Pump/Motor - Theory Of Operation; Pump Motor Electrical Evaluation - Oshkosh JLG 1930ES Service Maintenance Manual

SECTION 4 - HYDRAULICS
4.3

PUMP/MOTOR - THEORY OF OPERATION

The Power Module (see Figure 3-18., ZAPI Power Module
Location on page 3-24) is essentially a "low-side" switch
for the pump motor. The positive terminal of the pump
is tied to Battery Positive after the Line Contactor. The
negative terminal of the pump connects to the P Termi-
nal of the Power Module, which switches current
through MOSFET transistors to the Battery Negative.
For variable speed pump operation, the MOSFET transis-
tors switch On and Off at high frequencies (16kHz). The
Duty Cycle is varied to control the voltage applied to the
pump motor. When the MOSFET's spend 50% of the
period On and 50% Off, approximately ½ of the avail-
able Battery Voltage will be applied to the pump motor.
Similarly, the MOSFET are On continuously (100% Duty
Cycle) to apply all available Battery Voltage to the pump
motor (as in Lift Up at full speed).
When the Control System is energized, the voltage at
the P Terminal will be approximately +24V (referenced
to -B) when the pump is static. The P Terminal will be
approximately at +1V (referenced to -B) when the pump
is running at full speed (Lift Up from Ground Mode).
Series DC
Pump Motor
4-2

Pump Motor Electrical Evaluation

Several basic electrical tests can be performed on the
Pump Motor. Failure of one of these evaluations is signif-
icant and may indicate that the device is physically dam-
aged.
Refer to Figure 7-2., Resistance Measurement. Make all
measurements with a voltmeter set to resistance scale
(Ohms). Disconnect the Main Battery Disconnect and all
pump motor cables during this analysis.
• Resistance < 5 Ohms between Motor Termninals.
The internal windings are very low impedance and
should appear to be a short-circuit for an ordinary
voltmeter (other tests can determine if the windings
are truly shorted). High resistance can signal worn
brushes, a faulty commutator, or open windings.
• Resistance > 1 MegaOhms between Motor Termi-
nals and Motor Housing. The internal windings
should be electrically isolated from the motor hous-
ing. Low resistance may be an indication of a broken
motor terminal, damaged brush, faulty commutator,
or burned winding.
Common Difficulties
The following difficulties can be examined using the JLG
Analyzer, a voltmeter, and simple hand tools. Unless other-
wise noted, the Control System shall be energized in
Ground Mode during testing. For a convenient Ground
Reference, place the black meter lead on the negative post
of the left battery in the left-side battery compartment.
The vehicle should be placed on a firm, level surface for all
analysis.
Line Contactor
+B
P
-B
Power Module
24V
3121656