82 Frequency Converters and Motors 3.1.3 Running in Current Limit As seen, the relationship between motor shaft torque and motor current indicates that if motor current can be controlled, then the torque is also under control. If an application temporarily needs torque up to maximum it is essential that the FC is designed for continuous operation current up to the current limit, and not exceed it or trip. There are diff erent strategies for designing the FC to run in current limit situations. The most typical strategy is the fact that torque will be reduced when the speed is reduced.
But as we shall see later there can be applications where this strategy cannot be utilised and can even cause bigger problems. 3.2 Compensations It used to be diffi cult to tune a FC to a motor because some of the compensation functions, such as “start voltage”, “start compensation” and “slip compensation”, are diffi cult to relate to practice. These compensations are required because motor characteristics are not linear. For example an asynchronous motor requires a greater current at low speed to accomplish both magnetising current and torque-producing current for the motor. The built-in compensation parameters ensure optimum magnetisation and hence maximum torque:
• During start • At low speeds • In the range up to the rated speed of the motor In the latest generation of FC’s, the device automatically sets the necessary compensation parameters once the motor rating details of the motor have been programmed into the FC. These details include voltage, frequency, current and speed. This applies to approximately 80% of standard applications such as conveyors and centrifugal pumps. Normally, these compensation settings can also be changed manually for fi ne tuning applications such as hoisting or positive displacement pumps if required.
3.2.1 Load-independent Start Compensations Increase the output voltage in the lower speed range by manually setting an additional voltage, often called start voltage.