When starting a 3-phase motor, if the motor
shown at position B between L1-L2. A voltage
fails to start and gives a loud hum, you should
reading shows a good fuse in L1. Likewise, check
stop the motor by pushing the STOP switch.
the other two fuses between L1-L3 and L2-L3.
These symptoms usually mean that one of the
A no-voltage reading would show a faulty fuse.
phases to the motor is not energized. You can
If the line fuses check good and the main
assume that the control circuit is good. This is
contacts are closed, then check the voltage
because the main coil has operated and the
between terminals T1-T2, T2-T3, and T1-T3. The
auxiliary contacts are holding the main contactor
controller is faulty if there are no voltmeter
closed. Look for trouble in the power circuit (the
readings on all three of the terminal pairs. You
main contacts, overload relays, cable, and motor).
would then proceed to check the main contacts,
controller. However, if there is voltage at all three
LEAD-ACID STORAGE BATTERIES
terminals, the trouble is either in the motor or the
lines leading to the motor.
Storage batteries provide the power source for
the UPS battery system. Periodic inspection of
the storage battery is essential in maintaining
CONTROL CIRCUIT ANALYSIS
maximum efficiency and long life of the battery.
Batteries used for UPS systems are subjected to
Suppose the overload reset buttons have been
moderately heavy use. They require frequent
reset and the START switch closed. If the main
charging by the UPS battery charger or rectifier.
contacts do not close, check the control circuit.
A testing procedure follows.
1. Check for voltage in the controller at L1,
The ratio of the weight of a certain volume
L2, and L3.
of liquid to the weight of the same volume of
2. Place the voltmeter probes at points C and
water is called the specific gravity of the liquid.
D (fig. 10-10). You should have a voltage
The specific gravity of pure water is 1.000.
reading when the STOP switch is closed.
Sulfuric acid has a specific gravity of 1.830. A
You should have a no-voltage reading when
mixture of sulfuric acid and water is called
the STOP switch is open. The conditions
electrolyte. The specific gravity of electrolyte will
would indicate a good STOP switch and
vary from 1.000 to 1.830.
control circuit fuse.
As a storage battery discharges, the sulfuric
3. Next, check the voltage between points C
acid is depleted. The electrolyte is gradually
and E. The START switch is good if you
converted into water. This action is a guide in
get a no-voltage reading when the START
determining the state of discharge of the lead-acid
switch is open. The START switch is also
cell. The maximum specific gravity of electrolyte
good if you get a voltage reading when the
placed in a lead-acid battery should be 1.350 or
START switch is closed.
less. Generally, the specific gravity of the
4. Place the voltmeter probes at points C and
electrolyte in standard storage batteries is adjusted
F. A voltage reading with the START
between 1.210 and 1.220.
switch closed would indicate a good OL1.
It would also indicate one of the follow-
Measuring Specific Gravity
ing components is open: OL3, the main
coil, the control fuse, or the connection to
The specific gravity of the electrolyte is
measured using a hydrometer, which was
5. Place the voltmeter probes at points C and
discussed in chapter 3. Figure 3-40 shows a
G. Close the START switch. A no-voltage
hydrometer. The hydrometer used with portable
reading locates the trouble in the control
storage batteries has a float with a scale from
circuit to OL3.
1.100 to 1.300. When taking a reading, ensure that
the electrolyte in a cell is at the normal level. If
A faulty auxiliary contact is indicated under
the level is below normal, you cannot draw
the following conditions: (1) The system operates
enough fluid into the tube to cause the float to
only as long as the START switch is held in the
rise. If the level is above normal,' there is too much
ON position; and (2) when the switch is released,
water. Then the electrolyte is weakened, and the
the system will shut down.