Introduction
The application of frequency converters is very broad, whether in household appliances or in industrial equipment, their presence is evident. However, the specific roles and functions of frequency converters may not be very well understood by everyone.
Below, the editor will explain to the customers what constant power and constant torque mean in the inverter of a motor, what are the differences between the two, and how to adjust the speed for constant torque and constant power?
1. The meaning of constant power and constant torque:
Constant power refers to the motor output power remaining basically unchanged; as the speed increases, the torque becomes smaller.
Constant torque means that the motor's output torque remains essentially unchanged, while the output power changes with the speed.
Both DC motors and variable frequency motors have these two characteristics, where constant torque is the characteristic at low speeds (below the base frequency), and constant power is the characteristic at high speeds (above the base frequency).
The constant torque of a variable frequency motor means that the torque output within a certain frequency range of the variable frequency motor is constant, generally referring to below 50Hz as constant torque, and after exceeding 50Hz it becomes constant power; as the frequency of the inverter increases, when exceeding the motor's rated power, the motor's torque correspondingly decreases.
2. The difference between constant power and constant torque:
Constant power is generally applied to small loads, to protect the output power of the inverter during small load applications, this feature is scientifically applied in actual use.
For example, some short-term workloads such as electric drills, scraper conveyors, etc., have characteristics similar to constant power at startup, with high static friction but low speed, and after stabilization, they generally work in a constant torque mode, with the constant power performance of electric drills being more obvious.
This can be understood as seldom used constant power. The output F/V characteristics of the inverter, that is, the load torque output mode, are basically divided into two types: a constant relationship (corresponding to constant torque mode) and an exponential relationship (corresponding to the fan-type load mode), not divided into constant power and constant torque.
The definition of constant torque mode is that the magnitude of its torque (determined by the output current) depends on the inherent characteristics of the load, with little relation to its output frequency, and an exponential relationship (the exponent is generally greater than 1) has its torque related to the frequency in a squared, cubic, etc., relationship, or even fractional, indicating that when the frequency is high, that is, the load speed is fast, its torque correspondingly increases, such as for fans, pumps, etc.
3. How are constant power and constant torque achieved in variable frequency speed regulation?
Constant torque speed regulation: Keep the magnetic flux at a constant value, V/F=Q (magnetic flux) is a constant. Since the load torque is a fixed value, the motor's output torque value is also required to be a fixed value, looking at the formula: T=KIQ, now Q is constant, so the motor output torque is proportional to I, and the value of Q can be calculated through the nameplate V (rated voltage)/50HZ.
Therefore, when Q is determined and constant, and the rated current of the coil (regardless of whether there is a load, the maximum current that can pass through) is determined, the maximum torque that the motor can output can also be determined (that is, the maximum torque of the constant load that the motor can drive), and the motor's overcurrent capacity reflects the motor's overload (torque) capacity.
In constant torque speed regulation, only a certain frequency of voltage modulated by the inverter is needed to be supplied to the motor.
Here it refers to the magnetic flux being a fixed value, and the load torque is also a fixed value; because N and T are fixed values, so the input power P is also fixed. If F increases, the speed N increases, then the power P becomes larger, because the torque T will not increase due to an increase in speed. (This is also known as constant torque load, such as conveyor belts.
The characteristic of constant torque load is that the load torque is unrelated to speed, and the torque always remains constant or basically constant at any speed.
Applications include conveyor belts, mixers, extruders, and potential loads such as cranes and hoists.) Constant power speed regulation: The rated speed value is obtained when the motor is idling. The larger this value, the smaller the output torque will be, which is a characteristic of constant power speed regulation (formula T=9550*P/N rated speed).
When the output is at maximum power, the N value increases and F>50HZ, it is important to note whether the T value is decreasing, to avoid accidents caused by the T value
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