Motor Insulation; Motor Bearing Currents - GE AF-650 GP Design And Installation Manual

Installation Consideration
Illustration 7.6 Derating of Output Current versus Altitude at
T for Unit Sizes 4x, 4xh, 5x, and 6x
AMB, MAX
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,
and 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 extra cooling or derating of the
motor.
In the graphs in Illustration 7.7, the typical VT curve is
below the maximum torque with derating and maximum
torque with forced cooling at all speeds.
Illustration 7.7 Maximum load for a standard motor at 40 °C
driven by a drive
MG35M402
AF-650 GP™ Design and Installation Guide
─ ─ ─ ─
─•─•─•─
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Table 7.7 Legend to Illustration 7.7
Note 1) Over-syncronous speed operation results 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.
Nominal Mains Voltage
≤ 420 V
U
N
420 V < U
500 V < U
600 V < U
Table 7.8 Motor Insulation

7.7 Motor Bearing Currents

All motors installed with 150 hp or higher power frequency
converters 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 frequency converter, motor,
driven machine, and motor to the driven machine is
required.
Standard mitigation strategies:
1.
2.
DET-767c All rights reserved.
Typical torque at VT load
Max torque with forced cooling
Max torque
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
2a Ensure that the motor and load motor
are aligned.
2b Strictly follow the EMC Installation
guideline.
2c Reinforce the PE so the high frequency
impedance is lower in the PE than the
input power leads.
2d Provide a good high frequency
connection between the motor and the
frequency converter for instance by
shielded cable which has a 360°
connection in the motor and the
frequency converter.
7
= 1300 V
LL
= 1600 V
LL
= 1800 V
LL
= 2000 V
LL
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