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Servicing should only be performed by a Qualied Service Agent
VARIABLE FREQUENCY DRIVE – SUF 400 AND 500
The Ultra Force SUF 400 and 500 models have a variable frequency drive (VFD) that controls the rpm
of the blower motor. The VFD receives a signal from the central control board that instructs the VFD to
transmit the proper frequency to the blower to produce the proper blower speed and proper input.
Three conditions must be met for the frequency drive to start the blower:
1. 120VAC must be supplied to the VFD 120VAC input.
2. The ignition control board closes an enable circuit – terminals LI1 and +15V
3. The ignition control board sends a 1-10VDC instruction – terminals 0V and AI1. See table on the
next page for voltage and frequency information.