Why did my Vitamix shut off? Understanding motor heating (with data)

measuring-vitamix-motor-temperatureIf you’ve ever had your Vitamix abruptly stop working, most likely it was the thermal override kicking in. The thermal override turns the motor off before the motor burns out. Then, once the motor cools, you can run it normally again. It’s not a huge deal, but it’s best to avoid. (Depending on what kind of blending you do, you could easily use a Vitamix for decades without ever getting close to overheating. But, if you blend super thick foods, there’s a good chance it will happen at some point.)

This post will explore what causes motor heating and describe how to minimize it. I investigated by directly measuring the temperature of the motor in a variety of blending conditions.

Why should you avoid overheating the motor?

  • If the thermal override trips, you cannot run the Vitamix until it cools, which can take 20–60 minutes. If you’re in the middle of making something, the interruption is a hassle.
  • Keeping the motor cooler should prolong its life. Occasionally overheating the motor is fine, but if you do it regularly, it puts more wear and tear on the motor.

If you don’t want to read all the details, here are a few key points:

Why did my Vitamix shut off?

Thick blends can overheat the motor. It will shut itself off before it burns out, and it will not let you turn it on until it cools down. To avoid overheating, don’t blend more than 4 cups of thick blends like hummus, nut butter, or ice cream. When blending thick ingredients, run at maximum speed.

If the motor does overheat, you can use it again sooner by blowing air into the vents with a fan or hairdryer (on cold!). Some people speed cooling by putting the Vitamix base in the fridge, but, if you leave it in too long, you could run into condensation issues when you take it out.

The rest of this post will explain factors that contribute to motor heating, why fast speeds are helpful, and a special motor-cooling trick.

Background

Vitamix motors have the following properties:

  • The harder the motor works (e.g. with a thicker blend), the more it heats itself. This is because it draws more electric current at higher loads. (The amount of current draw has to do with the back EMF that forms in the motor.)
  • Thicker blends slow down the motor (see my speed measurement page for more details). This is why the motor sound has a lower pitch when blending thick things.
  • The motor is cooled by air blown by a fan on the bottom of the drive shaft, and the fan spins at the same speed as the motor.

You can see how a thick blend increases the chances of overheating: the motor heats more and has less cooling. For more information about the type of motors used in Vitamix machines, look up universal motors.

Factors that contribute to motor heating


You don’t need to obsess about these factors—just keep them in mind. The motor is most likely to overheat when multiple factors act together.

  • Thickness of blend (thicker = more heat).
  • Quantity of blend (more volume = more heat).
  • Blending time (longer time = more heat). You can get away with blending something really thick for a short time, but if you blend it for extended periods, it will overheat the motor. Similarly, if you make one recipe after another, the motor can be more likely to overheat, because it didn’t fully cool between uses.
  • Temperature of environment (hotter environment = less cooling by air) so you have to be more aware in the summer or if the kitchen is hot from a lot of baking.
  • Interestingly, humidity of the air does NOT matter.1

That brings us to motor speed, which I think is most interesting. Most Vitamix manuals have a statement like this one from the 7500 manual:

Tip: Blending too long at too low of a speed setting will overheat the machine and cause the Automatic Overload Protection to turn off the machine. Blending on higher speed maximizes the motor’s cooling fan to avoid overheating.

(Many people repeat a version of this and say that the cooling fan only runs when the Vitamix is set to high. But that is not true. The fan is connected to the motor shaft, so it runs every time the motor does. If you put your hand near the air outtake vents on the bottom sides of the base, you can feel air at all but the slowest speeds, where it is still blowing, but is less detectable.)

Vitamix’s speed recommendation is counterintuitive, and it’s what inspired me to investigate. I decided to measure the temperature of the motor windings. I opened up the 7500 base, stuck a temperature sensor to the stationary windings using thermal transfer tape, and ran a wire out an air vent so I could continuously monitor temperature.

Measurements

I made a batch of 8 cups of thick syrup (similar to what I used for my horsepower measurement) to get a high load that would be consistent. One thing to watch out for with syrup is that its viscosity changes significantly with temperature, and it heats up during blending. But as long as you keep this in mind, you can ensure the syrup temperature is consistent across comparisons.

Here’s a simple temperature trace of the motor blending thick syrup for 1 minute, and then staying off:

blend-heavy-load-in-vitamix-then-let-cool-temperature-plotYou can see that it’s about halfway cooled after half an hour and almost completely cooled after two hours, but it takes about 8 hours for the motor to fully return to its starting temperature. The important takeaway is that motor heating becomes a more significant issue if you blend multiple thick things within about an hour, because the motor won’t have time to cool in between. [Curious why there is a bump at ~105°F?2]

I wondered if lower speeds would heat more due to the slower cooling fan, which would explain Vitamix’s high speed blending recommendation. So I made a series of measurements comparing motor heating at high and low speed. However, for a given load, high speed always heated the motor faster than lower speeds. This seems contrary to Vitamix’s recommendation! At first I thought maybe I wasn’t seeing the lower speeds heat more because the motor was too close to room temperature. (The greater the temperature difference between motor and air, the greater the cooling advantage of higher speeds.) So I made measurements at elevated motor temperatures, but again saw the same thing: high speed heated the motor more than low speed. This means that the lower electric current at lower speeds more than compensates for the slower cooling fan.

Then I realized that in most actual thick blending situations, the Vitamix does not run against a constant load. When making hummus or nut butter, the motor encounters a high load, and then the blades often form an air pocket. In the air pocket the blades encounter almost no resistance. Then you collapse the air pocket with the tamper, so the motor ends up going between high and low loads.

I figured maybe the air pockets only form at high speed. I tested with a batch of almond butter that was identical to my recipe write-up, except I blended it at low speed (setting 2 of 10). Sure enough, no air pockets formed at low speed. Here are the temperature plots:

almond-butter-motor-heating-speed-comparison

The initial motor heating rate at low speed is less than at high speed. At that point the blades are chopping the nuts. But at high speed, as the nut pieces get tiny, they fly away from the blades and form an air pocket, and the motor starts cooling more. The low speed case ends up heating much more than the high speed one. (Also, the low speed almond butter never got fully smooth.)

This is the answer to the paradox of why high speed keeps the motor cooler. At high speed the blade is more likely to form air pockets, which allow the blade to intermittently spin at low load, which keeps it much cooler. (You can tell when the motor forms an air pocket because the sound changes to a high pitch whine.) There’s also a secondary aspect, which is that at high speed the motor will be able to finish the blending task faster, so it’s on for a shorter time.

The use of air pockets to cool the motor is a feature of Vitamix blenders that I didn’t know about until now. (The formation of an air pocket is not a problem because Vitamix machines come with a tamper that lets you collapse it without needing to stop the motor.) Now I’m wondering if Vitamix engineers designed the blades to form the air pocket for this purpose.

Advanced cooling trick

You can use this trick if you have a thick blend that is not forming air pockets or if you can sense the motor heating up. Sometimes you can smell the motor heating before the thermal override kicks in. (I noticed the motor smell when the windings got to ~167°F, and the thermal override kicked in at ~185°F.)

The trick is simple: stop the motor, remove the container, make sure that nothing is touching the drive socket, then turn the motor on high. Run it for ~60 seconds, and it will pull lots of air through without having to work hard.3 Just make sure nothing touches the fast-spinning drive socket. And remember to only move the container when the motor is fully stopped. Here’s a comparison of the motor cooling while turned off and cooling with 1 minute running with no container:

active-vs-passive-cooling-of-vitamix-motor

In this case, 1 minute of no-load blending cools the motor as much as waiting ~30 minutes without running. You can see the temperature quickly drops almost all the way to room temperature when running without the container. It increases after the motor stops because the thermometer measures the surface temperature of the windings. The temperature increase comes from conduction from the hotter internal windings.

One thing to watch out for is that the thermal override sensor also sits on the surface of the windings. So if you use this trick and immediately resume super heavy blending past the overheat temperature, the thermal override will be a bit delayed, and the motor could go above the ideal temperature cutoff. It shouldn’t be a problem if you wait a few minutes after the no container blend, or if you’re blending something only moderately thick. Also, if you use the trick preemptively, you can prevent the motor from getting close to the overheat temperature.

Summary

Thick blends cause blender motors to work harder, which makes them more likely to overheat. If you understand the basics of what’s going on, you can make thick blends without overheating. Running the motor at maximum speed reduces the chances of overheating by shortening blending time, and by periodically forming an air pocket around the blades, which allows the motor to efficiently cool itself. There’s one other thing I learned from these measurements: you don’t have to be paranoid about totally avoiding low speeds for thick blends—low speed will usually only be a problem if you leave it blending for extended periods over ~1 minute.

1. Interestingly, the heat conduction properties of air are not significantly sensitive to humidity.
I was surprised by this because, as humans, humidity influences our perception of temperature. But, apparently, that’s mainly due to confounding factors and clothing being less warm when it’s damp.

2. It doesn’t matter for this discussion, but I’m afraid the bump may be distracting. It is a repeatable feature of these temperature plots, and I think I know what causes it. Above that point the motor is hot enough that the other parts inside the base warm up as they absorb heat from the motor faster than they lose it to the environment. That bump is the point where the motor temperature has roughly equilibrated to the inside of the base. That temporarily slows the motor’s cooling rate. Then the rest of the base starts cooling, and the motor resumes its cooling.

3. For every combination of speed and load, there is a steady-state temperature that the motor would get to if you kept blending long enough. At that temperature, heat from the current in the windings is balanced by cooling by the fan. For high loads, that steady-state temperature is well above the overheat temperature. I measured the steady-state temperature of the motor at max speed with no container, and it runs about 14°F above ambient, which is much less than the overheat temperature. The current draw at maximum speed with no container is 2.6 A, compared to 12-15 A for high loads.


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Comments

Why did my Vitamix shut off? Understanding motor heating (with data) — 2 Comments

  1. > Universal motors have the following properties …

    None of the properties you mention are specific to “universal motors”. They are, in fact, general to *all* electric motors (other than the shaft fan). The shaft mounted cooling fan is simply a property of air cooled electric motors with shaft mounted fans.

    • Fair enough. I see your point, and I’ll make a minor edit. I was initially going to mention some properties specific to universal motors (high starting torque, voltage dependence), but ended up cutting those because they were not essential for understanding the rest of this piece.

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