Harbor Freight Rikon Impeller Upgrade – Is It Worth It? #3: Help! My motor's smoking!

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Blog entry by clagwell posted 12-03-2019 08:35 PM 1677 reads 0 times favorited 1 comment Add to Favorites Watch
« Part 2: Airflow Measurement Setup Part 3 of Harbor Freight Rikon Impeller Upgrade – Is It Worth It? series Part 4: Performance Curve Comparisons »

Well, no, it’s not really. I was just afraid that no one would look at a post with a boring title like “Temperature Rise”.

Temperature rise may be boring but it’s important to the life of a motor. In fact, it’s critical to the lifetime of nearly everything electrical and electronic. Insulation weakens with age. It ages faster at higher temperatures. The hotter a motor runs the shorter it’s life. The same for an extension cord.

When operated at it’s temperature rating the motor insulation will have lost no more than 50% of it’s mechanical strength in 20,000 hours. There’s a rule of thumb that a change of 10C in operating temperature results in a factor of two change in aging rate. That is, 10C hotter means half the lifetime.

A piece of wire has it’s temperature rating marked on the wire along with it’s diameter (gauge). Motors with a NEMA nameplate label their temperature rating as an insulation class with a letter designation. For example, a Class B motor is allowed 80C rise with a 40C ambient (40C is the default ambient temperature for ratings). Class F is 105C rise and Class A 60C. Nearly all US made new motors are Class F. Many of the Asian motors are Class A or B. When you see “HEAT 60C” on a Taiwan made motor that’s Class A.

Heat in the motor is mostly produce by current flowing through a resistance. That heat varies with the square of the current value. It’s produced in both the stator windings and rotor bars. It’s the temperature of the Copper stator windings that’s important here.

Let me repeat, it’s the temperature of the windings not the outside temperature of the motor that counts. The winding temperature can be much higher than the casing, especially on a TEFC (totally enclosed fan cooled) motor.

The usual way of measuring the winding temperature is the Resistance Method from the NEMA MG 1 spec. It’s not difficult but you can’t do it with a common Ohmmeter and it can involve some personal risk. For that reason I am not going to describe the method.


The Harbor Freight has no nameplate. The full load current and insulation class are unknown, so I started at a low current value and worked my way up. Each test takes an hour to reach a stable temperature and several to cool down. When I got to the third value of 15.8A the temperature rise was 85C so I stopped. Not knowing the temperature rating of the motor I really didn’t want to run it for an hour above that.

The curve fit is simple square law. Above 16A it’s an extrapolation. At some point above that, I don’t know where, the curve will get steeper because the resistance of the windings increases with temperature. The motor might actually go into thermal runaway and the curve turn into a hockey stick. Above 16A the curve should be seen as a minimum, temperature rise will be at least what’s shown.

-- Dave, Tippecanoe County, IN --- Is there a corollary to Beranek.s Law that applies to dust collection?

1 comment so far

View 1965scooper's profile


21 posts in 2920 days

#1 posted 12-04-2019 04:17 PM

Clagwell, you’ve earned my respect and admiration for conducting this analysis and providing everyone with a cogent summary. Alas, I’m an aging English major. Information like yours only increases my math anxiety and will cause nightmares involving issues with junior high school algebra.

So, pardon me for not fully understanding. I’ll just follow two rules:

1. When you buy cheap, you often end up buying twice.

2. When it gets hot, shut if off.

-- 1965 Scooper

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