Parallel Connection Of Motors; Motor Cables; Thermal Motor Protection; Brake Connection - Danfoss VLT 2800 Design Manual

Installation

3.4.6 Parallel Connection of Motors

3 3
Illustration 3.29 Parallel Connection of Motors
The frequency converter is able to control several motors
connected in parallel. If the motors are to have different
RPM values, use motors with different rated RPM values.
Motor RPM is changed simultaneously, which means that
the ratio between the rated RPM values is maintained
across the range. The total current consumption of the
motors is not to exceed the maximum rated output
current I for the frequency converter.
INV
Problems may arise at the start and at low RPM values if
the motor sizes are widely different. This is because the
small motors' relatively high ohmic resistance in the stator
calls for a higher voltage at the start and at low RPM
values.
In systems with motors connected in parallel, the
electronic thermal relay (ETR) of the frequency converter
cannot be used as motor protection for the individual
motor. For this reason further motor protection must be
used, e.g. thermistors in each motor or an individual
thermal relay. (Circuit breakers are not suitable as
protection).
NOTICE
Parameter 107 Automatic motor adaption, AMT cannot be
used when motors are connected in parallel. Parameter
101 Torque characteristic must be set to [8] Special motor
characteristics when motors are connected in parallel.
50
Design Guide

3.4.7 Motor Cables

See chapter 5.3 General Technical Data for correct
dimensioning of motor cable cross-section and length.
Always comply with national and local regulations on
cable cross-section.
NOTICE
If an unscreened/unarmoured cable is used, some EMC
requirements are not complied with, see
chapter 5.1.21 EMC Compliance.
To comply with EMC specifications regarding emission, use
a screened/armoured motor cable, unless otherwise stated
for the RFI filter in question. Keep the motor cable as short
as possible to reduce the noise level and leakage currents
to a minimum. Connect the motor cable screen to the
metal cabinet of the frequency converter and to the metal
cabinet of the motor. The screen connections are to be
made with the biggest possible surface area (cable clamp).
This is enabled by different installation devices in different
frequency converters. Avoid mounting with twisted screen
ends (pigtails), since these spoil the screening effect at
high frequencies. If it is necessary to break the screen to
install a motor isolator or motor relay, the screen must be
continued at the lowest possible HF impedance.

3.4.8 Thermal Motor Protection

The electronic thermal relay in the frequency converter has
received UL-approval for single motor protection, when
1-28 Thermal Motor Protection is set to ETR Trip and
1-24 Motor Current is set to the rated motor current (see
the motor name plate).
For thermal motor protection, it is also possible to use the
MCB 112 PTC Thermistor Card option. This card provides
ATEX certificate to protect motors in explosion hazardous
areas, Zone 1/21 and Zone 2/22. When 1-28 Thermal Motor
Protection, set to [20] ATEX ETR, is combined with the use
of MCB 112, it is possible to control an Ex-e motor in
explosion hazardous areas. Consult the Programming Guide
for details on how to set up the frequency converter for
safe operation of Ex-e motors.

3.4.9 Brake Connection

No.
Table 3.7 Brake Connection
The connection cable to the brake resistor must be
screened/armoured. Connect the screen to the metal
cabinet of the frequency converter and to the metal
cabinet of the brake resistor with cable clamps. Dimension
Danfoss A/S © Rev. May/2014 All rights reserved.
81 82 Brake resistor
R- R+ terminals
MG27E402