Specifications

Printed in the U.S.A.

AE8-1368 R2

Appendix C

Transformer Selection And Contactor Control

Power requirements for controlling CoreSense Diagnostics v2.11 compressor operation must be

provided by a Class 2 24VAC transformer. This voltage is used for powering the CoreSense Diagnostics

v2.11 module, crankcase heater relay, unloaders, pilot duty relay or contactor coil. This voltage can be used

for contactor coils providing the total load on the class 2 transformer does not exceed 100VA. When the

loading on a class 2 transformer exceeds 100VA, it will be necessary to use a pilot duty relay which meets

the electrical and mechanical characteristics to adequately carry the required loading. Transformer loading

should be evaluated from both the Steady State/Continuous/Sealed, and Inrush VA’s published by the

manufacturer. It is recommended that pilot duty relays be used when NEMA contactors are speciﬁ ed for the

compressor.

In selecting the VA of the class 2 transformer, the total load characteristics of the circuit must be evaluated.

These are the total steady state, (sealed) VA, and the total inrush VA of all the devices the transformer will

be operating at any given time. Table C-1 shows the steady state and inrush VA’s for devices such as:

CoreSense Diagnostics v2.11 control and sensor modules, crankcase heater relay, pilot duty relay and

unloader. If other devices than this are used, steady state and inrush VA must also be considered in the

transformer selection. The device manufacturers should provide this information.

The 5 steps for selecting the transformer are:

1. Determine your primary and secondary voltage requirements, as well as the supply frequency i.e. 60hz,

50 hz.

2. Determine the inrush and sealed VA of all components (including lights, etc.) that might be in thecircuit.

3. Determine the sum of all sealed VA values.

4. Determine the transformer total inrush VA by summing the component sealed VA and inrush VA.

Total Inrush VA = Σ Sealed VA + Σ Inrush VA

5. Take the sealed VA value from step (3) and the total inrush value from step (4).Refer to the transformer

regulation table, using the 95% secondary voltage with a 40% power factor.Select a nameplate VA rating

that satisﬁ es both the sealed VA and the total inrush VA of the transformer.

Table C-1

(Device Inrush And Sealed VA’s)

Device VA Inrush VA Sealed

CoreSense Diagnostics Modules 3.78 3.78

Crankcase Heater Relay .96 .96

**Pilot Duty Relay 24 10

*Unloader (One) 37.4 18

*6D compressors may use 2 unloaders

** Stancor Type 91 Relay PN 91-901