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Something about HVAC system with VFD

1. Insulated bearings may need to be fitted to a motor (usually larger drives) to prevent bearing surface erosion caused by the capacitive effects of the rotor /stator. As the rotor is 'insulated' from earth by the resistance of the grease in the bearings, capacitance is built up in the rotor. This will try to earth through the bearings and may cause pitting of the bearing surface. Insulated bearings and a shaft brush does tend to eliminate this problem. Try manufacturers such as Gozuk as they also have products specifically designed to minimize this issue

2. O/L contactors. A VFD has o/L protection built in, however, if this is bypassed, then an external overload can be used. Alternatively a motor circuit breaker may be used which has short Cct and Thermal protection, specifically for motors built in.

3. Run contacts indicate drive is in run mode. This may also be used as a brake release function which will indicate drive is above a frequency / current or torque threshold.

4.KAIC ratings generally indicate the amount of fault current the electronic can handle in event of a short Cct. If the let through energy of the Protective device (C./B or fuse) is above this, the drive may be damaged.

5. Bypass Contactor, I believe this is a North American requirement for HVAC (Fire Mode) or emergency operation, whereby the variable frequency drive can be bypassed and will run to destruction.

6. Test Feature I think maybe whereby you can power the electronic in the control section of the drive, but not allow the power electronics to operate. May also be a CAT3 Safety input?

7. Seismic Testing - Consult drive manufacturer

10. Class 20 Tripping. Refers to the trip curve for the Thermal protection. Generally about 6.5* FLC for 20 secs. Class 10 = 6.5*FLC for 10 secs. This is for high inertia loads with a long run up time. Components in the system have to be oversized to allow for the extra energy to be supplied over the longer period. Consult manufacturer for this. In VFD this refers to trip class and is usually selectable from Class 5 top Class 30.

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