When the frequency reference command is higher than the upper frequency limit, the operating frequency will be clamped to the upper limit. When the frequency reference command is lower than the lower frequency limit, the operating frequency will be clamped to the lower limit. Within the range from lower limit to upper limit, the frequency rises to the set value or upper limit following the configured acceleration time. This mainly prevents equipment damage caused by sudden inrush current and ensures stable output of the machine.

In closed-loop vector control, when operating in torque mode: The inverter operating frequency may exceed the upper frequency limit. However, with a large torque reference, the motor can run steadily at the upper frequency limit. When the motor is loaded and driven by the load (motor enters generating mode), the actual speed may go beyond the set upper frequency. Most inverters have multiple frequency protection limits. In torque mode, frequency is not the controlled target; the inverter regulates torque instead.

Maximum frequency setting for a 50Hz motor controlled by inverter

The inverter can be configured up to twice the rated frequency. But we need to consider the motor mechanical characteristics and operating duration. Normally it is set to 60Hz (around 1800 rpm). Although the inverter software can be adjusted as high as 135Hz, the motor hardware cannot withstand such speed.

A 50Hz rated motor can be forced to run at 100Hz, but this will cause severe wear on the motor. For such operation, variable-frequency motors are recommended. Insufficient torque usually comes from undersized inverter capacity. It is common practice to select an inverter with higher rated current, which also extends inverter service life.

The base frequency of a variable-frequency motor is 33.3Hz. Can we set the inverter base frequency to 50Hz?Yes. If the base frequency is set to 50Hz, it will be equivalent to a 6-pole motor characteristic.

For a 50Hz rated 6-pole motor, what is the minimum adjustable frequency via inverter? Why?

1. For conventional V/F inverters:

Operation below 5Hz may become unstable, especially under heavy load, and hard starting may occur. Continuous long-time operation of a standard motor at low frequency is not allowed. Standard motors rely on shaft-mounted fans for cooling; cooling capacity drops sharply at low speed and the motor may burn out. A variable-frequency motor with separate forced cooling is recommended in this case.

1. For vector control and higher-performance inverters:

Stable operation at 0.1Hz is achievable if the load is not extremely heavy. Vector inverters deliver high low-speed torque to meet torque requirements for most applications.