Use frequency converter to drive asynchronous motor speed control. After the induction motor is determined, the frequency converter is usually selected according to the rated current of the asynchronous motor, or the frequency converter is selected according to the current value (maximum value) in actual operation of the asynchronous motor. When the operation mode is different, the calculation method and selection method of the inverter capacity are different, and the conditions that the inverter should meet are also different. When the inverter capacity is selected, the rated current of the inverter is a critical amount. The capacity of the inverter should be selected according to the maximum operating current that may occur during operation. The operation of the inverter generally has the following methods.
First, the calculation of the required inverter capacity in continuous operation
Because the frequency converter transmits to the electric motor is the pulse current, its pulsation value is larger than the electric current when the power frequency supplies, therefore must leave the surplus capacity of the frequency changer to have the appropriate surplus. At this time, the inverter should satisfy the following three conditions at the same time:
formula:
PM, η, cosφ, UM, IM are motor output power, efficiency (take 0.85), power factor (take 0.75), voltage (V), current (A)
K: Correction factor of current waveform (take 1.05 to 1.1 in PWM mode)
P CN : rated capacity of the inverter (KVA)
I CN : rated current of the frequency converter (A)
When the IM is selected according to the maximum current in actual operation of the motor, the capacity of the inverter can be appropriately reduced.
Second, the choice of inverter capacity during acceleration and deceleration
The maximum output torque of the inverter is determined by the maximum output current of the inverter. Under normal circumstances, for short-term acceleration and deceleration, the inverter is allowed to reach 130% to 150% of the rated output current (according to the inverter capacity), so the output torque during short-term acceleration and deceleration can also be increased; Conversely, if only a small acceleration/deceleration torque is required, the capacity of the inverter can also be reduced. Due to the pulsation of the current, the inverter's maximum output current should be reduced by 10% before selecting it.
Third, the frequency converter capacity selection during frequent acceleration and deceleration
According to the acceleration, constant speed, deceleration and other current values ​​under various operating conditions, the following formula determines:
I 1CN =[(I 1 t 1 +I 2t 2 +... +I 5t 5 )/(t 1 +t 2 +...t 5 )]K 0
In the formula:
I 1CN: Inverter rated output current (A)
I 1, I 2, ... I 5: Average current in each operating state (A)
t 1, t 2, ...t5: Time in each operating state
K 0 : safety factor (take 1.2 when running frequently, 1.1 under other conditions)
Four, a drive more than one motor drive, and multiple motors running in parallel, that is, into a group drive
When a plurality of motors are operated in parallel with one inverter, after a small part of the motor starts to start, and additional motors start to be added, the voltage and frequency of the inverter have already risen, and the additional input motor will generate a large Starting current, therefore, the inverter capacity needs to be larger than when starting at the same time. Take the short-term overload capacity of the inverter as 150%, 1min as an example to calculate the capacity of the inverter. If the motor acceleration time is within 1min, the following two formulas should be satisfied:
If the motor accelerates above 1min
In the formula:
n T: number of parallel motors
Ns: number of simultaneous start
P CN1 : continuous capacity (KVA) P CN1 = KP Mn T /ηcos
P M : Motor Output Power
η: motor efficiency (approx. 0.85)
Cosφ: power factor of the motor (usually 0.75)
Ks: motor starting current/motor rated current
I M : Rated current of the motor
K: Positive coefficient of current waveform (taken from 1.05 to 1.10 in PWM mode)
P CN : Drive Capacity (KVA)
I CN : Inverter rated current (A)
The inverter drives multiple motors, but one of the motors may be connected to the inverter at any time or at any time. The rated output current of the frequency converter can be calculated as follows:
V. Calculation of the required inverter capacity when the motor is directly started
Generally, when the three-phase asynchronous motor is directly started with the power frequency, the starting current is 5-7 times of its rated current. When the motor with the motor power less than 10kW is directly started, the inverter can be selected as follows.
In the formula:
I K: locked rotor current (A) at rated voltage and rated frequency;
K g : Allowable overload of frequency converter K g =1.3~1.5
In operation, if the motor current changes irregularly, it is difficult to obtain the operating characteristic curve at this time. In this case, the motor current limit at the maximum torque output can be selected within the rated output current of the inverter.
VI. Calculation of Inverter Capacity at Large Inertia Load Start
Through the inverter overload capacity is usually more than 125%, 60s or 150%, 60s. When an overload capacity exceeding this value is required, the capacity of the inverter must be increased. In this case, the capacity of the inverter is generally calculated as follows:
Seven, the choice of frequency converter when light load motor
The actual load of the motor is smaller than the rated output power of the motor, and it is considered that the inverter capacity commensurate with the actual load is selected, but for a general-purpose inverter, even if the actual load is small, a variable frequency smaller than the inverter selected by the motor rated power is used. The device is not ideal. This is mainly due to the following reasons;
1. When the motor is at no-load, it also flows 30% to 50% of the rated current.
2. The starting current flowing at the start corresponds to the voltage and frequency applied by the motor, and has nothing to do with the load torque. If the inverter has a small capacity and this current exceeds the overcurrent capacity, it often fails to start.
3. If the motor capacity is large, the percentage of motor leakage reactance based on the capacity of the inverter becomes smaller, and the pulsation of the inverter output current increases. Therefore, the overcurrent protection capacity operates and often cannot operate.
4. When a general-purpose inverter for motor is started, its starting torque is usually smaller than that of the power-frequency power supply, and it may not be started depending on the starting torque characteristics of the load. In addition, the torque in the low-speed operating zone tends to be smaller than the rated torque. When the selected inverter and motor cannot meet the required starting torque and low-speed torque of the load, the capacity of the inverter and the motor is also reduced. Need to increase again.
The above describes the capacity calculation and selection method of the inverter in several different situations. When selecting the capacity, it is necessary to make full use of the overload capacity of the inverter, and not to overheat the device when the load is running.
Newcomers may find the formula cumbersome. In fact, many masters can choose the capacity based on experience, and there is no problem in the actual use of the process!
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