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How insulation resistance of a motor winding checked If the insulation resistance is low What is likely to cause?
LesegoNyoka
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∙ 12y ago
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Why must do the insulation testing in circuit?
Insulation resistance testing involves applying a voltage across a dielectric, and measuring the resulting current and resistance. However, in order to get a proper reading, any electrical charge still within the system needs to be taken out, so as not to mess with the readings.
What is the formula to calculate AC resistance if the DC resistance is known?
AC resistance is the term used to describe the elevated value of resistance due to the reduction in a conductor's effective cross-sectional area due to the the skin effect caused by an alternating current. The skin effect describes how AC current tends to flow towards the surface of a conductor, rather than being distributed across the entire cross-sectional area as is the case for a DC current.For 50/60 Hz supplies the skin effect is not great and, so, there will not be a great difference between the DC resistance and AC resistance of a winding. Accordingly, it will be very difficult to calculate the winding's AC resistance, as its value is likely to be masked by experimental error and the accuracy of the instruments.In theory, by using a wattmeter and an ammeter, the resistance of a load -in this case a winding- the readings can be inserted into the equation: R = P/I2. Using a DC supply will reveal the (DC) resistance value, and using an AC supply will reveal the AC resistance value. As explained, a combination of experimental error and instrument accuracy is very likely to mask any actual difference between the two resulting values.So, at mains' frequencies, the difference between AC resistance and actual (DC) resistance would be very difficult to determine with any degree of accuracy.
What will be change in resistance of a wire if you stretch it?
If you stretch a wire, it would become longer, and therefore thinner. The cross sectional area will decrease and so the resistance will increase. in the real world, if it's stranded wire, the strands will likely break. Solid wire would not stretch evenly, and in either case insulation will tear, so I wouldn't recommend stretching a wire.
What will happen if DC supply is given to the transfomer?
A capacitor resists a change in voltage. Initially, a capacitor given a DC voltage will appear to have very low resistance, but as current flows and time goes by, the resistance will increase as the voltage approaches the applied voltage. At equilibrium, the voltage across the capacitor will be equal to the applied voltage, the current will be zero, and the resistance will be infinity.
What happens if resistance decreased in earthing?
It will work properly! The resistance and impedance of the earth lead must be low, though the corollary is that it must be capable of passing any likely fault current safely.
Is the earth fault close to neutral end of an wye connected winding very common?
Probably not, because the potential (with respect to earth) of a point on the winding, close to its neutral end, will be relatively low. Accordingly, it is less likely that the insulation will fail -the most common cause of electrical faults.
Why must do the insulation testing in circuit?
Insulation resistance testing involves applying a voltage across a dielectric, and measuring the resulting current and resistance. However, in order to get a proper reading, any electrical charge still within the system needs to be taken out, so as not to mess with the readings.
Identify one item that needs to be disconnected prior to carrying out insulation resistance tests?
The voltage source needs to be disconnected prior to carrying out insulation resistance tests. Disconnect any electronic devices like Variable Frequency Drives, Soft Starters etc. These devices are likely to get damaged if meggar test is done keeping them in circuit.
What is the formula to calculate AC resistance if the DC resistance is known?
AC resistance is the term used to describe the elevated value of resistance due to the reduction in a conductor's effective cross-sectional area due to the the skin effect caused by an alternating current. The skin effect describes how AC current tends to flow towards the surface of a conductor, rather than being distributed across the entire cross-sectional area as is the case for a DC current.For 50/60 Hz supplies the skin effect is not great and, so, there will not be a great difference between the DC resistance and AC resistance of a winding. Accordingly, it will be very difficult to calculate the winding's AC resistance, as its value is likely to be masked by experimental error and the accuracy of the instruments.In theory, by using a wattmeter and an ammeter, the resistance of a load -in this case a winding- the readings can be inserted into the equation: R = P/I2. Using a DC supply will reveal the (DC) resistance value, and using an AC supply will reveal the AC resistance value. As explained, a combination of experimental error and instrument accuracy is very likely to mask any actual difference between the two resulting values.So, at mains' frequencies, the difference between AC resistance and actual (DC) resistance would be very difficult to determine with any degree of accuracy.
What will happen to the transformer if rated voltage is applied to the winding during short circuit test?
If rated voltage is applied to Transformer during S/C test, The secondary winding will burn out due ta heavy current flow through the winding. During S/C test the secondary winding is short circuited so the impedance between phase and neutral is very low(only winding resistance). But the voltage across the secondary winding is rated hence heavy current flows through the winding, as I=V/Z. it depends which rated voltage is applied. if you are talking about primary winding voltage, transformer should withstand the primary rated voltage it's been designed for (OR it has been poorly designed). Otherwise, if rated voltage is the insulation voltage between a winding and earth OR winding-to-winding, you just have to check if: 1 - it is higher than the maximum primary winding voltage the transformer can withstand (could be, could not be..). Then, you can guess if your transformer is likely to burn or not. 2 - your test setup (usually a HV generator connected between primary and secondary winding) can deliver the requested current for the setup. I guess this won't be the case, since HV testers are usually designed to generate high voltages, but very small output currents.
Which type of winding would most likely be used in a high output alternator?
delta
What is a health hazard an insulation worker is likely to encounter on the job?
Abestos
What happen if primary of transformer connected to dc?
Nothing noticeable. DC power is not transmitted between the coils of a transformer. There would be no current on the other side of the transformer, unless the power of the source was constantly modulated. bescause flux does not change its state.after some time excessive heat is produced and winding may burnAnswerA transformer's primary winding presents two forms of opposition to current flow. The first is resistance, which is dependent upon the length, cross-sectional area, and resistivity of the wound conductor. The second is reactance, which depends upon the inductance of the winding and the frequency of the supply. Resistance opposes both AC and DC currents, while reactance opposes only AC current. In the case of a transformer's windings, the resistance is relatively low while the reactance is very high. When an AC voltage is applied, the reactance is sufficiently high to limit the value of AC current flowing through the winding. However, the resistance is so low that a large current would flow if an equivalent DC voltage was applied instead. This current would likely overheat the winding's insulation, resulting in its breakdown, causing a short circuit(s) which could severely damage the transformer.
Will homeowners insurance pay for mold removal if it was caused by insulation in crawl space?
mold cannot be caused by insulation - likely location of insulation and cause of moisture will need to be determined for coverage analysis - contact your agent
What causes a melting plastic or wax smell from car?
The problem is most likely electrical. Wires could be bare and burning their insulation off. You should probably get your vehicle checked by a mechanical. If the wired continue to burn, it could start a fire that could burn your entire vehicle.
What is the most likely cause for resistance to vary in a circuit?
heat
Is a megohm test a valid test for checking an air conditioner compressor?
Yes. A megohm test is a way of determining the condition of the compressor's winding insulation. If the insulation is deteriorated or beginning to deteriorate, a megohm test will show this. Generally speaking, 20 megohms or less shows bad motor winding insulation. When AC voltage is applied to a compressor that has bad windings, it will likely blow a fuse as it will short to ground. Moisture in the system can cause a false megohm reading. Also, it is not advisable to test a system that is in a vacuum, as this also can cause a false reading. If a compressor is grounded, a regular multimeter set to ohms will determine this (one probe on any of the compressor terminals and one to the discharge or suction line pipe). A megohm meter will show that a problem is eminent even if the compressor is not yet fully grounded and blowing fuses.
