LEESON's IRIST - Inverter Rated Insulation System

Industrial electric motors, as a rule, are noted for being quality products that provide long, trouble-free operation. This is as true today as it ever was. The difference today is that, as more AC motors are being applied in adjustable-speed drive systems, there is a heightened awareness concerning potential dangers to motor windings. The danger is voltage spikes induced by the increasingly popular pulse width modulated (PWM) controls, or inverters, which use IGBT power transistors. This heightened awareness often focuses on the motor's insulation, sometimes the magnet wire insulation itself and sometimes the entire insulation system.

LEESON's approach, through IRIST , or Inverter Rated Insulation System, is clearly to focus on the total system and, even more, on the total motor product - from the initial engineering concept to the final manufacturing step and beyond. This includes extensive testing of all components, separately and in cooperation with component manufacturers. In addition, all stators are tested to ensure quality in manufacture, and life testing is done to guide future development.

The purpose of this article is to explain the elements of LEESON's "systems approach." Additional information and definitions of key terms are included in the Appendix at the end of the article.

IRIST Element No. 1 - Spike-Resistant Magnet Wire

This is the hottest topic today, though as we will see, it is only one component of the success of IRIST . The difference between standard and spike resistant wire is in the film or coating on the copper wire. By adding different inorganic and organic materials to the coating, it can be made to resist corona breakdown (deterioration due to ozone, the product of corona discharge). The adjoining charts tell the story. The dielectric strength and voltage at which corona begins for most wire types used in small and medium size motors are very similar. Increasing the thickness of the coating, by adding more layers, increases the dielectric strength, and of course the thickness of the wire, as you would expect. BUT, the life of the wire when voltage exceeds the corona inception voltage is quite different. Testing shows that coatings specially made to resist breakdown last much longer than simply adding more layers of standard coatings. This leads to the perception that the sole solution to creating a spike-resistant motor is to change the wire. The truth is, it's not that simple. Here are some complicating factors:

MEASURING VOLTAGE INSIDE THE MOTOR. While higher voltage controls, 400-600 volt, can cause high voltage spikes at the motor terminals, the magnitude and number of these spikes depends on the drive and the application. However, voltage that the magnet wire "sees," not voltage at the terminals, is the critical point. Tests are usually done by twisting two pieces of wire and applying the high test voltage from one wire to the other. This situation, of course, should never occur in a motor, by design. Voltages at the motor terminals divide among many coils of wire inside the motor, which then divide (although unevenly) among many turns in each coil. The goal in design is to ensure that the voltage between any two wires is below safe levels considering how they are insulated.

DIFFERENT WIRE FROM DIFFERENT MANUFACTURERS. Not only is the wire different, but manufacturers are introducing new versions or generations of wire. To further complicate the issue, there are no standards for testing or rating wire or any other insulation component for use on controls. LEESON has been working closely with wire manufacturers when developing new generation products to ensure that they work well with other materials and manufacturing processes. LEESON also performs its own tests to supplement and verify data from wire manufacturers.

FIRST TURN FAILURES. There is much discussion about uneven voltage distribution in the coils and the turn to turn, or first turn, failure in inverter-fed motors. LEESON's testing and experience has shown this to be a non-issue on small and medium sized motors, something that "can happen" but almost never does. Even those motor failures originally thought to be turn to turn, when analyzed, often prove to be from other causes. On very large motors it is an issue. Here, nearly all reputable motor manufacturers have ways of dealing with this problem.

WHY DO SOME MANUFACTURERS CLAIM MAJOR IMPROVEMENTS WITH THE NEW WIRE TYPES? As the previous graphs have shown, the advantage of the "new" wire is in situations where the wire is subjected to high spikes from the control. An insulation system, when properly designed, minimizes exposure of the wire to this high voltage. Inverter rated wire should extend motor life when used on a control in a drive system. It should provide a margin of safety as well. However, it should not be relied upon as being the only source of protection against possible voltage spikes. In fact, if changing the wire alone results in significant life improvements, that could be an indication of a more serious problem in basic motor design and manufacturing methods.

IRIST Element No. 2 - Placing Wire in the Stator Core

Techniques and processes used to insert or wind the wire into the stator core are more important than the coating used on the wire. The coating cannot be effective if it is scratched or nicked. Special attention must be paid not only to the equipment and processes used in motor manufacturing but also the trade-offs considered when developing new wire coatings. A balance must be achieved when considering corona resistance, flexibility, and abrasion resistance.

METHODS OF WINDING. Because the voltage is divided among the turns of wire, it is important that the winding be orderly and not have wires crossing randomly over each other. There are several ways to accomplish this using various types of coil winders and coil inserters, and even hand winding processes. The trade-off here is wire position versus wire damage. LEESON has selected quality automatic coil winders and inserters, built to our specifications, for smaller motor production. Larger motors are wound by hand or machine depending on design. In each case care is taken to ensure the best quality. This includes special training for production associates involved in winding inverter-rated motors.

Some have touted the advantages of "in slot" winders. In this winding method, the wire runs through "needles" that feed the wire directly into the slot through the narrow slot opening. But the wire must run back and forth the length of the stator and around fingers on each end for each turn. Compare this to wire coming smoothly off a spool onto a form and inserting the finished coils only once. There are clearly trade-offs. The stated benefit of in-slot winding is the ability (in theory) to "automatically position" or lay the wire in the slot in layers, keeping the beginning and end of the coil as far apart as possible. In practice, because the wire is free to move around, coils are not picture perfect. And, as mentioned, an orderly winding is the goal of any method.

The point is, there is no clear "best way" to wind stators in everyday production. If there was, everyone would use it. The key to success is to select a proven method, design for it, and perfect it. The results will speak for themselves.

INSULATION MATERIALS. The best winder cannot make up for poorly assembled stator cores or slot insulation that is not suited for the application. LEESON uses a variety of quality insulation materials (polyester films and laminates such as DMD, NMN) specifically tailored to the manufacturing process and insulation class.

IRIST Element No. 3 - Insulate All Critical Areas

SLEEVING. It is critical that coil leads be appropriately sleeved according to their location and the voltage they will be exposed to. These coil leads may have to run across coils from other phases were voltage differences are the highest. Relying only on the wire coating would be a mistake. In order to adequately protect these leads, it is often necessary for sleeving to extend from the lead connection into the stator slot.

PHASE INSULATION. Phase insulation can be the most difficult part of the entire stator winding process, and of critical importance. It is the only insulation component specifically designed to separate coils and wires of different phases (where the highest voltage differences are present). In the past this is the area where some manufacturers have cut corners. Thinner materials (or no insulation at all) or improperly positioned phase insulation may go unnoticed on motors intended for low voltage or strictly utility power. But today, more motors are being used with controls. While some manufacturers were adding back phase insulation into motors that didn't have it, LEESON was busy looking for ways to make our phase insulation, which was always in place, even better.

CONNECTION INSULATION. There are many ways to make and insulate the connections between motor leads and the stator winding or coils. LEESON has and will continue to look for and try improved methods. But for now, connections continue to be taped or sleeved to pad and protect them, providing a high level of electrical and mechanical strength. Connections poking through insulation are a common failure point for inverter motors, but another one you don't have to worry about with LEESON motors.

IRIST Element No. 4 - Varnish Control

The varnish must penetrate into the slots and between wires to be effective. In the case of inverter-rated motors, the varnish replaces the air surrounding and between the wires. This protects by minimizing the amount of air able to ionize or become ozone, and by keeping air farther away from the wire. A thicker varnish appearance on the outside of coil does not necessarily mean it has penetrated into the coil. Also, care must be taken to select the right varnish for the wire type used. Testing has shown that some varnishes actually reduce the life of inverter-rated wires, or not improve life as much as other varnishes, even though they are chemically compatible.

The bottom line is that IRIST represents an insulation system made up of quality class F and class H components designed to work together and implemented properly. It is a system that is inverter-rated in more ways than one.

APPENDIX

Useful Definitions

Control: Also called inverter or converter, is an electronic device that converts an input AC or DC power into a controlled output AC voltage or current (as defined in NEMA and IEEE standards).

Corona: A luminous discharge produced in the neighborhood of a conductor, without greatly heating it, due to ionization of the air surrounding the conductor caused by a voltage gradient exceeding a certain critical value.

Corona inception voltage: The lowest or beginning voltage at which continuous corona occurs.

Drive: The equipment used for converting electrical power into mechanical power suitable for operation of a machine. A drive is a combination of a power converter (control), motor, and any motor mounted auxiliary equipment (as defined in NEMA and IEEE standards).

dV/dt:Literally delta (change in) volts divided by delta (change in) time. It is the slope of or rate of change of voltage over time of a voltage pulse or waveform. It is normally measured in volts per microsecond (V/µs). A modern IGBT drive will have a value of 6000 to 9000 V/µs.

IGBT (isolated gate bipolar transistor): Power control devices used in modern PWM type inverters.

Nanosecond (ns): One billionth of a second.

Ozone: A colorless gas, with a penetrating odor. A form of oxygen, O³. (This gas will react with certain organic compounds.)

Peak voltage: The peak instantaneous value, normally the maximum value of voltage.

PWM (pulse width modulated): A control method that varies the pulse width to produce a desired waveform.

Rise time: The time interval of the leading edge between the instants the value reaches a specified lower and upper limit. This may be either from 10% to 90% (normally) of the peak value, or of the steady state value. Both definitions are used, thus causing confusion. NEMA uses the steady state value. Values of 70-100 ns (nanoseconds) are common for the latest IGBT controls; values of 200-300 ns are seen on older controls.

Voltage spike: A distortion (usually assumed to be of a relatively high voltage) in a voltage pulse of relatively short duration superimposed on an otherwise regular or desired waveform.

Explaining the Physics of the Inverter-Motor Connection

The short version of how an AC PWM variable frequency control works is as follows. Electronically the control first takes the line voltage and changes (or rectifies) this AC to DC voltage. Then, using power devices such as transistors or SCRs, the control produces a stream of pulses that "simulate" the voltage and frequency desired. The figure at right shows a sine wave (AC) line voltage, superimposed on pulsed inverter output, or "simulated" AC. The number and width of the pulses varies or is modulated (PWM) so that if you average (or mean, RMS) the pulses you would get the same value as the sine wave. Notice that the pulses are the same height. This is correct because the DC voltage the drive uses to make these pulses is nearly constant if the AC power to the drive is a constant value.

Now look at the figure on the right, representing an oscilloscope view of pulses from an inverter. The bottom pulses are those that emerge directly from the inverter. They look very square. The top pulses, however, look quite different. They show what pulses may look like at the motor end of the cable. The overshoot, or "ringing" high voltage spikes occuring at the motor end are the source of trouble for some insulation systems.

The cause of this "ringing" can be explained in several ways. It can be thought of as the electrical response of the "circuit" consisting of the inductance, resistance and capacitance of the motor and cable to the pulse. Or it can be thought of as the interaction of pulses reflected back from the motor with those coming from the control. Either way, the result is a peak voltage approximately twice as high (sometimes higher) as the pulse the control put out in the first place, with the addition of high frequency "ringing" besides.