When selecting circuit breakers that can protect electric motors from damage as a result of short circuits or excessively high loads, it is necessary to take into account the large value of the starting current, often 5-7 times higher than the nominal value. Asynchronous power units with a squirrel-cage rotor are subject to the most powerful starting overloads. Since this equipment is widely used for work in industrial and domestic conditions, the issue of protecting both the device itself and the power cable is very relevant. This article will discuss how to correctly calculate and select a motor protection circuit breaker.
Tasks of devices for protecting electric motors
Household electrical equipment is usually protected from large inrush currents in networks using three-phase circuit breakers that operate some time after the current exceeds the rated value. Thus, the motor shaft has time to spin up to the desired rotation speed, after which the force of the electron flow decreases. But the protective devices used in everyday life do not have precise settings. Therefore, the choice of a circuit breaker that allows you to protect an asynchronous motor from overloads and short-circuit overcurrents is more complicated.
Modern motor protection circuit breakers are often installed in a common housing with starters (the so-called motor starting switching devices). They are designed to perform the following tasks:
- Protecting the device from overcurrent occurring inside the motor or in the power supply circuit.
- Protection of the power unit from phase conductor breakage, as well as phase imbalance.
- Providing a time delay that is necessary so that the motor, which is forced to stop as a result of overheating, has time to cool down.
Control and protective automation for the engine on video:
- Shutting down the installation if the load is no longer supplied to the shaft.
- Protection of the power unit from long overloads.
- Protection of the electric motor from overheating (to perform this function, additional temperature sensors are mounted inside the unit or on its body).
- Indication of operating modes, as well as notification of emergency conditions.
It is also necessary to take into account that the circuit breaker for protecting the electric motor must be compatible with control and control mechanisms.
Selecting a circuit breaker
In the case of direct starting, when the engine is put into operation using an automatic motor and a contactor, you must first know its power. This information can be found either in the technical specifications for the engine, or in the passport data that is indicated on the nameplate.
The next step is to select an automatic machine based on the rated engine power. From various manufacturers you can find performance tables that indicate the rated operating current and the adjustment range of the circuit breakers depending on the engine power. In particular, the figure below shows the correspondence table for Allen Bradley motor protection circuit breakers.
And the last step is to set the required shutdown current using the range regulator. It is usually specified that it must be greater than or equal to the rated current of the motor. But it is desirable that the protection operation current exceeds the rated motor current by 10-20%.
That is, if the rated current of the motor is, for example, 10 A, we multiply this value by 1.1. We get 11 A. This current value is set by the regulator.
And I also wanted to say a few words about the design of the automatic motor. First of all, it should be noted that according to the control method, there are two types of machines - push-button and with a rotary switch. Also, the terminals can be either screw or with spring contact (used for motors with power up to 2 kW). You can also note the presence of a Test button on the front side of the case, which allows you to simulate the operation of the machine’s protection to check its functionality.
And in conclusion, I would like to note that operating motors without protective devices often leads to their failure due to overload, phase loss, voltage surges, etc. And this, in turn, leads to financial costs and equipment downtime. Therefore, motor protection circuit breakers are a necessary element and you should not skimp on them, especially since their prices are currently quite reasonable.
Calculation of an automatic machine for an electric motor
Until recently, the following scheme was used to protect electric motors: a thermal regulator was installed inside the starter, connected in series with the contactor. This mechanism worked like this. When a large current passed through the relay for a long time, the bimetallic plate installed in it was heated, which, bending, interrupted the contactor circuit. If the excess of the set load was short-term (as happens when starting the engine), the plate did not have time to heat up and trigger the machine.
Internal structure of the motor protection circuit breaker in the video:
The main disadvantage of this scheme was that it did not save the unit from voltage surges, as well as phase imbalance. Nowadays, the protection of electric power plants is provided by more accurate and modern devices, which we will talk about a little later. Now let’s move on to the question of how to calculate the machine that needs to be installed in the electric motor circuit.
To select a protective circuit breaker for an electrical installation, you need to know its time-current characteristics, as well as its category. The time-current characteristic does not depend on the rated current for which the AV is designed.
To prevent the circuit breaker from tripping every time the motor is started, the value of the starting current should not be greater than that which causes the device to immediately operate (cut-off). The ratio of the starting current and the nominal value is specified in the equipment passport, the maximum allowable is 7/1.
When calculating the machine practically, you should use the reliability coefficient, denoted by the symbol Kn. If the rated current of the device does not exceed 100A, then the value of Kn is 1.4; for larger values it is 1.25. Based on this, the value of the cut-off current is determined by the formula Iots ≥ Kn x Istart. We select the circuit breaker in accordance with the calculated parameters.
Another value that must be taken into account when selecting when the machine is mounted in an electrical panel or a special cabinet is the temperature coefficient (CT). This value is 0.85, and the rated current of the protective device should be multiplied by it when selecting (In/Kt).
YOURSELF A MASTER
The correct selection of a circuit breaker to protect an electric motor is of great importance for the equipment. Reliability of operation, protection of the engine from emergency operating conditions and wiring directly depend on the selection of the circuit breaker.
In this article we will outline the conditions for choosing a circuit breaker to protect an electric motor. In order to choose a circuit breaker you need to know:
— rated motor current;
— multiple of the starting current to the rated current;
— maximum permissible electrical wiring current.
Motor rated current
- this is the current that the electric motor has during operation at rated power. It is indicated on the electric motor passport or taken from the electric motor passport data tables.
Multiplicity of starting current to rated current
– this is the ratio of the starting current that occurs in the electric motor during start-up to the rated current. It is also indicated on the electric motor passport or in the electric motor tables.
Maximum permissible wiring current
- this is the permissible current that can pass through a wire or cable that is connected to an electric motor.
Conditions for correct selection of circuit breaker
to protect the electric motor:
— the rated current of the circuit breaker must be greater than or equal to the rated current of the electric motor. For example: electric motor current AIR112M4U2 In. dv. =11.4A, select the VA51G2534 circuit breaker for rated current In. = 25A and release current IN..ras. = 12.5A.
After this, we will check that the circuit breaker does not trip when starting the electric motor using the following condition:
Iu.e.>kzap. · kр.у · kр.п. ·In.dv ·ki
where Kzap . — safety factor that takes into account voltage fluctuations, Kzap. = 1.1;
kр.у - coefficient that takes into account the inaccuracy of the insertion of the operating current of the electromagnetic release of the circuit breaker, Kр.у = 1.2;
kr.p. — coefficient that takes into account the possible deviation of the starting current from its rated current, kр.п. = 1.2;
K i is the catalog multiplicity of the starting current of the electric motor;
In.dv - rated motor current, A.
Iу.е = 14 · In.ros = 14 · 12.5 = 175A
From the table of electric motors we find K i = 7.0 for the AIR112M4U2 electric motor.
We substitute in the condition and determine
175A > 1.1 1.2 1.2 7.0 11.4
175A > 126.4A
The condition is met, therefore the circuit breaker will not trip when the engine starts.
— the rated current of the circuit breaker must be less than the maximum permissible current of the cable that powers the electric motor. For example: the connection is made with an AVRG cable (3x2.5) which has a permissible current Iadd = 27A. For a water circuit breaker to protect the electric motor, the condition is met because Iadd = 27A > Ін. = 25A.
In this article, you learned how to correctly select the correct circuit breaker to protect the electric motor using the selection conditions.
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Modern electrical protection devices for power units
Modular automatic motors, which are universal devices that successfully cope with all the functions described above, are very popular.
In addition, they can be used to adjust shutdown parameters with high accuracy.
Modern automatic motors come in many varieties, differing from each other in appearance, characteristics and control method. As when selecting a conventional device, you need to know the value of the starting current, as well as the rated current. In addition, it is necessary to decide what functions the protective device should perform. Having made the necessary calculations, you can buy an automatic motor. The price of these devices directly depends on their capabilities and the power of the electric motor.
Features of protection of electric motors in industrial conditions
Often, when turning on devices whose power exceeds 100 kW, the voltage in the general network drops below the minimum. In this case, the working power units are not switched off, but their speed is reduced. When the voltage is restored to normal levels, the motor begins to gain speed again. In this case, its operation occurs in overload mode. This is called self-starting.
Self-starting sometimes causes false alarms. This can happen when, before the temporary voltage drop, the installation had been operating in normal mode for a long time, and the bimetallic plate had time to warm up. In this case, the thermal release sometimes trips before the voltage returns to normal. An example of a voltage drop in a car's electrical network in the following video:
To prevent the shutdown of powerful factory electric motors during self-starting, relay protection is used, in which current transformers are connected to the general network. Protective relays are connected to their secondary windings. These systems are selected using complex calculations. We will not present them here, since in production this task is performed by full-time power engineers.
