Thermo-mechanical stresses in electric motors arise from thermal expansion of the metals used in its construction. The copper in the coils has a coefficient of thermal expansion quite a bit higher than that of the steel (see Table II).
Coefficient of Expansion
|Copper||9.2 x 10-6/ °F|
|Steel||6.5 x 10-6/ °F|
|Aluminum||3.3 x 10-6/ °F|
|Brass||1.0 x 10-6/ °F|
The difference in thermal expansion is made much worse at motor start-up. The inrush current 5 or 6 times the full load current causes a rapid heating of the conductor during the first seconds of excitation. On the other hand, the stator core undergoes little heating during the initial period. The relative growth of the coils with respect to the stator structure causes mechanical stress in the end-turn region where the coil ends are constrained by surge rings and support brackets.
The growth of coils from thermal expansion and subsequent contraction during cooling tends to loosen coil end supports. The number of motor starts strongly influences such failures and a motor that is started and stopped frequently will be more at risk of an early winding failure. Often plant personnel do not recognize the stresses created by operating procedures. In many cases it is possible to change the procedure to minimize the stress factor.
Dielectric stress (sometimes called voltage stress) is the stress placed upon a material when a voltage is placed across it. The voltage used in calculating voltage stress is the peak voltage or RMS x √2&RADIC>.
If the coil conductors shown in the figure are at 4,000 volts with 0.075 inches of insulation between coils and core and the core is at ground potential, the nominal stress is calculated as,
stress = 4,000/75 = 53 volts/mil.
This value of voltage stress is fairly typical for electric motors that are rated up to 4,000 V. The value becomes much greater at higher voltages. Motors at lower voltage ratings have proportionately more insulation and work at very low stress levels. Since the nominal working stresses are well under the values of dielectric strength shown for various materials in Table III, it might be assumed that dangerous electric stresses are unlikely to occur in motor coils. However, that is not the case.
The 53 volts/mil calculation assumes that the resulting stress is uniformly distributed across the 75 mils of insulation. This uniformity of distribution can develop only if all of the insulation space is occupied by material of the same dielectric constant. The fact is that a voltage placed across the space between the stator iron and coil conductor material divides in such a way that the stress on each insulating material is inversely proportional to the dielectric constant for the particular material. If the space is filled by mica insulation with air voids, the electric stress on the air can easily exceed its low break-down strength with resulting discharges.
Partial discharges in electric motors
Most people have observed an electrical discharge. Such a discharge occurs whenever switch contacts interrupt current into a circuit. It also happens when two exposed and energized conductors contact each other. In general, an arc causes severe damage on surfaces where it impinges. Switch contacts must be made from special metal alloys in order to withstand repeated arcing. Surfaces not designed to withstand an arc but nevertheless subjected to one incur heavy damage.
Discharges may also occur between surfaces if one or both are insulated. Such a discharge is called a partial discharge. It is different from an arc between two conductors because the insulation on at least one surface reduces current to a very low value. However, it does have a similar effect of damaging the surfaces upon which it impinges although at a much slower rate in proportion to the magnitude of current.
A partial discharge occurs when the dielectric strength of an air space between two charged surfaces is exceeded. The voltage that will cause a discharge to occur can be calculated by the combination of air-gap and insulation thickness. This value of voltage is called the inception voltage. It is calculated by the expression,
Vi = (1+ t⁄&FRASL>kg)Vp
where Vi is the inception voltage (the voltage that will cause a partial discharge), k is the dielectric constant of the insulating material, t is the total thickness of the insulation material, g is an air-gap between the sides of two charged surfaces, Vp is the Paschen voltage corresponding to an air-gap g.
A partial discharge can occur between the side of a core slot and a loose coil side if an air-gap exists there. The stress is greatest on those coils subject to the highest voltage. This means that the first coil position (the coil connected to the incoming line) in each coil group is most vulnerable to slot discharges.
A discharge between the coil and slot does not constitute a ground fault. The level of current is too minute to trip ground fault protectors. However, there may be many discharges during each half cycle that the coil voltage exceeds the inception voltage. This produces a discharge pulse frequency of several thousand per second. The cumulative effect of millions of discharges is to eat a hole in the insulation wall.