How can a frequency converter drive multiple motors, and what precautions should be taken?
Equipment selection
1. Inverter Selection. When selecting an inverter, the first thing to consider is the operating conditions—whether one or more motors need to be started and stopped at any time during inverter operation.
If the motor does not need to start frequently during the operation of the frequency converter, but only needs to stop or remain stationary, then when selecting the frequency converter capacity, simply ensure that the rated power of the frequency converter is greater than the total power of all motors, and then increase the capacity by one level. In this case, the electrical design must adhere to the principle that the contactor should only be switched to engage the variable frequency motor when the frequency converter is stopped; and during the operation of the frequency converter, it is strictly prohibited to start or stop one or more devices individually.
If the motor needs to start and stop frequently during the operation of the frequency converter, special attention is needed when selecting the frequency converter capacity! First, calculate the total power of the motors that may need to be started and stopped frequently. Then, multiply this power by 5 to 7 (during frequency converter operation, starting a motor frequently is equivalent to direct starting, and the starting current is approximately 5 to 7 times the rated current). Finally, add this result to the total power of the motors that do not need to be started and stopped frequently. The sum is the theoretical power of the required frequency converter. If there are many devices that need to be started and stopped, then this power can be used as the selected power of the frequency converter, and there is no need to increase it by one level—because it is rare for multiple motors to start simultaneously. If there are very few devices that need to be started and stopped, then this power needs to be increased by one level before it can be used as the selected power of the frequency converter.
AC contactor selection
For motors that need to be started and stopped at any time, an AC contactor is required. When selecting an AC contactor, the general selection principle can be followed—select one level higher than the motor's rated current.
3. Selection of thermal overload relays
For inverters that operate in a multi-motor configuration, to protect the safety of each motor and the inverter itself, a thermal overload relay must, in principle, be installed in the main circuit of the motor. The selection of the thermal overload relay follows general selection principles—the rated current of the motor must be within the setting range of the thermal overload relay.
Precautions
When one frequency converter drives N motors, excessively long wiring can lead to significant distributed capacitance, resulting in large high-frequency currents and causing issues such as overcurrent, increased leakage current, and reduced current display accuracy. If the wiring is too long, an output filter is necessary. The following explanation uses a Fuji frequency converter as an example.
For motors below 3.7kW, the wiring length should not exceed 50 meters; for motors above 3.7kW, the wiring length should not exceed 100 meters. When driving multiple motors, the total wiring length should be calculated based on the total length of the wiring for all motors.
When there is a thermal relay between the frequency converter and the motor, especially in 400V series inverters, the thermal relay may malfunction even if the wiring is less than 50V. In this case, please use an output filter or reduce the carrier frequency of the frequency converter.
When driving multiple motors, if an output filter is configured, the total length of the motor wiring should not exceed 400 meters.
