IMO Precision Controls Jaguar VXS, Руководство пользователя

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9.1 Introduction
When a rotating mass is electrically braked to
a standstill by reducing the frequency of the
supply at the motor terminals, the motor effec-
tively assumes the characteristics of an asyn-
chronous generator for at least some period of
the deceleration time. The kinetic energy of
the load is converted by the motor to electrical
energy.
As the input-side rectifier of the inverter is not
designed to deliver current into the supply
system, the voltage of the inverter DC bus is
thus caused by the regeneration to increase,
and may rise to an unacceptable level if not
controlled. The inverter would then trip
(overvoltage trip code OU). Control of decel-
eration would be lost; the motor would be
freewheeling.
The DC bus can absorb, typically, between
3% and 5% of regenerative power, equivalent
to normal heat losses when driving.
To absorb higher levels of regenerative energy
it is necessary to convert excess DC voltage
into heat by delivering the regenerative cur-
rent to a resistor in short pulses controlled by
transistor switching.
The factors which influence the amount of
regenerative power to be dissipated are:
The speed of the motor and load in rpm;
the inertia of the motor and load, J
M
+ J
L
;
the deceleration rate,
∆ω
/
∆
t.
The rating of the resistor will depend on the
duty cycle, illustrated above.
9.2 Overhauling Loads
The braking components may be active not
only when a high inertia load is being deceler-
ated but also when the regenerative period has
to be constant and prolonged, as in a hoisting
application when the load, which may be up to
the maximum for which the inverter is rated,
must be lowered at a controlled speed.
In this case, the mass of the load would
attempt to overspeed the motor for a large part
of the duty cycle, again with the consequence
that the DC bus voltage would become exces-
sive. Obviously, it would be dangerous to
permit the inverter to trip, as the load would
then be out of control.
Careful consideration must be given to the
selection of the resistor to be of adequate
capacity for this duty.
A worked example of calculations for the
ohmic rating and power (capacity kW) of a
braking resistor is given on the following
page.
NOTES
1 For an external braking circuit, Function
33 must be set to 1.
2 Attention is drawn to the table of mini-
mum braking resistor values on the
following page.
55
9 Braking
Braking
time
Duty cycle
Cooling
time
T
0
Braking power
T
1
T
1
WARNING Inverter braking functions are
not a substitute for mechanical braking
devices designed to ensure safety.