11
Installation and maintenance - Three-phase induction motors
4850 en - 2018.11 / f
ASSEMBLY
2.4.3 - Other control systems
Frequency inverters, flux vector control, etc. Special
precautions need to be taken when standard induction motors
are being used for variable speed control, powered by a
frequency inverter or voltage controller:
The reference voltage (drive output or motor
input) is 400V at 50 Hz: The drive must deliver a
constant voltage/frequency signal to the motor in the
50 Hz operating range. Beyond the 25/50 Hz range,
ensure that the fan and bearing unit are suitable.
During prolonged operation at low speed, cooling efficiency is
greatly diminished. It is therefore advisable to install a forced
ventilation unit that will produce a constant flow of air
independently of the motor speed.
In prolonged operation at high speed, the fan may make
excessive noise. It is again advisable to install a forced
ventilation system.
1/3
2/3
1
0
1/3
2/3
1
N / Ns
Forced ventilation
(temperature rise)
Natural cooling
Forced ventilation
for N > 3600 min-1
Operating speed/
Synchronous speed
Effect of
cooling
P/PN = f (N/NS)
If the frequency exceeds 50 Hz:
a - Carefully check that all the components on a particular
transmission are properly aligned.
b - The voltage remains constant above 50 Hz.
c - The power supplied by the motor up to 60 Hz remains
constant (make sure that the power absorbed by the load does
not vary differently in this frequency range).
d - Check that the application speed does not exceed the
synchronous speed values:
- 2P --> 3600 min
-1
motors
- 4P --> 1800 min
-1
motors
- 6P --> 1200 min
-1
motors
e - For all other frequency and/or voltage limits, additional
precautions must be taken for derating, bearings, ventilation,
noise, etc: please consult Leroy-Somer.
Check that the vibration level of the assembled machine
complies with standard ISO 10816-3.
The user is responsible for protecting the motor and drive
equipment from hazardous currents and overvoltages in the
winding. These instructions cannot guarantee efficiency in all
cases.
2.4.4 - Permissible starting times and locked
rotor times
The starting times must remain within the limits stated below
on condition that the number of starts per hour is 6 or less.
Three successive cold starts and two consecutive warm starts
are allowed.
Permissible motor starting time in relation to the ratio I
D
/ I
N
.
2.4.5 - Earthing (see section 2.5.5)
2.4.6 - Power factor compensation capacitors
Before any work is carried out on the motor or in
the cabinet, check that the capacitors are
isolated and/or discharged (read the voltage at the
terminals).
2.4.7 - Motor protection devices
2.4.7.1 - On-line protection
Adjusting the thermal protection
It should be adjusted to the value of the current read on the
motor nameplate for the connected mains voltage and
frequency.
Thermal magnetic protection
The motors must be protected by a thermal magnetic device
located between the isolating switch and the motor. These
protection devices provide total protection of the motor against
non-transient overloads.
This device can be accompanied by fused circuit-breakers.
Built-in direct thermal protection
For low rated currents, bimetallic strip-type protection may be
used. The line current passes through the strip, which shuts
down or restores the supply circuit as necessary. The design
of this type of protection allows for manual or automatic reset.
2.4.7.2 - Built-in indirect thermal protection
The motors can be equipped with optional heat sensors.
These sensors can be used to monitor temperature changes
at “hot spots”:
- overload detection
- cooling check
- Monitoring strategic points for maintenance of the installation
25
20
15
Time (s)
10
5
5
6
7
Id/In
8
9
10
Hot start
Cold start
