Inverter-driven motors above 90 kW: dedicated bearings and end shields | NDR

Inverter-driven motors above 90 kW: why dedicated bearings and end shields matter

The inverter is now standard practice even at high power ratings. It regulates speed, cuts energy consumption and makes starts smoother. However, above 90 kW a standard motor connected to an inverter can develop a problem that is invisible at first: bearing currents. This is why large inverter-driven motors need to be selected with a few specific precautions.

What happens inside the bearings of an inverter-driven motor

An inverter does not feed the motor with a clean sine wave. It feeds it with very fast voltage pulses. As a result, a voltage builds up between the shaft and the frame.

When this voltage exceeds the resistance of the lubricant film, it discharges through the bearing. Each single discharge is tiny. But it repeats thousands of times per second, for every hour of operation.

Over time, the bearing raceways develop regular grooves, similar to the ridges of a washboard. The grease also degrades faster. The motor first becomes noisy, then starts to vibrate, and eventually the bearing fails.

Why the risk increases above 90 kW

On small motors the phenomenon exists, but it usually stays within limits. As frame size grows, however, the shaft gets longer and the rotor and winding get larger. Both the shaft voltage and the currents that can circulate inside the motor therefore increase.

These currents do not always stop at the motor. Through the coupling they can also reach the bearings of the gearbox or of the driven machine.

On a high-power motor, a bearing failure is expensive. It is not just the spare part: above all, it is the machine downtime. That is why the problem should be addressed when the motor is selected, not after the first failure.

The solutions: bearings and end shields designed for inverter duty

There is no single solution. In practice, the choice depends on the power rating, the inverter, the cable length and the type of machine. Several executions are available among the motors we offer.

Insulated bearings

The bearing has one ring coated with an insulating layer, which interrupts the current path. It is usually fitted on the non-drive end. At higher power ratings, or in particular cases, both ends are insulated.

Hybrid bearings with ceramic balls

Ceramic balls do not conduct electricity. This makes them the most effective choice when currents are high or the inverter operates at high switching frequencies.

Custom-made end shields

Alternatively, the insulation can be built into the end shield, in the bearing seat. The advantage is practical: the bearing stays standard, so maintenance and replacement remain simple. The end shield can also be adapted to the application, for example to house roller bearings when the radial load is high.

Shaft grounding

A grounding ring or brush discharges the current before it reaches the bearing. It is often combined with an insulated bearing on the opposite end, so the driven machine is protected as well.

Not just bearings

Under inverter duty, the winding matters too. Voltage peaks stress the insulation, so reinforced insulation is worth considering at high power ratings.

Then there is cooling. At low speeds, the shaft-mounted fan is not enough. In these cases, forced ventilation keeps the motor at the right temperature at any speed. Finally, if speed or position control is required, the motor can be fitted with an encoder.

Installation plays its part as well: shielded cables, correct earthing and, where needed, filters on the inverter reduce the voltages reaching the motor.

How we select the right execution

When a machine builder asks us for a motor above 90 kW for inverter duty, we start from a few questions. What is the power rating and at what speed does the motor run? Which inverter is used and how long is the cable? What machine is connected to the shaft?

On this basis, we recommend the most suitable execution among those available. This way, the motor arrives already prepared for inverter duty, with no later modifications and no surprises after the first months of operation.

If you are designing a machine with high-power inverter-driven motors, write to us at info@ndr.it: we will assess the most suitable solution together.