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Affinity User Guide
127
Issue Number: 5 www.controltechniques.com
8.2 Current limits
The default settings for the current limit parameters are:
•
113% x motor rated current for open loop mode
•
114% x motor rated current for RFC mode
There are three parameters which control the current limits:
•
Motoring current limit: power flowing from the drive to the motor
•
Regen current limit: power flowing from the motor to the drive
•
Symmetrical current limit: current limit for both motoring and regen
operation
The lowest of either the motoring and regen current limit, or the
symmetrical current limit applies.
The maximum setting of these parameters depends on the values of
motor rated current, drive rated current and the power factor.
The drive can be oversized to permit a higher current limit setting to
provide higher accelerating torque as required up to a maximum of
1000%.
8.3 Motor thermal protection
The drive models the temperature of the motor using the motor rated
current (Pr
5.07
), the thermal time constant (Pr
4.15
), whether low speed
thermal protection mode has been enabled (Pr
4.25
) and the actual
current flowing at any point in time. Pr
4.19
gives the estimated motor
temperature as a percentage of maximum temperature.
The temperature of the motor (Pr
4.19
) as a percentage of maximum
temperature, with a constant current magnitude of I, constant value of K
and constant value of Motor rated current (Pr
5.07
) after time t is given
by:
Percentage motor temperature (Pr
4.19
) = [I
2
/ (K x Motor rated
current)
2
] (1 - e
-t/
τ
) x 100%
This assumes that the maximum allowed motor temperature is produced
by K x Motor rated current and that
τ
is the thermal time constant of the
point in the motor that reaches its maximum allowed temperature first.
τ
is defined by Pr
4.15
. If Pr
4.15
has a value between 0.0 and 1.0 the
thermal time constant is taken as 1.0.
The value of K is defined as shown in Figure 8-1.
Pr
4.25
can be used to select two alternative protection characteristics.
If Pr
4.25
is 0 the characteristic is for a motor which can operate at rated
current over the whole speed range. Induction motors with this type of
characteristic normally have forced cooling. If Pr
4.25
is 1 the
characteristic is intended for motors where the cooling effect of motor
fan reduces with reduced motor speed below 50% of base speed/
frequency. The maximum value for K is 1.05, so that above the knee of
the characteristics the motor can operate continuously up to 105%
current.
Figure 8-1 Motor thermal protection
Both settings of Pr
4.25
are intended for motors where the cooling effect
of the motor fan reduces with reduced motor speed, but with different
speeds below which the cooling effect is reduced. If Pr
4.25
is 0 the
characteristic is intended for motors where the cooling effect reduces
with motor speed below 15% of base speed/frequency. If Pr
4.25
is 1 the
characteristic is intended for motors where the cooling effect reduces
with motor speed below 50% of base speed/frequency. The maximum
value for K is 1.01, so that above the knee of the characteristics the
motor can operate continuously up to 101% current.
When the estimated temperature in Pr
4.19
reaches 100% the drive
takes some action depending on the setting of Pr
4.16
. If Pr
4.16
is 0, the
drive trips when Pr
4.19
reaches 100%. If Pr
4.16
is 1, the current limit is
reduced to (K - 0.05) x 100% when Pr
4.19
reaches 100%. The current
limit is set back to the user defined level when Pr
4.19
falls below 95%.
The thermal model temperature accumulator is reset to zero at power-up
and accumulates the temperature of the motor while the drive remains
powered-up. If the rated current defined by Pr
5.07
is altered, the
accumulator is reset to zero.
The default setting of the thermal time constant (Pr
4.15
) is 89s, which is
equivalent to an overload of 150% for 60s from cold.
The time for the drive to trip from cold with constant motor current is
given by:
T
trip
= -(Pr
4.15
) x ln(1 - (K x Pr
5.07
/ Pr
4.01
)
2
)
Alternatively the thermal time constant can be calculated from the trip
time with a given current from:
Pr
4.15
= -T
trip
/ ln(1 - (K / Overload)
2
)
The maximum value for the thermal time constant can be increased up
to a maximum value of 3000s to allow an increased overload if the motor
thermal characteristics permit.
0.00
0.70
1.00
Pr
=
0
Pr
=
1
4.25
4.25
1.00
1.01
Base speed/
frequency
50% of
base speed/
frequency
15% of
base speed/
frequency
K
Fire Mode - Important Warning.
When Fire Mode is active the motor overload and thermal
protection are disabled, as well as a number of drive
protection functions. Fire Mode is provided for use only in
emergency situations where the safety risk from disabling
protection is less than the risk from the drive tripping -
typically in smoke extraction operation to permit evacuation
of a building. The use of Fire Mode itself causes a risk of fire
from overloading of the motor or drive, so it must only be
used after careful consideration of the balance of risks.
Care must be taken to prevent inadvertent activation or de-
activation of Fire Mode. Fire Mode is indicated by a flashing
display text warning "Fire mode active".
Care must be taken to ensure that parameters Pr
1.53
or
Pr
1.54
are not inadvertently re-allocated to different inputs or
variables. It should be noted that, by default, Pr
1.54
is
controlled from digital input 4 and changing Pr
6.04
or Pr
8.24
can re-allocate this digital input to another parameter. These
parameters are at access level 2 in order to minimize the risk
of inadvertent or unauthorized changes. It is recommended
that User Security be applied to further reduce the risk
(section 5.10
Parameter access level and security
page 97). These parameters may also be changed via serial
communications so adequate precautions should be taken if
this functionality is utilized.
WARNING
Summary of Contents for Affinity
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