CHAPTER 8 MOTOR TESTING

8.1       GENERAL

The general requirements for the testing of all rotating electrical machines, including motors, are laid down in BS 4999 :1976, Part 60.

As far as induction motors are concerned, the principal requirements are:

·                     Manufacturer’s Tests

·                     On-site Test

·                     Insulation Resistance Testing.

8.2       MANUFACTURER’S TESTS

Manufacturer’s tests are given three classifications: ‘Basic’ (formerly called ‘Type Tests’), ‘Duplicate’ and ‘Routine Checks’.  Basic tests are mainly to prove a new design.  They include exhaustive tests to ensure that the design meets the specification and all other performance requirements.  They are normally carried out only on the ‘first of class’ motor, and a test certificate is usually provided to confirm the tests.  Basic tests may, on special request, be repeated on the first machine of a new, large order, but this is not usual.

Duplicate tests are for performance.  They are applied to a motor that is of the same design and construction as one previously made (and in no way altered) and which has already undergone basic tests.  The duplicate tests are to ensure that the motor is still in accordance with the original design.

Routine checks are tests to show that each motor has been assembled correctly, is able to withstand the appropriate high-voltage tests and is in sound working order both electrically and mechanically.

The three classes of test are listed in Table 1.

TABLE 1 - MANUFACTURER’S TESTS

Test	Basic	Duplicate	Routine
Resistance of windings (cold)	X	X	-
No-load losses and current	X	X	X
Locked rotor - current	X	X	-
- torque	X	-	-
Secondary induced voltage    (wound-rotor type only)	X	X	X
Temperature rise	X	-	-
Power factor and efficiency	X	-	-
Momentary overload	X	-	-
High voltage	X	X	X
Vibration	X	-	-

Most of these tests are self-evident, but two are further explained overleaf.

8.2.1    Locked Rotor Test

This test is carried out with the rotor locked and at a voltage reduced so that the stator and rotor currents do not exceed their normal full-load values.  It simulates the standstill situation at the moment of starting without motion actually taking place.  By measuring the current and torque at this reduced voltage, the actual standstill current and torque when starting at full rated voltage may be calculated.

8.2.2    High Voltage Test (also called a ‘Withstand’ test)

At this test a high voltage is applied between the frame and all the motor windings (the stator windings only in the case of squirrel-cage motors), with all other conductors, metal and auxiliary (i.e. heater) circuits bonded to the frame.  The actual voltage applied is in accordance with Table 2 and is sustained for one minute.  It may be at any frequency between 25Hz and 100Hz.  It is primarily an insulation test for the motor’s windings and is included also in the routine checks to ensure that there has been no fault during assembly of any individual motor.

TABLE 2 - HIGH VOLTAGE TESTS

Windings	Test Voltage (rms)
Motor stator windings:
	500V + twice rated voltage
>1 000V, <1kva span="span">	1 000V + twice rated voltage
1 – 10 000kVA	1 000V + twice rated voltage, min 1 500V
>10 000 kVA and
<2 000v="000v" span="span">	1 000V + twice rated voltage
2 000 — 6 000V	2.5 times rated voltage
6 000 — 17 000V	3 000V + twice rated voltage
>17 000V	Special agreement
Rotors of wound-rotor machines: (non-reversing motors)	1 000V + twice open-circuit standstill  voltage with normal stator voltage applied.  Min 1 500V

8.3       ON SITE TESTS

Any motor installed on an offshore or onshore installation may be assumed to have undergone its full routine check tests, and its prototype a full basic or duplicate test.  On-site tests are therefore only needed to check the original installation and commissioning, and thereafter to ensure that no deterioration has taken place.  The remainder of this chapter deals only with tests for deterioration.

Deterioration can occur for many reasons: among them are entry of dampness or water leakage in the motor or cable-entry box, overheating of the windings due to overloading or prolonged stalling, or mechanical faults such as vibration or bearing failure.

Both dampness and overheated windings can cause reduced insulation resistance of the windings.  After drying out, the motor should be insulation-tested to ensure that insulation resistance has been restored.  Deterioration can be progressive, especially when a motor is little used, and a regular programme of insulation resistance testing every motor should be drawn up and the results logged.  After temperature correction (see para 8.4) the resistance levels should be plotted, and, if there is progressive deterioration, this will be immediately apparent.

After repairs to a motor set an insulation resistance test should always be carried out before reconnection if the motor or its connections have in any way been interfered with.

High voltage withstand tests should never be needed on site unless a major overhaul has been carried out, in which case it would be an engineering or manufacturer’s concern.

8.4       INSULATION RESISTANCE TESTING

Instruments such as a ‘Megger’ are provided to operate at 250V, 500V or 2 500V, and the correct one must be used depending on the rated voltage of the motor to be tested.  Normally motors over 415V and high-voltage motors would require a 2 500V megger.  Motors in the 100V range would need the 500V instrument; a higher-voltage instrument might itself damage the insulation.

When the tester is connected and the handle wound up, the voltage should continue to be applied until the needle settles down to a steady value; this might take one minute or more.

When testing the insulation resistance of a winding, all other conductors, metalwork (stator and rotor) and auxiliary circuits such as for temperature protection and heaters should be connected to the frame with light wire (fuse wire will do), and the test voltage applied between winding and frame.  Where the three phase windings are independent and

FIGURE 8.1

INSULATION RESISTANCE TEMPERATURE COEFFICIENT

brought out to six terminals (fairly rare), it is advisable to make a test also between pairs of windings.  However, most motors are star-connected with their star-point internal and permanently made, so inter-phase tests are not possible.

Insulation resistance is very dependent on temperature, and, in order to compare one reading with another, it is necessary to reduce the value to a common temperature.  This is usually 40°C.  Unlike the resistance of a conductor, which rises with temperature, the resistance of insulation falls rapidly with increase of temperature.

The graph of Figure 8.1 is used to make this correction by means of a ‘temperature coefficient’.

For example, if the observed reading (R_t) is 10 megohms when taken at 70°C, then, using the graph, the temperature coefficient (K_t) is 8.0, and the corrected reading (R₄₀) at 40°C is then 10 x 8.0 = 80 megohms.  It can be seen from this example that the correction is considerable when the winding is hot at normal working temperatures.

The recommended minimum value of insulation resistance for items of plant is given in manufacturers’ literature.  As a guide where precise information is not available, the minimum acceptable value (R_m) for a motor stator winding is given by

R_m = (kV + 1) megohms when corrected to 40°C,

where kV is the motor’s rated voltage in kilovolts. Thus: