Lightest ~3KW DD motor?

What is the lightest DD motor that can handle around 3KW?

The Crystalyte HS3540 has been used to 3KW by the Stealth Fighter without any problems and weighs 16.4 lbs. The H3540 I guess is just a newer version of that motor? It weighs a bit less at 13.2 lbs. The H2540 weighs even less but these are not popular and I'm not sure how much less power it can handle.

I am planning to use ferrofluid with the motor to reduce heat. It will be used in a city with some hills so I expect a lot of low speed high torque starts which will make it run less efficient and hot.

I am also planning on using it with a sine-wave controller to minimize noise.

Mid-Drive using an Astroflight or similar motor would be maybe 3-5 pounds lighter but I don't like the noise.

The lower numbered motor series is narrower which produces less torque for the same conditions.

You should go to the http://ebikes.ca/simulator and try out different combinations of motor, battery, controller, conditions, and see which ones will do what you need them to do.

If they dont' have the torque to get you to teh speed you want in the time you want, without so overpowering them that they are no longer in their efficient zones, you don't want them even if they are lighter. Otherwise they're just wasting your battery power as heat, even if they dont' overheat in the process.

Using too small a motor far enough outside it's efficient zone, you'd have to carry more battery to make up for it's losses to heat, which will negate having the smaller motor.


(a motor is designed for a certain amount of phase current thru it; beyond taht maximum the losses from inefficiency climb, and all those losses create heat. if you stay below that current, you won't create nearly as much heat, which both helps the motor survive and stay more efficient, and it saves you battery weight for the same range/capabilities.)


If you're going to be doing a lot of startups from a stop at max power, then as long as the motor can cool down between them, it'll be fine.

If it doesn't get the chance to cool down, that's where problems start.

I'm still new to electric motors so please confirm if my thinking is correct:

The power rating of an electric motor is temperature limited before a component fails (magnet, insulation, or etc. fails).

Temperature is a factor of:
1. The heat capacity of the components (transient only)
2. Heat transfer to the environment
3. The electrical power input and efficiency (heat generation)

So if I use a motor that is not rated for what I need, it will overheat because:
1. There is not as much mass to absorb transient spikes in heat production during acceleration/hill climb
2. There is less surface area due to shorter length/smaller diameter of windings and smaller motor casing area
3. Efficiency will not be as high for a smaller motor? Where does the loss in efficiency come from? Ohmic losses? What determines the maximum phase current? Is it saturation of the core material?

Seems like the effect of 1 will be small if I choose a motor with aluminum stator arms, and 2 can be mitigated by using external fins / FF if the winding wires don't melt first.

lagfish said:

The power rating of an electric motor is temperature limited before a component fails (magnet, insulation, or etc. fails).

Click to expand...

Basically, yes.

Temperature is a factor of:
1. The heat capacity of the components (transient only)
2. Heat transfer to the environment
3. The electrical power input and efficiency (heat generation)

Click to expand...

Basically, yes.

So if I use a motor that is not rated for what I need, it will overheat because:
1. There is not as much mass to absorb transient spikes in heat production during acceleration/hill climb
2. There is less surface area due to shorter length/smaller diameter of windings and smaller motor casing area
3. Efficiency will not be as high for a smaller motor?

Click to expand...

Yes to all of those.

Where does the loss in efficiency come from? Ohmic losses? What determines the maximum phase current? Is it saturation of the core material?

Click to expand...

Loss in efficiency is from various things, you might look at the "definitive tests on heating / cooling hub motors" thread for a lot of discussion of heat, efficiency, etc., that will probably tell you more than I can.

Ohmic losses in wire increase with heat, so the hotter it gets the worse that problem becomes. I don't know how the heat affects the magnetic fields, saturation, etc.

Saturation would be the main limiter of max phase current--once it's saturated, you're not really increasing torque/etc just turning the current into heat. This is why you see some experiments/builds where people run bursts of say, 10kw thru a motor meant for much lower power, and it gives some extra power than at it's rated max, but not 10kw worth--and it heats up like crazy (or requires significant cooling to prevent that).

Seems like the effect of 1 will be small if I choose a motor with aluminum stator arms, and 2 can be mitigated by using external fins / FF if the winding wires don't melt first.

Click to expand...

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The first only up to the point at which the whole motor is already hot. The second to some degree at least, as long as there is airflow to cool the fins and outside of motor.

Best is to minimize heat creation in the first place, because all of the heat is wasted power.

All wasted power means a bigger battery (and more weight/cost) to do the same job.

FWIW, good aerodynamics will also help lower power usage while riding at speeds above around 15MPH.

The answer keeps coming up, "it depends" because the load on the motor makes all the difference. The simulator has the input to take load into account.

In my general experience, three situations tend to overheat motors.

One is repeated, rapid, starts and stops, or at least braking hard and then grabbing full throttle often enough. This happens on racetracks with lots of turns, and on dirt trails that have lots of tight corners. The big load is when the motor is asked to go from 0-30 mph, or even 10-30 mph extremely often. The high load/ low rpm portion of the ride is when its running very inefficient, and heats up the motor fast. Sometimes the heavy motor works better then, because it has more thermal mass to soak up that heat for longer. Also, with hub motors, smaller wheel for the win. The larger the wheel, the more time it spends at low rpm, per acceleration.

Another is simple weight, and usually some hills. Takes a lot hill and a lot of weight to bother a motor that can handle 3000w. For example, 2000w (in efficient rpm, and a big enough motor for 2000w) is plenty for 400 pounds total weight up an 8% grade, that is miles long. So if you weigh 200 pounds, your bike, battery, and other cargo can weigh another 200, and even with 26" wheel, you can climb that hill at 15 mph, which while not peak efficiency, will not overheat the motor in most weather. Its a matter of yes, the motor is heating quite a bit, but it can radiate that heat out at a similar rate. The result is a motor temp that reaches an equilibrium no matter how long the hill is. If the motor core can stay below about 250 F, it won't even brown the windings let alone melt down.

Lastly, saturating the motor will overheat it fast. This is what happens when you try to run 3000w through a 500w rated motor, like the typical 9 continent type with the 28 mm wide magnets. They can handle 1500w easy, or even 2000w pretty well. But run 3000w through that smaller motor and you overheat in about 45 min. But it works,, sort of, fine for that first 30 min. Works just fine, if you tend to run only 1000w continuous, and just call for more sparingly.

A general rule of thumb that seems to work is, the motor rating x 3 works good if the load is reasonable. Meaning no trailer full of cargo, or body weight over 200 pounds.

So the 500w rated motors of all types can generally stand a 1500w max watts, intermittently with no big heat issues.

A larger motor like the 3540 will be rated more than 500w, and thus will handle 3000w max watts ok, and larger load weights. It will have more magnet and copper, so it pulls harder. This means it takes less time to accelerate, shortening the time in inefficient rpm. And it will mean that if you weigh 300 pounds, still no problem, even if you need to climb the rocky mountains.