3 Installation
3.3 Assembly
DB1 variable frequency drive
11/20 MN040031EN
Eaton.com
53
The required maximal thermal resistance R
th
of the cooling system depends
on the power loss P
V
of the variable frequency drive and the difference bet-
ween the temperature T
CP
on the variable frequency drive cooling surface
and the ambient temperature T
AMB
in the control panel. The greater the tem-
perature difference, the less cooling is required.
The power loss P
V
is calculated from the efficiency
η
F
of the variable
frequency drive and the power delivered to the motor.
The power delivered to the motor is calculated from the motor voltage U
M
,
the motor current I
M
and the motor's power factor cos
ϕ
.
Overall, one obtains for the power loss P
L
:
The thermal resistance value calculated in this way must also be effective.
If, for example, a heat sink is used with the calculated thermal resistance and
with a larger cooling surface than that of the variable frequency drive, it is
assumed that the effective thermal resistance is greater in this case. In this
instance, you should contact the manufacturer of the heat sink.
In other cases, existing surfaces such as a mounting plate should be used as
cooling surfaces. If the thermal resistance is unknown, measurements can
determine whether the existing cooling type is sufficient. During these mea-
surements, the temperature on the variable frequency drive cooling surface
must be measured under normal operating conditions (ambient temperature,
motor load, closed control panel doors).
The temperature rise will be delayed due to the thermal time constant.
If the temperature exceeds the maximum permitted value, the measure-
ments must be interrupted and cooling improved.
The DB1 variable frequency drive self-monitors its internal temperature and
switches off if required.
The value calculated above for thermal resistance R
th
is the maximum per-
mitted value in the relevant application. The lower the thermal resistance,
the lower the temperature on the cooling surface and inside the variable
frequency drive.
During installation, it is important to ensure that the cooling air can circulate
adequately and that there are no hotspots.
R
th
=
T
CP
- T
AMB
P
V
P
V
=
√
3
·
U
M
· I
M
·
cos
ϕ
· (1 -
η
F
)