the single-phase split-phase motor capacitor can be calculated by the empirical formula C=35000I/2PUfcos&

Such as I=250W/220V=1.2A

C=35000x1.2/2x1x50x220X0.8=24uf

Can choose 350V30uf capacitance.

Starting Capacitor Table

Figure8. Starting Capacitor Table

How to calculate the voltage across the run capacitor of a single-phase motor?

① First of all, you must know the impedance value of the secondary winding. You can measure the resistance value by measuring the DC resistance with a multimeter. Then, the secondary winding is connected to a 12V AC voltage and the current value is measured. According to the winding impedance equal to the resistance and reactance in series, it can be calculated by phasor Out the winding inductance value.

② In normal operation, the capacitor is connected in series on the secondary winding, that is, the three equivalent parameters of winding resistance, winding reactance, and capacitance are connected in series and then connected to 220V voltage. It is easy to calculate the phasor according to the formula of the series circuit. Calculate the voltage on the capacitor.

③ When a single-phase motor is running, the voltage at both ends of the capacitor is generally above 300VAC, so the capacitor voltage is generally selected for a capacitor with a withstand voltage of 400V or more, and a capacitor with a voltage of more than 450V is better.

④ For the calculation of the capacitance withstand voltage, please refer to Article 2. First measure the resistance R and reactance XL of the secondary winding, and then select the capacitance C according to the power of the motor to calculate the capacitive reactance Xc.

The actual voltage across the capacitor during operation: Uc= Xc*220/(R+jXL-jXc); the withstand voltage value of the capacitor: Uce=1.3~1.5Uc.

Single Phase Motor Starting

Figure9. Single Phase Motor Starting

Detailed selection guide of start capacitor and run capacitor

Single-phase motor capacitor selection.

Withstand voltage formula: U (capacitance) is greater than or equal to 1.5*U

Single-phase run capacitor formula: C=1950×I/U×cosφ (using a capacitor, which is both a start capacitor and a run capacitor, is commonly used for small-capacity motors such as electric fans and washing machines)

Start capacitor capacity formula: C=3500*I/U*cosφ (a capacitor is only used when starting, disconnected during normal operation, and switched with a transfer switch or a centrifugal switch.

Dual-value capacitor run capacitor capacity formula: C=1200*I/U*cosφ (use 2 capacitors, one for operation and one for startup)

Dual-value capacitor start capacitor capacity formula: C=(2~3)*C (run capacitor)

C: Capacitor capacity: I: motor rated current, U: motor rated voltage, cosφ: power factor 0.7.

Generally, there is no need to calculate. The run capacitor is 2~3μF per 100W, and the start capacitor is 2~3 times the run capacitor. The capacitor selection of the motor has strict requirements on the voltage, and it must be equal to or greater than 1.5 times the rated voltage of the motor. For a power supply with a rated voltage of 220V, the rated voltage of the capacitor cannot be lower than 400V. The capacitance value has a certain broadness, it doesn't matter if it is larger or smaller, especially the start capacitor, which can be selected at 2-6 times the working capacitor.

How to choose the capacitor of single-phase asynchronous capacitor start motor

We can calculate according to the following formula

Phase start capacitor capacity:

C=350000*I/2p*f*U*cosφ

In the formula:

I---current;

f-frequency;

U---voltage;

2p-the larger power factor is 2, and the smaller power factor is 4;

cosφ---power factor (0.4~0.8).

Split-phase start capacitor withstand voltage:

The capacitor withstand voltage is greater than or equal to 1.42*U.

Run capacitor capacity:

C=120000*I/2p*f*U*cosφ

In the formula: I---current;

f-frequency;

U---voltage;

2p-take 2.4;

cosφ---power factor (0.4~0.8).

Run capacitor withstand voltage:

The withstand voltage of the capacitor is greater than or equal to (2~2.3)*U.

start capacitor capacity of double-value capacitor motor:

C=(1.5~2.5)*operating capacitor capacity.

Withstand voltage of start capacitor:

The capacitor withstand voltage is greater than or equal to 1.42*U.

4.2 Notes about Replacing the Start Capacitor

The start capacitor is an important part of the electronic circuit. Once the start capacitor is broken, the motor cannot be started. The damaged start capacitor will only make a buzzing sound when it is energized for a short time, causing the current to surge, and long-time energization will cause severe overheating and even burn the motor, so it should be replaced immediately. And it is not difficult to judge that the start capacitor is broken. Most of the damaged start capacitors are bulging, and the surface will be burnt due to excessive current, and the rotor speed will be slow and weak. Of course, the most intuitive and accurate way is to use the capacitance setting of a multimeter to measure the quality.

Once we have confirmed that the start capacitor has failed, the things that should be noted when replacing the start capacitor:

① After the start capacitor is discharged, there will still be part of the residual charge that cannot be discharged for a while, and an artificial discharge should be performed again.

② Since the failed start capacitor may have poor lead contact, internal disconnection or fuse blown, etc., part of the charge may not be discharged. Therefore, the maintenance personnel should wear insulating gloves before touching the failed start capacitor. Using the short-circuit wire to short the two poles of the faulty capacitor first, and then it can be removed and replaced by hand.

③ If multiple start capacitors are used in series, they should be discharged separately.

Replace the Start CapacitorFigure10. Replace the Start Capacitor

④ When handling or replacing a malfunctioning start capacitor, disconnect the power supply of the start capacitor, disconnect the switch or unplug the plug, and discharge the start capacitor.

⑤ When discharging, first connect the grounding terminal of the grounding wire, and then use the grounding rod to discharge the start capacitor several times until there is no discharge spark or discharge sound, and then fix the grounding terminal.

⑥ It should also be noted that general users often ignore the instruction manual, and the precautions for use must be carefully understood and followed during installation. As we all know, the impedance of a capacitor is inversely proportional to frequency. As the frequency increases, the loss also increases. Measures should be taken to limit the harmonics and inrush current in the circuit. Capacitors always generate heat, so pay special attention to ventilation and cooling.

After the reactive power compensation device is installed, during the trial operation, the system should be tested, and measures should be taken in time if over-voltage, over-current, oscillation, and harmonics are found, which is very necessary for the normal operation of the capacitor.

V How to Test the Motor Start Capacitor without a Multimeter

A DC voltmeter can be connected to the capacitor in parallel, and an insulating shaker can be used to charge the capacitor (note the + and-poles)

(1) See if the voltage can rise to the rated voltage of the capacitor:

① 0V, the capacitor is short-circuited.

② Slowly rise to the rated voltage of the capacitor, it proves that the capacitor is good.

③ Raised quickly to the rated voltage of the capacitor, and the insulation resistance is about the internal resistance of the DC voltmeter, then the capacitor is open.

(2) When it is stable at the rated voltage value of the capacitor, look at the insulation resistance of the capacitor:

①The insulation resistance is close to the internal resistance of the DC voltmeter, so the capacitance is good.

②If the insulation resistance is less than the internal resistance of the DC voltmeter, it means that the leakage of the capacitor is large, and it is easy to generate heat and cannot be used.

VI Frequently Asked Questions about Start Capacitors

6.1 Which Motors are Served by the Start Capacitor?

Although some electrical appliances seem to have similar principles, they are different in the selection of motors, such as electric fans and air conditioners. Most electric fans use single-phase motors. Single-phase motors have only one 220v live wire and one neutral wire, while air conditioners often uses the three-phase motor, which has three wires, 220v live wire, neutral wire, and 380v live wire. The most obvious difference between a single-phase motor and a three-phase motor is that the number of start capacitors is different. A single-phase motor is equipped with a start capacitor, while a three-phase motor has no start capacitor.

6.2 Why Does a Three-phase Motor do not Need a Start Capacitor?

Because the three-phase motor itself has three running windings and can generate a magnetic field by itself, the appearance of the magnetic field can effectively replace the start capacitor, so the three-phase motor is generally not equipped with a start capacitor. However, the start capacitor still plays an irreplaceable role in a single-phase motor, because there is only one running winding in a single-phase motor, which cannot form a rotating magnetic field, and the operation of electrical appliances can only rely on the start capacitor.

In addition to the start capacitor in a single-phase motor, there is also a run capacitor. Although these two capacitors work together, the function of the start capacitor is much greater than that of the run capacitor, so once the start capacitor is damaged, the fan will make a lot of noise , The blade speed is reduced. If this happens to your electric fan, you might as well try to replace a start capacitor, the problem should be solved.

6.3 The Relationship Between Start Capacitors and Motor

At present, single-phase motors with low-power motors, the start capacitor is connected in series with the starting coil, and then connected in parallel with the running coil to work at the same time. In order to speed up the start-up time of the high-power motor, a large capacitor to help start is added. After the motor is started, the additional large start capacitor is disconnected by the centrifugal switch. The smaller capacitor connected in series with the starting coil is responsible for the phase shift required during normal operation. Electric current, the power supply machine is operating normally.

Is there a single-phase motor that is connected to the starting coil and connected in parallel with the running coil in the circuit from start to run and does not require other large capacitors to help start? Low-power motors are always used in the circuit. High-power motors have to add additional capacitors due to their large power and large starting distance.

Torque-speed characteristic

Figure11. Torque-speed characteristic

6.4 What are the Functions of Start Capacitors, Run Capacitors, and Centrifugal Switches in Single-phase Motors?

The start capacitor is used for phase separation, and the purpose is to make the current in the two windings produce a phase difference close to 90 ゜ to generate a rotating magnetic field, allowing the motor to run quickly in a static state.

There is an automatic clutch switch in the motor. When the motor is started, the motor will continue to run due to inertia. When the speed reaches a certain speed, the start capacitor will be separated by centrifugal action and automatically connected to the run capacitor, and the motor will enter the normal working state. .

The function of the run capacitor is to keep the current in the two windings with a phase difference of 90° to generate a continuous rotating magnetic field.

For motors with start capacitors, the rotational torque generated by the start capacitors is larger than that of the run capacitors, which is more suitable for starting with a load. Motors without start capacitors are not suitable for starting with a larger load.

VII Quiz

The starting capacitor of a single phase motor is

(A) Electrolytic capacitor

(B) Ceramic capacitor

(C) Paper capacitor

(D) None of the above.

Answer: A

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