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.
The main differences between variable frequency motors and ordinary motors are as follows:
First, ordinary motors can only operate near the power frequency for extended periods, while variable frequency motors can operate for long periods at frequencies significantly higher or lower than the power frequency. For example, the power frequency in our country is 50Hz. If an ordinary motor operates at 5Hz for a long time, it will quickly malfunction or even be damaged. The advent of variable frequency motors has solved this shortcoming of ordinary motors.
Secondly, the cooling systems of ordinary motors and variable frequency motors are different. The cooling system of an ordinary motor is closely related to its speed; that is, the faster the motor speed, the better the cooling system, and the slower the motor speed, the less effective the cooling. Variable frequency motors, however, do not have this problem.
Adding a frequency converter to a regular motor can enable variable frequency operation, but it is not a true variable frequency motor. If it operates in a non-power frequency state for a long time, it may damage the motor.
01 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 operate 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%-20%.
02 Insulation strength problem of electric motor
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.
03 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 these electromagnetic force waves to avoid the natural vibration frequencies of the motor's structural components.
04 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.
05. In response to the above situation, the variable frequency motor adopts the following design.
Minimize stator and rotor resistance as much as possible to reduce fundamental copper loss in order to compensate for the increased copper loss caused by higher harmonics.
The main magnetic field is designed to be unsaturated for two reasons: first, higher harmonics will deepen magnetic circuit saturation; second, the output voltage of the frequency converter can be appropriately increased at low frequencies to improve output torque.
The structural design mainly focuses on improving the insulation level; fully considering the vibration and noise issues of the motor; and adopting forced ventilation cooling, that is, the main motor cooling fan adopts an independent motor drive mode, and the role of the forced cooling fan is to ensure the cooling of the motor at low speeds.
The distributed capacitance of the coil in a variable frequency motor is smaller, and the resistance of the silicon steel sheet is larger. This reduces the impact of high-frequency pulses on the motor and improves the inductance filtering effect.
Ordinary motors, i.e., power frequency motors, only need to consider the starting process and the working conditions at one point of power frequency (WeChat Official Account: Electromechanical Networks) before designing the motor; while variable frequency motors need to consider the starting process and the working conditions at all points within the variable frequency range before designing the motor.
To accommodate the large number of harmonics in the sinusoidal AC current simulated by the PWM width modulation waveform output by the frequency converter, a specially made variable frequency motor is essentially a reactor plus a regular motor.
01. Structural differences 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, 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.
02 Design differences between conventional motors and variable frequency motors
1. Electromagnetic Design
For ordinary asynchronous motors, the main performance parameters considered during design are overload capacity, starting performance, efficiency, and power factor. However, for variable frequency motors, since the critical slip is inversely proportional to the power supply frequency, they can start directly when the critical slip is close to 1. Therefore, overload capacity and starting performance are no longer important considerations. The key issue to address is how to improve the motor's adaptability to non-sinusoidal power supplies.
2. Structural Design
In structural design, the main considerations are the impact of non-sinusoidal power supply characteristics on the insulation structure, vibration, noise, and cooling methods of variable frequency motors.
