dv/dt and Reflected-Wave Overvoltage on Long Motor Cables

dv/dt and Reflected-Wave Overvoltage on Long Motor Cables

The cable between a frequency inverter (VFD) and an asynchronous motor is a component that is often overlooked, yet it is decisive for motor life. The cable is assumed to merely carry current; however, as the distance between inverter and motor grows, the cable begins to behave like a line that transmits a voltage wave. Two concepts come to the fore here: dv/dt, which describes how fast the voltage rises, and the reflected wave that appears on long cables. When these two phenomena combine, overvoltage spikes reaching almost twice the inverter output can appear at the motor terminals. At DRG Motor, this article explains in detail this mechanism, its threshold values and the measures to take, so you can protect our IE3/IE4/IE5 class asynchronous motors.

What Is dv/dt?

dv/dt is a quantity expressing how fast the voltage changes with time. While mains voltage follows a smooth sine curve, the output of a frequency inverter consists of sharp-cornered pulses. Each of these pulses leaps from zero to its full value in a very short time. This rapid rise means a high dv/dt.

Modern inverters continuously raise the switching speed to improve efficiency. Faster switching gives better efficiency and smoother current; but it also increases the rise rate of the voltage. High dv/dt is therefore a natural side effect of modern frequency inverters, and it is the factor that triggers the real problem on long cables.

Why Does a Cable Behave Like a Transmission Line?

At low frequencies and on short cables we can think of a conductor as a simple wire. But when the voltage changes very quickly, because each metre of cable carries a small inductance and capacitance, the cable turns into a transmission line. In this case the voltage does not propagate instantly along the cable but travels as a wave moving at a certain speed.

When a wave travelling in a medium meets a different impedance, part of it reflects back. Just as a wave returns when you shake a rope whose end is tied to a wall. In a motor cable too, the voltage wave sent from the inverter output reflects back when it reaches the motor terminal.

The Reflected Wave and Voltage Doubling

The input impedance of the motor is much higher than the characteristic impedance of the cable. Because of this mismatch, the voltage wave reaching the motor terminal reflects back almost entirely. When the reflected wave adds onto the incoming wave, the two waves sum and the voltage at the motor terminal can rise to roughly twice the value at the inverter output.

So while a certain voltage exists at the inverter output, the motor at the end of the cable can see much more. This sudden spike becomes a pulse that stresses the design limits of the motor winding. This is exactly the mechanism by which reflected-wave overvoltage forms.

Stress on the First Winding Turn

When a high-dv/dt voltage pulse reaches the motor winding, the voltage does not distribute evenly across the winding. Because the pulse rises so fast and takes time to spread along the winding, a large part of the voltage is loaded onto the first few turns of the winding.

As a result, the first winding turns are exposed to a far higher voltage stress than the average. These repeated pulses gradually wear the insulation on the first turns. When the insulation fatigues and weakens, a turn-to-turn short circuit appears and the winding fails. This failure most often begins in the first coil groups at the supply end of the motor, because the highest voltage stress accumulates right there. When searching for the root cause of a winding failure, looking at which turns the damage concentrates in gives a strong clue as to whether the problem stems from the reflected wave. For this reason, insulation class and winding quality are critically important in inverter-driven motors.

The Cable Length Threshold

For the reflected wave to double the voltage, one condition is required: the incoming pulse must not yet have completed its rise before the wave reflected from the motor terminal returns to the inverter. This happens once you exceed a certain cable length.

When the cable is short, the reflected wave returns very quickly and is damped before the voltage rises significantly. As the cable lengthens, the wave takes longer to travel back and forth, and the voltage spike reaches its full value. As the inverter's switching speed increases, this threshold length shortens; that is, with faster inverters even shorter cables can cause problems.

Which Applications Carry More Risk?

The reflected-wave problem comes to the fore in installations where the inverter and motor are physically far apart. The following conditions increase the risk:

• Remote motor location: the inverter panel being tens of metres away from the motor.
• High-switching-speed inverter: very fast-rising voltage pulses.
• Continuously running systems: the pulses repeating millions of times over years.
• Standard-insulation motor: winding insulation not specially reinforced for inverters.

The coming together of these conditions is one of the most common hidden causes of premature winding failures in three-phase industrial motors.

Solution 1: The dv/dt Filter

One of the most common solutions is to place a dv/dt filter at the inverter output. This filter rounds the corners of the voltage pulses, lowering their rise rate. When dv/dt drops, the voltage rises more smoothly and both the effect of the reflected wave and the stress on the first winding turns are markedly reduced.

The dv/dt filter is a relatively compact and economical solution. On medium-length cables and in most standard applications it is sufficient to protect the motor insulation.

Solution 2: The Sine Filter

When more comprehensive protection is required, a sine filter is preferred. This filter converts the inverter's pulsed output into an almost smooth sine wave. The motor thus sees a clean voltage as if it were fed from the mains.

The sine filter is the strongest solution in installations with very long cables and in applications where protecting the motor insulation is critical. It also reduces the high-frequency noise propagating over the cable, providing a benefit in terms of noise and vibration. The filter choice should be made according to cable length and motor power.

Solution 3: Inverter-Duty Insulation

Alongside filters, the motor itself being resistant to inverter supply is a decisive advantage. Inverter-duty insulation equips the winding with reinforced materials and construction against repeated high-voltage pulses. Such a motor withstands reflected-wave spikes with a much higher safety margin.

When choosing a motor that will run on an inverter, having insulation designed for this pulsed supply is one of the most lasting protection methods in the field. The right motor choice often also reduces the need for additional filters. For this reason, stating from the outset that the motor will run on an inverter, and requesting an appropriate insulation structure, eliminates many potential problems at the very beginning.

The Role of Cable Selection and Grounding

Another dimension of managing overvoltage is cable selection. The shortest possible cable route reduces the reflected-wave risk from the start. Using a screened motor cable and terminating the shield properly at both ends keeps high-frequency components under control. Good grounding is needed to balance both overvoltage and high-frequency noise, and it is a part of the maintenance steps that should not be neglected.

Driving Multiple Motors from a Single Inverter

In some applications a single inverter drives several motors in parallel. In this case the separate cable to each motor significantly increases the total cable length and the reflected-wave risk multiplies. In such installations an output filter becomes almost mandatory. In parallel-motor applications, cable lengths and filter selection must be planned with particular care.

How to Recognise the Symptoms

Winding stress caused by the reflected wave usually progresses slowly and its symptoms become clear only as failure approaches:

• Repeated winding failure: failures concentrated especially in the first turns at the supply end of the motor.
• Appears only on long-cable installations: the same motor running flawlessly with a short cable.
• Linked to the inverter: a problem never seen on mains supply appearing with the inverter.
• Drop in insulation resistance: the winding insulation resistance falling over time in periodic measurements.

Early Diagnosis and Monitoring

Because reflected-wave damage is cumulative, regular monitoring is the best way to catch failure in advance. Periodic measurement of the winding insulation resistance shows the downward trend over time and allows intervention before failure occurs. Predictive maintenance and energy monitoring approaches make such hidden stresses visible and prevent unplanned shutdowns.

The Assurance That Comes From Design

The most robust approach against the reflected-wave problem is selecting the motor to be suitable for inverter operation from the outset. When reinforced insulation, quality winding workmanship and the right filter combination come together, the motor runs safely even on long-cable installations. DRG Motor's IE3/IE4/IE5 class asynchronous motors are offered with these requirements of modern drive-fed applications in mind.

A Holistic View: Cable, Filter and Motor Together

Reflected-wave overvoltage is a matter for the whole drive line, not a single component. Inverter selection, cable length and type, filter and motor insulation must all be evaluated together. It is hard to solve the problem permanently by merely replacing the motor or merely adding a filter; the right result is obtained with an engineering view covering the entire line. When planning an installation, if the motor location, panel position and cable route are addressed together from the very start, most of the problem is prevented before damage occurs. A common field mistake is choosing the motor and inverter correctly but separately, while the cable and filter between them are an afterthought; yet when these three components are designed as a single system, the motor runs both longer and more reliably.

dv/dt and the Reflected Wave: Two Sides of the Same Coin

These two concepts are often mentioned together, because one is not fully understood without the other. dv/dt describes how fast the voltage rises; the reflected wave describes how this fast pulse multiplies on a long cable. If the voltage rose slowly, that is if dv/dt were low, the pulse would not yet have completed its rise as the reflected wave travelled back and forth, and the voltage would not multiply significantly. High dv/dt is therefore the precondition for the reflected wave becoming harmful. For this reason most solutions aim first to lower dv/dt; once the rise rate falls, the reflected-wave problem largely eases on its own.

Pulse Repetition and the Fatigue Effect

A single overvoltage pulse usually does not damage the motor immediately, because quality insulation is designed to withstand one-off high voltages up to a certain margin. The real problem is the constant repetition of these pulses. Because the inverter produces thousands of pulses per second, over years the motor winding is exposed to this stress billions of times. Just as bending a wire repeatedly at the same point eventually breaks it, repeated voltage pulses gradually fatigue the insulation too. This accumulation of fatigue is the fundamental reason an apparently healthy motor suddenly suffers a winding failure.

The Contribution of Ambient Conditions

While reflected-wave stress alone wears the winding, adverse ambient conditions accelerate the process. High temperature increases insulation ageing; moisture and dust form a conductive layer on the winding surface, amplifying the effect of voltage pulses. For this reason, in inverter-driven motors it matters that the insulation is durable not only electrically but also against ambient conditions. A motor with the right protection class offers a longer life against both pulsed supply and harsh environments.

The Basic Difference Between Mains and Inverter Supply

An asynchronous motor fed directly from the mains sees a smooth, continuously changing sine voltage. There are no sudden corners or fast jumps in this curve; therefore neither high dv/dt nor a reflected wave forms. When the same motor is connected to an inverter the supply changes completely: the voltage now consists of sharp-cornered, very fast-rising pulses. This pulsed structure is the one and only true cause of reflected-wave overvoltage on long cables. So a motor that has run flawlessly on the mains for years can begin to suffer winding stress when moved to an inverter with a long cable. Understanding that the problem lies not in the motor itself but in the form of supply is the first step toward the right solution.

How to Prioritise the Right Solution

It is not necessary to apply the most comprehensive solution in every installation; what matters is scaling the risk correctly. On a motor with a short cable, low power and inverter-duty insulation, an additional filter is often unnecessary. On medium-length cables a dv/dt filter is enough to keep the voltage spike within safe limits. In very long-cable, high-power applications or where protecting the motor insulation is critical, a sine filter is the safest choice. The right combination is determined by evaluating cable length, the inverter's switching speed, motor power and the criticality of the application together. Getting this evaluation right from the start protects against both unnecessary cost and future failures.

DRG Motor for Long-Cable Systems

In installations where the inverter and motor are far apart, choosing the right motor is the first step to minimising the dv/dt and reflected-wave risk. At DRG Motor, we stand by you with our IE3/IE4/IE5 efficiency-class asynchronous motors and application support. For the most suitable motor and insulation solution for your long-cable drive system, explore our electric motor products and reach our technical team via drgmotor.com. To better understand inverter-driven applications, you can also read our what is an electric motor and industrial electric motors content.