The frequency drive (VFD) is the most common way to adjust the speed of an asynchronous motor, and it delivers major energy savings in applications such as pumps, fans and conveyors. However, drive operation creates a different electrical environment from connecting the motor directly to the grid. The pulse-width-modulated (PWM) voltage the drive produces feeds the motor not with a sine voltage but with rapidly switched sharp pulses. This gives rise to two hidden problems: additional heating from switching harmonics and bearing currents from common-mode voltage. These two effects, if no proper measures are taken, heat the motor winding, internally erode the bearing and cause failure much earlier than expected. In this guide we cover the symptoms, risks and protection methods of VFD and harmonic-induced additional heating and bearing currents, together with practical solutions such as insulated bearings, du/dt or sine filters, grounding brushes and cable selection. The aim is to select the right motor and take the right measures to run a motor long and trouble-free in a drive system.

Switching harmonic and bearing current risk in an asynchronous motor connected to a frequency drive (VFD)

Why Does the Motor Behave Differently with a Drive?

A motor connected directly to the grid is fed with a smooth sinusoidal voltage. A frequency drive, by contrast, produces voltage by rapidly switching it on and off; the resulting wave reaching the motor, although on average resembling a sine, contains high-frequency harmonic components and very fast-rising voltage edges (high du/dt). This causes additional losses and voltages in both the magnetic circuit and the bearings of the motor. We covered when a frequency drive is needed and how to select it in detail in our frequency drive (VFD) with asynchronous motor article. The efficiency gain in a drive system can be found in our VFD energy savings in pumps and fans content.

Switching Harmonics and Additional Heating

The harmonic components in the PWM voltage increase the iron and copper losses of the motor. These additional losses turn into heat and make the motor hotter than when running the same load fed from the grid. Especially when running at low speed, since the motor own cooling fan slows down, cooling decreases and the heating becomes more pronounced. For this reason, in motors that will run continuously at low speed with a drive, the temperature rise class and cooling method must be chosen carefully. We covered the temperature rise class in our temperature rise class (Delta T 80K) article and cooling methods in our cooling methods IC411 and IC416 content.

The Cooling Problem at Low Speed

The cooling fan of a standard asynchronous motor is attached to the shaft end and rotates at the same speed as the motor. When the drive lowers the speed the fan also slows; while the cooling flow drops, the motor heat load can continue at full load. This is a serious risk in applications requiring full torque continuously at low speed (constant-torque load). The solution is to select a motor with external (forced) cooling or to size the motor larger with a derating. We detailed the constant-torque and variable-torque distinction in our motor selection in variable-speed applications article and temperature monitoring in our protection with PT100 and PTC thermistor content.

Derating: Selecting the Motor One Size Larger

One practical way to compensate for the additional losses and reduced cooling in a drive-fed motor is to select a motor at a power slightly above the rated load. This keeps the motor always running below the thermal limit and extends its life. Oversizing also has a cost; a motor chosen too large runs inefficiently at low load. To strike the right balance, load ratio and efficiency should be evaluated together. We covered load ratio and correct sizing in our motor load ratio and correct sizing article and power derating at high ambient temperature in our derating at high altitude and hot environment content.

How Does Bearing Current Form?

The instantaneous sum of the three phases of the voltage wave the drive produces is not zero; this creates a common-mode voltage fluctuating relative to the motor body. This voltage builds up a voltage on the motor shaft. Since the only conductive path between shaft and body is the bearings, when the built-up voltage exceeds a certain threshold it discharges by puncturing the bearing oil film. These small but repeated electrical discharges (EDM-like sparks) open micro-pits on the bearing surface. Over time these pits form a grooved pattern called fluting on the bearing raceway; the result is noise, vibration and early bearing failure. We covered bearing type and life in our bearing type and life: insulated bearing article.

Fluting and EDM Erosion

Fluting is the most recognised symptom of bearing current. The washboard-like grooves forming at regular intervals on the bearing raceway carry the marks of electrical discharge under a microscope. This damage is not mechanical wear but electrically induced erosion; therefore it cannot be prevented by better lubrication alone. The symptoms can be found in our motor failures: symptoms and causes article and the factors affecting bearing life in our bearing greasing and lubrication content.

How to Recognise the Symptoms?

The field symptoms of bearing current are usually: bearing noise that increases over time in drive operation, a high-frequency hum, shorter-than-expected bearing life and an increase in vibration. When these symptoms appear, vibration and noise analysis is done to distinguish whether the problem is mechanical or electrical. We detailed the vibration acceptance values in our vibration and balance ISO 10816/20816 acceptance values article and noise sources in our noise sources: magnetic, mechanical and aerodynamic content.

Circulating Currents at High Power

One type of bearing current is the discharge (EDM) current dominant at low power; another is the circulating current that comes to the fore in high-power motors. The circulating current arises from a voltage induced by high-frequency magnetic flux around the shaft and forms a loop through shaft-bearing-body-bearing. For this reason, in high-power motors this loop is usually cut by fitting an insulated bearing to one end. We covered the correct selection in high-power cast iron body motors in our cast iron or fabricated steel body article and the high-power supply plan in our supply of high-power motors above 90 kW content. When evaluating which measure is needed at which power, the motor frame size and power are decisive.

Protection against bearing currents with insulated bearing, grounding brush and sine filter

Protection 1: Insulated Bearing (NDE)

One of the most common solutions is fitting an insulated bearing to the non-drive-end (NDE, fan side) of the motor. The insulated bearing can be ceramic-ball (hybrid) or have an insulating-coated outer ring; both cut the electrical path between shaft and body, preventing the discharge current from passing through the bearing. In high-power motors fitting an insulated bearing to one end is usually recommended; if fitted to both ends, leakage of current from the shaft to the end equipment (via the coupling) must also be prevented. We covered this subject in our insulated bearing article and bearing life in cast iron body motors in our bearing and bearing life content.

Protection 2: du/dt and Sine Filter

Filters added to the drive output reduce both heating and bearing current. A du/dt (output reactor) filter softens the rise rate of the voltage edges; this protects both the winding insulation and the reflection voltage on long cables. A sine filter brings the output closer to a true sine, noticeably reducing harmonics and the common-mode effect. In facilities using long motor cables these filters are often mandatory. We covered the strength of the winding insulation class in our winding and insulation class (F/H) article and voltage tolerance in our voltage tolerance and grid fluctuation content.

Cable Length and Reflection Voltage

As the cable between drive and motor lengthens, the fast voltage pulses reflect at the cable end, creating voltage overshoot at the motor terminals. These overshoots stress the winding insulation and accelerate insulation ageing in the long run. The solution is to limit cable length, use shielded motor cable and add a du/dt filter. We covered the terminal connection and cable entry in our terminal box and cable connection article and cable selection by rated current in our rated current: cable, fuse and contactor selection content.

Protection 3: Grounding Brush and Shaft Grounding

A shaft grounding ring discharges the common-mode voltage built up on the shaft to the body through a low-resistance path; thus the current flows through the brush, not the bearing. This method is very effective, especially when used together with an insulated bearing. Good shaft grounding and general grounding improve both bearing current and electrical safety. We detailed grounding and electrical safety in our grounding and electrical safety article. Drive parametering and the choice of switching frequency also affect the common-mode effect; we covered drive setting in our drive parametering and commissioning content.

Drive Switching Frequency

As the drive switching frequency rises the noise decreases but high-frequency common-mode currents and cable effects can increase; as it falls, motor noise and harmonics increase. The correct switching frequency is balanced according to the application, cable length and use of filters. This balance directly affects both heating and bearing current risk. Low-noise motor selection can be found in our noise and vibration: low-noise motor selection article.

Correct Motor Selection: Inverter Duty

A motor that will run continuously with a drive should be selected as inverter duty. These motors have reinforced winding insulation, an insulated bearing where needed and an external cooling option. High-efficiency IE3 and IE4 motors maximise the saving potential together with the drive. We covered the efficient motor and drive combination in our high-efficiency motor + frequency drive article and the efficiency advantage of the IE4 motor in our IE4 2-pole 3000 rpm pump and fan content. For our product families you can visit our homepage.

Comparison with Soft Starting

The drive is used not only for speed control but also for soft starting. In loads that frequently stop and start or have high inertia, the drive reduces the inrush current and the mechanical shock. We covered starting methods in our star-delta or soft starter article and the source of inrush current in our starting current (LRA) content.

Efficiency Class and Saving in Drive Operation

In a drive-fed motor the real saving comes both from the efficiency class and from the affinity-law gain of reducing speed. In variable-torque loads such as pumps and fans, when the speed is reduced by a small ratio the power consumption falls by a much larger ratio; that is why the VFD delivers the highest saving in these applications. We covered the effect of efficiency class on long-term cost in our total cost of ownership (TCO) article and the IE4 threshold in pumps and fans in our IE4 threshold in pumps, fans and compressors content. To calculate field efficiency realistically, our nameplate vs field efficiency article is also useful.

Frequently Asked Questions

Is an insulated bearing needed in every drive-fed motor?

No, it is not mandatory in every motor, but the risk grows with power and application. In low-power, short-cable systems the bearing current usually stays low. However, in medium- and high-power motors, in facilities with long cables and in continuously running critical applications, measures such as an insulated bearing, a shaft grounding brush or an output filter are strongly recommended. When deciding, motor power, cable length, running time and the criticality of the application should be evaluated together.

Why does the drive heat the motor?

The harmonic components in the PWM voltage the drive produces increase the iron and copper losses of the motor; these additional losses turn into heat. Also, when the motor runs at low speed its own cooling fan slows down, so cooling decreases. When these two effects combine, the motor becomes hotter than when running the same load fed from the grid. The solution is to select an inverter-duty motor, use external cooling if needed, select the correct temperature rise class and size the motor appropriately in applications requiring continuous full torque at low speed.

Does lubrication prevent fluting damage?

No. Fluting is erosion caused by electrical discharge; it is not mechanical wear. Better grease or more frequent lubrication does not prevent this damage. The real solution is to prevent current from passing through the bearing: electrical measures such as an insulated bearing, a shaft grounding brush and an output filter must be applied together. Lubrication only slows normal mechanical wear; it does not eliminate the bearing current problem.

Get a Quote

Let us determine together the inverter-duty motor with the right temperature rise class and, where needed, an insulated bearing for your drive-fed application. Share your power, speed, cable length and operating profile; we will quickly offer you a suitable solution and protection recommendations. To request a quote, reach us through our contact page or call now: +90 (532) 345 49 86.

VFD Compatibility and Protection Checklist

  • Is the motor inverter duty with reinforced insulation?
  • If full torque is needed continuously at low speed, has external (forced) cooling been planned?
  • Has the temperature rise class been chosen to cover the additional drive loss?
  • Has an insulated bearing been requested for the NDE end at medium/high power?
  • Is a shaft grounding brush needed and has it been evaluated?
  • Has a du/dt or sine filter been selected according to cable length?
  • Is shielded motor cable and the correct gland being used?
  • Has the drive switching frequency been set according to application and cable?
  • Are the shaft and body grounding low-resistance and solid?
  • Have temperature (PT100/PTC) and vibration monitoring been planned?