In the discussion about the relationship between motor speed and power, different situations will have different relationships.
Some motors with the same power size have a faster speed and some have a slower speed.
The speed of the motor depends on the number of pole pairs of the motor, for example, a 2-pole motor has a synchronous speed of 3000 rpm and a 10-pole ac motor has a synchronous speed of 600 rpm.
What is the relationship between the speed and power of industrial frequency electric motors?
In the industrial frequency motor, the ac motors with the same center height can be designed into different number of poles according to the demand of actual working condition.
Different speed can correspond to different power; under the condition of the same center height, generally the slower the speed corresponds to the smaller the power.
if two electric motors with the same power, the less pole ac motor corresponds to the small dynamic torque, and the multi-pole slow electric motor corresponds to the large dynamic torque.
What is the relationship between the motor’s speed and power of the variable frequency drive motor?
As we discussed that the same variable frequency drives motor is in a constant torque state at low speed, that is, the dynamic torque of the ac motor remains unchanged when the ac motors frequency changes.
So that the electric motor is a process of gradually increasing power as the speed becomes faster (the power of the ac induction motor is the product of speed control and torque), and in accordance with the relationship between current and torque, the variable speed drive motor basically maintains a stable state in the process of constant torque operation.
For this characteristics, the inverter motor must take forced ventilation structure to ensure that its temperature rise in the low speed state meets the requirements.
When the frequency of the electric motor reaches a certain value, its power will reach its rated power, and if it runs according to the constant torque, the problem of overload is bound to occur, so the electric motor must run according to the constant power when it exceeds the working frequency frequency.
At this time, the dynamic torque is decreasing in the process of speeding up the electric motor, and the corresponding current of the electric motor is also decreasing.
Comprehensive above, different speed motors can be set to different power, only the corresponding electric motor specifications are different; and the same power, specifications of basically the same size of the motor, high speed is small torque, low speed is large torque.
And for the same inverter motor, low frequency state constant torque operation, speed and power is positively correlated, high frequency operation according to constant power, the higher the speed the smaller the torque.
V/f variable frequency control mode, how does the ac motor operate?
V/f inverter, by the changing characteristics of voltage and frequency, directs the performance of the electrical motors to change or remain at a specific value.
In the actual setting, the voltage and frequency are adjusted according to a specific ratio to achieve constant torque at low motor speed control state and constant power at high full speed state.
V/f control inverter is to control the output voltage of the inverter while changing the frequency, so that the flux of the electric motor is kept certain, and the efficiency and power factor of the electric motors do not drop in a wider speed regulation range.
The ratio of voltage to frequency change is set according to the actual characteristics of the electric motors, and this setting will be pre-set in the storage reserve device of the inverter, which is selected by means of a transfer switch or a scale dial.
For the principle of setting the ratio of voltage to frequency, the general direction will not change the voltage decreases proportionally when the frequency decreases and increases proportionally when the frequency increases.
But in the actual change process, the DC resistance is constant, while the AC impedance will change with the change of frequency.
When the frequency decreases, if the voltage is reduced completely proportionally, because the AC impedance becomes smaller while the DC resistance remains unchanged, it will cause the torque generated at low speed to have the inevitable tendency to decrease.
Therefore, given V/f at low frequency, the output voltage should be increased a bit to obtain the starting torque to meet the requirements, this compensation is to enhance the starting performance.
Three-phase asynchronous inverter speed control motor, one of the more widely used products, will be marked in the motor’s nameplate information motor and operating characteristics
such as: 5-50Hz for constant torque characteristics, 50-100Hz for constant power characteristics.
As a frequency converter variable speed control method, V / f control is relatively simple, The high efficiency motor are mostly used for general-purpose inverter, fans, pumps, energy-saving operation of machinery and production line table drive, etc.
The synchronous speed of an asynchronous motor is determined by the frequency of the power supply and the number of poles of the electric motors.
When the frequency of the electric motor power supply is changed, the synchronous speed of the electric motor changes with it.
When the ac motor is running under load, the rotor speed of the induction motor is slightly lower than the synchronous speed of the electric motor.
Keeping VFD controls are the most basic control method of asynchronous motor frequency control, which controls the output voltage of the inverter while controlling the motor power frequency change, and makes the ratio of the two V/f constant, so that the flux of the electric motor is basically kept constant.
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