Alright next on the list of topics to talk about are capacitors. These small and rather obscure components attached to an electric motor may keep you scratching your head wondering what they are all about, don't worry – I'm here to debunk some mysteries.

What is a capacitor? 

So what is a capacitor? A capacitor is a common component in any electronic system. They are used as a short term storage of a charge that can also quickly discharge. Unlike a battery they can nearly instantly discharge their entire contents and then very rapidly charge up again to full charge.

A capacitor is actually a very simple, the ones we use in pumps motors are about 5-10cm long and shaped like a round cylinder. There are 2 terminals on the top and these attach to the 2 poles of the capacitor. These poles consist of a metal foil, usually aluminium and there is a non conductive material in between these sheets usually a plastic or metal oxide material, then this “sandwich” is covered in resin coated paper and rolled up inside a plastic or metal case so it is compact.

We have a wide range of replacement capacitors IN STOCK in our webshop. 

Power factor what is that?  

So the reason that capacitors are used in an electric motor is all down to a strange concept call power factor. Power factor is a ratio between the “real power” used by an electrical circuit and “apparent power” delivered into a circuit from a source.

• Real power is the amount of current and voltage currently used at any particular time by a circuit. If there is an electrical motor in this circuit this can rise and fall depending on if the motor is starting, under load or running with no load.
• Apparent power is the amount of current and voltage entering the circuit. This is usually higher than the real power due to losses and inefficiencies in the load causing some of the current to be sent back out of circuit without being utilised.

See in the diagram above, when an AC power supply is “in phase” the voltage and current rise and fall in sync with each other and all the available power can be used. When an “inductive load” is connected to the power supply this can cause a drop in power factor. What we mean by an "inductive load” is a coil of wire windings that creates an electro-magnet when it is powered up. This electromagnet “inducts” a charge into another piece of metal and makes the metal move. What happens when this electromagnet is powered up is a condition known as lagging power factor, where the rise and fall of the current falls very slightly behind the rise and fall of the voltage. What happens then is that some of the current actually reverses back out of the power circuit and works against the main current. This drops the overall power by 10-15% .

Now the exact opposite of a lagging power factor condition is when there is a leading power factor occurring. This is what occurs when a “capacitive load” is applied to the power circuit instead of an inductive load. A capacitive load is a lot less common than an inductive load, usually this is created by connecting 1 or more capacitors to a circuit or a mechanical device like a synchronous condenser. In this instance the current rises and falls very slightly ahead of the voltage and this causes a drop in overall power by 5-10%.

As most electrical components with large current draw are inductive loads, mainly because they contain electric motors, then the majority of the problems with power factor in an electrical circuit are with lagging power factor. Therefore to correct the problem with lagging power factor, the most common solution is to connect a capacitor and bring the power factor back to 1.0. This is part of the reason why capacitors are used in electric motors but the main reason is to advance the current slightly ahead of the voltage. An electromagnet is created by a flow of current not voltage and if the voltage lags the current, then this creates a magnetic force slightly “ahead” of the motors position and pulls the rotor to make it start spinning and then accelerate. This is really only important for single phase motors as three phase motors have the benefit of each phase working as a team. At a certain position phase 1 is pulling, phase 2 is resting and phase 3 is pushing, then as the Sine wave changes the roles swap around according to the voltage that is applied to each phase.

How to select and size the correct capacitor 

Now the important part, selecting the right capacitor for your pump motor. It's not just about picking any old capacitor off the shelf and calling it a day, we can help guide you to the correct size and type using our pump part selection tool. Different motors need different sized and sometimes types of capacitor so work properly. If you put in a capacitor that is too big or too small, then it can overload the start winding or the run winding. This can melt things and destroy your motor as well as it not spinning at the correct speed.

Capacitors are sized by the amount of microfarad (µF), by the maximum voltage they can handle and if that voltage is DC or AC. Microfarad is the measure of the amount of capacitance or charge that the capacitor can store on its plates. Generally most motor capacitors are rated between 2µF and 150µF, with some larger motors 7.5kw or larger needing up to a 250µF capacitor to start or run properly. The voltage that a capacitor is rated to means the maximum voltage that can be run through the unit under normal operating conditions and for it to operate correctly and have a long service life. Most pump motor capacitors are rated to 450AC as this is "maximum" voltage and normal single phase power is 240V RMS (root mean square) so there are moments when voltage is alot higher than 240V as voltage alternates through its sine wave.  There are also specialised motor start capacitors that are rated to 250VAC so are designed to handle these voltage spikes too, they should only be used for start winding applications. 

Now capacitors can be connected to both AC or DC power supplies depending on what your requirements are. If the label on a capacitor rates it for AC voltage then you need to ensure that if you are using it for DC voltage that it will be large enough. This is due to AC voltage being measured as the RMS (root mean square) voltage of the supply. As AC voltage is constantly changing from 0 to maximum back to zero and then a negative of the maximum, you need to measure the “average” level of voltage when all parts are converted to positive. So RMS voltage is a measurement of the “equivalent” DC voltage to created the same current with the given resistance.

So if a capacitor is rated for 240V AC this is equivalent to only 152V DC and so you can’t use it on a DC power supply higher than 152V and expect any sort of service life. This can also be important if you are buying a capacitor and there is only DC rated units available, because you can use any capacitor rated over 155V DC easily for 240V single phase applications.

There are 2 types of capacitor generally in small single phase pump motors, start capacitors and run capacitors. Generally speaking both “types” of capacitors are made from the same materials, though occasionally start capacitors have some heavier duty internals so they can handle the current flow from multiple or hard starting conditions. Most of the time “start” capacitors start at around 30µF and for larger motors can go up to 250µF. For run capacitors are smaller and can be from 2µF up to around 50µF.

There are also 2 types of connection terminals, either spade terminals to attach cables to or wire loom (or cable leads) where there is a short fixed cable that attaches to terminals on the motor. It is important to check which type you have before ordering as it affects how you attach it to your motor. 

How do you know if your Capacitor is broken?

If you are having problems with your motor not running or starting properly, then a lot of the  time one of the Capacitors can be to blame. The most commonly seen fault is a loud buzzing sound and the motor not starting. This is caused by the motor winding not able to spin enough to swap into the next magnetic field and then pushing against it as the capacitor hasn’t done its job. The other sign could be just silence, turn the power on and no spinning and no action.

Remember at this point to always unplug your motor from the power point so its safe and if you are unsure of what you are doing to call in a qualified and licensed electrical technician to test and repair your motor. If you have the experience and have turned off the power supply, then check the capacitor inside the motor cover. Sometimes it is obvious, like the capacitor has melted and there is a pool of black sludge and melted metal and it stinks. Sometimes it looks fine but isn’t working. The capacitor then needs to be disconnected from the motor (remembering which wires you need to reconnect to) and tested with a good multimeter. If your multimeter tests for capacitance in µF, then the capacitor needs to be with its label spec. ie if it says 25µF +/-5% that means it needs to test between 23.75µF and 26.25µF. If its outside this range, then the capacitor needs to be thrown out and replaced with a new one. So check the label and get in contact through our website shop and we will ship a new capacitor out to you, Australia wide.

At Rural pumps, we have a large range of both starting and run capacitors available in stock. We also have access to more unusual or specialised capacitors that are not part of our usual stock, so if you can’t find a replacement in our webshop, then take a photo text, email it or upload it to our website contact page and we will be straight back to you with answers on the parts you need to get your pump running again.