Rewiring Door Engines for 120V Operation


The original 32V (or other voltage) DC door engines on a railway car can be made to operate on 120V AC by the addition of a transformer, rectifier and some other components.

The drawing Electric_DoorEng can be downloaded as a PDF. It shows a schematic of the original door engine (electrical and pneumatic) plus schematics detailing how to rewire the door engine to operate on 120V.

Changes to the Door Engine

A 120V to 36V stepdown transformer is fed through a 1A circuit breaker (for safety sake) to supply the correct operating voltage for the door engine. This transformer must be able to supply 4A, since the solenoids draw quite a bit of power. A heavy steel bracket is fabricated to hold the transformer securely and it is mounted out of the way.

The output of the transformer is passed through a full-wave, bridge rectifier to produce DC. The DC has ripple but that is irrelevant in this case.

The original time delay relay is replaced by a universal voltage, programmable time delay relay. This relay provides the timing function for determining how long the door remains open. Incidentally, since this relay is socketed and since the door engine is a non-critical component, this relay may be considered as a spare for other more critical time delay relays on the car, if the same universal relay is chosen to fill all timing requirements.

The two solenoid valves are clamped by 38V, high wattage MOVs across their coils. These MOVs absorb the huge reverse spikes generated by the solenoid valves when they open, thereby protecting the components of the door engine (particularly the bridge rectifier) as well as any electronic gear nearby from radiated EMI.

At the time that the door engine is being rebuilt, the limit switches can be replaced by the switches shown in the drawing, if need be. These limit switches must work properly and be adjusted so that the door opens the correct amount but not too much, and so that the closing door cannot trap someone and pin them. There are large forces involved and not having the limit switches work properly could result in damage to the door and/or an injury to a passenger or crew member.

A terminal block for all of the external leads can be added (or the existing one reused) to terminate all of the leads and allow them to be easily connected to the outside world. Remember to keep the 120V AC and 32V DC separate at all times by using a proper barrier terminal strip.