The term "upsizing the inverter capacity" mentioned means selecting an inverter with a higher rating than the motor rated power, in contrast to matching the inverter capacity exactly to the motor rated capacity. The main scenarios are summarized as follows:

01 The rated current of the inverter is insufficient

As stated above, the rated current of some inverters is lower than the rated current of motors with the same power rating. In such cases, an inverter of one size up shall be considered according to the motor load conditions.

02 The motor may undergo short-time overload operation

The so-called "short time" for a motor is defined relative to its thermal time constant, ranging from several minutes to dozens of minutes. In contrast, the typical overload capability of an inverter is 150% rated current for 1 minute. Compared with the motor, the inverter offers very limited overload margin. Therefore, if the motor is likely to run under overload for a short period, the inverter capacity needs to be increased.

03 Special requirements for acceleration and deceleration time

The acceleration duration of a motor is a relative parameter related to inertia (GD²).

For instance, an acceleration time of 5 seconds may be too short for a high-inertia load (large GD²), yet excessively long for a low-inertia load (small GD²).

Generally, for loads that start and stop under heavy load conditions, upsizing the inverter capacity shall be considered in the following situations: (1) Fast startup and shutdown are required; (2) Frequent jogging operation is involved.

04 Presence of impact loads

A typical example is a motor connected to the load via a clutch. Normally, the motor runs first, and then the load is driven once the clutch engages. At the moment of clutch engagement, the motor speed drops, slip increases, and the motor current rises sharply, which may trigger overcurrent protection. For such applications, the inverter capacity should be upsized.