Sometimes, in HVAC systems, a single phase electric motor needs to stay and run at a constant pace without any ups and downs. For this, the electric motor uses start capacitor and run capacitor. A start capacitor produces required torque by creating a high phase shift for starting up, and the run capacitor creates a rotating magnetic field to give an auxiliary current to the electric motor. They are also called as HVAC capacitors. Let’s check some more details about the Start Run Capacitors.
Start Run Capacitor
The electric motor start capacitor is usually bigger than the run capacitor. It also has a capacitance value larger than that of a run capacitor, i.e. 430-516 uF. Instead of using a set number, the capacitance rating of a start capacitor arrives in range. The main uses of an electric motor start capacitor are to increase the starting torque of the motor and providing access to the motor to have a quick on or off cycle. Until the motor reaches to a 3/4 of its full capacity, the motor start capacitor stays active in the circuit. The voltage rating of a start capacitor is 330V, 250/220V, 165V, or 125/110V.
Different to the start capacitor, the run capacitor keeps the auxiliary coil powered and running at both start-up and normal running phases. It starts operating once the circuit is started. The motor run capacitor provides a great efficiency to a motor run. It has a very less capacitance value than an electric motor start capacitor, i.e. 1.5–100 uF.
Generally, a run capacitor is made from a polypropylene material. It is made for providing continuous work while the electric motor is in running mode. The voltage rating of a run capacitor is 370V or 440V.
Dual Run Capacitor
The dual run capacitor is nothing but a combination of two capacitors in a single package. The dual run capacitor works as same as using two single capacitors side by side. The only purpose of using the combination is to save the space and cost of the unit.
In large air conditioning systems, for supporting two electric motors, i.e. a fan motor and a compressor motor, a dual run capacitor is used. It ideally saves space and cost of two single packaged run capacitors. And so it is also called an AC compressor capacitor and such a system is called dual capacitor air conditioner.
Other applications which use run capacitors are washing machines, dishwashers, dryers (tumble), etc.
Working of Start Run Capacitor
The combination of start or run capacitors which also called as a ‘dual capacitor’ has three leads. You can split them as two separate capacitors too. The main function of a start capacitor is to provide a needed torque to the fan motor to have a spinning start, on the other hand, a run capacitor provides an extra torque to the motor whenever needed in the running mode.
If the start capacitor you have used does not function well, then the motor will not start, and if the run capacitor lacks in performance, then there are the chances of running amperage going high which results in a high temperature of the motor as well as reducing the life of the motor. The start capacitor is energized only for a few second due to their high capacity, and this is one of the strongest reasons for failing of a start capacitor mostly before any other electric motor components.
Generally, a dual capacitor has three connections; COM, FAN, and HERM. COM connects with a contactor to give power to the capacitor, FAN connects with the condenser motor, and HERM connects with the hermetically sealed compressor.
Are you buying an HVAC capacitor?
Remember that whenever you use a new capacitor, install it only with a new motor. If you are looking to buy a start run capacitor online, then AMAZON is the best place to go, and if you want to purchase it offline, then you can find them in any HVAC supplementary stall. But keep in mind that any capacitor you buy, it must match with the motor you are attaching with.
How to replace a start run capacitor?
If you see a leaky capacitor or if it has a swollen look, then change it instantly. Whenever you replace a capacitor in the motor, do it carefully. The voltage can go high if required, but it should never get lower. Also, the uf should remain same throughout.
To replace a start run capacitor,
- First turn off the HVAC system. Double check it using a meter.
- Remove the side panel of the electric supply of the system.
- Now find the start run capacitor in the unit. If it is a dual run capacitor, then you need to replace it whole. And if it is two capacitors placed with the fan motor, then you need to replace the fan motor capacitor.
- Check the capacitance value and voltage of the capacitor, and then connect it to the motor. Connect one leg at one time to ensure a correct connection.
Testing the capacitor
The capacity of a capacitor is measured in MicroFarad, and most of the technicians use analog or digital multimeter for it. Place the multimeter probes across the capacitor terminals to measure the capacitance value.
In the end
You can use a set of run capacitors in place of a run capacitor. Yes, you can replace the start capacitor with a run capacitor. But, the only thing is you need a whole bunch of run capacitors to start a motor because it needs a very high torque to start which start capacitor provides easily. Using multiple run capacitors will also acquire more space in your system. If you have a mounting space available, then you can use multiple run capacitors in place of a start capacitor. You cannot replace the run capacitor with a start capacitor, though. Because it cannot charge up constantly as the run capacitor does, so it will only give performance only for a few time.
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