Maximum and rated power kW. What is the rated power of an electric motor and how is it calculated
One of the natural characteristics of an electric motor is its rated (effective) power ( Pnom), which for alternating and DC is the mechanical power on the shaft.
This is the engine power with which it could operate in nominal mode - mode efficient work for a long time (at least several hours). Rated power is measured in watts (kW) or horsepower (hp) and is indicated on the plate electric machine along with other main characteristics.
, engine power is fully developed. When the engine is loaded to rated power for a relatively short period of time, it can be considered that it is not being used to its full potential. In such a situation, it may be advisable to briefly overload it, the limit of which is determined by the overload power of the engine.The manufacturer’s electric motor data sheet always indicates the rated power values. Pnom, voltage Unom, power factor cosϕnom, rated angular speed of the motor ωnom.
Rated power calculation
Equivalent current method
It is applicable for calculating the rated power, subject to obligatory observance during operation of the invariance of power loss indicators in the motor windings, consisting of constant and variable power values, resistance of the rotor and stator windings, and mechanical friction losses. Knowing the rated power factor, equivalent current and rated voltage, it is possible to calculate the rated power of the electric motor:
Pnom ≥ Iek ∙ Unom ∙cosϕnom,
where Iek is the equivalent current indicator,
Unom – rated voltage,
cosϕnom is the rated power factor, which increases with increasing power and rated angular speed of the rotor, and also depends on the load. For most electric motors it is 0.8-0.9.
Equivalent moment method
Electric motors of any type are proportional to the product of current and magnitude magnetic flux torque. The equivalent torque method for calculating rated power is used in cases where the conditions of the applied load directly determine the torque required from the motor, and not the current. For synchronous and asynchronous machines AC, the power factor cosϕ is approximately taken as a constant value:
Pnom = Mvr ∙ ωnom,
where Mvr is the torque value,
ωnom – rated angular speed of the motor.
Determination of rated power experimentally
Indicated in the passport or device label rated power will be equal to this value only at the optimal load on the shaft, determined by the manufacturer for the nominal mode. What to look for if for some reason your passport has not been preserved or the inscriptions on the plate have been erased?
Practical measurements will help:
- It is necessary to completely turn off all other sources of electricity consumption: lighting, electrical appliances, etc.
- In case of use electronic meter, you should connect the engine under load for 5-6 minutes, the electronic display will display the load value in kW.
The disk meter measures in kWh. You should record the latest readings and turn on the engine for 10 minutes, accurate to the second. After stopping the electric machine, subtract the recorded readings from the obtained value and multiply by 6. The resulting number will be the active mechanical power of the engine.
When using this method, it is important to select the load correctly, since if it is insufficient or overloaded, the determined indicator will be far from the rated power of the electric motor.
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This is the engine power with which it could operate in nominal mode - the mode of efficient operation for a long time (at least several hours). Rated power is measured in W (kW) or horsepower (hp) and is indicated on the electrical panel along with other main characteristics.
At loads less than Pnom, the engine power is fully developed. When the engine is loaded to rated power for a relatively short period of time, it can be considered that it is not being used to its full potential. In such a situation, it may be advisable to briefly overload it, the limit of which is determined by the overload power of the engine.
In the electric motor passport, the manufacturer always indicates the rated values of power P nom, voltage U nom, power factor cosϕ nom, rated angular speed of the motor ω nom.
Rated power calculation
Equivalent current method
It is applicable for calculating the rated power, subject to obligatory observance during operation of the invariance of power loss indicators in the motor windings, consisting of constant and variable power values, resistance of the rotor and stator windings, and mechanical friction losses. Knowing the rated power factor, equivalent current and rated voltage, it is possible to calculate the rated power of the electric motor:
P nom ≥ I eq ∙ U nom ∙cosϕ nom,
where I eq is the equivalent current indicator,
U nom – rated voltage,
cosϕ nom – rated power factor, increasing with increasing power and rated angular speed of rotor rotation, and also depending on the load. For most electric motors it is 0.8-0.9.
Equivalent moment method
Electric motors of any type have a torque proportional to the product of the current and the magnitude of the magnetic flux. The equivalent torque method for calculating rated power is used in cases where the conditions of the applied load directly determine the torque required from the motor, and not the current. For synchronous and asynchronous AC machines, the power factor cosϕ is approximately taken as a constant value:
P nom = M time ∙ ω nom,
where M wr is the value of the torque,
ω nom – rated angular speed of the motor.
Determination of rated power experimentally
The rated power indicated in the passport or plate of the device will be equal to this value only at the optimal load on the shaft, determined by the manufacturer for the rated mode. What to look for if for some reason your passport has not been preserved or the inscriptions on the plate have been erased?
Practical measurements and an electricity meter will help:
It is necessary to completely turn off all other sources of electricity consumption: lighting, electrical appliances, etc.
If you use an electronic meter, connect the engine under load for 5-6 minutes, the electronic display will display the load value in kW.
The disk meter measures in kWh. You should record the latest readings and turn on the engine for 10 minutes, accurate to the second. After stopping the electric machine, subtract the recorded readings from the obtained value and multiply by 6. The resulting number will be the active mechanical power of the engine.
- For low-power engines, you can calculate the number of revolutions of the counter disk, for each of which the value of full revolutions in power units is indicated. Simple calculations will help determine the required power value.
When using this method, it is important to select the load correctly, since if it is insufficient or overloaded, the determined indicator will be far from the rated power of the electric motor.
Rated active power of the electric drive () – this is the power consumed from the network at the rated load of the electric device at which it must operate long time in steady state without exceeding the permissible temperature.
For long-term operation of the electric drive 
equal to the passport value 
:
.
For receivers operating in intermittent mode, the rated power is determined from the nameplate power by bringing it to a long-term operating mode (PV = 1) in accordance with the formulas:
Or 
,
Where 
nameplate value, p.u.; 
– switching factor, calculated from the ED load graph, see formula (2.1).
For electric motors, the power consumed from the network is called connected power 
and is determined by the expression:
,
Where – rated power developed on the motor shaft, kW;
– nominal efficiency of the electric motor, p.u.
Rated reactive power of the electric unit
(
) – reactive power consumed by it from the network at rated active power and rated voltage.
For an electric generator operating in long-term mode, the value calculated by the formula
,
Where 
corresponds to nominal 
EP ( – passport value).
For an electric generator operating in intermittent mode, the value 
calculated by the formula
.
Nominal apparent power of the electric drive
.
12. Design power (definition)
One of the main stages in the design of facility power supply systems is the correct determination of the expected (calculated) electrical loads both individual electric power supply units and load nodes at all levels of the power supply system.
Load design values are loads corresponding to such a constant current load ( 
), which is equivalent to the actual time-varying load for the greatest thermal impact (without exceeding permissible values) on the element of the power supply system.
There are various methods for determining design electrical loads, which in turn are divided into main ones; and auxiliary.
The calculated electrical loads include the calculated values of active power ( 
), reactive power ( 
),
full power (
) and current ( 
).
13. RMS power (definition)
RMS value of the active power of an individual electric unit for the considered period of time
,
Where 
– root mean square value of the active power of the electrical receiver, kW; 
– active power consumed by the electric device for the considered period of time 
(determined from the load graph by active power), kW; – time interval for which it is determined , min, h.
If there are graphs of reactive power consumption, the root mean square value of reactive power is determined in a similar way.
Root mean square value of reactive power of electric power supply for the considered period of time
,
Where 
– root mean square value of the reactive power of the electrical receiver, kVAAr; 
– active power consumed by the electric power plant for the period of time under consideration (determined from the reactive power load graph), kVAr; – time interval for which it is determined , min, h.
In the absence of reactive power consumption schedules, the rms value of reactive power
,
Where – corresponds to the nominal 
EP ( – passport value).
Based on the known root-mean-square values of the active and reactive power The rms values of the total power and current are determined.
Root mean square value of the total power of the electric generator for the considered period of time
,
Where 
– root mean square value of the total power of the electric unit, kVA.
Root mean square value of the electric current for the considered period of time
,
Where 
– root-mean-square value of the electric current, A; 
– rated voltage of the electrical unit, kV.