Month: June 2018

How does a typical motor know how fast to run?

Typical induction motors are slaves of the electrical cycle rate of the entering power (measured in hertz ).

Our power in the US makes one full rotation from positive electrical peak to negative peak 60 times per second or 60hz (50hz in many other countries)

This means that the generators at the power plant would have to run at 3600 RPM if they only had two poles of power 2 poles (60 cycles per second x 60 seconds per minute = 3600 rotations per minute) in reality, power plants generators can run at different speeds depending on the number of magnetic poles within the generator. This phenomenon is replicated in motor design.

The more “poles” you have in a motor the shorter the distance the motor needs to turn per cycle.

In a 2 pole motor it rotates all the way around every cycle, making the no-load speed of 2 pole motor in the US 3600 RPM.

A 4 pole motor only goes half the way around per cycle, this makes the no-load (Syncronous) RPM 1800

6 pole is 1200 no load (no slip)

8 pole is 900 no load (no slip)

So when you see a motor rated at 1075 RPM, it is a 6 pole motor with some allowance for load and slip.

An 825 RPM motor is an 8 pole motor with some allowance for slip.

A multi-tap / multi-speed single phase motor may have three or more “speed taps” on the motor. These taps just add additional winding resistance between run and common to increase the motor slip and slow the motor.

This means  a 1075, 6 pole motor will run at 1075 RPM under rated load at high speed. Medium speed will have greater winding resistance than the high speed and therefore greater slip. Low speed will have a greater winding resistance than medium and have an even greater slip.

Variable speed ECM (Electronically commutated motor) are motors that are powered by a variable frequency. In essence the motor control takes the incoming electrical frequency and converts it to a new frequency (cycle rate) that no longer needs to be 60hz. This control over the actual frequency is what makes ECM motors so much more variable in ten speeds they can run.

So in summary. There are three way you can change a motor speed.

  • Change the # of poles (more = slower)
  • Increase slip to make it slower, decrease slip to bring it closer to synchronous speed
  • Alter the frequency (cycle rate)

— Bryan

I was watching this video on the HVACR Videos channel by my friend Chris Stephens and I was impressed by how completely he diagnoses the system both visually and with his tools. Take a look.

I say this all the time, but a good, efficient technician is one who sees everything and can also quickly diagnose the main thing

First off… and this isn’t shown in the video. Chris knows what type of appliance this is, what sequence of operation it runs in and how the controls work. He likely knows all of this because he has a ton of experience on these freezers. If he didn’t then the first course of action would be to do a bit of reading up on the manufacturer’s site or app so that you have the basics before proceeding.

So let’s explore everything Chris found and how he found it

  • Failed door seal
  • Dirty Condenser Coil
  • Frozen Evaporator Coil
  • Defrost Heater inoperable
  • Failed Defrost Limit

The main thing wrong with this machine is a failed defrost limit but Chris was able to diagnose the problem quickly because he used these skills of perception –

  • Cycling a defrost mode (prior knowledge of the equipment or reading the manual)
  • Noticing how quickly the suction line froze (visual and touch)
  • Visual observation of the frozen coil (visual)
  • Looking at the wiring diagram to find the circuit (Diagram reading)
  • Testing for voltage to the defrost heater (electrical diagnosis)
  • Measuring amperage on the heater circuit to see if it is working (electrical diagnosis)
  • Looking at the diagram again and finding a limit inline (diagram Reading)
  • Finding the high limit failed open (electrical diagnosis)

In addition to these efficient diagnosis skills, Chris was thinking about –

  • Proper authorization for repair (getting approval before defrost)
  • Protecting the equipment and workspace (covering the control with a bag)
  • Noting and quoting other repairs needed for optimum performance (condenser coil cleaning, door seal)

These additional diagnosis items for optimum performance took no additional time and will result in a more profitable repair ticket and a piece of equipment that will run better, with lower power consumption and less likelihood of an expensive callback.

You will notice that his first instinct wasn’t to hook up gauges or use fancy tools. In this case, he really didn’t need them and it would have only wasted time. When he returns he will make the repairs and observe temperatures only to gauge the progress of the machine to ensure it’s going to get to and maintain temperature.

In refrigeration, especially small refrigeration like this you need to think about –

  • Coil cleanliness (Condenser & evaporator)
  • Fan and compressor operation (is everything running when it should and going off when it should)
  • Defrost type and function (is it coming in and out of defrost)
  • Product and boxes aren’t in the way of proper condenser or evap airflow
  • Door seals are in good shape
  • Door fits properly and hinges aren’t bent or damaged
  • Box achieves and maintains the proper temperature for the product contained within

Of course, there is a time to connect gauges or probes and refrigeration is no exception, but the best techs use their senses first, knowledge of the equipment or reading second and their tools only once they know exactly what they are looking for.

Great job Chris! and I would suggest subscribing to his channel HERE

— Bryan

When I first started in the trade I used to advise customers with hot attics to install powered attic ventilators (PAVs) to “suck” that hot air out of the attic. It just made sense to me at the time… if the attic is hot, get the hot out!

When I started learning more about design someone enlightened me that when you blow air into a space the same amount has to go out and when you such air out of a space (like an attic) it has to come from somewhere. Ideally, the air would come from soffit or gable vents but in most houses, there are also a lot of gaps from the attic into the home and a lot of that air will come from the inside and waste energy.

Nowadays I’m a “fan” (pun intended) of either encapsulating and conditioning the attic or using large, well-vented soffits (will often require baffles to keep the insulation out) and ridge vents.

My friend Neil Comparetto just made a video on this topic that illustrates it nicely

Many of us are aware that X13 and Fully variable motor failure has peaked over the last few years and I’m sure there are multiple reasons for that. One of the reasons is fairly simple and can be traced back to two simple installation and service practices that can be easily implemented.

  1. Seal all air handler / Furnace / Coil penetrations
  2. Use drip loops on wires entering the motor


Eliminating “Straws”

Straws are openings in the cabinet that are unsealed that “suck” moisture into the system and can cause condensation on the interior surfaces. These can be copper penetrations, drain port openings and electrical penetrations.

This is a bigger factor on fan coil systems and package units than it is on furnace/coil systems because in a fan coil or package unit warm/moist air can more easily be drawn in after the coil and before the blower.

When unconditioned air enters into the system due to these penetrations it can cause mold, short circuits, and corrosion. This moisture can also gather on wires and drip into electrical connections causing issues with motors and control panels.

Make sure to seal any penetrations into the conditioned compartments of equipment with proper rubber grommets or in some cases silicone or thumb gum can be used.


Wire Drip Loops 

Anytime a wire enters a plug, board or motor it is best to either locate the connection facing down to prevent water from entering or make a drip loop before the connection point. This allows moisture to drip off of the wire before entering the connector or device.

These issues have been identified as causes of X13 and ECM motor failure and checking these two areas can be very helpful in preventing future failure.

— Bryan

  

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