English (GB)
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6.1 Frequency converter operation
6.1.1 Grundfos motors
Three-phase Grundfos motors can be connected to a frequency
converter.
If a temperature transmitter is required, Grundfos offers a Pt100
or Pt1000 sensor for the submersible motor.
To avoid damage to the pump part, the motor must stop when the
pump flow falls below 0.1 x nominal flow.
Depending on the frequency converter type, it may expose the
motor to detrimental voltage peaks.
We recommend that you protect other motors against voltage
peaks higher than 850 V.
The above disturbance can be abated by installing an RC filter
between the frequency converter and the motor.
Possible increased acoustic noise from the motor can be abated
by installing an LC filter which will also eliminate voltage peaks
from the frequency converter.
We recommend that you install an LC filter when using a
frequency converter. See section 6.7.6 Frequency converter
operation.
For further details, contact your frequency converter supplier or
Grundfos.
6.1.2 Other motor makes than Grundfos
Contact Grundfos or the motor manufacturer.
6.2 Motor protection
6.2.1 Single-phase motors
Single-phase MS 402 motors incorporate a thermal switch and
require no additional motor protection.
Single-phase MS 4000 motors must be protected. A protective
device can either be incorporated in a control box or be separate.
Franklin 4" PSC motors must be connected to a motor-protective
circuit breaker.
6.2.2 Three-phase motors
MS motors are available with or without a built-in temperature
transmitter.
The following motors must be protected by a motor-protective
circuit breaker with thermal relay, or a MP 204 and contactor(s):
• motors with a built-in and intact temperature transmitter
• motors with or without a defective temperature transmitter
• motors with or without a Pt100 sensor.
MMS motors have no built-in temperature transmitter.
The Pt100 and the Pt1000 sensor are available as an accessory.
6.2.3 Required settings of motor-protective circuit breaker
For cold motors, the tripping time of the motor-protective circuit
breaker must be less than 10 seconds at 5 times the rated
maximum current of the motor. During normal operating
conditions the motor must be running at full speed in less than
3 seconds.
In order to ensure optimum motor protection, the motor-protective
circuit breaker should be set as follows:
1. Set the motor-protective circuit breaker to the rated maximum
current of the motor.
2. Start the pump and let it run for half an hour at normal
performance.
3. Slowly grade down the scale indicator until the motor trip point
is reached.
4. Increase the setting by 5 %.
The highest permissible setting is the rated maximum current of
the motor.
For motors wound for star-delta starting, the motor-protective
circuit breaker should be set as above, but the maximum setting
should be rated maximum current x 0.58.
The highest permissible startup time for star-delta starting or
autotransformer starting is 2 seconds.
The motor should not run at a frequency higher than
the rated frequency (50 or 60 Hz) during frequency
converter operation. In connection with pump
operation, never reduce the frequency (and
consequently the speed) to such a level that the
necessary flow of cooling liquid past the motor is no
longer ensured.
Warning
MS 402 motors for supply voltages up to and
including 440 V (see motor nameplate) must be
protected against voltage peaks higher than
650 V (peak value) between the supply terminals.
Warning
When the motor has been thermally switched off, the
motor terminals are still live. When the motor has
cooled sufficiently, it will restart automatically.
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