Various types of dc motors for traction application

[ElectronS]

i am doing a research on which motor is better suited for Small EV or kart conversion using a fixed gear (single stage).

the options available are : PMDC , SepEx , Series , BLDC . ( No ac motors )

I have found so many information regarding BLDC motors and their advantages , but no useful info on the other types .

So assume 4 motors of each type that can rated continuously (S2 duty) to work on 96v 100A , what which motor is better for traction ? my initial guess is Series or SepEx , since you can design the fixed ratio gearbox for maximum torque required (hills). and get extra speed when torque is not needed (flat road) by "Field Weakening" ( something like electrical gearing) . while with PMDC or BLDC you cannot do that .

1-is my logic correct ?
2- what are the criteria in choosing one of these motors ?
3- how much you can weaken the field and how much extra speed you will get (to stay in safe zone) , most motors shops donot provide much scientific information for an engineering or academic approach for example Netgain motors.

 

[Punx0r]

Chances are you're over-thinking this. While there are advantages to each type of motor, they're going to be small given the application. Regardless of type you need enough motor to do the job when appropriately geared, so think torque and speed requirements to give the performance you want for your cart.

The other half of the equation is the motor controller. PMDC brushed controllers are cheaper than BLDC, sepex are rarer, so dunno how they compare. Series wound motors do give high starting torque but are less efficient so run hot. They're good for low-duty applications like engine starting and winches. Also, reversing a series-wound motor is not as simple as swapping the negative and positive wires.

S2 is not continuous duty (S1 is continuous) - it's a short-term rating where the motor isn't run long enough to reach thermal equilibrium and is allowed to cool back to ambient before being used again.

 

[neptronix]

There are controllers that can phase weaken BLDC motors.. many controllers also can do phase current multiplication relative to the battery current to give really amazing initial torque and thus do many of the interesting things you're mentioning.

You'll notice that roughly 95% of the motors used here are BLDC.. induction motors are not usually used because the cost and complexity of the controller tends to get high..

What motor is best has to do with your requirements... especially when it comes to cost, size, power handling capability, rpm range, efficiency, energy density etc.

 

[ElectronS]

Chances are you're over-thinking this. While there are advantages to each type of motor, they're going to be small given the application. Regardless of type you need enough motor to do the job when appropriately geared, so think torque and speed requirements to give the performance you want for your cart.

.

So there is no significant advantage of going into series or sepEx motor instead of PM , as i suspected the choice usually boils down to availability , price , size ... my misconception is from reading too much theory books and especially this phrase " " Series motors are used traction applications" . and comparing speed/torque curves

doing some calculations yields that you need a BIG motor if you are not using a multiple speed gearbox .
is there a thread here or calculator that tells you how to size motor for weight , incline angle , speed and most importantly wheel friction ???

 

[Punx0r]

Good for traction purposes could mean a lot of things: railways, golf karts etc. No modern electric car or motorcycle that I know of uses a series wound, sepex or brushed DC motor - they're exclusively BLDC (mostly) or AC-induction (rarer).

At the hobby level of EV building a big part of what is the "best" motor for an application is driven by price and availability of motor and controller.

I would go for a fixed ratio reduction on your drive. It's the simplest, cheapest and usually most efficient. If you need multiple ratios then it would indicate the motor is unsuitable for the job (rather than a case of not being "big" for the job). That's the prevailing opinion amongst the experts on this forum and increasingly in production and racing EVs.

I really would be looking at a BLDC motor ideally, but if I saw a bargain on a brushed motor and controller I'd probably use that

 

[fechter]

Availability and cost are primary considerations if you are just making one. A brushed motor may be more cost effective in the size you want depending on where you can get the motor. BLDC motors are available in many sizes now, but really big ones tend to be expensive. You also should consider price and availability of the controller, as the system requires both.

I have seen some nice induction motor/controller setups used on golf carts. Old school carts tend to use sep-ex or series wound motors. These are less efficient, but may be cheaper.

 

[ElectronS]

thanks for the valuable info guys .

maybe i am doing it wrong ( my calculations )

So how to choose a suitable motor and gearbox for a kart that weighs 800Kg maximum (with passengers) , top speed 65km/h , wheel diameter is 60cm , maximum incline 20 degree.

any rule of thumb ?

 

[riba2233]

BLDC motors are actually AC motors. Only motors with brushes are true DC motors.

 

[fechter]

Wow, that's a heavy cart. I'd look for a used motor from a small EV car. I'm sure there's a way to calculate the power needed based on what's given. If I take a while I could do it. Once you know the needed power level, then you can search for motors in that range. That sounds like more than your typical golf cart. Possibly two golf cart motors might be close.

How about a Nissan Leaf motor?

 

[Miles]

is there a thread here or calculator that tells you how to size motor for weight , incline angle , speed and most importantly wheel friction ???

https://endless-sphere.com/forums/viewtopic.php?p=312555#p312555