Electric motor types

I am starting this thread to discuss the types of electric motors and their particular characteristics. There are many claims about electric motors, but in general, these motors are treated as if they are all the same. I want to keep this thread about the technical aspects of electric motors. There are many philosophical and political opinion related to them. However, there are other forums where those arguments are appropriate. Let's keep this technical and civil.

 

Ok then, I'll start with some thoughts about my planned solar electric drive for a 6.3 meter cabin motor katamaran I currently build, started 2017.

As our goal is to go slow and silent with our boat, enjoy the nature and to have only little need to fill up our batteries at shore grid, we have to skimp with electric energy. For this reasons we decided to use a electronically commutated (brushless) DC motor with permanent magnets. At its best operating point it will convert 90 % of the electric power input into mechanical power output (torque times angular velocity). This motor needs a three phase AC input which is provided by the motor controller powered with DC by the traction batteries of the boat. (The controller produces another few percent of energy losses.) The motor is rated 3000 Watt (3 kW) at 48 Volt (48 V), max. current is about 97 Ampere (97 A). (One motor each at the demihulls).

To discuss the characteristics of this kind of motors I'll add a graph which shows the typical values.

 

Normally I'm able to read diagrams like this but in this case I'm not shure what voltage and current is shown. Because the voltage is set to 48 V constantly this should be the battery voltage. So the current should be the battery output current (= controller input current). While the manufacturer is selling controllers also, the motor is offered and sold without a controller. And the 3 phase motor input has a different (lower) voltage and a different (higher) current. Are the diagram data "created" by a virtual ideal controller with no losses?

 

Though I have very little technical knowledge, I want to follow along, so I will post this one question.

If you need to supply three-phase AC power, why a DC motor? My understanding is that poly-phase AC induction motors can be more efficient than 90%, though admittedly, I'm pulling this out of long term memory.

You're losing energy converting DC to AC, then the motor's controller converts it back to DC?

-Will

 

May be I wasn't clear enough. Cause this motors are considered as further developments of the ordinary brushed DC motors they are called BLDC motors (brushless direct current). The board batteries provide DC current and the AC flows only between controller and motor, no back converting to DC. (The "better" trolling motors have the controller included in the motor case, so you often can't tell if it is a brushed or a brushless motor from its appearance.)

 

Yes, the voltage is shown at a constant 48V which makes me believe this is likely bench data and the motor was being fed a constant power supply. In real life, depending on battery type and solar charge input, this voltage can vary.

The Amp value could be the load drawn by the motor or motor controller.

Looks like max watts in is about 4750W with a max output of about 4250W.

The curve of the Amp draw versus RPM and efficiency value leads me to further believe this is a bench test without controller or loading.

Once you add a controller and start loading a motor it will behave differently. This is why controller design is so important for brushless electric boat motors.

For comparison, here is the test data of one of my 24V 1.44kw brushless motors under load pushing a small inflatable dinghy with two passengers aboard. I’m pretty sure this test was performed using a FLA battery based on the voltage sag recorded under load.



Looking at the chart you’ll note a peak efficiency of about 81% drawing about 31A while the controller is still Pulse Width Modulating.

And you can see that while operating the motor beyond this point produces more thrust, it greatly reduces the RPM and draws considerably more current reducing overall efficiency.

And keep in mind most hulls start to work against you as you get to this point, requiring a much more powerful motor and bigger prop to achieve planing if possible, so you really want to design for efficiency up to the point of where your hull design starts to become a serious limiting factor.

The point before this where the motor is still modulating drawing the least amount of current while providing the highest prop RPM is what I call the cruising sweet spot.

If you can figure this out for a motor, you can design the rest of your system (batteries, panels, discharge rating, props, etc..) around it to give you the most efficient cruise performance while still leaving some extra thrust in the “tank” for when you really need it.

As mentioned, the chart above is real world data under load. It was performed on a calm lake in a boat hull design that eventually really starts to work against you, with a trolling motor style prop that is designed to move large displacement vessels slowly.

Since my hulls are more efficient and I run dual motors and found I wasn’t working them to their max, I decided to switch to larger diameter more aggressive pitched 2 blade props. The bigger props increased my top cruising and max speeds while giving me better bite and control in ocean conditions, and loaded the motors to just within their max specs. The down side is at slow speeds you can feel them churning/chopping more than my old 3 blade props, and the boat is a little less responsive maneuvering forward to reverse, whereas before they performed more like a thruster.

The other downside is I should have had my LifePo4 batteries built with a slightly higher max discharge rate as the new props at full speed load the motors to their max rating which exceeds my 100A max discharge battery rating when I encounter shaded areas long enough, which in turn causes the BMS in the batteries to disconnect and shut down power. This then causes my motors to go into “Limp mode” due to the low voltage as they are designed to limit amperage draw to protect the batteries in such circumstances. It takes less than a minute to reset it all, but if I had given myself a bit more headroom of say 20A I could play with even bigger props to further optimize my cruising speed. For now I’m maxed out.

 

To expand on Heimfried's response, any brushless motor system which uses DC as the input has a controller which converts the DC input to AC. It is impossible to run a motor with only DC going through all the coils. The commuter and brushes in a conventional DC motor convert the current going through the armature coils into AC.

 

Thank you for proving data.

That would be my guess assuming the manufacturer wants to make the motor looks as good as possible.

It took me a minute to understand the graph with torque along the abscissa. It is clear if efficiency and avoiding heat in the motor is important you don't want to "over prop" the motor (propeller with larger diameter and/or pitch). Load the motor so it slows below the speed for peak efficiency and efficiency drops off rapidly. The electric energy not converted into mechanical power will become heat in the motor.

 

Since we are talking electric boat motor designs, another important thing to consider is system voltage. Powerful E motors that provide performance on par with an ICE motor have typically required high voltage which is dangerous.

Here’s a company that has come up with an interesting way to provide high KW power at safer DC voltage levels by designing an independently controlled 12 phase stator system in the inverter and motor making it capable of handling the 1000+amps needed to produce the 50kw (65hp) the motor is capable of providing at only 48V.

https://molabo.eu/

 

Additional to the diagram in #2 here is the data table for the same motor.

 

SolGato,
the table you provide could belong to the Caroute N200 motor.
High Efficiency Brushless Electric Boat Motor http://www.caroute-outdoor.com/
Afaik is Caroute the (real) manufacturer of a whole lot of trolling motors which come to the market at different brands. I'm quite shure that my Haswing motors (1.44 kW 24 V) are from Caroute also.
As the controller is inside the motor housing I guess the table data show the electric power input (DC) of the whole unit (controller included). This will be one of the reasons for the lesser efficiency at the sweet spot (81 %) compared with the top efficiency of 92 % of my motor without any controller losses.

As far as I noted (#3) that I had problems to interpret the diagram it was regarding the input voltage and current cause the contradiction of one phase DC against 3 phase AC.

The meaning of sweet spot (optimaler Betriebsbereich) and its connection to properties of hull and prop are quite clear to me.

 

This is fundamental to understanding DC motors - they are AC! So I hope no one minds me repeating DCockey's comment here.

 

In different sources I read that it is most common to produce the data of electric motors in diagrams using torque along the abscissa.
When I was confronted with this kind of diagrams for the first time it was not easy for me to understand the meaning of it in practical terms. I hope it is better now.
I think this kind of presentation is also connected to the way obtaining this data.
My guess is the motor to be tested is set up at a bench with a controller (at WOT) and a break
(acting as a load) to measure the torque output. Other quantities measured are input voltage and current and rpm.
The torque is the independent variable to be set with the force on the break. The other measurements are depending of the actual torque.
Start of the process is with (nearly) no torque, then the break is tightened in steps.

 

Yes, it is for Caroute motors, and yes they are an actual manufacturer who builds for other brand names.

As I stated the data is tested under load in an actual boat, not on a bench using constant voltage, but factoring in pushing a hull through water using a prop drawing from a FLA battery.

It is far more useful information with respect to real life boat application of a motor.

Other boat specific motor manufacturers I’ve communicated with use water tanks for testing and a scale to record thrust.

Universal motor manufacturers typically just provide bench data.

One big problem with the data you have posted on the Golden motor that makes it hard to apply is the RPM.

What prop do you plan to spin at 4000+rpm?

Unless you are planning to power a blender or small lightweight speed boat, a gear reduction likely needs to be used to reduce the RPM to allow the motor to spin an appropriate sized prop to move a boat of substantial size and displacement. This will introduce significant loss.

Consider that a equivalent 3.5hp ICE Tohatsu reaches a max 5-6000rpm but uses a 2:1 reduction to spin it’s prop, and you should probably plan on doing something similar to ensure you don’t damage the motor or overload the motor controller, and are able to operate it somewhat efficiently.

Most of the home built conversions I’ve seen using motors like the Golden motor have struggled with motor controller issues, specifically the need for substantial cooling. Many have tried less expensive controllers only to burn them up.

Elco is an example of a motor manufacturer using a similar style motor to the Golden in conjunction with a reduction. Take a look at their operating RPM’s and performance moving various hulls and you get a good idea of what needs to be done to strike a balance between reliability and affordability. (Nevermind, I forgot Elco is using PMAC motors)

With regard to heat, it is interesting to note that upon studying the owners manual for the Navy line from EPropulsion, I discovered the motors have oil in the lower end and a closed cooling system in the top. This is smart. Having a background in electronics I can tell you the number one killer of components is heat.

For component based propulsion systems with separate controllers like one might use as an inboard in a boat, check out the packages Thunderstruck offers. They have appropriately matched motors and controllers and gear reductions that are designed to give good reliability and efficiency.

https://www.thunderstruck-ev.com/electric-sailboat-kits-and-accessories-inboard-motor-ev/

I’ve said it before and I’ll say it again, the motor is just one part of the puzzle. The controller does the majority of the work and is a huge factor in reliability and the ability to take advantage of a motors potential for efficiency. This is why buying a pre-packaged matched system like a trolling motor or outboard can have a lot of advantages.

 

I know the problem with my motor rpm. Since 2017 I've been reading a lot about drive trains on boats. So it is clear to me, that I go for a prop with a relative large diameter turning at at relativ low rpm. To achieve this I could slow down the motor rpm strongly with the controller (acting as throttle). But this would decreasing the efficiency significantly and create more heat to dissipate. So my plan at now is to build a experimental electric outboard motor with two mechanical reductions: 1. (at transom) a timing belt 2:1 (similar to the one shown at the thunderstruck site you linked, but shaft vertically oriented), 2. (under water) a bevel gear 2:1. Both reductions will naturally eat some energy, but the prop will operate at 1000 rpm. I think this will overcompensate the gear losses.