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Motor start capacitor

Motor start capacitors are designed for starting AC induction motors. Start capacitors have two essential roles:

1.They reduce sparking between the contacts.

2. They provide rapid increase of the magnetic field (i.e. compression of the magnetic field).

Motor start capacitors as well as all other capacitors have the following basic parameters that characterize them and by which they can be selected: capacitance – it is measured in Farads (most often Micro-farads) and depends on the material of the dielectric, the method of manufacturing, the ambient temperature, etc. Rated voltage – it is measured in volts and is perceived as a normal operating voltage of the capacitor under ideal environmental conditions. Capacitor tolerance –it is typically + / – 5% or + / -10%. It is displayed by the manufacturer, along with the rated voltage and the capacitance, on the capacitor’s housing. Tangent of the loss angle. Its value shows the amount of losses in the dielectric. The tangent of the loss angle depends exclusively on the material of the non-conductive part of the capacitor – this is the part between the two poles.Material of the housing. There are a great many types of capacitors depending on the material which their housing is made of. Most common are capacitors with aluminum housing, with plastic housing, as well as capacitors coated with different types of resins and bitumen. Types of terminals. Capacitors may have different terminals. Some of them have flexible wires coming out of their housing, others have rigid pins, etc. The degree of protection is also a basic parameter. IP00 means lack of protection, IP64 means waterproof, etc. You can learn more about capacitors in the article “Capacitors”.

In order to clarify the work of a start capacitor, we will take as an example a capacitor with metal-paper construction. It consists of plates of metallized paper, which are separated from one another by two sheets of insulating material (e.g. wax paper). These plates, together with the insulating material (the dielectric) are rolled up and placed in a metal housing. One plate is connected to the grounded metal enclosure and the other is taken out to a contact circuit-breaker. When there is voltage at the terminals, the current can flow inside, but not through the capacitor. The voltage charges the capacitor to a value commensurate with the value of this voltage. When the capacitor is charged, the current stops.
If you now remove the wires through leakage, the voltage at the poles will gradually begin to decrease. If the charged capacitor is connected to the network, a discharge will proceed with direction opposite to the direction in which the charge current flew. In order to understand the operating principle of the capacitor, let us consider a system of motor start with a removed capacitor. When the contacts are open, the force lines intersect both the primary and the secondary windings and thus the electromotive force is accumulated in the primary winding. This results in generating electromagnetic field that is so strong that it can penetrate the air gap between the contacts. An electrical arc which maintains a current flow with very high amperage is generated, causing severe scorching of the contact bodies. This resembles the work of a simple hydraulic system depicted in Figure 1.


The water flowing through the pipe at a high speed leads to rapid raise of the pressure if the valve closes quickly. This pressure, however, is gradually reduced if there is a place where the water can go when the valve is closed (the bell-shaped extension). The presence of an air chamber in the system allows the water to go there when the valve is suddenly closed, and after a while the air pressure will push the water back through the pipe. The capacitor operates as a buffer, similar to the air cap in the hydraulic system. When the contacts begin to open, the capacitor absorbs the self-induction current, therefore when it is fully charged, the contacts can open without the risk of sparks. Then the capacitor is discharged in a direction opposite to the direction of the initial current.

Circuits of single-phase induction motors with start capacitors

The work of each induction motor requires the presence of rotating electromagnetic field. It is easy to fulfill this condition by connecting an induction motor to the three-phase network: we have three phases shifted from each other by 120 °, creating a field that is changed cyclically in the space between the rotor and the stator. The problem stems from the need to use an asynchronous motor in domestic conditions, where the voltage is single-phase and is 220 VAC. It is not very easy to generate a rotating magnetic field in such network, for this reason single-phase asynchronous motors are not as common as three-phase ones. However, they are used in household fans, pumps and other devices. We need, of course, to have in mind that the power of the single-phase circuit is not very high, and the electrical characteristics of single-phase motors, in general, are substantially inferior to the characteristics of three-phase induction motors, and their power rarely exceeds 1 kilowatt.
The rotor of single-phase induction motors runs as short-circuited, as because of their low power these machines need no regulation in the rotor circuit. The stator circuit consists of two windings connected in parallel in the circuit. One of them is a run winding and ensures the work of the motor at 220 volts, and the second one is known as the auxiliary or start winding. An element, ensuring the de-phasing of the current in the windings, is connected to the second winding, which is necessary for creating a rotating electromagnetic field. In 99% of the cases this element is a start capacitor, but there are still electric motors with connected resistors or inductors having the same purpose. Depending on the coupling circuits capacitor electric motors are divided into several groups: with start capacitor, with start and run capacitor and only with run capacitor.

sheme Cp and Cp
The most common is the case of a single-phase induction motor with an additional winding and a capacitor connected to the circuit only during the start time, and then it is turned off as we already explained in the article about the capacitors in this blog. If this is a run capacitor, it is permanently connected to the circuit. Electrical machines, whose circuits are implemented with start capacitors, have good starting torque at the beginning of the working cycle, but during the working process the parameters of such motors worsen, because the field created by the run windings appears to be elliptic, not circle. Motors with run capacitor have good run characteristics which are maintained throughout the working process, at the expense of bad start characteristics. Obviously, the elaboration of a compromise circuit with start and run capacitors, where the motor has fairly good start and run characteristics, is inevitable.

Circuits with start capacitor are obligatory for heavy starting modes and circuits with run capacitor are used where the need for high starting torque is not so significant. Practically, you decide which of the circuits to choose because all terminals, both motor and capacitor ones, are led to the terminal box and there, by simply changing the locations of the wires, you can implement one or another circuit option. If you have to choose capacitors, you can observe the following values: the run capacitor should be about 0.7-0.8uF per kilowatt, and the start capacitor – 2.5 times larger capacitance. If you are wondering how to find out which terminals belong to the start winding and which to the run winding (if they are not labeled, of course), you can get orientation by their section: the start one has significantly larger section.

There are also cases, which are not very rare, where the start and the run windings are connected inside the motor housing and are drawn out with a common terminal. In this case we can not implement a reverse because we must not swap the places of the start winding. In practice, in this case we will have three power terminals in the terminal box. Which terminal belongs to the start winding and which belongs to the run winding can be determined only by continuity tests. The biggest resistance will occur between the terminals of the start and the run windings, and the resistance between the common terminal and the start one is greater than that between the common terminal and the run one.

Connection of a three-phase induction motor to a single-phase network through a capacitor.

In practice, we often need to use a three-phase motor in the single-phase domestic network. We can use the following circuit, which provides 75% of the power of the three-phase machine.


How to calculate the capacitor? If the motor is calculated with phase voltage 127 VAC (127h3 = 380VAC) then: C = 2800.I / U = 12,72. I uF, where U and I are the rated current and voltage in the single-phase network.

If the motor is calculated with phase voltage 220 VAC, then: C = 4800I / U = 21,82. I uF and the calculation should always consider some reserve.

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