Motor Capacitor- Types, Features, Applications and Testing

The AC motor capacitor is a type of capacitor that is specially designed to operate the ac motors or compressors. It is mostly used in fans, water pumps, freeze, oxygen concentrators, etc. Based on the application they are mainly of two types. They are run and start capacitors. However, there can also be a dual-type of the capacitor, that operates in both run and start mode.

In this blog, we will be discussing the differences between these types of motor capacitors. Also, we will be calculating the required capacitance for the given electric motor.

How many types of Motor Capacitor are there?

1. Run Capacitor

Run capacitor is a type of motor capacitor that is necessary for the running of the single-phase AC electric motor. So, it has got a 100% duty cycle, which implies that the run capacitor is in continuous use.

They are most probably the polymer, propylene film capacitors. The most commonly available voltage ratings are the 370 and 440 VAC. The most common capacitance values found in the run capacitors are the 1.5, 2.5, 5, 10, 15, 20, 25, 30, 40, 50, and 60 µF.

2. Start Capacitor

Start capacitor is a type of motor capacitor that is necessary for starting the single-phase AC electric motor. After turning on the motor, it remains passive.

They are mostly, non-polarized aluminum electrolytic capacitors with non-solid electrolytes in them.

The voltage rating is lower than 370 VAC. The most commonly found voltage ratings are 125, 165, 250, and 330 VAC. Similarly, the capacitance rating is above 70 µF.

Run Capacitor Vs Start Capacitor

Some of the common differences between the run and the start capacitors are as follows.

Motor Run Capacitor Motor Start Capacitor
Operates on voltage above 370 VAC. Operates on voltage below 370 VAC.
Generally, it has a low capacitance value (below 70 MFD). Generally, it has a high capacitance value (above 70 MFD).
Mostly, the exact capacitance is stated in the outer surface of the capacitor. Mostly, the range of capacitance value is stated in the outer surface of the capacitor.
The capacitor is always in use The capacitor is used only once, for turning on the motor.

(Note:- Rating can also be given in MFD instead of µF. Here MFD means Micro Farad, which is also the same thing. )

Can I use a run capacitor as a start capacitor?

Yes, you can use a run capacitor as a motor start capacitor. But, due to the low capacitance of the run capacitor, it will not be sufficient to turn on the motor. In such a case, you need to add it in the parallel combination. In the parallel combination, the overall capacitance increases. Thus, the overall capacitance will be sufficient for the motor.

We can not use a start capacitor as a motor-run capacitor. We know that the start capacitor operates at a higher capacitance. So, using it as a run capacitor will cause a heating effect on the motor. Thus, it will reduce the lifespan of the motor.

Single Phase Motor Capacitor Calculation

Before doing a calculation, you need to know if the capacitor is either run or start type. After knowing the type, you can use the formula stated below.

For Start Capacitor

C (μF)     = (I x 1000000)/(2ΠFV)

              = (I x 1000000)/(6.285FV)

where, I = current

F = frequency of the AC current, which is mostly 50 Hz or 60 Hz (based on the country)

V = Rating voltage which you can see in the outer surface of the single-phase AC motor.

Π= 22/7 =  3.14

Use the formula shown below to calculate the value of the current

I = Power/(V x P.f)

where V = voltage

P.f = power factor

(Note:- If you find the power factor in the machine use it in the formula. Else, assume that the value of the power factor is 0.8 for the given motor.

If the power is given in horsepower (HP), then convert it into watt using a formula 1 HP = 746 Watt)

Now, let us take an example


Power = 1 HP

P. f = 0.8 (assume)

Voltage (V) = 220 VAC

Frequency (F) = 50 Hz

Now Current (I) = Power/ (Voltage X P.f)

= (1X 746)/(220 X 0.8)

= 4.238 A


C (μF) = (I x 1000000)/(2ΠFV)

= (4.238 X 1000000)/ (2 X 3.14 X 50 X 220)

= 4238000/ 69080

= 91.349 μF

Hence, 1 HP motor start capacitor has an approximate value of 91.349 μF.

(Note:- You may not be able to find a 91.349 μF start capacitor in the market. In such a case, choose a capacitor with slightly higher capacitance.)

For Run Capacitor

C (μF) = (Power X Eff ( in %) X 1000)/ (V² X F)

where Eff = efficiency (if the value is given in the motor, then put it in the formula. Else assume that the efficiency of a single-phase motor is 80 %. Also, while using this formula, put the value of efficiency in percentage)

V= rated voltage (given in the motor)

F = frequency of AC current

Now, let us take an example

Power = 1 HP

Eff = 80% (assume)

V = 220 VAC

F = 50 Hz


C (μF) = (Power X Eff X 1000)/ (V² X F)

= (1 X 746 X 80 X 1000)/(220² X 50 )

= 59680000/2420000

= 24. 661 (μF)

Hence, 1 HP motor run capacitor has an approximate value of 24.661 μF.

(Note:- You may not find a 24.661 μF run capacitor in the market. In such a case, choose the run capacitor with slightly higher capacitance.)

Motor Capacitor Sizing Chart

As seen in the above calculation, the motor capacitor can depend upon a number of factors such as supply AC frequency, voltage, the efficiency of the machine, etc. Hence, I am unable to give you the general sizing chart of the motor capacitor.

Different motors may have different sizing charts. However, the chart can be determined for different powered motors using the formulae shown above.

Motor Capacitor Failure Symptoms

The basic symptoms of motor capacitor failure are as follows.

  • The capacitor can be broken, ruptured, or swollen. There can also be leakages from it.
  • In the case of the motor start capacitor, the motor may not turn on. Or, there can be some delay before starting on the motor.
  • In the case of the motor-run capacitor, the motor may overheat or may not spin with full efficiency.
  • The capacitors may emit smoke due to overheating. Also, the motor may turn off.

Testing of Motor Capacitor

First of all, inspect the capacitor externally. If the capacitor is externally ok, then you can check it by using the following ways.

Before you carry out any test, make sure to look at the reading on the surface of the capacitor.

First Method

The first method is quite risky. Besides that, the result that you obtain will also not be reliable. So, this is the least recommended method. In this method, take an AC power source. Connect the two cables to the two terminals of the capacitor. Then connect the other ends of the cables to the power supply. Now, turn on the power supply for a fraction of a second. During this time, the charging of the capacitor takes place

(Note:- The charging should be within a fraction of the second. If you charge it for a second or more, the capacitor will damage. )

After that, remove the two terminals from the power supply. Then, short the two terminals. Here the process of discharging takes place. So, you will see the sparks with a noise, during the process of discharging.  If it does not produce any kind of sparks along with the noise, then the capacitor can be defective.

As stated before, this method is not effective. The method only shows the charging and the discharging feature of the capacitor. However, it fails to measure the capacitance value.

Second Method

The second method is just similar to the first one. However, it bears a low risk.

For this method, supply a dc power supply to the given motor capacitor for a short period of time. During this time, the capacitor will charge. After being fully charged, apply a load (such as dc buzzer) to the same two points. During this, the discharging of the capacitor takes place. Hence, the load will activate (eg, the buzzer will make a noise).

Third Method

The third method needs the multimeter (clamp meter).

First of all discharge the capacitor. After that, put the multimeter on a Ω  scale. Connect the two probes of the multimeter to the two terminals of the capacitor. Then, observe the value on the display. The capacitor begins to charge up. Hence, the value of resistance begins to rise up. At a certain point, the display will show the OL symbol which denotes the overload. This indicates that the capacitor has been fully charged.

After that interchange the two probes in the point of the capacitor. Then observe the display. Here the value will decrease from the OL to 0. This marks that the capacitor has been discharged. After that, the value will again increase in opposite direction. Thus, the charging will occur on the opposite plate of the capacitor.

Fourth Method

The fourth and the best method is to use a multimeter having the feature of measuring capacitance. 

For this, connect the two terminals of the capacitor to the multimeter. Set the multimeter in the capacitance mode. Then, observe the result on the display of the meter. If the obtained value lies within the range of the rated value of the capacitor, then it is working properly. Else, it may be defective.

Fifth Method

First of all, measure the supply voltage. Then connect the two terminals of the capacitor to the two points of the power supply with the help of a wire. Then, place the hook of the clamp within the live wire. After that, turn on the power supply. The clamp meter will give you the reading of the current in Ampere. After 5 seconds, turn off the power supply

(Note:- If the capacitor is a run type, then you may apply the power supply for 5 to 10 seconds. However, if the capacitor is a start type, then you should not apply the power supply for more than 5 seconds. Else, the start capacitor may damage. )

After, obtaining the voltage and the current, you can use the following formula for the calculation of the capacitance.

(Note:- Do not remove the capacitor with bare hands. Since it carries a charge, you may get an electric shock. You can short the two pins and discharge them. After that, remove the capacitor)


  • Capacitance in MFD (50 Hz) = 3100 * Ampere / Voltage
  • Capacitance in MFD (60 Hz) = 2650 * Ampere / Voltage

where MFD denotes microfarad

After obtaining the value, compare it with the actual value shown on the outer surface of the capacitor. If the obtained value lies outside of the required capacitance, then replace the capacitor with the same one.

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