Technical Note: Controlling multiple motors with one vfd

In the HVACR world, controlling multiple motors with one drive is a common practice.

When one drive controls multiple motors, the output frequency of the drive will be the same for all motors, and their mechanical speed (rpm) will be the same.

Some essential technical considerations are:

1. All motors must have the same characteristics: the same Hp, Voltage, rpm, and NEMA design. Ideally, they will be made by the same manufacturer.
2. The drive current must be equal to the sum of the individual currents. It usually is suggested to add a 20% design factor.
3. For instance, if each Motor draws 15 Amps, the selected drive must be 3x15x1.2 = 54 Amps as a minimum.
4. The cable distance between the VFD and the Motor is critical for PWM drives. The longer the cable distance, the higher the risk of having dV/dt problems due to reflected waves. An output line reactor will smooth the output waveform by adding extra impedance. Please have a look at eaton.com/dh1 for recommendations.
5. When calculating the total cable distance from the VFD to the motors, add all cable sections (the three cables going to each Motor plus the distance to the drive.
6. To adequately protect and isolate each Motor, individual MPP must be used. The VFD will monitor and protect the total current feeding all motors but cannot see each motor individually.
7. It is important to reconnect the motors to the drive only if the VFD has been turned off. Starting one of the motors with the drive already working might generate an inrush current that can damage the VFD inverter.

One important consideration about the MPP for each motor. Many cooling tower manufacturers already include an MPP in their machines. In this case, the MPPs in the VFD can be saved, provided that the Buyer informs the Vendor when placing the order. There are some savings by doing this. One single set of power cables can be run from the VFD to the machine and then locally, daisy chain the machine MPP. Depending on the installation, this is a good possibility.

I suggest evaluating cable distances and VFD panel location; every case is different.