Driving motors with frequency converters has become an irreversible trend. In practice, however, improper matching between the converter and the motor often leads to various issues. When selecting a frequency converter, it is essential to fully understand the load characteristics of the driven equipment.
Industrial production machinery can be classified into three types: constant‑power loads, constant‑torque loads, and fan/pump loads. Each type imposes different requirements on the frequency converter.
Constant‑Power Loads
Machine tool spindles, rolling mills, paper machines, and winders/unwinders in plastic film production lines typically require torque that is roughly inversely proportional to speed – these are constant‑power loads. However, the constant‑power nature applies only within a certain speed range. At very low speeds, mechanical strength limitations cause the load to revert to constant‑torque behaviour.
For motors, constant‑flux speed control gives constant‑torque operation, whereas field‑weakening speed control gives constant‑power operation.
Fan and Pump Loads
For fans, pumps, and oil pumps, torque varies with the square of speed as the impeller rotates, while the required power is proportional to the cube of speed.
When the required air or flow rate decreases, using a frequency converter to adjust speed can achieve significant energy savings. However, because power demand rises rapidly with speed at high speeds, fan and pump loads should not be operated above the mains frequency.
Constant‑Torque Loads
In these loads, the load torque (T<sub>L</sub>) remains constant or essentially constant at any speed. When a frequency converter drives a constant‑torque load, it must provide sufficient torque at low speeds and have adequate overload capacity. If stable low‑speed operation is required, attention must be paid to motor cooling to prevent overheating and burnout.
Below are important technical points to consider when matching a frequency converter with a motor:
Current and temperature rise
When a mains‑rated motor is driven by a frequency converter, the motor current increases by 10–15% and temperature rise increases by about 20–25%.
Harmonic effects
Driving high‑speed motors with a converter produces more high‑order harmonics, which increase the converter’s output current. Therefore, the converter should be selected one size larger than for a standard motor.
Wound‑rotor motors
Compared to standard squirrel‑cage motors, wound‑rotor motors are more prone to overcurrent tripping, so a slightly larger converter capacity is recommended.
Geared motors
When using a frequency converter with a geared motor, the operating range is limited by the lubrication method of the gearbox. Overspeeding may cause oil starvation.
Selection basis
Choose the converter based on the motor’s current rating; the motor’s rated power serves only as a reference.
Power factor and efficiency
The harmonic‑rich output of a converter reduces both the power factor and efficiency of the motor.
Long cable runs
If the converter must be installed far from the motor, the cable’s effect on performance must be considered, and specialised cables may be required. To compensate, the converter should be upsized by one or two steps.
Special operating conditions
High ambient temperatures, frequent switching, high altitudes, etc., derate the converter’s capacity, so upsizing by one step is advisable.
Synchronous motors
Compared with mains operation, a converter driving a synchronous motor reduces the output capacity by 10–20%.
Fluctuating loads
For loads with large torque fluctuations (e.g., compressors, vibrators) or peak loads (e.g., oil pumps), the converter should be chosen from a larger rating class, based on a thorough understanding of the mains‑powered operation.