## Posts Tagged ‘electric motor’

### Transistors – Voltage Regulation Part III

Tuesday, August 7th, 2012
When my daughter was seven she found out about Ohm’s Law the hard way, although she didn’t know it.  She had accidentally bumped into her electric toy train, causing its metal wheels to derail and fall askew of the metal track.  This created a short circuit, causing current to flow in an undesirable direction, that is, through the derailed wheels rather than along the track to the electric motor in the locomotive as it should.

What happened during the short circuit is that the bulk of the current began to follow through the path of least resistance, that of the derailed wheels, rather than the higher resistance of the electric motor.  Electric current, always opportunistic, will flow along its easiest course, in this case the short, thick metal of the wheels, rather than work its way along the many feet of thin metal wire of the motor’s electromagnetic coils.  With its wheels sparking at the site of derailment the train had become an electric toaster within seconds, and the carpet beneath the track began to burn.  Needless to say, mom wasn’t very happy.

In this instance Ohm’s Law was at work, with a decidedly negative outcome.  The Law’s basic formula concerning the toy train would be written as:

### I = V ÷ R

where, I is the current flowing through the metal track, V is the track voltage, and R is the internal resistance of the metal track and locomotive motor, or in the case of a derailment, the metal track and the derailed wheel.  So, according to the formula, for a given voltage V, when the R got really small due to the derailment, I got really big.

But enough about toy trains.  Let’s see how Ohm’s Law applies to an unregulated power supply circuit.  We’ll start with a schematic of the power supply in isolation.

## Figure 1

The unregulated power supply shown in Figure 1 has two basic aspects to its operation, contained within a blue dashed line.  The dashed line is for the sake of clarity when we connect the power supply up to an external circuit which powers peripheral devices later on.  The first aspect of the power supply is a direct current (DC) voltage source, which we’ll call VDC.  It’s represented by a series of parallel lines of alternating lengths, a common representation within electrical engineering.

And like all electrical components, the power supply has an internal resistance, such as discussed previously.  This resistance, notated RInternal, is the second aspect of the power supply, represented   by another standard symbol, that of a zigzag line.

Finally, the unregulated power supply has two output terminals.  These will connect to an external supply circuit through which power will be provided to peripheral devices.  Next time we’ll connect to this external circuit, which for our purposes will consist of an electric relay, buzzer, and light to see how it all works in accordance with Ohm’s Law.

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### Food Manufacturing Challenges – HACCP Design Principle No. 7

Sunday, November 27th, 2011
 Ever overdraw on your checking account or max out a credit card?  It’s not hard to do if you’re not keeping track of things.  How can we manage household expenses without some sort of record keeping?       Away from home, in the business sector, record keeping becomes even more important.  In fact, it’s the very thing covered by HACCP Design Principle No. 7.       Principle 7:  Establish record keeping procedures. – This HACCP principle requires that all food manufacturing plants maintain records to show they implemented a HACCP plan, are following all principles, and the plan is working effectively.      Let’s look at an example.  In keeping with the directive of HACCP Design Principle 7, the engineering department of a food manufacturing plant must keep records for each design project.  The design record for a new cookie forming machine would contain things like engineering calculations to determine strength requirements of machine parts and supports, as well as power requirements for the electric motor that drives the machine.  This design record would also contain documentation concerning materials selected to construct the machine, as well as dimensioned mechanical drawings of the machine and its parts.  These dimensioned drawings will show all physical dimensions of the machine and its constituent parts.       The record would also contain test results and analysis of the results.  Lastly, the design record must include a risk analysis of potential hazards that could result.  Other activities include identification of CCPs, establishment of critical limits, and other factors in accordance with HACCP Design Principles 1 through 5.  In other words, the record must be complete, bearing witness to an effective adherence to HACCP Design Principles 1 through 5.       Principle 7 also encompasses guidelines set in place through Design Principle 6, which calls for the establishment of procedures to govern Principles 1 through 5.  A complete record would contain the procedures themselves, along with any revisions.  It would also contain documentation that the procedures were reviewed and approved by management along the way.      Finally, of what use would records be if they were incomplete, disorganized, and outdated?  A document control system not only establishes procedures, but assigns responsibilities to personnel within the department for filing design records to make sure that everything is up to snuff.  This system would encompass everything, from the creation of engineering documents, to their timely entry into the record keeping system.      We have now exhausted our discussion on HACCP Design Principles.  We’ll switch to a new topic next time, examining some basic concepts behind the control of industrial equipment and machinery.  ____________________________________________

### Industrial Ventilation – Local Exhaust Ventilation Fans

Sunday, May 22nd, 2011