Testing of Three Phase Induction motor after manufacture or after installation on site are made in accordance with standard Specifications.
Routine testing of induction motor
- Insulation resistance test
- Winding resistances,
- Temperature rise, and
- Losses.
- Winding resistance measurement
- The direction of rotation
- Vibration testing on No load
- Current balance
- Temperature rise test
- Noise
- No-load running current measurement
Further tests on particular types of induction motor (e.g. commutation, starting) are required to obtain design data.
Insulation Resistance Test
`Megger’ is used for testing of the insulation resistance between windings and from windings to frame/ ground and must be performed before any live connections are made.
The insulation resistance (in mega-ohms) should not be less than 1M ohm, or less than V/(1+S) for a machine of rating S (in kilovolt-amperes),
where V is the rated voltage
If the measured insulation resistance is found low, then `dry out’ the machine to improve resistance.
The winding continuity having been checked, an insulation test should immediately precede a h.v. test and also be made prior to energizing the machine for the first time.
Temperature rise
The permissible temperature rise of a machine on rated load depends on the insulation class.
Temperatures are measured at a number of points (particularly at or near likely `hot spots’)
and, where possible, during a heat run with the machine operating at rated load, until a steady temperature has been reached.
The rated load may be a maximum continuous rating, or a short-time rating, or some special rating based on a duty cycle.
The duration of the heat run varies from 2 h for small machines to 8 h or more for large ones.
Temperature readings are taken (where feasible) every 15 or 30 min.
The following three methods of temperature measurement are in use.
Thermometer Mercury or alcohol thermometers may be used, the latter being preferable especially on large machines, as eddy currents in the mercury caused by stray fluxes may cause high readings; also mercury from a broken thermometer can cause damage to certain alloys.
Good contact must be made between the thermometer bulb and the surface concerned, and the bulb should be well covered by a non-heat-conducting material such as felt or putty.
The thermometers should be located on the stator core and windings and read at intervals throughout the run, and then affixed to the rotor core and windings and to the commutator, if any, as quickly as possible after shut-down.
They should be placed where temperatures are likely to be highest; however, only the surface temperature is measured, and not the true hot-spot temperature.
The resistance temperature coefficient of the winding material can give an average winding temperature..
Back-to-back:
If two similar machines are available, one as a motor can drive the other as a generator. The net input is then the total loss, which can be accurately measured.
This method can be applied for heat runs with comparative economy.
HV tests (high voltage)
The final test carried out before shipping a machine is the h.v. test in which a specified voltage at a frequency between 25 and 100 Hz is applied for 1 min between windings and earth and between windings.
The specified voltage is usually (twice rated voltage).The purpose of the test is to ensure that the insulation has a sufficient factor of safety to guard against fortuitous voltage transients which may occur in practice.
The test should, however, only be carried out once as repeated applications may damage the insulation. I
t may be desirable in some cases to repeat the test after the machine has been assembled on site, in which case a voltage of not more than 80% of the original test voltage should be applied
Routine test of induction motor
The following Tests are related specifically to three-phase induction motors.Most of the necessary data are obtained from the no-load and short-circuit tests.
No load: The motor is run on no load at rated voltage and frequency, and the current and power input are measured.
The input power supplies core and mechanical losses plus the stator I2R loss; the rotor I2R loss on no load can be neglected.
Short circuit (locked rotor): The short-circuit current and power are measured by holding the rotor stationary and applying a low voltage to the stator, it being usual to adjust the voltage to obtain full-load current.
In the case of a cage motor the starting torque may also be measured.
If the motor is connected for star-delta starting, the actual starting current and torque may be measured direct.
The short circuit current at full voltage is calculated by assuming the current to be proportional to voltage, although on some machines the short-circuit current is actually higher because
the leakage.
