VLT® 2800 Series
Installation
105
Motor current, I
M, N
has been programmed to
the rated motor current (see motor nameplate).
■
Parallel connection of motors
The adjustable frequency drive is able to control
several motors connected in parallel. The combined
total current consumption of the motors is not to
exceed the maximum rated output current (I
INV
) for
the adjustable frequency drive. If the motors are to
have different rpm values, use motors with different
rated rpm values. Motor frequency is changed
simultaneously, which means that the ratio between
the rated rpm values is maintained.
Problems may arise at start-up and when operating
at low rpm if the motor sizes are widely different.
This is because the small motors
’
relatively high
resistance in the stator calls for a higher voltage
at the start-up and at low rpm.
In systems with motors connected in
parallel, the electronic thermal relay (ETR)
of the adjustable frequency drive cannot
be used as motor protection for the individual motor.
Protection must be provided individually for each motor.
■
Motor cables
See Technical data for correct sizing of motor cable
cross-section and length. Always comply with national
and local regulations on cable cross-section.
NOTE
If an unshielded/unarmoured cable is used,
some EMC requirements are not complied with,
see
EMC test results
in the Design Guide.
If the EMC specifications regarding emission are
to be complied with, the motor cable must be
shielded/armoured, unless otherwise stated for the
RFI filter in question. It is important to keep the
motor cable as short as possible so as to reduce the
noise level and leakage currents to a minimum. The
motor cable shield must be connected to the metal
cabinet of the variable frequency drive and to the metal
cabinet of the motor. The shield connections are to
be made with the biggest possible surface area (cable
clamp). This is enabled by different installation devices
in different variable frequency drives. Mounting with
twisted shield ends (pigtails) is to be avoided, since
these spoil the shielding effect at high frequencies. If
it is necessary to break the shield to install a motor
isolator or motor relay, the shield must be continued
at the lowest possible HF impedance.
■
Brake connection
NOTE
Voltages up to 850 VDC occur on the terminals.
No. 81 82
Brake resistor
R-
R+ terminals
Connect the brake resistor wiring to drive terminals 81
and 82. The connection cable to the brake resistor
must be shielded/armored. Connect the shield to
both the ground of the drive and the brake resistor by
means of cable clamps. Dimension the cross-section
of the brake cable to match the brake torque.
■
Ground connection
Since the leakage currents to ground may be higher
than 3.5 mA, the adjustable frequency drive must
always be connected to ground in accordance with
applicable national and local regulations. To ensure
that the ground cable has good mechanical connection
to terminal 95, the cable cross section must be a
minimum 7 AWG (10 mm
2
). To improve protection
MG.28.A8.22 - VLT is a registered Danfoss trademark
17
