MIL-HDBK-1003/3
APPENDIX D (Continued)
a) VFD operating more than one motor. The total peak
currents of motor loads under worst operating conditions must be
calculated. The VFD must be sized based on this maximum current
requirement. Additionally, individual motor protection must be
provided here for each motor.
b) Load is spinning or coasting when the VFD is
started. This is very often the case with fan applications.
When a VFD is first started, it begins to operate at a low
frequency and voltage and gradually ramps up to a preset speed.
If the load is already in motion, it will be out of sync with the
VFD. The VFD will attempt to pull the motor down to the lower
frequency, which may require high current levels, usually causing
an overcurrent trip. Because of this, VFD manufacturers offer
drives with an option for synchronization with a spinning load;
this VFD ramps at a different frequency.
c) Power supply source is switched while the VFD is
running. This occurs in many buildings, such as hospitals, where
loads are switched to standby generators in the event of a power
outage. Some drives will ride through a brief power outage while
others may not. If your application is of this type, it must be
reviewed with the drive manufacturer for a final determination of
drive capability.
d) Hard to start load. These are the motors that dim
the lights in the building when you hit the start button.
Remember, the VFD is limited in the amount of
overcurrent it can produce for a given period of time. These
applications may require oversizing of the VFD for higher current
demands.
e) Critical starting or stopping times. Some
applications may require quick starting or emergency stopping of
the load. In either case, high currents will be required of the
drive. Again, oversizing of the VFD may be required.
f) External motor disconnects required between the
motor and the VFD. Service disconnects at motor loads are very
often used for maintenance purposes. Normally, removing a load
from a VFD while operating does not pose a problem for the VFD.
On the other hand, introducing a load to a VFD by closing a motor
disconnect while the VFD is operational can be fatal to the VFD.
When a motor is started at full voltage, as would happen in this
case, high currents are generated, usually about six times the
full load amperes of the motor current. The VFD would see these
193
Previous Page
