Ordinary electric motors are designed for constant frequency and constant voltage, and cannot fully adapt to the speed regulation requirements of frequency converters. Therefore, they cannot be used as variable frequency motors.
I. The main impacts of frequency converters on motors are on motor efficiency and temperature rise.
Inverters can generate harmonic voltages and currents of varying degrees during operation, causing motors to run under non-sinusoidal voltage and current conditions. The high-order harmonics within them will increase the stator copper loss, rotor copper loss, iron loss, and additional losses of the motor. The most significant increase is in rotor copper loss. These losses will cause the motor to generate extra heat, reduce efficiency, and decrease output power. The temperature rise of a typical motor will generally increase by 10% to 20%.
II. Insulation Strength Issues of Electric Motors
The carrier frequency of the frequency converter ranges from several thousand to tens of kilohertz, which means that the stator winding of the motor has to withstand a very high rate of voltage rise, which is equivalent to applying a very steep surge voltage to the motor, putting the inter-turn insulation of the motor to a relatively severe test.
III. Harmonic Electromagnetic Noise and Vibration
When a conventional electric motor is powered by a frequency converter, the vibrations and noise caused by electromagnetic, mechanical, and ventilation factors become more complex. The harmonics in the frequency converter interfere with the inherent spatial harmonics of the motor's electromagnetic components, creating various electromagnetic excitation forces and thus increasing noise. Because electric motors have a wide operating frequency range and a large speed variation range, it is difficult for the frequencies of various electromagnetic force waves to avoid the natural vibration frequencies of the motor's structural components.
IV. Cooling issues at low speeds
When the power supply frequency is low, the losses caused by the high-order harmonics in the power supply are large; secondly, when the speed of the variable motor decreases, the cooling air volume decreases proportionally to the cube of the speed, which causes the motor heat to not be dissipated, the temperature rise increases sharply, and it is difficult to achieve constant torque output.
The difference between ordinary motors and variable frequency motors
1. Higher insulation class requirements
Generally, the insulation class of variable frequency motors is F or higher, with enhanced insulation strength to ground and coil insulation, and special consideration should be given to the insulation's ability to withstand impulse voltage.
2. Variable frequency motors have higher requirements for vibration and noise control.
Variable frequency motors must fully consider the rigidity of the motor components and the whole system (Public Account : Pump Manager), and try to increase their natural frequency to avoid resonance with various force waves.
3. Different cooling methods for variable frequency motors
Variable frequency motors generally use forced ventilation cooling, meaning that the main motor cooling fan is driven by an independent motor.
4. Different protection measures are required.
For variable frequency motors with a capacity exceeding 160kW, bearing insulation measures should be adopted. This is mainly because they are prone to magnetic circuit asymmetry, which can generate shaft current. When combined with currents generated by other high-frequency components, the shaft current will increase significantly, leading to bearing damage. Therefore, insulation measures are generally required. For constant power variable frequency motors, when the speed exceeds 3000 rpm, high-temperature resistant special grease should be used to compensate for the temperature rise in the bearings.
5. Different heat dissipation systems
The variable frequency motor cooling fan is powered by an independent power supply to ensure continuous cooling capacity.
