Frequency converters can achieve one-to-two or even one-to-many operation. In practical applications, one-to-many frequency converters are often used in situations where the motor power is not large (within 7.5KW ), but there are many motors—such as frequency conversion drives in production lines (where there are many small-power motors), drive motors in roller kilns, etc.
Case Study: A factory has many kilns with this type of drive system—one 11kW Fuji inverter drives 15 0.55kW cycloidal pinwheel geared motors. Moreover, these motors may start and stop at any time—motor protection switches are installed locally on each motor to allow for immediate start and stop for maintenance of the chain and other mechanical transmission components. The average distance between the motors and the inverter is approximately 30 meters. This system has been operating normally for many years without any abnormalities.
While a one-to-one inverter configuration is reliable, a single kiln might have a dozen or so motors, so how many inverters would be needed for several kilns? Wouldn't the control room become an inverter warehouse? Furthermore, production costs, maintenance workload, noise levels, and temperature rise would all become issues. Therefore, using a one-to-many inverter configuration is more cost-effective, reduces failure rates, and simplifies operation and maintenance.
So how does a frequency converter achieve a one-to-many function? The following example only shows the primary electrical schematic. The secondary electrical schematic needs to be designed according to the control requirements, and will not be elaborated here.
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 or disengage the variable frequency motor when the frequency converter is stopped; and during frequency converter operation, 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, motors that start frequently are essentially directly started, 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.
2. Selection of AC contactors
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 motor's rated current must be within the relay's setting range.
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 shall not exceed 50 meters; for motors above 3.7kW , the wiring length shall not exceed 100 meters. When driving multiple motors, the wiring length shall be calculated based on the total length of wiring for each motor.
When there is a thermal relay between the frequency converter and the motor, especially in 400V series inverters, the thermal relay may fail to operate 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.
