bonitron M3612EC, Customer Reference Manual

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User’s Manual
25
7.
A
PPLICATION
N
OTES
7.1.
A
PPLICATION
C
ONSIDERATIONS
There are some issues that should be considered when designing the complete
system.
7.1.1.
D
RIVES
Most variable frequency AC drives are suitable for use with the M3612EC.
Some things to consider are listed below.
Check the manual for the drive you are using, or call the technical support
line for the drive manufacturer if you have questions on this hookup.
7.2.
S
YSTEM
V
OLTAGE AND
S
OURCE
I
MPEDANCE
The M3612EC is intended to be used with an input reactor for the reduction of peak
input currents and bus voltage ripple and ripple current. The reactor adds inductive
impedance to the circuit to reduce these factors, and adds to the total input impedance
of the system. Other sources of inductive impedance in your installation are the main
incoming transformer and the conductors to that transformer. If the total input
impedance is too high, it can cause low voltage at the input of the power source, and
therefore the DC bus of the drive. This is referred to as a “soft” source, and will have
a transformer closely matched to the load and high impedance.
If your system is running hot, or blows input fuses under normal loads, an input reactor
can reduce the RMS input currents and the capacitor bank ripple current to allow the
system to operate within tolerances.
Transformers are rated in kVA and percent impedance. In order to see what kVA your
system needs, you can roughly multiply the horsepower by 1000. Your transformer
should be rated higher than this. In other words, a 50 horsepower system would
require at least a 50kVA transformer, if not larger. If the transformer has 5%
impedance, the voltage drop to the output of the transformer at full load will be 5%.
For instance, a 480V transformer with 5% impedance can only have 456VAC at the
terminals at full load. If the transformer is much larger than the required kVA, or has
a low percent impedance, the source is considered to
be “stiff” and may not require
further consideration.
If the input impedance is too high to the system, the input to the power source can
drop to the point where the DC bus of the drive will be out of specifications. When the
DC bus falls too low, the motor can lose power or run hotter than usual. If the DC bus
falls low enough, the drive will trip and not operate the motor. This typically is shown
as an “Undervoltage” fault on the drive.
There are situations where the existing system impedance may be high enough that
the input reactor may be bypassed. The following steps will help determine if you
should bypass the reactor.

Measure the AC voltage
at the input to the reactor
(transformer side) with the unit
disabled or turned off. If the AC voltage is lower than 95% of the nominal value,
the transformer taps should be adjusted to raise the incoming voltage. If the AC
voltage is between nominal and 110% continue to the next step.

Enable the power supply and check the voltage at the output of the reactor (System
side) while the system is running at full power. If the voltage drops to below 90%
of the nominal value, check the DC bus voltage of the drive, and make sure it is
well within the operating range of the drive’s specifications.