I. Definitions of Torque and Speed
Definition of torque
According to the definition of torque in GB/T2900.61-2008 Electrical Engineering Terminology (Physical and Chemical), torque is the sum of the moments of a set of forces whose resultant force is zero. Torque is the basic load form of transmission shafts in various working machines and is closely related to factors such as the working capacity, energy consumption, efficiency, service life, and safety performance of power machinery.
In the electrical engineering of the FF157 reducer , torque, and moment are the same. Under the condition of fixed power, torque is inversely proportional to engine speed; the faster the speed, the smaller the torque, and vice versa. In motors, they all refer to the driving "moment" generated by the rotor windings that can drive mechanical loads. "Moment" is the product of the applied force and the distance between the fulcrum and the direction of the force; its SI unit is Newton-meter (Nm).
Definition of rotational speed
GB/T2900.36-2003 Electrical Engineering Terminology - Electric Traction defines rotational speed as the speed at which a motor rotates. It is represented by the symbol "n"; its international standard unit is rps (revolutions per second) or rpm (revolutions per minute). When the unit is r/s, it is numerically equal to the frequency, i.e., n = f = 1/T, where T is the period of the circular motion. The linear velocity corresponding to a point on the circumference is: v = 2π*R*n, where R is the radius of rotation corresponding to that point.
II. Formulas for Calculating Motor Speed and Torque
Gear reducer motor speed formula: n=60f/P
n = rotational speed, f = power supply frequency, P = number of pole pairs
Motor torque formula: T=9550P/n
T is torque, in N·m; P is output power, in kW; n is motor speed, in r/min.
For detailed derivation, please refer to: Derivation and Memorization Methods of Torque and Power Calculation Formulas.
The meanings of the two parameters:
1kg = 9.8N: The gravitational pull of the Earth on a 1-kilogram object is 9.8 Newtons.
9.8 N·m: The force acting perpendicularly at a position 1 meter away from the center of the grinding disc is 9.8 N.
III. How to rationally select an electric motor using motor power and torque calculation formulas
We know that selecting an F157 reducer motor that is too large or too small will cause malfunctions. If the motor power is too small, it will result in a "small horse pulling a large cart" phenomenon, causing the motor to be overloaded for a long time, leading to insulation damage due to overheating, and even burnout of the motor. If the motor power is too large, it will result in a "large horse pulling a small cart" phenomenon, where its output mechanical power cannot be fully utilized, resulting in low power factor and efficiency. This is not only detrimental to users and the power grid but also leads to energy waste.
Therefore, the selection of motor power should be based on the power required by the production machinery, and the motor should be operated under rated load as much as possible. To correctly select the motor power, the following calculations or comparisons must be performed.
The required power of the motor is calculated using the following formula:
P=F*V/1000
P = calculated power (kW), F = required pulling force (N), working machine linear speed (m/s)
For continuous operation under constant load, the required motor power can be calculated using the following formula:
P=P/n1n2
In the formula, n1 is the efficiency of the production machinery; n2 is the efficiency of the electric motor, i.e., the transmission efficiency.
When selecting an electric motor, the rated power of the selected motor should be equal to or slightly greater than the calculated power.
