How a starting capacitor differs from a working capacitor

One of the main problems when using electric motors in the home consists in the fact that almost all sufficiently productive models are designed for inclusion in a three-phase network, and in apartments, apartment buildings and garage cooperatives are stretched single-phase. But this problem, too, can be solved. Three-phase motors are included in the single-phase network through capacitors.

Such a connection leads to neither a decrease in the performance of the electric motor, nor to a decrease in its reliability. Losses, if any, are negligible. But, again, you should not connect high-powered electric motors to the household network – the fact that it is possible in principle, does not guarantee the stability of at least the same wires or sockets.

In order to connect a three-phase motor to a single-phase system, a system of two capacitors, one operating capacitor and one starting capacitor, must be used. The difference between the two is quite noticeable. And in this article, we’ll look at how a starting capacitor differs from a running capacitor.

Motor start capacitor

Differences between a start capacitor and a run capacitor

The main difference between these elements of the network is in their purpose. So:

  1. The operating capacitor is used when there is a phase shift. It can also be called the “first” capacitor. It is used continuously throughout the entire service life of the motor – and is therefore not deleted from the circuit. Generally connected in series with the auxiliary winding. Because it is used in phase switching, its capacitance must be relatively small. This helps to avoid overheating of the motor, a slowing down of the power increase and a braking of the torque;

  2. A starting capacitor is used when starting an engine. After the motor reaches the necessary frequency and power, it is removed from the circuit. Capacity increases the starting torque of the motor, allowing it to reach its normal operating mode more quickly.

Let’s consider these capacitive elements in more detail – in terms of performance and electrical characteristics.


Starting capacitor


Where to use

In asynchronous motors

In asynchronous motors

How it’s connected

Parallel to the operating one

In series with an auxiliary winding

What is it for

To create a starting magnetic field that increases the motor’s starting torque

To generate the rotating electromagnetic field needed to drive the rotor

When a capacitor is used

During the entire running time of the motor

When starting the motor

What conditions are designed for

For a 220 V motor, a 220 V capacitive element is needed as standard

For “hard” with overvoltage. A capacitive element of 500 to 600 V is needed for a 220 V motor

Suitable type

Paper or oil-filled


The aforementioned difference in operating conditions is due to elementary physical processes that occur during the operation of the parts. The operating capacitor is connected into the motor winding, which is a simple oscillating circuit. The result is that during certain periods of time, voltages are applied to the outputs of this circuit which are 2 to 2.5 times higher than those applied to the inputs. Because of that, parts that can’t withstand such stress simply burn out.

Starting parts operate under less severe conditions. The voltage that is applied to these elements practically does not exceed the mains voltage – and if it does, then only slightly, about 1.15 times. You can ignore that and use 220-volt versions – especially if you consider their short-lived operation while turning on the cycle of the machine or other device.

As a consequence, as the capacitors included in series with the winding, you must choose options that can withstand prolonged exposure to high voltages. Practice shows that they are paper or oil-filled versions (MBGH, MBGO brands). Moreover, judging by the experience of domestic users, the Russian-made elements are characterized by greater longevity and reliability.

However, they are not without their disadvantages. In particular, MBGH and MBGO are distinguished by their large size. Because of this it is not possible to connect them in compact devices. You can, of course, use a more compact oxide, but in this case it is necessary to install diodes in a certain scheme.

Electrolytic models, although they can be designed for significant operating stresses, are used only as starting. This is due to the following characteristic of electric motors. In networks, where they are included, when they work there is a reactive voltage. Electrolytic containers under its influence boil over very quickly, which leads to damage to the device itself, as well as to equipment, and is a source of danger to maintenance personnel.

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