These disadvantages are practically eliminated by use of the split-core AC volt-ammeter. See Fig. 5-57. This instrument combines an AC voltmeter and AC split- core ammeter into a single pocket-size unit with a con- venient range switch to select any of tile–multiple voltage ranges or current ranges. See Fig. 5-58. With the split-core ammeter, the line to be tested does not have to be disconnected from its power source.
This type of ohmmeter uses the transformer princi- ple to connect the instrument into the line. Since any conductor carrying alternating current will set up a changing magnetic field around itself, that conductor can be used as the primary winding of the transformer.
The split-core ammeter carries the remaining parts of the transformer, which are the laminated steel core and
Fig. 5-57 Clamp-on volt-ampere-ohmmeter with rotary scale.
(Amprobe)
Fig. 5-58 The clamp-on volt-ampere-ohmmeter with parts labeled.
To locate the grounded portion of the winding, dis- connect the necessary connection jumpers and test.
Grounded sections will be detected by a full-line volt- age indication.
Testing for Opens
To determine whether a winding is open, connect test leads as shown in Figs. 5-61 and 5-62. If the winding is open, there will be no voltage indication. If the circuit is not open, the voltmeter indication will read full-line voltage.
the secondary coil. To get transformer action, the line to be tested is encircled with the split-type core by sim- ply pressing the trigger button. See Fig. 5-59. Aside from measuring terminal voltages and load currents, the split-core ammeter-voltmeter can be used to track down electrical difficulties in electric-motor repair.
Testing for Grounds
To determine whether a winding is grounded or has a very low value of insulation resistance, connect the unit and test leads as shown in Fig. 5-60. Assuming the available line voltage is approximately 120 V, use the unit’s lowest voltage range. If the winding is grounded to the frame, the test will indicate full-line voltage. A high-resistance ground is simply a case of low-insulation resistance. The indicated reading for a high-resistance ground will be a little less than line voltage. A winding that is not grounded will he evi- denced by a small or negligible reading. This is mainly due to the capacitive effect between the winding and the steel lamination.
Split-Core AC Volt-Ammeter 135 Fig. 5-60 Testing for and open with test leads.
Fig. 5-59 Find the location of a grounded phase of a motor.
(Amprobe)
Fig. 5-61 Isolating an open phase. (Amprobe)
Fig. 5-62 Locating an open in a motor. (Amprobe)
Checking for Shorts
Shorted turns in the winding of a motor behave like a shorted secondary of a transformer. A motor with a shorted winding will draw excessive current while running at no load. Measurement of the current can be made with- out disconnecting lines. This means you engage one of the lines with the split-core transformer of the tester. If the ampere reading is much higher than the full-load ampere rating on the nameplate, the motor is probably shorted.
In a two- or three-phase motor, a partially shorted winding produces a higher current reading in the shorted phase. This becomes evident when the current in each phase is measured.
Testing Squirrel-Cage Rotors
In some cases, loss in output torque at rated speed in an induction motor may be due to opens in the squirrel- cage rotor. To test the rotor and determine which rotor bars are loose or open, place the rotor in a growler as shown in Fig. 5-63. Set the switch to the highest current range. Switch on the growler and then set the test unit to the appropriate current range. Rotate the rotor in the growler and take note of the current indication whenever the growler is energized. The bars and end rings in the rotor behave similarly to a shorted secondary of a trans- former. The growler winding acts as the primary. A good rotor will produce approximately the same current indi- cations for all positions of the rotor. A defective rotor will exhibit a drop in the current reading when the open bars move into the growler field.
Testing the Centrifugal Switch in a Split-Phase Motor
A faulty centrifugal switch may not disconnect the start winding at the proper time. To determine conclu- sively that the start-winding remains in the circuit,
place the split-core ammeter around one of the start- winding leads. Set the instrument to the highest current range. Turn on the motor switch. Select the appropriate current range. Observe if there is any current in the start-winding circuit. A current indication signifies that the centrifugal switch did not open when the motor came up to speed. See Fig. 5-64.
Test for Short Circuit Between Run and StartWindings
A short between run and start windings may be deter- mined by using the ammeter and line voltage to check for continuity between the two separate circuits. Dis- connect the run- and start-winding leads and connect the instrument as shown in Fig. 5-65. Set the meter on voltage. A full-line voltage reading will be obtained if the windings are shorted to one another.
Test for Capacitors
Defective capacitors are very often the cause of trouble in capacitor-type motors. Shorts, opens, grounds, and insufficient capacity in microfarads are conditions for which capacitors should be tested to determine whether they are good.
To determine a grounded capacitor, set the instru- ment on the proper voltage range and connect the instru- ment and capacitor to the line as shown in Fig. 5-66. A full-line voltage indication on the meter signifies that the capacitor is grounded to the can. A high-resistance ground will be evident by a voltage reading that is somewhat below line voltage. A negligible reading or a reading of no voltage will indicate that the capacitor is not grounded.
To measure the capacity of the capacitor, set the test unit’s switch to the proper voltage range and read the line-voltage indication. Then set to the appropriate
Fig. 5-63 Testing a squirrel-cage rotor. (Amprobe)
Fig. 5-64 Testing a centrifugal switch on a motor.
current range and read the capacitor-current indica- tion. During the test, keep the capacitor on the line for a very short period of time, because motor-starting electrolytic capacitors are rated for intermittent duty.
See Fig. 5-67. The capacity in microfarads is then computed by substituting the voltage and current read- ings in the following formula, assuming that a full 60-Hz line was used:
An open capacitor will be evident if there is no cur- rent indication in the test. A shorted capacitor is easily detected. It will blow the fuse when the line switch is turned on to measure line voltage.
Microfarads 2650 amperes volts
= ×
Split-Core AC Volt-Ammeter 137 Fig. 5-65 Test for finding a winding short circuit. (Amprobe)
Fig. 5-66 Test for finding a grounded capacitor. (Amprobe)
Fig. 5-67 Measuring the capacity of a capacitor. (Amprobe)