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Overload again and again and again!

n the ate 1990s I worked for a company that manufactured subsystems for rai transit cars: door operators heating ventiating and air conditioning HVAC and other products. I was hired to be a fireman—not the rea type but to take care of technica probems that pagued any project. About a month after my hiring my boss came into my cubice and tod me he had a probem. One of our products—an HVAC system—was tripping the Auxiiary Power Unit APU and that I shoud investigate. He aso tes me the design is from a company that we acquired and none of the peope who designed it are working with us. I understand this means I am on my own.

Do you have a memorabe experience soving an engineering probem at work or in your spare time? Te us your Tae.
et me give you some background information on subway cars and tramways. Most cars are DC powered. Power comes in at 600 V to 750 V depending on the system and is sent to the traction system and to the APU. The APU converts the DC to threephase AC so it is an inverter. In our case the 60 kVA APU was powering ony one thing the HVAC unit.
My boss tod me that I had to be at our HVAC factory in upstate New York by the next morning to witness the fina test on a unit that was being deivered.

Next morning I’m on the factory foor ooking at the HVAC unit. It is a top mounted unit; it wi be instaed on the tramway’s roof. It is about 3.5 meters ong and 2.5 meters wide. A separate contro box is instaed in the car ceiing for easy access. The technician pugged the contro box into a ow votage DC power suppy then connected the high votage to the factory 208 V threephase 60 Hz turned on the DC suppy switched on the AC circuit breaker and ran through the fina test. I see no sparks smoke or other signs of overoad or stress or oud humming. I tak with the technician and he tes me he has not seen any indication that the units manufactured to date cause overoads during the tests.The next day I reported my findings to both my boss and the project manager. My next action is to go to the transit authority faciity and investigate.

A coupe of days ater I am in a tramway car at the transit authority main repair shop. It is a new transit system: new cars new tracks and new shop. I meet with the transit authority technicians the APU technician and the car buider representative. They a confirm that the HVAC is repeatedy overoading the APU. The APU is designed specificay for the HVAC oad for these cars. The transit authority requirements for the APU the HVAC and a other systems are a written ceary in the request for bid documents. The APU technician informs me that the APU monitors its output current and after three overoads it wi shut down permanenty. Afterwards the technician has to insert a USB thumb drive with specia software to reset the APU. To demonstrate the technician powers the car on and then off and the third time he powers it up the APU trips. We repeat the test a few times with the same resut.

We came to the concusion that to determine if the APU or the HVAC is the source of these overoads we wi have to take measurements in a car. This means having access to a ot of test equipment a test car personne and other resources. I te them I wi go back and discuss this with my boss and the project manager to coordinate with a invoved.
The next day my boss and the project manager both agree that measurements in a car are the way to go. The project manager informs me that the HVAC contro box I requested is in my cubice. I te them that I wi check it out. I hope to find a simper soution than one that requires days of expensive onsite tests.

The HVAC contro box is the size of a tower type PC. It has two connectors. The CPU reads the temperature then sends commands to the contactors and that’s it. The CPU writes the command words into two 8bit atches; the atches contro transistor buffers the buffers contro ow power reays. Finay the reays send power to the contactors cois mounted in the HVAC unit itsef. This way the high votage is present ony on the top mounted HVAC unit and not in the contro box.
The turnon reset circuit is compex. The CPU has its own reset circuit and the two output atches each have a separate RC network connected to the output enabe pin. I connected oscioscope probes to the atch output enabe pin and to the write pin. The CPU writes data into the atch after the output enabe is reeased! At power on the atches are in an undefined state and the output coud turn a the reays on. This means the fan motor the compressor motor and a the heating eements are on. This amounts to twice the rated APU power. So this is ikey the origin of the overoad. It aso expains why this was never seen during the fina systems test at our factory. The test technician aways appies DC for a few second before he can reach the AC switch.

I measured the time the CPU takes to write into both atches. I computed a new worst case vaue for the resistor in the output enabe RC network. I changed the circuit from two separate RC networks to a singe RC network with a onger time constant driving the two output enabe pins. I aso added a diode in the pace eft free by removing one of the resistors. This wi give a faster discharge to the capacitor if power is ost momentariy. In case of a short duration power oss the atch wi ose its content but the capacitor may not have time to discharge sufficienty. The diode takes care of that case.

About Husnain Ahmad

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