GE AF-650 GP Series, Design And Installation Manual

Page: 98 / 166

Figure 7.5
Figure 7.6
7.5.5 Derating for Running at Low Speed
When a motor is connected to an adjustable frequency
drive, it is necessary to make sure 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.
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.
Figure 7.7 Maximum load for a standard motor at 104° F [40
°
C]
driven by an AF-600 FP drive
─ ─ ─ ─
Typical torque at VT load
─•─•─•─
Max torque with forced cooling
‒‒‒‒‒
Max torque
Table 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
overloading the motor.
7.6 Motor Insulation
For motor cable lengths
≤
than 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 AC line 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 AC Line Voltage Motor Insulation
U
N
≤
420 V
Standard U
LL
= 1300 V
420 V < U
N
≤ 500 V Reinforced U
LL
= 1600 V
500 V < U
N
≤ 600 V Reinforced U
LL
= 1800 V
600 V < U
N
≤ 690 V Reinforced U
LL
= 2000 V
Table 7.8
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.
Installation Consideration AF-650 GP
TM
Design and Installation Guide
DET-767A 7-7
7 7