Motors Flashcards
- With reference to electrical motors:
a) List FOUR maintenance actions that are necessary for good operation. (8) b) Describe how to test for electrical faults within in an electrical motor. (8)
a) Make sure motor is tightly secured and is free from vibration.
Ensure regular insulation resistance readings are taken and values obtained recorded. Make sure that the motor shaft is turning freely.
Make sure the bearings are greased regularly but not overly.
Make sure motor body is kept clean; free from dust and moisture residues.
Make sure motor is dissipating heat correctly and it is well ventilated.
Make sure motor’s electrical connections are tightly secured and correct labels are secured. Regularly check the state of motor windings for integrity.
b) Insulation Resistance Test: This test is carried with the aid of an appropriately rated IR Tester (Megger). Before applying any electrical test, appropriate safety paperwork such as Risk Assessment and Permit to Work is completed; this will ensure that the motor is isolated and it is safe for the test to be carried out. To prove the basic operation of the tester, short the two probes together and press the ‘test’ button. The display should read 0Ω. For an IR test on a three phase machine, measure and log the phase-to-phase insulation resistance values. Three readings should be measured as U-V, V-W, W-U. Measure and log the phase to earth insulation resistance values. Three readings should be measured as U-E, V-E, W-E. Continuity Test: Appropriately rated multimeter set to ‘continuity’ is used, interconnections are removed so we have both the ends of each winding. Test probes are applied, in turn, to each end such as U1 and U2 and checking the multimeter to give continuity indication. Balance Test: This test is carried out with the aid of an appropriately rated multimeter. Resistance readings for each winding are obtained. A good motor will show that each winding resistance is similar to other windings.
- a) Describe how you would overhaul an electric motor that has been flooded with sea water. (8)
b) Explain how to check the insulation resistance of the motor, stating the minimum acceptable value. (8)
a) The motor should be removed from its fixings and dismantled into rotor and stator. Both the rotor and stator should then be bathed in fresh water to clean off the salts and other matter. They should then be allowed to dry before re-varnishing the stator. Once the varnish has dried, electrical tests should be carried out; a megger test, balance and continuity test. If all tests produce acceptable results the motor can be reassembled with new bearings and commissioned for service.
John Willison IAMI Revision 60
b) Before conducting the test it is ensured that the motor has been safely isolated and that certified and appropriately rated test instruments are being used.
On the terminal block of the motor each winding’s connections are separated. With the aid of a megger (500V DC for 440V AC motor), two sets of readings are obtained; 3 between phases and earth (UE, VE, WE). It is also ensured that obtained insulation resistance values are recorded for trending purposes. This allows engineers to predict any maintenance requirements. Minimum acceptable value is 1MΩ.
- With reference to the testing and maintenance of large electric motors:
a) State the minimum satisfactory value of insulation resistance. (2)
b) State the instrument AND the voltage to be used for the insulation resistance test. (4) c) State the regular maintenance that is necessary for good electric motor operation. (10)
a) 1MΩ.
b) A megger & 500V DC for a 440V AC motor.
c) Make sure motor is tightly secured and is free from vibration.
Ensure regular insulation resistance readings are taken and values obtained recorded. Make sure that the motor shaft is turning freely.
Make sure the bearings are greased regularly but not overly.
Make sure motor body is kept clean; free from dust and moisture residues.
Make sure motor is dissipating heat correctly and it is well ventilated.
Make sure motor’s electrical connections are tightly secured and correct labels are secured. Regularly check the state of motor windings for integrity.
- With reference to small 3 phase electrical motor, driving a transfer pump that is not used on a regular basis, failing to rotate when the start button is pressed:
a) Outline the procedure to determine the possible cause (8)
b) Explain the procedure to remove the motor should it be faulty and require changing (8)
a) There may be two general reasons for non-starting of the motor; mechanical and electrical. Mechanically, bearings may have become seized, or the gears may have been seized, in both the cases motor will overheat rapidly producing unusual noise as it is trying to rotate the pump. Ensure that the power supply is switched on and also is available at the motor terminals. This can tested with the aid of an appropriately rated potential indicator. If the power supply is proved to be available at the terminal block then this is indicating that the fault is inside the motor. Inside the motor, few electrical tests are carried out such as continuity test to see if the phase windings are continuous (electrically) and are not damaged. Once continuity of the windings has been proved, then other tests such as megger should be carried out.
b) Prior to work being carried out, motor and its attached pump is safely isolated both mechanically and electrically as per ship’s SMS requirements. Electrical isolation will involve
John Willison IAMI Revision 61
switching off the power, securing switch off so that it cannot re-energised even accidently, posting notices to let other engineers know of the work being carried out. Ensure correct PPE, tools, sufficient lighting, appropriate and certified lifting equipment are available. Motor is then uncoupled from the pump, safely stowed to a designated storage space. Work area itself secured until the fitting of new motor to protect the pump.
- State the procedure to be carried out if you found someone you suspected of having received an electric shock. (16)
First of all, I will call for help, for example letting the bridge know of the situation with brief details of the situation and asking for assistance.
I will quickly assess the situation to see if it is safe for me to approach the casualty; I will approach with extreme caution.
I will separate – with extreme caution – the casualty from the source of electric power. Depending on the situation, this could by switching off the power mains, if not possible, I may also use insulating material such as wood to remove the casualty from the source of electric power.
Once, removed from the source of electric power, I will assess the condition of casualty and provide necessary first aid that could be CPR procedure or even help with blood circulation through casualty’s body.
If conscious and breathing, I will put the casualty in recovery position and continue monitoring his/ her condition.
Use of defibrillator may be required.
If required, we might call for shore-side medical assistance.
