Two significant events in the motor world are having a major effect on the way synchronous AC motors are designed, purchased and applied. One is driven by technology, the other by politics and economics.
The insulated-gate bipolar transistor and the simplicity of the AC synchronous motor have made inverter drives the first choice for precise, adjustable speed and torque control. They are basically maintenance-free, efficient, and cost-effective compared to mechanical, electromagnetic, or DC motor drives. However, the AC motor insulation pays the price for this performance and convenience.
High-voltage spikes are the result of high switching frequencies (up to 20 kHz) and rapid rise times (dV/dt) that are characteristic of insulated-gate bipolar transistors. Further amplification from transmission line reflections and impedance mismatches cause a damaging corona effect. Together with thermal stresses from non-sinusoidal wave-form harmonics, they cause premature insulation failure in motors that lack suitable designs, material selections, and manufacturing methods.
Energy policy act impact
The motor efficiency mandates of the Energy Policy Act of 1992 (EPAct) do not specifically cover motors used in adjustable-speed applications. However, merely labeling a NEMA Design A or B motor as being suitable for inverter duty on adjustable-frequency, adjustable voltage power does not, by itself, exempt it. If it meets the other EPAct criteria (and most do) it must also meet the new efficiency standards.
The criteria for EPAct-covered motors as defined in NEMA Standards Publication MG 1-1987 include:
- single-speed, poly-phase T Frame,
- 1 through 200 hp,
- 3,600, 1,800, or 1,200 rpm,
- foot-mounted, squirrel-cage induction motors, NEMA Designs A and B,
- continuous rated, and
- operating on 230/460 volts, constant 60 Hz line power
Knowing how to recognize an inverter-duty motor that can deliver long life and high efficiency brings maximum value for your adjustable-speed motor dollar.
The corona effect occurs when the electrical potential between two conductors reaches a level where the air around the conductors loses or gains electrons and becomes charged. When this happens, there may be a discharge through the air between the conductors if the applied voltage exceeds the dielectric strength of the insulation.
Corona effect was not a concern in low-voltage motors (600 volts or less) operating on utility-generated, 60 Hz, pure sine wave power. The special materials and steps taken to prevent corona insulation damage in medium and high-voltage (600 V+) and formed-coil motors were not necessary.
Because voltage spikes as high as 2,600+ volts occur in a 575 volt system, corona prevention is a major consideration for the survival of low-voltage inverter-duty motors.
A single voltage spike is normally not enough to create a full catastrophic discharge within the windings of an inverter-duty motor. It can, however, lead to a partial discharge that can cause steady degradation of the organic material used in the magnet wire insulation. It can also affect the varnish separating the coils.
These partial discharges generate heat, radiation, and mechanical and chemical energies. When partial discharges cause sufficient accumulated insulation damage, the applied voltage (including spikes and reflections) causes rapid insulation failure.
Corona inception sites
The level at which these partial discharges occur is the <I>corona inception voltage<I>. When the applied voltage exceeds the corona inception voltage, air-containing voids in the insulating material become sites for partial discharges, insulation damage, and eventual failure. Voids include bubbles in the insulation, crevices between turns that did not fill with varnish, and cracks from mechanical damage.
Depending upon the nature of the site and the cumulative number of partial discharges, failure may occur in days or even hours.
Insulation failure locations
Partial discharges can occur anywhere within the insulation system. Ground insulation failures have been noted, as well as failures between adjacent phases (simultaneously at different potentials) and between coils in the same phase. However, turn-to-turn failures within the same coil are the most common.
The greatest potential difference lies between the first and last turn of a coil. Studies show that the voltage difference between randomly touching turns can reach 40 to 90 percent of the terminal voltage.
Through random coil winding, the first and last turn of a coil may touch. This creates an ideal site for insulation failure from partial discharges. Positioning the coils carefully within the slot nearly eliminates the chance of turn-to-turn failures.
What to do about it
Take two basic approaches to combat premature failure of inverter-duty motors. The external approach tries to prevent harmonics and over-voltage spikes from reaching the motor. The internal approach calls for motor materials and designs that withstand any voltage stresses the motor may encounter.
External solutions include:
- modifications to the drive controller,
- a variety of power conditioning accessories (filters, reactors, isolation transformers), and
- restrictions on the distance (cable length) between the drive and the motor to limit voltage build-ups as voltage waves reflect back and forth along the line.
While power conditioning and other external approaches improve chances of inverter-duty motor survival, they add cost. They also may reduce efficiency, affect torque, and apply stress to other drive components. Restricting cable length often requires installation compromises and only solves part of the problem.
Internal solutions include:
- basic electromagnetic design changes (such as stator slot configurations and effective winding turns) made to increase low-speed torque and tighten speed control.
- improved insulation materials and manufacturing techniques that withstand the thermal effects of power harmonics and the stresses of high voltage spikes.
Recently advances in magnet wire (winding) insulation include an added inorganic component that does not degrade under transient voltage spikes. While this may postpone insulation breakdown, the organic components continue to degrade from the effects of corona inception.
Building a motor with a corona inception voltage level (at operating temperature) well above any spike voltage the motor is likely to see is the comprehensive solution. This can be done with proper design, materials, and manufacturing methods. It avoids limiting drive selection, adding power conditioners, or forcing installation compromises.
In addition to basic electromagnetic changes, the inverter-duty motor design should limit the motor's internal operating temperature rise. As temperature increases, corona inception voltage decreases along with insulation life in general. Rotor slots should be configured to minimize skin-effect heating. Cast-aluminum rotors should be free of voids to minimize electrical resistance, heat generation, and energy loss (increasing motor efficiency).
Ventilation systems should be designed for maximum heat transfer. This includes using good heat-conducting materials such as extruded aluminum and assuring good mechanical contact for thermal transfer to external cooling surfaces.
Maximizing cooling surface areas is also critical. For example, stagger-stacking stator laminations exposes side surfaces as well as perimeter surfaces to the flow of cooling air. Windings should include built-in thermostat protection.
The bearing system should be designed for rigid support and long life. Entry of contaminants should be restricted to prevent heat build-up (and reduction of efficiency). Motors should be dynamically balanced to precision limits for smooth operation at any speed.
Premium active materials (iron and copper) should be the materials of choice. Stator laminations should be stamped from high-grade electrical steel to minimize eddy-current losses and heat build-up caused by inverter-induced current harmonics (more efficiency issues).
High-grade (Class F, H, H+) slot, phase, and lead insulation materials along with premium (Class H) varnish should be used liberally in combination with the design that minimizes internal heat rise. This gives the motor the superior thermal cushion it needs to withstand additional heating caused by low-speed operation. A good indication of the built-in thermal headroom is the duration of the warranty the manufacturer for an inverter-duty motor offers.
Magnet wire should be handled carefully to prevent insulation damage. It should be wound properly to provide maximum physical separation between the first and last turns of a coil.
In-slot winding diminishes the possibility of the highest-voltage turns in a coil from touching the lowest-voltage turns. It automatically positions each turn within the stator slot with computer-controlled consistency. This technique may be compared to a level wind on a fishing reel.
In-slot winding also minimizes handling damage because it dispenses wire from its original packaging directly into the stator slot. It controls the shape of the coil noses (ends) and minimizes the need for post-winding coil-nose forming that can damage the insulation.
The more commonly used coil winding and insertion method defeats the benefits of improved insulation materials by scraping or marring them during winding, insertion, and forming. Turns within the coil also may shift during insertion, increasing chances of positioning the highest-voltage turns close to the lowest-voltage turns.
Wound stators, with insulating materials in position, should be dipped in varnish and baked to cure the varnish. Dip-and-bake cycles stabilize the coils to prevent abrasion damage from coil and lamination deflection. Double dipping and baking greatly minimizes the potential for internal voids where the partial corona discharges occur.
Measuring corona inception voltage
Manufacturers of high- and medium-voltage motors that operate on constant 60 or 50 Hz sine-wave power have long been concerned with measuring corona inception voltage. They do it by opening sections of the winding and applying increasing sine-wave voltage until the corona effect reveals weak points in the isolated sections of the winding. However, applying the same test to an inverter duty motor does not take into account the effects of rise time and pulse duration of the inverter wave forms. It cannot evaluate a complete winding that has been connected for operation.
A new method measures the corona inception voltage of an inverter-duty motor directly. A high-voltage DC power supply and high-voltage switching module generate short variable-voltage pulses with rise times similar to typical pulse width modulated drives. These pulses are applied to each phase of the motor stator at a constant width and frequency small enough to prevent excessive heat build-up.
An oscilloscope monitors the current generated by the voltage pulses as the pulses voltage level increases. Before partial discharges occur, the current waveform is solid and clear. When the voltage exceeds the corona inception voltage of the insulation system, the current wave form becomes fuzzy and unstable.
The lowest measured corona inception voltage for a given electrical phase becomes the corona inception voltage of the motor at the testing temperature. Testing the corona inception voltage of comparable (frame size) motors allows evaluation of various materials, internal designs, and manufacturing methods.
Such tests confirmed the ability of proper design, in-slot winding, high-grade insulation materials, and sufficient varnishing to raise the corona inception voltage of inverter-duty motors above the levels commonly encountered with today's adjustable speed drives.
Adjust speed, not outlook
Choosing a properly designed and manufactured inverter-duty motor allows you to design for the adjustable speed and energy efficiency requirements of the process--instead of the limitations of the motor.
Voltage overshoots and transmission line effects
The high voltage spikes that cause early insulation failure in inverter-fed motors result from:
- voltage overshoots produced in the inverter
- voltage wave reflections occurring in the transmission line between the inverter and the motor that reinforce and amplify the overshoots.
The variables affecting the height of the spikes include:
- rise time of the voltage as the current is switched on and off in the inverter,
- characteristic impedance of the cable,
- line impedance which is a function of cable length, and
- impedance of the motor.
Transient inverter voltage overshoots can occur at both the beginning and end of each pulse as shown in Figure X. They occur between 20 and 100 times per cycle with the fast switching employed in today's pulse width modulated inverter drives. It is the repetitions that result in cumulative insulation damage.
Because the amplitude of the reflected wave is proportional to the length of the motor cables, the longer the cable length, the greater the chance the reflected pulse will reach a level that damages the insulation.
There is a worst-case maximum cable length beyond which only minimal increases occur--approximately 40 to 60 feet depending on the variables mentioned above. It is quite common in industrial applications for the drive and motor to be separated by distances greater than that.