VFDs are incorporated into motor control to improve process efficiency and bring down maintenance episodes, but some plant managers experience the opposite. Why? Quite certainly, this is the result of a mismatch between motors and drives.
One may think of variable speed drives to be the culprit, but the fact is, they don’t make motors fail. It is incorrect planning and selection that is the root cause. Modern drive technology should not be sidelined due to fears of motor failures.
The cost of VFDs are continually dropping due to technological breakthroughs, while their efficiency is on the rise. Moreover, the devices are becoming more compact, incorporating faster switches, i.e. Insulated Gate Bipolar Transistors, that bring down switching losses. New drives also have revamped Pulse Width Modulation (PWM) algorithms that have made the motor’s current nearly sinusoidal with respect to the voltage. This makes the operation beneficial at lower speeds, as harmonics are reduced.
All these features make VFDs a necessity for motor control, e.g. new installations of AC induction motors. This has resulted in a sky-rocketed demand for VFDs, giving way to the problem of motor/drive incompatibility, which has the potential to neutralize the benefits of such an installation.
Winding Stresses and Bearing Failures
The incompatibility problem can be filtered down to two areas, winding stresses and bearing failures.
When the drive’s pulses interact with the motor/drive leads, high-voltage peaks are generated. If the motor doesn’t have sufficient insulation to sustain these peaks, winding stresses can surface. How does this happen? Current in the inductor leads is initiated from a rectangular pulse from the VFD. This results in the motor’s capacitance value reaching that of the peak of rectangular pulse, which in turn causes the voltage to drop to zero in the inductor, while the current still flows through it. The resultant energy stored in the inductor will force the current to continue to flow, leading to a proportional increase in the capacitor’s voltage until the current stops.
When the current stops, the motor’s capacitance voltage is greater than the peak of the VFD’s pulse. Next, the voltage across the leads reverses and current starts flowing into the VFD, effectively reenergizing the leads and making the capacitor voltage go below the peak. The cycle continues until the leads’ resistance dissipates the energy stored in the inductor, with the process occurring at high frequency, i.e. 0.5 – 4 MHz
The culprit behind this overshoot and uncontrollable ringing is the energy stored in the leads. As the length of the motor’s leads increases, so does the inductance, which increases the charge stored in the motor’s capacitance. As a consequence, the overshoot increases as well. As a rule of thumb, longer leads directly translates into greater overshoot. For example, 10 feet of lead length can have a 43% overshoot.
Typical VFDs in production have a rise time ranging from 0.025 – 1 microsecond. The quick transition causes lead to store energy, generate overshoot and distribute the voltage unevenly. Faster rise time creates greater overshoot, and modern VFDs tend to have this adverse feature, something that wasn’t present in older models.
Rise time of motor effects voltage distribution
The motor’s first coil acts as a filter for the remaining windings, making it prone to have more voltage across it. If the rise time is faster, then the percentage of peak terminal voltage will be larger on this coil. Typical dual voltage 3-phase motors have two sets of coils, so external series/parallel connections can be allowed. Each of the sets may have four or more coils. In such an arrangement, there would be equal voltage across each set as well as each coil.
But when VFDs are brought into the equation then the scenario changes. Motors featuring taps have been part of experiments, and it has been identified that the first coil in each phase has higher peak phase voltage than the rest. In fact, it is possible for the first coil to have 10 times as much voltage when operated across the line. But if the first coil has several turns then the voltage across the initial turn remains low.
Peak voltage of the motor must be known
Knowing this will allow selection of a motor that can handle peak voltage. At the terminals of a motor operating in conjunction with a VFD, the peak voltage depends on the drive’s input voltage and the magnitude of overshoot at the motor. Irrespective of the rise time or inductance value, the overshoot will not exceed twice the peak of the rectangular input, except for some rare conditions.
Modern PWM VFDs rectify the mains’ input, giving a DC bus voltage that is equivalent to the square root of 2 times the rms input voltage.
Some may refer to the NEMA process, stating the Standard MG1, section 126.96.36.199, where it says that motors rated at 600 volts must be able to withstand a 1600 volts peak. But in practice, it is very much possible for a 575 volts motor to have a peak over 1600 volts. Owing to this fact, NEMA is revising its standards.
Failure Mechanism Corona
An electric field is developed around conductors when high voltage is present across them, bypassing the insulation. If the voltage becomes high enough, even for short periods time, it can breakdown air insulation leading to corona losses. The discharge is visible as bright sparks and has severe adverse effects.
It results in production of Ozone, which is a highly reactive gas and can breakdown the actual insulation of cables. Moreover, corona can result in the generation of nitrogen oxide which has similar properties, while finally the charged air particles can mechanically damage the insulation.
Surely, corona doesn’t occur at every voltage level and there are some limitations that need to be met. The specific level is known as “corona inception voltage”, which is dependent on the spacing, temperature, humidity and type of insulation used. Once this threshold has been reached, there is a substantial reduction in voltage when corona discharges occur.
Corona is one of the major limiting factors for voltage that can be applied to windings. Motors designed for high-voltage operation make use of specific technologies such as layered insulation, mica tapes, semi-conductive layers, etc. to mitigate such issues. Other ways to deal with corona discharge is lowering the temperature and increasing the diameter of wires.
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