An electric motor, also known as a motor, is an extremely common component in modern industry and daily life, and is the primary device for converting electrical energy into mechanical energy. Automobiles, high-speed trains, airplanes, fans, robots, automatic doors, water pumps, hard drives, and even our most ubiquitous mobile phones all contain electric motors.
Many people new to electric motors or just learning about motor drives may find the subject difficult to understand, even daunting when faced with related courses, earning it the nickname "credit killer." The following piecemeal sharing will help beginners quickly grasp the principles of asynchronous motors.
★ The principle of an electric motor: The principle of an electric motor is very simple. Simply put, it is a device that uses electrical energy to generate a rotating magnetic field in a coil, which in turn drives the rotor to rotate. As anyone who has studied the law of electromagnetic induction knows, a current-carrying coil will rotate in a magnetic field. This is the basic principle of an electric motor, which is knowledge from junior high school physics.
★ Motor Structure: Anyone who has disassembled a motor knows that it mainly consists of two parts: the stationary stator and the rotating rotor, as detailed below:
1. Stator (stationary part)
Stator core: An important part of the motor's magnetic circuit, on which the stator windings are placed;
Stator winding: This is the coil, the electrical part of the electric motor, connected to the power supply, used to generate a rotating magnetic field;
The frame serves to secure the stator core and motor end covers, and also provides protection and heat dissipation.
2. Rotor (rotating part)
Rotor core: An important part of the motor's magnetic circuit, the rotor windings are placed in the core slots;
Rotor windings: Cut the rotating magnetic field of the stator to generate induced electromotive force and current, and form electromagnetic torque, thereby causing the motor to rotate;
★Several calculation formulas for electric motors:
1. Electromagnetic related
1) Formula for induced electromotive force of electric motor: E=4.44*f*N*Φ, where E is the electromotive force of the coil, f is the frequency, S is the cross-sectional area of the conductor (such as iron core) wrapped around the coil, N is the number of turns, and Φ is the magnetic flux.
We won't delve into how the formula is derived; our main focus is on how to utilize it. Induced electromotive force (EMF) is the essence of electromagnetic induction. When a conductor with induced EMF is closed, an induced current is generated. This induced current experiences an Ampere force in a magnetic field, producing a magnetic moment that drives the coil to rotate.
As can be seen from the formula above, the magnitude of the electromotive force is directly proportional to the power supply frequency, the number of coil turns, and the magnetic flux.
The formula for calculating magnetic flux is Φ=B*S*COSθ. When a plane with area S is perpendicular to the direction of the magnetic field, the angle θ is 0, and COSθ is equal to 1. The formula then becomes Φ=B*S.
Combining the two formulas above, we can obtain the formula for calculating the magnetic flux intensity of the motor as: B=E/(4.44*f*N*S).
2) Another formula is Ampere's force formula. To know the force on a coil, we need this formula: F = I * L * B * sinα, where I is the current intensity, L is the conductor length, B is the magnetic field strength, and α is the angle between the current direction and the magnetic field direction. When the conductor is perpendicular to the magnetic field, the formula becomes F = I * L * B (for an N-turn coil, the magnetic flux B is the total magnetic flux of the N-turn coil, and it is not necessary to multiply by N).
Knowing the forces acting on the system, we know the torque, which equals the torque multiplied by the radius of action: T = r * F = r * I * B * L (vector product). Using the formulas power = force * velocity (P = F * V) and linear velocity V = 2πR * rotational speed per second (n seconds), we can establish a relationship with power, resulting in formula number 3 below. However, it's important to note that this uses the actual output torque, so the calculated power is the output power.
2. The formula for calculating the speed of an AC asynchronous motor is: n = 60f/P. This is very simple. The speed is directly proportional to the power supply frequency and inversely proportional to the number of pole pairs (remember, one pair). Just apply the formula directly. However, this formula actually calculates the synchronous speed (rotating magnetic field speed). The actual speed of an asynchronous motor will be slightly lower than the synchronous speed. Therefore, we often see that a 4-pole motor is generally around 1400 RPM, but not reaching 1500 RPM.
3. The relationship between motor torque, wattmeter speed, and motor speed: T = 9550P/n (P is motor power, n is motor speed). This can be derived from point 1 above, but we don't need to learn the derivation; just remembering the formula is enough. However, please note again that the power P in the formula is not the input power, but the output power. Due to motor losses, the input power is not equal to the output power. However, textbooks often idealize this and equate input power with output power.
4. Motor power (input power):
1) Formula for calculating the power of a single-phase motor: P=U*I*cosφ. If the power factor is 0.8, the voltage is 220V, and the current is 2A, then the power P=0.22×2×0.8=0.352KW.
2) Three-phase motor power calculation formula: P=1.732*U*I*cosφ (cosφ is the power factor, U is the load line voltage, and I is the load line current). However, U and I in this case are related to the motor connection method. In a star connection, since the common terminals of the three coils with voltages 120° apart are connected together, forming a zero point, the voltage applied to the load coil is actually the phase voltage; while in a delta connection, each coil is connected to a power line at both ends, so the voltage applied to the load coil is the line voltage. If we are using the commonly used 3-phase 380V voltage, the coil voltage is 220V in a star connection and 380V in a delta connection. P=U*I=U^2/R, so the power of the delta connection is three times that of the star connection. This is why high-power motors use star-delta reduced voltage starting.
Once you've mastered and thoroughly understood the formulas above, you'll no longer be confused about the principles of motors, nor will you be afraid of studying high-stakes courses like motor drives.
★Other components of the motor
1) Fan: Usually installed at the tail of the motor to dissipate heat from the motor;
2) Junction box: Used to connect to a power source, such as an AC three-phase asynchronous motor; it can also be connected in a star or delta configuration as needed.
3) Bearings: Connect the rotating and stationary parts of the motor;
4. End covers: The front and rear covers on the outside of the motor, which serve a supporting function.
