Can one frequency converter control two motors?
Theoretically, frequency converters can support “one-to-two” or even “one-to-many” configurations, but careful design and operation are required in practical applications. Here is a detailed explanation:
When can “one-to-two” be implemented?
Under the following conditions, consideration may be given:
- The motor power is the same or extremely close: The rated power, current, speed, and other parameters of the two motors must be identical.
- The motor operating conditions are fully synchronized:
It must be started and stopped simultaneously.
The load characteristics are identical during operation (e.g., both are fans or both are pumps, with consistent load curves).
The same rotational speed (i.e., the same output frequency) is required.
The requirements for acceleration and deceleration times are identical.
- Motor and Inverter Capacity Matching:
The rated power of the frequency converter should be greater than the sum of the rated powers of the two motors. It is generally recommended to select a frequency converter with one or two power levels higher than the total power to ensure sufficient starting and overload current.
The rated output current of the frequency converter must be greater than the sum of the rated currents of the two motors.
How to wire in a one-to-many configuration? (Common solutions)
- Direct parallel connection (simplest but highest risk):
Connect the two motors directly in parallel to the U, V, and W output terminals of the frequency converter.
Disadvantages: If one motor fails (e.g., short circuit or overload), it will directly affect the other motor and the frequency converter. Additionally, individual motor protection (e.g., thermal protection) cannot be implemented.
- By connecting the output contactor or switch in parallel (slightly better):
A contactor is connected in series in the circuit of each motor to enable individual start-stop operation. However, switching the contactor is strictly prohibited when the frequency converter is outputting power! This can cause overcurrent due to sudden current changes, which may easily damage the frequency converter. The contactor must only be switched after the frequency converter has stopped outputting.
- One-to-two dedicated solution (Recommended):
Install an independent output reactor for each motor to balance current, suppress harmonics, and reduce mutual interference.
Equip each motor with an independent thermal relay or motor protector, and connect the protection signal to the inverter’s control circuit to achieve overload protection for individual motors.
When configuring the inverter parameters, the motor protection type must be set to “External Thermal Protection,” and the internal electronic thermal protection function should be disabled (or its threshold adjusted to the sum of both motors).
Major Risks and Drawbacks
- Unable to operate independently: Cannot run two motors at different speeds or at different times.
- Protection Challenges: The electronic thermal protection function inside the frequency converter is designed for a single motor model and cannot detect the actual temperature rise of each motor. If one motor is overloaded while another is lightly loaded, the frequency converter may fail to promptly protect the overloaded motor, leading to its burnout.
- Current imbalance risk: Even if two motors are of the same model, their characteristics may have slight differences. Additionally, factors such as varying cable lengths can lead to uneven current distribution between the two motors, resulting in one motor drawing more current than the other.
- Wide impact of failures: Any short circuit or malfunction in a motor or cable can cause the entire system to shut down or even damage the frequency converter.
- High starting current: Starting two motors simultaneously results in a cumulative demand for total starting current and torque, placing greater stress on the frequency converter and power grid compared to operating a single motor.
In what situations should “one-to-two” be avoided?
The power difference between the two motors is significant.
Operating conditions vary (e.g., different startup/shutdown times, varying speed requirements, and load fluctuations).
For applications requiring high reliability.
The motor is far from the frequency converter, resulting in significant cable length differences.
No professional electrical personnel were involved in the design and commissioning.
A better alternative
“1-to-1” solution: Each motor is equipped with an independent frequency converter. This is the most ideal, reliable, and flexible control solution. Although the initial cost is high, it offers easy maintenance and strong reliability.
- Use a dedicated multi-pump controller: For parallel applications such as water pumps and fans, a specialized frequency converter control cabinet supporting “one-to-many” operation can be employed. It features built-in logic control to sequentially soft-start each pump and utilize a single frequency converter for speed regulation, while the remaining operate at standard frequency. This represents a more intelligent and mature solution.
suggestion:
For most applications, especially those requiring independent control or high reliability, it is strongly recommended to use the “one-to-one” configuration.
If “one-to-two” is indeed considered, it must be ensured that:
The motor parameters and operating conditions are completely identical.
Select a frequency converter with sufficient power (it is recommended to choose one with a higher capacity).
Add an output reactor and an external independent thermal protection device.
Installation and parameter settings shall be performed by professionals.
