Variable Frequency Drive - Bth 300 Thru 500 - A.O. Smith BTH 120 Service Handbook

VARIABLE FREQUENCY DRIVE – BTH 300 THRU 500
The BTH 300 - 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 +24V
3. The ignition control board sends a 1-10VDC instruction – terminals 0V and AI1.
VFD (Variable Frequency Drive)
Servicing should only be performed by a Qualified Service Technician
VFD Display
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