Building A Simple Motor Direction Tester

build motor direction tester

A simple motor directional tester.

Has this happened to you? Have you ever removed a motor from like a fan, waited a couple days, and then forgotten how to wire it so that it rotated properly, clockwise or counter clockwise? I have, plenty o’ times which is why I assembled this stupid and simple little tester for around $10.

Not much is more frustrating than installing a new motor into its frame, wiring it up, turning it on and then seeing it spin in the wrong direction. Because then you have to turn it off, disconnect the stubborn wires again, and remove it from the frame. And then reverse the procedure to re-assemble it. Fun times.

Here is what sometimes happens to me.

I will find a motor that needs to be replaced. Maybe because of the way it’s installed, or maybe because it’s cold out I don’t feel like making trips, for whatever reason, I remove the motor. Almost always I will crudely draw out the wiring diagram when removing it for ease of installation later. There are times when I forget to note which way it is wired to rotate, other times it takes so long to get parts by the time I do, I have forgotten.

The motor directional tester here is for 120V and below but you can make them rated for whatever you want as long as the components you use are rated for it, like 600V, etc.

Not only will this motor tester show you the direction you have it wired to spin, it can also show you if the motor is good or bad period. There should be a few pics to look at to help you along in building your own. It’s fairly convenient and it fits easily coiled up inside my tool box. Click ahead to check it out.

Why Motor Rotation Is Important

how to build a motor tester

Here the motor tester is connected and juiced up.

Well, the rotation of fan motors is important because that is how they were designed. The motor turns a certain direction and the way the fan blades are attached and designed, is how it moves the air.

If we were replacing the motor of a rooftop exhaust fan for example, usually those fans create a low pressure area in the center of the fan, building air is drawn to the lower pressure area, and then the fan blades take an imaginary hold on that air there, and fling it outward onto the roof.

If the blades spun the other way due to the motor spinning the opposite direction from the design, it would draw air in and try to push it into the building. Theoretically.

Since the fan wasn’t designed to do that, move air that direction, 1) that portion of the HVAC system will not operate properly and 2) it will put an improper strain on the motor, bearings, sheaves, etc. This means more maintenance work for you and higher repair costs for the facility.

Pro Tip

  • And you can easily avoid having to guess at the rotation direction prior to re-installation if you take good notes when you are removing it. Just a tip.

How To Build This Motor Tester

which way should a motor rotate

The 3 prong end – hot, neutral, & ground.

It’s straightforward and easy. Putting this device together should cost less than $10 total and take less than 30 minutes of your time. For me, in maintenance and engineering, it is a testing device that overall saves me time.

What You Need:

  • Switch – properly rated
  • Switch Box
  • Grounded 3 Prong Cord from Something
  • 3-5′ Length of 3 Conductor Wiring (the stuff you make cords with.)
  • Handful of Wire Connectors

For the grounded cord, I just used one of those power cords from a TV or computer monitor. First make sure, it does have the three prongs – a hot, neutral, and ground. If it does, just snip off the end – not the 3 prong end but the end that plugs into the device, not the wall outlet. After you snip it off (usually square,) just cut back the insulation as you normally would when stripping wires.

The wire connector end.

The wire connector end.

You want enough wiring wiggle so that you can manipulate the wires as you need and so you have enough to make the connections but not so much that it doesn’t all fit into the switch box. Usually 2″ or so extra is enough for now. You can almost trim more off later but you can never add wiring back, just like lumber.

Measure twice, cut once.

As far as the wire connectors go, figure out which type of connections most of the motors you work with use. I like using the slip on connectors, very fast and convenient.

We probably aren’t supposed to use solid copper wire for these wire connectors but it works as long as you seriously put some man strength into it and really crimp down on them. Give them a bite baby.

Putting It Together

Single pole switch

The single pole switch with the cover removed.

If we were to stretch this thing out on the floor, straight from end to end and look at it from left to right, the “components” would be arranged in the following order:

Looking at it left to right:

  • Wire Connector Ends
  • 3-5′ of 3 Conductor Wire, attached to the
  • Switch, single pole, which is mounted inside a
  • Switch Box. Coming out of the switch box on the right side is the
  • 3 Prong Cord, attached to the hot side of the switch.

Using The Tester

The wires from our motor tester landed on the motor terminal block.

The wires from our motor tester landed on the motor terminal block.

Since you just put this thing together, you probably are able to figure out what to do with it, or how you can use it.

To start with, you can use it to do a simple check if a motor you have is good or bad.

Not only if it runs period but also if the bearings are bad. Sometimes, people will order a motor and end up not needing it. That motor may end up back on the shelf in the parts room. A few months or years pass and some new guy wonders if this open motor is even good.

This is when you would bust out your motor tester and be a hero. Yep – it runs good and sounds good. Next.

Or, you can use it as a directional tester. We talked about why it’s such a pain to find out later that you wired it incorrectly.

You would hook up the tester, plug it in, and stand back.

Most motors require that they rotate in a certain direction in relation to:

  • You looking down onto the shaft.

Not from behind the motor but looking directly down onto the shaft itself. If it spins clockwise, that’s a clockwise rotation. And vice versa.

If you connect your tester, turn it on, and it spins clockwise when it was supposed to turn counter clockwise, reverse some wires. Most motors have the schematic mounted and will tell you if any leads, and which ones, can be swapped to make the motor rotate in the other direction.


motor wiring diagram

Look towards the bottom right, and in the center.

There you have it, a simple device that you can use to 1) see if your motor is good or bad and 2) test the direction of rotation.

If you have an improvement to this motor tester or you have one that blows this little bastard out of the water, let us know. I’d be really excited to hear about it and see it, don’t be shy.

If you have a comment, or question, or see a serious flaw in this, feel free to leave it or send a message.

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