Start Capacitor in AC Induction Motor
An AC motor needs a start capacitor to provide the initial burst of energy needed to start the motor's rotation.
When an AC motor is first turned on, it needs an extra jolt of electricity to overcome the inertia and get the motor turning. Without this initial burst of energy, the motor might not be able to start at all, or it may take a long time to get up to speed.
A start capacitor helps to provide this initial burst of energy by storing electrical energy and then releasing it in a powerful surge when the motor is first turned on. Once the motor is up to speed, the start capacitor is no longer needed, and it is automatically disconnected from the circuit.
It's important to note that not all AC motors require a start capacitor. Some motors, such as split-phase motors, have built-in starting mechanisms that don't require a separate capacitor. However, for certain types of motors, such as single-phase induction motors, a start capacitor is an essential component.
What's the function about motor capacitor?
A motor capacitor is an electrical component that is used to store and release electrical energy in an alternating current (AC) motor. Its primary function is to provide a boost of power to start the motor, and to help maintain a constant voltage supply during operation.
When an AC motor is first started, it requires a high amount of energy to overcome the initial resistance and get the rotor turning. The motor capacitor provides this initial burst of power, allowing the motor to start smoothly and quickly.
Once the motor is running, the capacitor continues to provide a steady supply of energy to help maintain a constant voltage level. This helps to improve the efficiency and performance of the motor, as well as reduce wear and tear on the motor's components.
There are two types of motor capacitors: run capacitors and start capacitors. Run capacitors are used to provide a continuous supply of energy to the motor, while start capacitors are used only during the startup process to provide the initial boost of power.
Single-coil AC induction motors
AC induction motors usually use two or more coils to generate a rotating magnetic field, which produces torque on the rotor. When a single coil is used, it will generate a pulsating magnetic field, which is enough to sustain rotation, but not sufficient to start the motor from a standstill. Motors with a single coil have to be started by using an external force, and can rotate in either direction. The direction of the rotation depends on the external force. If the motor was started in a clockwise direction, it will continue to rotate and build up speed in the clockwise direction, until it reaches a maximum speed which is defined by the power source frequency. Similarly, it will continue rotating counter-clockwise if the initial rotation was counter-clockwise. These motors are not practical due to their inability to reliably start rotation on their own.
Start capacitor AC induction motors
One way to improve on the single coil design is by using an auxiliary coil in series with a motor starting capacitor. The auxiliary coil, also called starting coil, is used to create an initial rotating magnetic field. In order to create a rotating magnetic field, the current flowing through the main winding must be out of phase in respect to the current flowing through the auxiliary winding. The role of the starting capacitor is to lag the current in the auxiliary winding, bringing these two currents out of phase. When the rotor reaches sufficient speed, the auxiliary coil is disconnected from the circuit by means of a centrifugal switch, and the motor remains powered by a single coil creating a pulsating magnetic field. In this sense, the auxiliary coil in this design can be regarded as a starting coil, since it is only used during motor startup.
Start/run capacitor AC induction motors
Another way to further improve on the single-coil single-phase induction motor design is to introduce an auxiliary coil, which remains powered not only during the motor startup phase, but also during normal operation. As opposed to an AC motor using only a motor start capacitor, which creates a pulsating magnetic field during normal operation, AC motors using a motor start capacitor and a motor run capacitor create a rotating magnetic field during normal operation. The function of the motor start capacitor remains the same as in the previous case - it gets disconnected from the circuit after the rotor reaches a predetermined speed by means of a centrifugal switch. After that point, the auxiliary winding remains powered through a motor run capacitor. The figure below describes this type of design.
circuit diagram of motor start and motor run capacitor
A run capacitor uses the charge in the dielectric to boost the current which provides power to the motor. It is used to maintain a charge. In AC units, there are dual run capacitors. One capacitor provides power to the fan motor. The other sends power to the compressor. Run capacitors measure in at approximately 7-9 micro-Farads. The value or rating of the run capacitor must be accurate. If the value is too high, the phase shift will be less than perfect and the winding current will be too high. If the capacitor value/rating is too low, the phase shift will be higher and the winding current will be too low. If run capacitors are not ideal, then the motor could overheat and the true torque will not be enough to drive current.
Some single phase AC motor designs use motor run capacitors, which are left connected to the auxiliary coil even after the start capacitor is disconnected by the centrifugal switch. These designs operate by creating a rotating magnetic field. Motor run capacitors are designed for continuous duty, and remain powered whenever the motor is powered, which is why electrolytic capacitors are avoided, and low-loss polymer capacitors are used instead. The capacitance value of run capacitors is usually lower than the capacitance of start capacitors, and is often in the range of 1.5 µF to 100 µF. Choosing a wrong capacitance value for a motor can result in an uneven magnetic field, which can be observed as uneven motor rotation speed, especially under load. This can cause additional noise from the motor, performance drops and increased energy consumption, as well as additional heating, which can cause the motor to overheat.