16
Information
Variable Speed Operation
CONSTANT HORSEPOWER LOADS
– Applications include coil
winders, band saws, grinders, and turret lathes. Operation
requires the motor to deliver the same horsepower rating,
regardless of shaft speed. Torque increases at low speed and
decreases at higher speed. Most general purpose motors can
deliver constant horsepower up to 1 1/2 times base speed
(consult a factory representative to verify performance).
However, many constant HP applications require operation to
twice base speed, and some, such as coil winders, up to 4
times base speed.
MOTOR GROUNDING
- Frames and accessories of all motors
must be grounded in accordance with the National Electric
Code (NEC) Article 430. Refer to NEC Article 250 for general
information on grounding. Proper grounding of inverter-driven
motors is essential to protect personnel and livestock from
inverter-sourced common mode voltages, which may reach
hazardous levels on the frame of ungrounded or poorly
grounded motors.
LOW INPUT VOLTAGE
– If, due to lower utility supply voltage,
the input voltage from the VFD to the motor is lower than the
motor’s rated voltage, de-rating of the motor’s base frequency,
horsepower, full load RPM, and constant HP RPM is required.
The revised values can be calculated by multiplying by the ratio
of the voltage change. For example, to operate a 460 volt motor
from an inverter fed by 50 or 60 HZ, 400 volt utility power, the
multiplier is 400/460 or 0.87.
The VFD can be reprogrammed to match the new base point
values, allowing the motor to provide rated torque at rated
current from the new base speed down to its original minimum
Constant torque speed. The motor’s CHP range will begin at the
new base frequency and will be shortened by the same ratio as
described above.
OVERSPEED CAPABILITY
– Maximum safe mechanical speed
capability is a function of bearing size and type, lubrication,
rotor balancing technique and specifications, air gap, enclo-
sure, frame construction and connection to the driven load.
In addition, consideration must be given to ambient noise levels,
as operation above base speed will increase motor noise and
vibration, and reducebearing life. Under nocircumstances should
bearing hub temperature exceed 100° C. Belted loads should
not exceed 60 Hz operating RPM by more than 25% (NEMA “TS”
shafts are not suitable for belted loads). Due to external cooling
fans, TEFC (and Explosion Proof Fan Cooled) motors are limited
to 4000 RPM maximum speed.
Maximum Safe Mechanical Speed Limits
(ODP, TENV, DPFV OR TEBC ENCLOSURES)
60 Hz base frequency
Frame Size
2-Pole
4, 6 or 8-Pole
56-184
7200
5400
213-256
5400
4200
284-286
5400
3600
324-326
4000
3600
364-365
4000
2800
404-449
3600
2800
5000 Fr
N/A
CALL
6800 Fr
N/A
CALL
