1
Quick calculation of rated current of electric motor
(1) Quick calculation formula:
Rated current of motor (A): "Double the motor power", that is, "one kilowatt is two amperes". It usually refers to a commonly used 380V three-phase asynchronous motor with a power factor of around 0.8. "Doubling the kilowatt rating" means the rated current of the motor.
(2) Empirical formula:
Rated current of motor (A) = Motor capacity (kW) × 2
The results of using the above quick calculation formulas and empirical formulas are consistent. The calculated rated current is very close to the actual current value on the motor nameplate, which meets the practical requirements. For example, for a Y132S1-2, 10kW motor, the rated current calculated using the quick calculation formula or empirical formula is: 10×2=20A.
2
Selection of circuit breakers for electric motors
Low-voltage circuit breakers are generally divided into two main categories: molded case circuit breakers (also known as modular circuit breakers) and frame circuit breakers (also known as universal circuit breakers). Molded case circuit breakers are commonly used for motors of 380V 245kW and below. Circuit breakers can also be classified according to their application, such as those for protecting power distribution lines, protecting motors, protecting lighting circuits, and providing residual current protection.
2.1 Selection Principles for Circuit Breakers for Motor Protection
(1) The long delay current setting value is equal to the rated current of the motor.
(2) Instantaneous setting current: For circuit breakers protecting squirrel-cage motors, the instantaneous setting current is equal to (8 to 15) times the rated current of the motor, depending on the model, capacity, and starting conditions of the protected squirrel-cage motor. For circuit breakers protecting wound-rotor motors, the instantaneous setting current is equal to (3 to 6) times the rated current of the motor, depending on the model, capacity, and starting conditions of the protected wound-rotor motor.
(3) The return time of 6 times the long delay current setting value is greater than or equal to the starting time of the motor. Depending on the weight of the starting load, the return time can be selected from 1s, 3s, 5s, 8s, or 15s.
2.2 Quick Calculation Formula for Circuit Breaker Trip Unit Setting Current
"Instantaneous motor power, 20 times the kilowatt power."
"Thermal trip unit, according to rated value"
The above rule of thumb refers to the selection of the electromagnetic trip instantaneous trip setting current of a circuit breaker controlling and protecting a 380V three-phase squirrel-cage motor, which can be 20 times the rated current of the motor. For thermal trip units, the selection should be based on the rated current of the motor.
3
Selection of fuses for electric motors
When selecting the type and capacity of a fuse, one should consider the protection characteristics of the load, the magnitude of the short-circuit current, and the operating conditions of the application.
Most small and medium-sized electric motors use light-load full-voltage or reduced-voltage starting, with a starting current typically 5 to 7 times the rated current. The power supply capacity is relatively large, with low-voltage distribution main transformers ranging from 1000 to 400 kVA (including parallel operation capacity). The system impedance is low, resulting in a large short-circuit current during short-circuit faults. Furthermore, working environments such as kilns and grinding mills often have poor ventilation, leading to high ambient temperatures. Therefore, the breaking capacity and rated current of the fuses selected should be appropriately increased compared to general industrial applications.
3.1 Empirical Formula for Rated Current of Melt
The rated current of the fuse (A) = the rated current of the motor (A) × 3
3.2 Quick Calculation Formula for Rated Current of Melt
"Melt protection, 6 kilowatts"
This quick calculation formula refers to a 380V squirrel-cage motor that starts under light load with full voltage or reduced voltage, and has a low operating frequency. It is suitable for squirrel-cage motors of 90kW and below.
If the actual motor starts frequently or has a long starting time, the result calculated by the above empirical formula or quick calculation method can be appropriately increased, but not too much. In short, the goal is to ensure that the fuse does not blow when the motor starts, and that it reliably blows to cut off the power supply in the event of a short circuit, thus achieving the purpose of short circuit protection.
4
Selection of contactors for electric motors
(1) Select according to usage category:
In actual production, the vast majority of squirrel-cage motors are selected according to the AC-3 usage category.
(2) Determine the capacity level:
When determining the capacity of a contactor, i.e., the rated current of its main contacts under rated voltage and other technical conditions, the following points should be noted:
1) The impact of work schedule and work frequency:
When selecting a contactor, it is important to consider whether the controlled object is for continuous or repetitive short-time operation. For high-frequency operation, it is also necessary to consider increasing the rated current capacity of the contactor. Contactors with silver, silver alloy, or silver-inlaid contacts should be selected whenever possible, such as the KSDZ-U series.
2) Impact of environmental conditions
The production process takes place in a harsh environment with severe dust pollution, poor ventilation, and high workplace temperatures. Therefore, it is advisable to use derating technology when selecting contactors.
5
Motor wiring
Electric motor wiring rules
1.5 plus two, 2.5 plus three
4 plus four, 6 plus six
Add five after age 25, and subtract five after age 50.
120 wires, matched with 100-number wires
This rule of thumb is for directly selecting the wires based on the capacity of a three-phase 380V AC motor.
"1.5 plus two" means that a 1.5mm² copper core plastic wire can be used with motors of 3.5kW and below. Since a 4kW motor is close to the selection range of 3.5kW, and this formula has a certain margin, the wire selected for motors below 4kW in the quick reference table is 1.5mm². "2.5 plus three" and "4 plus four" mean that 2.5mm² and 4mm² copper core plastic wires can be used with 5.5kW and 8kW motors, respectively.
"Adding six after 6" means that starting from 6mm², it can be used with motors with a power output "six times greater". That is, 6mm² can be used with a 12kW motor, and selecting a similar specification will allow you to use an 11kW motor. 10mm² can be used with a 16kW motor, and selecting a similar specification will allow you to use a 15kW motor. 16mm² can be used with a 22kW motor. There is also an 18.5kW motor, which also uses 16mm² copper core plastic wire.
"Add five after 25" means that starting from 25mm², the increment changes from six to five. That is, 25mm² can be fitted with a 30kW motor, 35mm² with a 40kW motor, and a similar specification can be fitted with a 37kW motor.
The phrase "increasing by five after 50" means that starting from 50mm², the size decreases in size, increasing by five in each increment. Specifically, 50mm² can be fitted with a 45kW motor (50-5). 70mm² can be fitted with a 60kW motor (70-10), and a similar size can be fitted with a 55kW motor. 95mm² can be fitted with an 80kW motor (95-15), and a similar size can be fitted with a 75kW motor.
The phrase "120mm² copper core plastic wire can be matched with a 100kW motor" means that a 120mm² copper core plastic wire can be matched with a 100kW motor, and the phase specification is a 90kW motor.
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