In some applications, instead of a single motor, two motors driving the same load together are preferred. This approach offers important advantages such as redundancy, modularity and part-load efficiency. But for two motors to share the same load in a balanced way, careful engineering is required. In this article we address parallel operation and load sharing in IE4 motors through speed, slip and correct motor selection when two motors drive one load, from a technical and purchasing perspective. You can explore matched motor pairs and stock options on our homepage.

Parallel operation means two or more motors mechanically driving the same load (through a common shaft, gearbox or conveyor). The most critical issue in this arrangement is that the motors share the load in a balanced way. If one motor is loaded more than the other, both that motor is stressed and the system efficiency drops. The key to balanced sharing is that the motors are compatible in speed, slip and character.

Below we explain step by step why parallel operation is preferred, how motors connected to a common shaft behave, the conditions for balanced load sharing and the control methods with a drive.

Two IE4 electric motors driving a common load in parallel

The Advantages of Parallel Operation

Two IE4 electric motors driving the same load in parallel provide three basic advantages: redundancy, modularity and part-load efficiency. Redundancy means that if one motor fails, the other can continue to carry part of the load; this reduces downtime in critical processes. Modularity offers the flexibility of adding a motor to the system as the load increases or taking a motor out of service when the load decreases.

The part-load efficiency advantage is especially important in variable-load applications. When the load is low, one of the two motors can be taken out of service so that the other runs in a higher (and more efficient) load band. This way, the system avoids the inefficient operation of a single large motor at low load. The high efficiency of the IE4 class makes this advantage even more pronounced.

One Large Motor or Two Small Motors?

Whether to use one large motor or two small motors in an application depends on the application's load profile and criticality level. In constant and continuously loaded applications, a single large motor is usually more economical. But in variable-load, highly critical or staged-capacity applications, a two-motor parallel solution is more advantageous. This decision should be made by evaluating initial investment, efficiency and operational continuity together.

Motors Connected to a Common Shaft Turn at the Same Speed

Two motors mechanically connected to a common shaft must physically turn at the same speed, because they drive the same shaft. But there is a critical point here: asynchronous motors show slip under load and, even though they turn at the same speed, they can be loaded differently. Because of the relationship between slip and load, the motor with lower slip (that is, the one that slows down less at the same load) tends to be loaded more.

We can explain this as follows: the common shaft turns both motors at the same speed. At this fixed speed, the torque-speed curve of the low-slip motor corresponds to a higher torque value; therefore this motor carries the larger part of the load. The high-slip motor produces lower torque at the same speed and is loaded less. This leads to unbalanced load sharing if the motors are not sufficiently similar to each other. To see the slip and speed relationship in more detail, our asynchronous motor pole selection content will be useful.

The Role of Slip in Load Sharing

Slip shows how far below the synchronous speed an asynchronous motor drops under load. In parallel operation, slip is the determinant of load sharing. If two motors have the same slip character, they produce the same torque at the same speed and share the load equally. If their slip characters differ, the sharing becomes unbalanced. For this reason, matching the slip values of motors that will run in parallel is the basic condition for balanced sharing.

Motors Must Be Matched for Balanced Sharing

For balanced load sharing, motors that will run in parallel must be as similar to each other as possible. Ideally the motors should have the same series, the same power (kW), the same speed and the same full-load slip. Choosing two motors of the same series from the same manufacturer with the same characteristics ensures that the slip and torque characters are compatible; this is the safest way to share the load equally.

  • Same series and same manufacturer: torque-speed and slip characters are compatible.
  • Same power (kW): the motors have equal capacity.
  • Same speed and pole count: they run at the same synchronous speed.
  • Same full-load slip: they produce the same torque at the same speed.
  • Same efficiency class (IE4): the overall system efficiency stays high and balanced.

Running motors of different power or different slip in parallel leads to one motor being overloaded and the other underloaded. This means both efficiency loss and early wear of the overloaded motor. For this reason, motor matching in parallel operation is the most critical step of the design.

Matched IE4 motor pair common shaft and coupling connection

Control with a Drive: Master-Follower and Droop

The load sharing of motors running in parallel can be controlled much more precisely with a variable frequency drive. There are two common methods: master-follower and droop control. In the master-follower method, one motor (master) sets the speed reference and the other motor (follower) follows the master's torque. This method balances the load very precisely and performs well during dynamic load changes.

In droop control, each drive slightly reduces the speed reference as the load increases; this artificial slip makes the motors share the load in a balanced way by themselves. The droop method is simpler because it requires no communication between motors, but it is not as precise as master-follower. Which method to choose depends on the application's precision need and the drive infrastructure.

Starting Sequence

If parallel motors start at the same time, this leads to a high current draw from the grid. For this reason, the motors are usually brought into service sequentially (in stages); first one motor starts, then the other comes into service. Sequential starting both limits the starting current and allows load sharing to be established in a controlled way. In drive systems, this sequence is easily managed by the control software, which is why a drive solution is recommended in applications requiring precision.

Mechanical Connection Types

Parallel motors can share the load with different mechanical arrangements. The most common arrangement is two motors connected to a common shaft or gearbox with couplings; in this case both motors physically turn at the same speed. Another arrangement is two motors driving the same conveyor or machine through separate gearboxes; here the motors are mechanically connected to each other through the machine. A third arrangement is each motor driving the same load (for example, from the two ends of a long conveyor) from a separate drive point.

In every mechanical arrangement, the motors must be compatible in character for the load sharing to be balanced. The more rigid the mechanical connection, the stronger the obligation for the motors to turn at the same speed, and the more critical the matching of slip character becomes. In more flexible connections, small speed differences can be tolerated. For this reason, mechanical design and motor matching must be considered together.

Coupling and Alignment

In parallel motors connected to a common shaft, coupling selection and alignment are important. The smallest alignment error between motors creates extra vibration and bearing load. Flexible couplings reduce this load by tolerating small misalignments. Correct alignment and a suitable coupling selection support both the balanced sharing and the long life of motors running in parallel. Precise alignment during commissioning is a step that should not be skipped in parallel systems.

Monitoring Load Sharing

In a parallel system, to verify that the load sharing is truly balanced, the motors' currents should be monitored. The current drawn by each motor is measured with a clamp meter or the drive screen and compared. If the currents of the two motors are close to each other, the load is being shared in a balanced way. If one motor's current is noticeably higher, that motor is being loaded more and the reason should be investigated.

The cause of unbalanced sharing is usually a difference in the motors' slip character, an alignment error or a problem in the control setting. Regular current monitoring catches the imbalance early and prevents the wear of the overloaded motor. In drive systems, this monitoring can be done automatically by the control software, and the sharing setting is corrected when needed. Monitoring load sharing is the assurance of the healthy operation of the parallel system.

Part-Load Efficiency and Energy Saving

One of the strongest economic advantages of parallel operation is the efficiency it provides at part load. In a variable-load application, a single large motor runs inefficiently at low load, because when the load falls below 50 percent both efficiency and power factor drop. In a two-motor parallel system, when the load is low, one motor is taken out of service and the other runs in a higher, more efficient load band. This staged operation noticeably increases the average efficiency.

For example, in a plant where demand varies over a wide range during the day, two medium-power IE4 motors provide high efficiency over a much wider load range than a single large motor. At low demand one motor runs, at high demand two motors run. This flexibility makes the already high efficiency of the IE4 class even more effective under real operating conditions. To examine the effect of power and speed selection on efficiency in more detail, our article on understanding HP-kW motor power is a useful resource.

Staged Capacity Increase

Parallel operation is also advantageous for plants where a capacity increase is expected in the future. A system that initially runs with a single motor can increase its capacity by adding a second motor when demand rises. This modular approach makes staged growth according to need possible, instead of investing in a large motor from the start. So the initial investment is optimised and the system adapts flexibly to growing demand.

Frequently Asked Questions

Why should two motors running in parallel have the same characteristics?

Two motors connected to a common shaft turn at the same speed, but if their slip characters differ, they are loaded differently. The low-slip motor is loaded more and overstressed. For balanced sharing, the motors must have the same series, the same power, the same speed and the same full-load slip. Motors with the same characteristics share the load equally, both preserving efficiency and preventing overloading.

Is parallel operation possible without a drive?

It is possible; two matched motors can run in parallel without a drive by being connected to a common shaft. But in this case, load sharing depends entirely on the motors' slip character; if the motors are not sufficiently similar, the sharing becomes unbalanced. For more precise and controlled sharing, master-follower or droop control with a drive is preferred. The drive also manages the starting sequence and dynamic load changes.

What is the biggest advantage of parallel operation?

The biggest advantages of parallel operation are redundancy, modularity and part-load efficiency. When one motor fails, the other can carry part of the load; as the load increases, a motor can be added to the system; when the load drops, one motor can be taken out of service so that the other runs in its efficient band. The high efficiency of the IE4 class makes the part-load advantage in particular even more valuable.