Electric motor question

Although the electric motor in question is the small one of an old corded electric string trimmer/edger, it has me wondering about the electric motor/s of hybrids and EVs.

In the case of my fourteen year old B&D corded trimmer, despite being careful not to run it non-stop, it began running hot this year. Last time using it a few days ago it suddenly made a deep chugging noise so I released the trigger within a couple of seconds. Checked to see if the string lines were messed up (again :roll_eyes:), they weren’t and the housing was only warm, not hot. Gave it and me a few minutes rest before trying it again. Motor then made a high pitched whine similar to a jet engine turbine whine but the spool no longer spins.

So either some connection to the spool is broken or binding, or the electric motor is failing. (I’ll get it sorted out and if need be will get a new, this time cordless, trimmer.)

But all this has me wondering about several things:

  • What is the key design difference between small electric motors that do not tolerate continual “on” use while others can run non-stop for anywhere from hours to weeks?

  • When an electric motor of any size or function fails what is the most common cause? Do they simply seize up, “burn out”, or something else?

  • Are small electric motors such as those on yard tools and some mowers, on sewing machines, etc. basically the same in form and function as a vehicle alternator or the small generators on early decades of cars?

I’m looking for a layman’s explanation as I know very little about electrical systems.

How Electric Motors Work | HowStuffWorks

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A big difference is that string trimmers often use brushed motors, whereas electric cars generally use either brushless DC (old) or induction AC (new!) motors.

The simple difference there is that brushed motors have parts called brushes (shock!) which are there to conduct electricity to other parts of the motor. Those brushes touch a rotating part, which causes mechanical wear. And they also electrically wear - ever notice the sparks when you pull the trigger of an electric drill? That’s a small spot of high voltage, and that causes wear on the brushes.

Brushless motors don’t have that wear part. Older electric cars used brushless DC motors in part for better longevity. Today’s electric/hybrid cars most commonly use the AC motors because they have much better performance. That was one of the big revolutions Tesla accomplished with the Model S. It used AC motors that gave it blistering performance compared to oldschool electrics (and, honestly, compared to just about everything else on the road too).

A notable exception is the Prius, which is well known for being almost glacial when accelerating on the electric motor only, and that’s because its brushless DC motor just isn’t that powerful. Same story with the really old electric cars from the 80’s, etc. It’s all their fault that electric cars got a reputation for being slow and unimpressive.

But regardless of whether the car uses AC or brushless DC, the motor doesn’t have those brushes that wear out.

There’s also the bit that an electric car motor is designed for much heavier duty use than the motor in a weed whacker. No one really cares if they have to go out and buy another $40 weed eater every 5 years or so. People would get furious if they had to go out and buy a new electric car every 5 years.

The motor shaft in a weed whacker is a lot weaker than the motor shaft in a car. And, unlike a car (unless you’re a particularly bad driver) you aren’t slamming the motor into rocks, fences, and the dirt over and over again. It sounds kinda like the shaft might be broken in yours, because it sounds like the motor is spinning but the business end is not.


It mostly has to do with heat dissipation. Things that will limit the run time are bearings, motor running at max capacity, fans and heat sinks etc. These are the differences between continuous run and those that have a duty cycle.

An alternator is a three phase unit. It would work as a motor only if feed three phase power and the rectifier pack was removed. The rectifier (diode) pack converts the three phase AC to DC. Also motors are designed as AC or DC and the two cannot be interchanged. Not enough room here to explain all that.

As for your string trimmer, they get old. The insulation around the wires gets old and brittle but mostly the bearings dry out and seize.

If you only trim and not do other things, an 18-20V trimmer will work fine. If you go with a 40-60v trimmer, you can add accessories like a pole saw, hedge trimmer, rototiller/cultivator, blower etc to it. The higher voltage units with brushless motors have long duty cycles and the power of most gas units, but they cost more.


Cooling and bearings

Bearings fail… If they are brushed motors, the brushes and commutators rings can burn up/wear out.

Not really.

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for those who might not know what electric motor brushes look like.


I’ve wondered about how they’ll cool the motors in EV’s. Strictly air cooled, I guess…?

We get a lot of large (around a ton or so and bigger) electric motors in the scrap. I always kinda wonder what killed them. Some have 4” diameter shafts, so they’re not light duty. Lots of times the shaft still spins freely as if the bearings are still good, so it’s a mystery.

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Thank you everyone! I appreciate all your responses.

To resurrect my old tag line … still reading, still learning. :slightly_smiling_face:


I think so. While some other parts may have liquid cooling, the motors don’t, because they convert almost all the electrical energy into mechanical energy. This is in contrast to a gas engine, where about 2/3 of the chemical energy is converted into heat.

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Never had to worry about overheating a weed eater, look at electric lawnmowers, no problems I have heard, Furnace blower motor never had a concern, starter motor for a car don’t crank longer than 10 seconds. I imagine it has to do with the power needed, and what it was designed for

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Aha, per some research, the most common causes on the particular model trimmer I have of the motor running but not engaging the spool is one of three things:

  • broken plastic ratchet piece $3 part
  • broken pulley belt
  • broken line connector between motor and spool gear

So… will try disassembly per the instruction manual, try diagnosis, and/or ask help from a young neighbor who recently offered help with anything mechanical if I need help. Maybe I can avoid the cost of a new trimmer. Worst case, I still end up buying a new one.

Nothing ventured, nothing gained. Although, admittedly, I am my father’s daughter and like him have a tendency to fix things beyond repair. :roll_eyes:


You never know what you can do if you don’t try I have been known to fix things beyond repair,Good luck I also use your tag line but I also add you never get to old to learn till you are on you death bed wicth hopefuly be many years down the road.for us.:grinning_face_with_smiling_eyes: :grinning_face_with_smiling_eyes: :grinning_face_with_smiling_eyes:

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Sounds like the hub/spool has detatched from the motor shaft.

Small motors typically have bearings with an oil soaked wick.
Over the years the wick runs out of oil and the bearing runs dry.
In a harsh environment the winding insulation can break down.
And brushes can wear out, if the motor has them.

No. Sewing machines, hand drills, blenders, reasonably priced vacuum cleaners and many other devices use “universal” motors.
They can actually run on AC or DC voltage. They use brushes.
They are cheap to make, relatively compact and light, spin at high speeds, and cheap to make.

The corded lawn mowers I’ve seen use AC induction motors, a Tesla invention.
Their biggest plus is they have only one moving part.
With 60Hz AC power they can’t run fast like universal motors (limited to 3600 rpm).

My father would never, ever hire someone to fix something without first trying to fix it himself.


There must be at least a 20 different styles of electric motors. Which style is used depends on what the weight, size, and power requirements are for the application. Getting electrical power to the stationary part of the motor is no problem but somehow electrical power has to also get to the rotating part of the motor, and that’s the key difference between the different styles. Smaller, lighter weight motors which require a lot of power (like for electric drills) would use brushes. For something like a Water Pik (to power a small pump, and weight and size are not that important) they’d probably use an induction motor. How tolerant the design is to extended use depends on those factors too. Most box fans (like the 20 inch size you’d use in your house) use induction motors; i.e. no brushes For that application, motor size and weight are not critical, and the airflow through the fan cools the motor windings.

Burn out is the usual final symptom, although something else seizing up could be the actual cause of the burn out. Burn out: The motor’s windings (the wires that loop in circles that make the magnetic fields) are coated with a sort of paint which insulates them from one another, and if they get too hot the paint melts which shorts the winding out.

An alternator uses a slip-ring, while a generator uses a commutator to get power from the stationary part to the rotating part. Alternator’s and generators have some similarity to motors, but the designs constraints are not the same.


I remember the days when an electric motor shop would rewind the coils in an electric motor. I doubt that this is done any more for motors used on equipment around the house. Back in the late 1940s, the motor burned out on the 1939 Coldspot refrigerator my parents had. The appliance serviceman removed the motor and had it rewound and then reinstalled it. This was before the days of hermetically sealed compressors. The motor and compressor were on top the refrigerator. There were oil cups at each end of the motor and my dad would lift the top off the refrigerator about every three months and oil the motor.
I do remember our well pump motor was called a repulsion-induction motor. It started as a series wound motor with the field coil in series with the windings in the armature. There were brushes that provided the power to the armature. As the motor came up to speed, a centrifugal switch kicked off the power to the armature windings and the motor ran as an induction motor.


I dunno but every electric motor has a duty cycle that determines how long it can run. Bearings, windings, etc. Heavier the longer it can go. Rather than analysis though, just toss it out. I got a new BD for less than $100. I chose BD because I have their batteries. Took two other weed chompers to the junk yard. I was sick and tired of starting and maintaining the gas unit, plus too heavy to carry around.

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I confess I would be happy to dispense with the hassle and limitations of being tethered to dragging heavy heavy-duty extension cords all around the place.

I have a few weeks to sort out options before I need to edge again and my neighbor often kindly trims around my house when he mows. We both tend to mow parts of each others yards as a courtesy.

I had an electric string trimmer powered through an extension cord. Mrs. Triedaq bought me a Ryobi battery powered trimmer. I used it for at least 10 years. The battery was a sealed lead acid which I replaced a couple of times. When the final replacement battery gave out, I found it was cheaper to buy a Worx brand battery powered string trimmer at Walmart than buy a replacement battery for the Ryobi. The Worx has a 40 volt lithium Ion battery. It is much lighter than the Ryobi and I can go almost a whole season on a single charge.
I would like to buy a battery powered leaf blower so I don’t have to.drag around a power cord. When I replace my gasoline powered mower, it will be with a battery powered mower.

When my Toro gas mower eventually needs replacing I’ll get a battery powered mower if I’m still doing yardwork. By then I may give up and pay someone to handle the outside work or have downsized to a condo by then.

I may even give in and outsource the yardwork and/or downsize soon and simply sell off or donate the yard tools. I’m too young by age to give in but the lifelong arthritis and asthma are making it scary difficult some days to mow this steep, rough, 80x120 foot yard. My ambition continues to outpace my abilities.


A quote many of us live by! That is how you learn to do new things and it tends to keep you young!

I fully intended to cut my own lawn when I moved to Florida. A few things changed that… Even a self propelled lawnmower has difficulty with St. Augustine grass, it gets “heart attack” hot in the summer when most of the grass cutting is required, lawn care companies are plentiful and cheap and the clincher…I hate cutting grass! So I don’t.