Materials chosen for electrical insulation have a very high intrinsic electrical resistivity and they also have a high dielectric strength.

This intrinsic high resistivity (and also the dielectric strength) degrades with time, electrical stress and its environment, as the properties of the material change in response to these factors.

When breakdown occurs as a result of electric stress, this intrinsic high resistivity remains, but it is effectively short circuited by the low value of resistance at the breakdown site, i.e.

1/R = 1/R_I + 1/R_B                . . . . . (1)

where R is the resulting insulation resistance, R_I is the intrinsic resistance of the insulation (intrinsic resistivity multiplied by a factor for the dimensions of the insulation) and R_B is the resistance of the breakdown path. This equation can be written

R = (R_I x R_B ) / (R_I + R_B) . . . . . (2)

and thus even though the intrinsic insulation resistance remains high, the low value of the breakdown resistance dominates.

In motors which are run intermittently, the heating/cooling cycles eventually crack the varnish on the conductors as a result of the differential thermal expansion coefficients of the varnish and the underlying metallic conductor. These microcracks have no significant effect on either the dielectric strength or the insulation resistance of the sections of the windings in the stator slots, as the ground (slot) insulation is not affected, but as they expose the actual conductors they have a major effect at the ends of the windings, which are subject to environmental influences, such as dusts and moisture.

If the dusts are conductive, either by their nature or by the absorption of moisture, they form a resistive path from the conductors to ground and thus, although their resistance may be orders of magnitude higher than the breakdown resistance RB discussed above, they affect the intrinsic insulation resistance in the same way. Moisture by itself has a similar effect.

The purpose of monitoring motor insulation resistance is to obtain a warning when the resistance is reduced to the level at which an insulation failure may occur as the motor is energized, creating a hazard for both the operator and the installation. Thus the need for this safety related information is immediate.

Motor insulation monitors are designed to provide this immediate safety information rather than to obtain information on the intrinsic quality of the insulation related to the possible length of its future operational life.

Typically the insulation resistance of a motor may vary from 1,000+ Megohms when new to 100 Megohms or so after being in service for some time. The effect of microcracking of the conductor insulation and contamination of the exposed ends of the windings reduce the resultant insulation resistance to 1 Megohm or less when the insulation is on the point of failure.

Experience in the marine industry, a more severe environment that is usually found in industry, suggests that motors at the 1 Megohm level can be recovered by cleaning the insulation, drying it thoroughly and revarnishing it, because the intrinsic insulation resistance has not vanished, it has been compromised by the microcracks and the conductive contamination of the ends of the windings, so when the contamination is removed and the microcracks are filled with new insulation, the intrinsic insulation resistance is regained.

Furthermore, it is clear that, if a motor can be started safely, the heat generated by its operation will dry out any moisture from the ends of the windings and thus raise the insulation resistance to a safe operating level. For this reason, our standard insulation alarm levels are in the range of 5 Megohms to 1 Megohm and we recommend that the highest available setting be used, to give early warning of an insulation problem whilst the motor can still be started safely.

Ohms Law states that the current I flowing though a resistance R is directly related to the voltage V across it and is usually written as

V = I x R                         . . . . . (3)

From this, if the voltage is known and the current is measured, the resistance may be determined.

However, if the required value of the resistance may vary significantly the errors introduced by measuring the current directly will influence the accuracy of the determination.

This source of error may be overcome by connecting a standard resistance in series with the unknown resistance and applying a voltage across them. Then, as the same current flows through both resistances the voltages across them are given by

V _standard = I x R _standard                                         . . . . . (4)

and

V _unknown = I x R _unknown                                         . . . . . (5)

So

V _standard / V _unknown = R _standard / R _unknown     . . . . . (6)

In other words, by comparing the voltages across the standard resistance and the unknown resistance, the value of the unknown resistance is easily determined. Note that the current drawn by the indicating device used to compare the two voltages must be insignificant compared to the current flowing through the resistors under comparison, if an accurate result is to be obtained. Thus, for one percent accuracy, the indicator current cannot exceed one percent of the comparator current and so on.

If the value of the unknown resistance is one of particular interest, such as the insulation resistance of a motor winding at which an alarm must be raised, choosing a standard resistance of the same value eliminates the need to measure the voltages, we only have to compare them to determine if they are equal, so the absolute values of the voltages are unimportant.

Before the era of transistors (ca. 1955) and the invention of integrated circuits (ca. 1960) voltage comparators were laboratory instruments reliant on Weston standard cells (or their equivalents) for their accuracy and were thus unsuited for use outside the laboratory. Field measurement of resistance was done using Ohm’s Law (equation 3) and thus required the measurement of current. At that time the most sensitive moving coil meter rugged enough for field use had a full scale deflection of 100 microamps. With the mid-scale resistance set at 10 Megohms, the voltage required would be

V = 50 µA x 10 Megohms = 50 x 10^-6 x 10 x 10⁶ = 500 volts     . . . . . (7)

so insulation resistance measurements were done at that voltage. When this point was forgotten with the advent of solid state devices, other reasons for the continued use of 500 volts were devised, such as "you can’t measure 10 Megs insulation resistance with 24 volts" - a valid point if you are using a moving coil meter to measure the current through the resistor, as 24 volts across 10 megohms produces a current of only 2.4 microamps - 2.4% of full scale deflection, hardly enough to move the pointer off zero.

However, if a modern op-amp voltage comparator is used, the maximum current required to operate the op-amp is 200 picoamperes (200 x 10^-12 amperes, four millionths of the current in equation (7)). So as noted above, the current through the standard resistor and the insulation resistance should be not less than 20,000 picoamperes to avoid the op-amp current compromising the accuracy of the measurement. This means that the device would make the necessary comparison with a measuring voltage of only 400 millivolts (0.4 volts), so the 500 volt "requirement" is meaningless.

In choosing 24 volts as our monitoring voltage the following factors were considered

• AC or DC

• safety of personnel

• freedom from nuisance alarms and

• the cost of the insulation monitor.

Insulation resistance monitors use DC voltages because the impedance of the insulation would be measured if AC voltages were used, in which case the capacitive impedance would dominate the measurement, as it is orders of magnitude lower than the insulation resistance.

For safety reasons the DC voltage should be low enough that the charge accumulation on the capacitance of a new, high insulation resistance motor circuit will be insufficient to cause injury to personnel either by direct electric shock or by physical injury resulting from a startle reaction to an unexpected electric shock.

To confer freedom from nuisance alarms caused by electrical transients affecting the measuring circuitry the voltage seen by the comparator at the alarm level should be significantly in excess of any possible transients, say five to ten times.

The cost of the insulation monitor depends on the measuring voltage as it affects the cost of the control transformer and other components.

Taking all of these factors into account, we selected 24 volts DC as our measuring voltage for insulation resistances up to ten megohms, because

• 24 volts is too low to penetrate human skin, which breaks down at around 250 volts, so

• the charge in the capacitance of a new, high insulation resistance motor circuit charged to 24 volts will not affect personnel,

• 24 volts is a convenient voltage at which to operate solid state circuitry and produces 12 volts across both the standard resistor and the motor insulation resistance at the alarm level,

• 12 volts is at least an order of magnitude higher than any transient affecting the measuring circuit and

• the control transformer and its associated components are significantly less costly that if we had chosen a much higher voltage.

Although it is quite practical to measure 100 megohm insulation resistance with 24 volts, as shown above, some customers are more comfortable, for whatever reason, with a higher voltage, such as 400 - 500 volts DC.

Such voltages can be used safely for occasional insulation resistance measurements as there is usually sufficient time for the resultant charge on the motor circuit capacitance to decay to insignificance, but when such voltages are used for continuous, automatic insulation monitoring, the motor circuit should be grounded momentarily to dissipate the residual charge if the monitor is disconnected to permit work on the motor circuit.

The three models utilizing 400 volts DC monitoring voltages are offered by MSE of Canada Limited.

 

 

Using MotoSafe and FailSafe INSULATION MONITORS with

VARIABLE FREQUENCY DRIVES and SOLID STATE CONTACTORS.

    When monitoring the insulation resistance of individual motors or generators it is essential that the unit wiring be isolated from all ground connections. This is particularly important when a variable frequency (or any other solid state) drive is involved, as its reverse current leakage is orders of magnitude greater than the insulation resistance monitoring current. Ideally, a contactor should be installed between the motor/generator and the drive and wired to open when the drive is not powered. In this situation the coil voltage can provide the isolation voltage for the monitor.

    In low voltage applications (up to 600V) often there is no contactor between motor and the drive. In such application an insulation resistance monitor can be installed without additional changes if there is an isolating contactor between power line and the drive and the drive has floating control (no leakage to the ground through the drive’s control). At the beginning of the run up to full speed, the motor line voltage (or the generator output voltage) will be below the MotoSafe monitor isolation voltage threshold, so the isolation voltage must be derived from a source which is activated when the drive is energized, such as the motor running light, to avoid false alarms.

    For low voltage machines, the MotoSafe Models M603INDS, LM602IND and M1200 have the insulation monitoring "sense" connection separate from the isolation voltage input to permit wiring as described and for medium voltage machines both the Type MHV and the Type MHV-H embody the same separation of "sense" and "isolation voltage" connections.

    With other models the result can be achieved by wiring normally closed relay contacts in series with the monitor connections to the motor (or generator) winding and energizing the relay coil from an output of the drive with is only energized when the drive is operating, such as a running light.

    The relay contact voltage rating should match the motor/generator voltage, although this requirement is not absolute, as the current flow is limited by the high resistance of the isolation circuits in the monitors and is much too low to permit an arc to be struck or maintained. Using a four-pole relay and wiring two poles in series for each connection confers an additional measure of safety.

    The machine windings may be meggered in the usual way, as described in the Installation Instructions.

MOTOR INSULATION MONITORING

on three phase MEDIUM VOLTAGE power systems

Insulation monitors are designed to complete the circle of protection around electric motors by monitoring the status of the winding insulation when the motor is not energized and raising an alarm if deterioration is detected. To do this, a connection to the motor winding is necessary.

As the motor energizing voltage with respect to ground is too high for a direct connection to be made to the MotoSafe Insulation Monitor in the instrument compartment of the motor control centre, an intermediate device is needed to reduce the voltage to a safe level.

Under normal circumstances the three phases of medium voltage power systems are at equal voltages with respect to ground, whether the power system is resistance grounded or ungrounded, but they are displaced in phase from each other by 120 degrees. Thus, a theoretical "neutral" point exists between the three phases and it is at ground potential. So, if the neutral point can be accessed at the motor, it is the ideal point for connecting the MotoSafe Insulation Monitor to the motor winding.

Several different ways of accessing the neutral point exist, but the simplest and most direct way is to connect one end of each of three high voltage, high megohm resistors of equal value together and then to connect the free ends of the resistors to the three phase conductors at the motor terminals. In this way, the common connection point of the three resistors - the "star" point or "Wye" point - has the same potential as the neutral point, it is normally at ground potential and is therefore a suitable connection point for the "sense" terminal of the MotoSafe Insulation Monitor. In the following discussion, the terms "star point" and "Wye point" are synonymous with "sense terminal".

The MSE Intermediate Resistor Blocks are devices of this type. The advent of film type resistors with strong ceramic rod substrates made possible high voltage, high megohm resistors of reasonable length, with significant power dissipation capabilities. The maximum power dissipation in MSE Intermediate Resistor Blocks is less than five watts. The enclosure design and the design of the resistors with their high resistance values, high power ratings, maximum continuous operating voltages not less than twice the Intermediate Resistor Block rating and high temperature silicone coatings, renders them short-circuit proof.

The enclosures are designed to ensure that the devices meet the dielectric strength, basic impulse level and partial discharge requirements of CSA Standard C22.2 No. 31- M89 and ANSI C37.20.2 at the appropriate voltage levels and they are all Certified and Listed accordingly.

The values of the resistors are chosen to assure personnel safety for all conditions of the power system as they limit the possible current flow from the star point to ground to a level well below the 5 milliamperes considered to be the maximum safe allowable shock current level resulting from contact with an energized AC power line. For MSE Intermediate Resistor Blocks to 7.2 kV the resistors are 10 Megohms and for units to 13.8 kV the resistors are 20 Megohms.

In addition, the "sense" terminals on all MSE MotoSafe and FailSafe monitors are fully shrouded, so inadvertent contact with them is virtually impossible.

In the following discussion we are concerned only with conditions which persist for a time which is significantly longer than a normal human heart beat, i.e longer than three quarters of a second. To constitute a hazard to the person, any condition shorter than that requires the coincidence of the following three points

The probability of such a triple coincidence occurring is so low that it may be ignored, as the only person able to contact the star point would be the electrician on duty.

Condition # 1 - no fault on the power system.

This is the normal condition, the star point is at ground potential and no hazard exists.

Condition # 2 - one phase grounded.

With a low-resistance grounded power system, this condition is cleared by the overcurrent relay, usually within a few tenths of a second, so no hazard exists.

Ungrounded or high-resistance grounded power systems are used so that when this condition occurs there is little or no damage and the system may be operated until it is convenient to shut down the circuit to clear the fault, which may be hours or days later. However, under these conditions the system is effectively a corner-grounded system and the neutral point and the star point are no longer at ground potential, they are at 58% of the phase to phase voltage with respect to ground. If the star point is grounded during this condition, eg. if a person touches it, the current flowing from the star point will be

I_{(sp-g) =} 1.73 x V_{(N - N )} / R

So, for the current to be no greater than 5 milliamperes on a 5 kV power system, the resistance R must be at least 1.73 Megohms.

Condition # 3 - loss of one phase.

The loss of one phase, eg. as a result of a single fuse blowing, is a very serious condition on a system feeding balanced three phase loads such as motors as the consequent current imbalance results in motor burn out in a relatively short time. Depending on the type of motor and the nature of its load, the time may be long enough to permit an orderly shut down of the affected feeder or it may be so short as to demand an immediate shut down. In either situation, the neutral point of the power system as a whole will not be affected, it will still be at (or close to) ground potential, but the star point of the Intermediate Resistor Block will be at the mid-point of the phase to phase voltage of the remaining phases and so the neutral-to-star point voltage will be

V_{(n - s)} = 0.5 x V_{(N - N )} x tan 30^o

which, for a 5 kV power system, will be 1.44 kV.

However, if in these conditions the star point is shorted to ground, eg. by a person touching the "sense" terminal of the MotoSafe Insulation Monitor, the star point will be at the same potential as the neutral point of the system, i.e. ground, and the current flowing will be

I_(SP-G) = V_{(N - n)} / R = V_{(N - N )} / R/3

For a 5 kV power system and R = 1.73 Megohms, the current will be 1.67 milliamperes - 33% of the maximum allowable shock current of 5 milliamperes.

The table below shows the voltages and currents in MotoSafe Intermediate Resistor Blocks under the power system conditions described above.

It is clear that, under any of the possible fault conditions affecting the motor circuit, the maximum shock current which could flow through a person making contact with the "sense" terminal of any of the MotoSafe Motor Insulation Monitors for medium voltage applications is less than 25% of the maximum allowable safe shock current of 5 milliamperes.

MSE of Canada Ltd.

Engineering Department

 

 

MotoSafe Products

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