Preventing Voltage Feedback Problems

May 7, 2001
/ Print / Reprints /
ShareMore
/ Text Size+
Figure 1. Electrical diagram of a typical commercial refrigeration system.


Evaporator fan with a 230-V motor.
My last two columns covered the topic of voltage troubleshooting as it applies to electrical circuits. An important point to remember is that the majority of service problems are electrically based, but which usually cause mechanical problems. One way to head off potential problems is to prevent voltage feedback.

Note: A thorough review of these last two articles will help in understanding the concept of voltage feedback covered in this article.

Figure 1 shows an electrical diagram of a typical commercial refrigeration system. The diagram includes:

  • A 230-V defrost timer motor;
  • A 230-V defrost termination solenoid (DTS);
  • A defrost limit switch;
  • 230-V evaporator fans;
  • 230-V defrost heaters;
  • Temperature activated defrost termination/fan delay switch;
  • Low pressure control (LPC);
  • High pressure control (HPC);
  • A 230-V compressor contactor assembly; and
  • A 230-V compressor/potential relay assembly.
  • Notice that all of the power-consuming devices (loads) in Figure 1 are 230 V. If some of these loads were changed to 115-V loads, problems could arise from voltage feedback.

    Let’s say the evaporator fans are 115-V devices; the circuit in the defrost mode would have the contacts between 2 and 4 of the time clock being open, and defrost circuit contacts between 1 and 3 being closed.

    Now let’s say there are now two power-consuming devices (loads) in series being fed voltage by line 2 (L2). The two loads in series are the 230-V compressor contactor coil (CC1) and the 115-V evaporator fan. Even though the contacts between 2 and 4 would be open, there is a feedback circuit from L2 through CC1, through the evaporator fans to neutral (ground) and back.

    The feedback circuit has a complete path for current to flow because the current can alternate from line L2, through the compressor contactor coil and evaporator fan motor, to neutral (ground). This can happen at 60 cycles per sec (60 Hz).

    Because CC1 and evaporator fan motor are in series with one another when in defrost, both power-consuming devices will see a lower voltage than they are rated for. The lowered voltage each will experience will depend on their individual impedance (ac resistance). CC1 will probably be humming and getting hot, and the evaporator fan may be running slowly.

    Both loads will eventually burn out because the low-voltage condition creates a high-amperage condition. This situation is what is referred to as voltage feedback. The voltage from L2 is feeding both power-consuming devices when in defrost mode.

    Voltage feedback often happens when power-consuming devices with different voltage magnitudes (230 and 115) are in the same circuit. In this situation, one remedy would be to install an evaporator fan with a 230-V motor (see Figure 2).

    With both CC1 and the fan being 230 V, there cannot be a voltage feedback because L2 would be feeding itself, and there would be no potential difference in voltage.



    Figure 3. Isolation relay in a commercial refrigeration circuit.

    ISOLATION RELAY

    Another scheme to prevent voltage feedback is to install an isolation relay into the circuit (Figure 3). The isolation relay consists of a 230-V relay coil and a set of normally closed contacts.

    The 230-V isolation relay coil is wired into the defrost circuit off of terminal No. 3. When the system initiates defrost, the isolation relay coil will be energized and will open a set of normally closed contacts in series with the evaporator fan.

    This will isolate the 115-V evaporator fan from any voltage feedback that may occur during the defrost cycle.

    Tomczyk is a professor of hvacr at Ferris State University, Big Rapids, MI.

    Publication date: 05/14/2001

    Did you enjoy this article? Click here to subscribe to The NEWS Magazine

    You must login or register in order to post a comment.

    Multimedia

    Videos

    Image Galleries

    2014 Energy Efficiency Forum

    Highlights from the 25th annual Energy Efficiency Forum in Washington, D.C.

    Podcasts

    NEWSMakers: Jeremy Begley

    Jeremy Begley, home-performance specialist and Web and social media manager with National Heating and Air Conditioning Co. in Cincinnati discusses how and why you should add home-performance philosophies to your contracting repertoire. Posted on Sept. 12.

    More Podcasts

    ACHRNEWS

    NEWS 09-15-14 cover

    2014 September 15

    Check out the weekly edition of The NEWS today!

    Table Of Contents Subscribe

    Venting R-22

    The NEWS reported that a man received prison time for venting R-22. Should EPA step up enforcement?
    View Results Poll Archive

    HVACR INDUSTRY STORE

    plumbing-hvac.gif
    2014 National Plumbing & HVAC Estimator

    Every plumbing and HVAC estimator can use the cost estimates in this practical manual!

    More Products

    Clear Seas Research

     

    Clear Seas ResearchWith access to over one million professionals and more than 60 industry-specific publications, Clear Seas Research offers relevant insights from those who know your industry best. Let us customize a market research solution that exceeds your marketing goals.

    DON'T MISS A THING

    Magazine image
     
    Register today for complete access to ACHRNews.com. Get full access to the latest features, Extra Edition, and more.

    STAY CONNECTED

    facebook icontwitter iconyoutube iconLinkedIn i con