Motor Insulation; Motor Bearing Currents - GE AF-600 FP Series Design And Installation Manual

Installation Consideration
7.5.5 Derating for Running at Low Speed
When a motor is connected to a frequency converter, it is
necessary to check that the cooling of the motor is
adequate.
The level of heating depends on the load on the motor, as
well as the operating speed and time.
Constant torque applications (CT mode)
Variable (Quadratic) torque applications (VT)
In VT applications such as centrifugal pumps and fans,
where the torque is proportional to the square of the
speed and the power is proportional to the cube of the
speed, there is no need for additional cooling or de-rating
of the motor.
In the graphs shown below, the typical VT curve is below
the maximum torque with de-rating and maximum torque
with forced cooling at all speeds.
Illustration 7.7 Maximum load for a standard motor at 40 °C
driven by an AF-600 FP drive
─ ─ ─ ─
Typical torque at VT load
─•─•─•─
Max torque with forced cooling
‒‒‒‒‒
Max torque
Table 7.8 Legend to Illustration 7.7
Note 1) Over-syncronous speed operation will result in the available
motor torque decreasing inversely proportional with the increase in
speed. This must be considered during the design phase to avoid
over-loading of the motor.

7.6 Motor Insulation

For motor cable lengths ≤ the maximum cable length
listed in the General Specifications tables the following
motor insulation ratings are recommended because the
peak voltage can be up to twice the DC link voltage, 2.8
times the mains voltage, due to transmission line effects in
the motor cable. If a motor has lower insulation rating it is
recommended to use a du/dt or sine wave filter.
TM
AF-600 FP Design and Installation Guide
Nominal Mains Voltage
≤ 420 V
U
N
420 V < U
500 V < U
600 V < U
Table 7.9 Motor Insulation

7.7 Motor Bearing Currents

All motors installed with 150 HP or higher power drives
should have NDE (Non-Drive End) insulated bearings
installed to eliminate circulating bearing currents. To
minimize DE (Drive End) bearing and shaft currents proper
grounding of the drive, motor, driven machine, and motor
to the driven machine is required.
Standard Mitigation Strategies:
1.
2.
3.
4.
5.
6.
7.
8.
9.
DET-768b
Motor Insulation
Standard U
≤ 500 V Reinforced U
N
≤ 600 V Reinforced U
N
≤ 690 V Reinforced U
N
Use an insulated bearing
Apply rigorous installation procedures
- Ensure the motor and load motor are
aligned
- Strictly follow the EMC Installation
guideline
- Reinforce the PE so the high frequency
impedance is lower in the PE than the
input power leads
- Provide a good high frequency
connection between the motor and the
frequency converter for instance by
screened cable which has a 360°
connection in the motor and the
frequency converter
- Make sure that the impedance from
frequency converter to building ground
is lower that the grounding impedance
of the machine. This can be difficult for
pumps
- Make a direct earth connection between
the motor and load motor
Lower the IGBT switching frequency
Modify the inverter waveform, 60° AVM vs.
SFAVM
Install a shaft grounding system or use an
isolating coupling
Apply conductive lubrication
Use minimum speed settings if possible
Try to ensure the line voltage is balanced to
ground. This can be difficult for IT, TT, TN-CS or
Grounded leg systems
Use a dU/dt or sinus filter
= 1300 V
LL
= 1600 V
LL
= 1800 V
LL
= 2000 V
LL
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