Aurora Consulting Engineering LLC
609 Highview Ct, Neshanic Station, NJ 08853
(908) 369-3297
www.AuroraConsultingEngineering.com
EMI-Induced Chattering of Zero Speed Relays
(by D. L. Lin)
Abstract: An improperly grounded metal cover for a controller equipment can cause substantial
electromagnetic disturbances to interfere with the operation of other equipment. This short report
describes the phenomena, its mechanism and how to fix it.
A decelostat controller on a commuter railcar is used to provide control of the brake when slip or
slide conditions occur. The test configuration for the controller is the same as during the revenue
service, i.e., the metal cover is closed.
When the controller was scanned at either 12” or 0” distance by a low power cell phone, all
equipment operated normally under stress. However, when the high power (about 5W) hand held
radio was used, the chattering of the zero speed relays up in the ceiling of the railcar was heard
when the radio was approaching the controller. At far away distances, there was no chattering.
The chattering was repeatable every time the radio was getting close to the controller from far
away. (The relay is a safety device to ensure the train is stopped so that the signal for “opening
the doors” can travel through the relay and enact the door opening mechanism).
It was during the attempt to measure at what distance the chattering started that it was
discovered the root cause of the problem. A 25-ft yellow measuring tape, with its metal end
touching the cover and the other hand held by a human hand, was used for the distance
measurement. When the radio was getting closer to the controller, the chattering sound was
nowhere to be heard. Removing the measuring tape, the chattering returned. Putting the
measuring tape back, with its tip close to but not touching the cover, the chattering was still there.
It was concluded that the grounding of the cover through the measuring tape to the human body
stopped the chattering.
MECHANISM OF THE ELECTROMAGNETIC INTERFERENCE (EMI)
What happened was that, without proper grounding, the cover is a piece of floating metal. A
floating metal can intercept the electromagnetic field and re-emits in all directions. Any nearby
cables or wires, inside or outside the metal case, would pick up the signals and transmit to
devices connected to them. The magnitude of the picked-up signal depends on the distance and
the relative orientation between the cable and the floating metal. The following sections describe
the circuits that are connected to the zero speed relay by two wires from the decelostat controller
box.
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Aurora Consulting Engineering LLC
609 Highview Ct, Neshanic Station, NJ 08853
(908) 369-3297
www.AuroraConsultingEngineering.com
CIRCUITS THAT ACTIVATE ZERO SPEED RELAY
The decelostat controller has two wires connecting the controller in the electric locker to the zero
speed relay on the ceiling. When the signal strength of the hand held radio increases as the
radio gets closer to the controller, the power of the re-emitted radiation from the floating cover
also gets stronger. The connecting cable, acting as an antenna, picks up the radiation and sends
the interference up to the zero speed relays in the ceiling. When the interference level exceeds
the threshold of the relay, it activates the relay and generates the audible sound.
IDENTIFICATION OF CAUSE OF PROBLEM AND ITS FIX
After the floating cover was identified as the cause of the problem, the reason why the cover was
floating was investigated. It was discovered that the top edge of the cover has two pins going into
two eyes (“hinges”) on the top of the controller case. Since the case is grounded, it is obvious that
the cover becomes floating if there is poor electric contact between the pins and the hinge eyes.
Electrical contact was restored by scrubbing the pins and the inner surface of the eyes with metal
files. Once scrubbed clean and the cover installed, the EMI symptom went away.
MITIGATION
It is recommended that the electric contact between the cover and the case of the decelostat
controller be checked (show good electrical conductivity) during acceptance of railcar to eliminate
the source of EMI.
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