JOHNSON CONTROLS
113
SECTION 4 - VSD OPERATION
FORM 160.84-OM1
ISSUE DATE: 9/21/2017
4
that the internal temperature of the OSCD does not ex-
ceed a safe operation level. Three Current Transform-
ers monitor the output current from the OSCD power
unit and are used to protect the drive and motor from
overcurrent conditions. Another 3 Current Transform-
ers monitor the input current to the OSCD to provide
current limit, ground fault detection, and information
for the harmonic current filtering.
OPTISPEED COMPRESSOR DRIVE DETAILS
(0490A, 0612A 0730A AND 1278A AMP
DRIVES)
The 0490A, 0612A and 1278A amp drives have the
same basic function as the 0490 and 0774 amp drives,
but the precharge of the drive is different.
The precharge is now completed through a precharge
and supply contactor, which are control by the drive
logic board. During the precharge period the drive log-
ic board will command the precharge contactor to close
for 12 seconds. During this time, the voltage will in-
crease across the bus capacitors, and the current will be
limited by the precharge resistors. This 12 second pe-
riod is called the precharge time. After 12 seconds has
passed, a small relay will close, and cause the supply
contactor to close. Shortly, after this time the precharge
contactor will open. The drive is now precharged and
ready to run. All of the precharge faults will remain the
same as when the Aux. power device is used to pre-
charge the drive.
The 0612A and 0730A amp drives contain only one
power assembly. It is very similar to the assembly used
on the 0490 and 0490A amp drives. The power assem-
bly contains 2 bus capacitors, with 6 power devices at-
tached to both of the outer faces of the bus capacitor.
A pair of power devices share a gate driver board. A
power supply board mounted on top of the power as-
sembly provides power to the gate driver boards.
The 1278A amp drive contains 3 power assemblies like
the 0774 amp drive, but 2 capacitors and more power
devices are required on each assembly. Due to the size
of the 1278A amp drive, it has 3 doors. The center and
right doors cannot be opened unless the circuit breaker
is placed in the off position, and the left door is open.
Harmonic Filter Benefits
The OptiSpeed Compressor Drive (OSCD) system
now includes an input Harmonic filter and high fre-
quency filter trap designed to meet the IEEE Std 519,
“IEEE Recommended Practices and Requirements for
Harmonic Control in Electrical Power Systems”. The
Harmonic filter provides a means to improve the input
current waveform drawn by the OSCD from the AC
line, thus reducing the possibility of causing electrical
interference with other sensitive electronic equipment
connected to the same power source, and reduce power
loss in the customer’s switch gear and source trans-
former.
In addition, the power factor of the system with this
Harmonic filter corrects the OSCD’s input power factor
to nearly unity over the full load range of the product.
Some passive filters may over correct the power factor
at lighter loads. Over correcting the power factor may
cause voltage ringing that may affect other equipment.
Harmonic Filter General Information
The Harmonic Filter is no longer an option for this
style of OSCD. The function of the Harmonic Filter is
now integrated into the input of the OSCD. The input
rectifier has fast switching transistors instead of SCR’s
and diodes. The OSCD can now control the input cur-
rent waveform to a near sine wave shape by controlling
how the rectifiers are turned on and off. The Harmonic
Filter of the past injected harmonic current into the in-
put of the OSCD to correct the current waveform.
Since the Harmonic Filter is basically the input to
the OSCD there is no need for the additional, pre-
charge, power unit, Harmonic Filter logic board, and
contactors for the Harmonic filter of yesterday. The
Harmonic Filter does not require its own pre-charge
time, thus allowing the chiller to start or restart sooner.
This is an important benefit to many customers. The
reductions in parts counts will improve the reliability
of the OSCD, and ease of repair. See Figure 45 on page
119 and Figure 46 on page 120.
A line inductor is still needed to limit the rate of change
in the input current. Without the line inductor the input
current cannot be properly controlled and harmonic
currents would be generated.
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