DC motor maintenance

It is American-made DC motor.
 Insulation improvement repair for the failure.
Since this type of removable is stator coil, it is easy rewinding.
When poured into the varnish to the rotor coil, because the center of rotation is eccentric,
There is a need to take a dynamic balance again.

DC High-Potential Test

The DC hi-pot test is applied at above the rated voltage of a transformer to evaluate the condition of winding insulation. The DC high-voltage test is not recommended on power transformers above 34.5 kV; instead the AC hi-pot test should be used. 
Generally, for routine maintenance of transformers, this test is not employed because of the possibility of damage to the winding insulation. However, this test is made for acceptance and after repair of transformers.
If the hi-pot test is to be conducted for routine maintenance, the AC test values should not exceed 65% of factory AC test value. The routine maintenance AC voltage value should be converted to an equivalent DC voltage value by multiplying it by 1.6, that is, 1.6 times the AC value for periodic testing (i.e., 1.6 × 65 = 104% of AC factory test value). The DC hi-pot test can be applied as a step-voltage test where readings of leakage current are taken for each step. If excessive leakage current is noticed, voltage can be backed off before further damage takes place. For this reason, the DC hi-pot test is considered to be a nondestructive test.
 Some companies conduct the AC hi-pot test at rated voltage for 3 min for periodic testing instead of the 65% of factory test voltage. The hi-pot test values for DC voltages are shown in Table 1.1.
The procedure for conducting this test is as follows (refer to Figure 1.1a and b for test connections):

FIGURE 1.1
Transformer high voltage (hi-pot) test connection: (a) high winding hi-pot test connection and
(b) low winding hi-pot test connections. 


Transformer must have passed the insulation resistance test immediately
prior to starting this test.
• Make sure transformer case and core are grounded.
• Disconnect all high-voltage, low-voltage, and neutral connections,
low-voltage control systems, fan systems, and meters connected to the transformer winding and core.
• Short-circuit with jumpers together all high-voltage bushings and all low-voltage bushings to ground as discussed under “Insulation resistance measurements.

Connect hi-pot test set between high-voltage winding and ground.
Gradually increase test voltage to the desired value. Allow test voltage duration of 1 min, after which gradually decrease voltage to zero.
• Remove low-voltage to ground jumper and connect hi-pot test set
between low-voltage winding and ground. Also connect the shortcircuited
high-voltage winding to ground. Gradually increase test voltage to desired value. Allow the test voltage duration of 1 min, after which gradually decrease voltage to zero.
• If the preceding two tests do not produce breakdowns or failures, the transformer is considered satisfactory and can be energized.
• Remove all jumpers and reconnect primary and secondary connections and other system equipment that may have been disconnected.
The following are some cautions and considerations in performing hi-pot
tests:
In liquid-filled transformers two insulation systems are in series, that is, solid insulation with oil or synthetic fluid. When AC or DC hi-pot test voltage is applied, the voltage drops are distributed as follows:

 Table 1.1
 

When using DC hi-pot test voltage on liquid-filled transformers, the solid insulation may be overstressed.
Insulation that may be weakened near the neutral may remain in service due to lower stress under operating conditions. However, when subjected to hi-pot test voltage, it may break down and require immediate repair. The weakened insulation may usually be detected by the measurement at lower voltages.
If a hi-pot test is to be conducted for routine maintenance, consider the following in advance: (1) assume that a breakdown will occur, (2) have replacement or parts on hand, (3) have personnel available to perform work, and (4) is the loss of the transformer until repairs are made beyond the original routine outage.

DC Testing Methods

After seeing how insulation behaves when DC voltage is applied to it, let us now take a look at the various tests that are conducted with this voltage. Two tests can be conducted on solid insulation with the application of DC voltage:

• Insulation resistance testing
• High-potential (Hi-pot) voltage testing

 Insulation Resistance Testing

This test may be conducted at applied voltages of 100–15,000 V. The instrument used is a megohmmeter, either hand cranked, motor driven, or electronic, which indicates the insulation resistance in megohms. An electronic megohmmeter is shown in Figure 1.1. The quality of insulation is a variable, dependent upon temperature, humidity, and other environmental factors.
Therefore, all readings must be corrected to the standard temperature for the class of equipment under test. The megohm value of insulation resistance is inversely proportional to the volume of insulation being tested. As an example, a cable 100 ft. long would have one-tenth the insulation resistance of cable 1000 ft. long, provided other conditions were identical. This test can be useful in giving an indication of deteriorating trends in the insulation system. The insulation resistance values by themselves neither indicate the weakness of the insulation nor its total dielectric strength. However, they can indicate the contamination of the insulation and trouble ahead within the insulation system if a downward trend continued in the insulation resistance values.

FIGURE 1.1
(a) Electronic megohmmeter, 5000 V and (b) 15 kV DC dielectric test set. (Courtesy of Megger, Inc., Valley Forge, PA.)

Insulation resistance measurement values can be accomplished by four
common test methods:
• Short-time readings
• Time-resistance readings (dielectric absorption ratio [DAR] test)
• Polarization index (PI) test
• Step-voltage readings  

High-Potential Voltage Test 

A DC hi-pot voltage test is a voltage applied across the insulation at or above the DC equivalent of the 60 Hz operating crest voltage (i.e., DC value = 1.41 times RMS value). This test can be applied as a step-voltage test. When the highpotential voltage is applied as a dielectric absorption test, the maximum voltage is applied gradually over a period of 60–90 s. The maximum voltage is then held for 5 min with leakage current readings taken each minute. When this test is applied as a step-voltage test, the maximum voltage is applied in a number of equal increments, usually not less than eight, with each voltage step being held for an equal interval of time. The time interval between each step should be 1–4 min. At the end of each interval, a leakage current or insulation resistance reading is taken before proceeding to the next step. A plot of test voltage versus leakage current or insulation resistance can then be drawn to indicate the condition of the insulation system. Routine maintenance tests are conducted with a maximum voltage at or below 75% of the maximum test voltage permitted for acceptance tests, or at 60% of the factory test voltage. 

Dielectric absorption test: The dielectric absorption test is conducted at voltages much higher than the usual insulation resistance test values and can exceed 100 kV. This test is an extension of the hi-pot test. Under this test, the voltage is applied for an extended period of time, from 5 to 15 min. Periodic readings are taken of the insulation resistance or leakage current. The test is evaluated on the basis of insulation resistance. If insulation is in good condition, the apparent insulation resistance will increase as the test progresses.
The dielectric absorption tests are independent of the volume and the temperature of the insulation under test.

DC Voltage Testing of Insulation

When DC voltage is applied to an insulation, the electric field stress gives rise to current conduction and electrical polarization. Consider an elementary circuit as shown in Figure 1.1, which shows a DC voltage source, a switch, and an insulation specimen. When the switch is closed, the insulation becomes electrifi ed and a very high current fl ows at the instant the switch is closed. However, this current immediately drops in value, and then decreases at a slower rate until it reaches a nearly constant value. The current drawn by the insulation may be analyzed into several components as follows:
Capacitance

 • Charging current
• Dielectric absorption current
• Surface leakage current
• Partial discharge current (corona)
• Volumetric leakage current
Capacitance charging current: The capacitance charging 
current is high as the DC voltage is applied and can be calculated by the formula:


FIGURE 1.1
Electrical circuit of insulation under DC voltage test.

where
ie is the capacitance charging current
E is the voltage in kilovolts
R is the resistance in megohms
C is the capacitance in microfarads
t is the time in seconds
e is Napierian logarithmic base

The charging current is a function of time and will decrease as the time of the application of voltage increases. It is the initial charging current when voltage is applied and therefore not of any value for test evaluation. Test readings should not be taken until this current has decreased to a sufficiently low value.

Dielectric absorption current: The dielectric absorption current is also high as the test voltage is applied and decreases as the voltage application time increases, but at a slower rate than the capacitance charging current. This current is not as high as the capacitance charging current. The absorption current can be divided into two currents called reversible and irreversible charging currents. This reversible charging current can be calculated by the formula:
where
ia is the dielectric absorption current
V is the test voltage in kilovolts
C is the capacitance in microfarads
D is the proportionately constant
T is the time in seconds
n is a constant
 
The irreversible charging current is of the same general form as the reversible charging current, but is much smaller in magnitude. The irreversible charging current is lost in the insulation and thus is not recoverable. Again, sufficient time should be allowed before recording test data so that the reversible absorption current has decreased to a low value. be eliminated by the use of stress control shielding at such points during tests.

Surface leakage: The surface leakage current is due to the conduction on the surface of the insulation where the conductor emerges and points of ground potential. This current is not desired in the test results and should therefore be eliminated by carefully cleaning the surface of the conductor to eliminate the leakage paths, or should be captured and guarded out of the meter reading.

Partial discharge current: The partial discharge current, also known as corona current, is caused by overstressing of air at sharp corners of the conductor due to high test voltage. This current is not desirable and should be eliminated by the use of stress control shielding at such points during tests. This current does not occur at lower voltages (below 4000 volts), such as insulation resistance test voltages.
Volumetric leakage current: The volumetric leakage current that flows through the insulation volume itself is of primary importance. This is the current that is used to evaluate the conditions of the insulation system under test.
 Sufficient time should be allowed for the volumetric current to stabilize before test readings are recorded. The total current, consisting of various leakage currents as described above, is shown in Figure 1.2.

 

Figure 1.2:

Various leakage currents due to the application of DC high voltage to an insulation system.