Speed / Torque / Gearing questions

[DaveAK]

I've got limited knowledge on each of these and haven't paid too much attention to it. With my bike it was pretty easy, I know the motor RPM so could gear it to get the top speed I was after. I know that if I want a little more acceleration I'll lose some off the top end, and vice versa. I'm not designing for the drag strip or race track, so I don't need to put much more thought in to it than that.

Now my summer project is a little different. It's not a bike but I hope you'll help me out anyway. :) It's going to be a slow speed tracked or multiwheel vehicle deigned for pulling a load. Most of the work will be done at probably 5mph with a 15mph top end just for getting from point A to point B.

Here's the motor spec:

CONTINUOUS RATING: @ 48 Volts
•3100 rpm
•2.8 ft lbs torque
•45 amps armature
•9.5 amps field
•2.3 HP

PEAK RATING:
•4500 rpm
•14.3 ft lbs torque
•350 amps armature
•20 amps field
•8.5HP

If I run at 48V I can gear it to run at 15mph @ 3000rpm, and 5mph @ 1000rpm (approx. 7.7:1 ratio). Or if I run at 24V I could gear it to give 15mph @ 1500rpm and 5mph @ 500rpm, right? (3.8:1 ratio) This assumes that 24V will give half the rpm.

This is where I start to get confused and need some pointers. I'm designing for slow speed pulling power, so torque is the critical factor.

1) Whats the relationship between the motor specs, voltage, torque and gearing to get me the best setup?

2) To get torque you need amps right? So would running at 24V or 48V be better for providing the low speed torque I'm looking for? I'll need to keep the amps to a reasonable level for continuous operation.

3) How does gearing effect torque at the wheel vs. motor torque?

Ultimately I can just as easily run this at either 24V or 48V, or even 36V, but I just can't get my head around figuring which would be best, and what the trade offs would be.

[Nuts & Volts]

1.) You have a sepex motor correct? The torque will be directly proportional to the armature current if the field current is held constant. The speed will be proportinal to the applied voltage if the field current is held constant as well. If the field current is variable then the torque/armature current relationship is still proportional, but that proportion changes (ex from 1 Nm/A to .5 Nm/A, etc). The RPM/V will change the opposite of the torque constant (1 up the other down). Anyone please correct me if I am wrong, I am using what I know about PM motors to tackle this one


2.) If your batteries can still deliver max amps in a 48V configuration then do that. But if not parallel some of the cells and create a 24V battery that will deliever twice the current rating.

3.) The gear acts as a torque multiplier. Multiply motor torque times the gear ratio and you get wheel torque (if going from a small to a big sprocket)

[DaveAK]

Thanks! 2 & 3 is pretty much what I was thinking. Not sure what the batteries will stand though. I'll be using some kind of deep cycle lead battery, dependent on cost.

As for point 1, I get what you're saying, but field and armature current are controlled by the controller. Now it does have two modes of operation, speed control and torque control. I'll have to read up on the options and see what the effects are. I know my bike is set up for speed control, so I haven't paid much attention as to what torque control does.

[Nuts & Volts]

Ok yea I mean I assume that the controller takes care of everything. I would the modes act like this. The Speed mode takes a throttle input and sets the motor to run at that speed and the torque is varied to stay at that speed. The Torque mode should take a throttle input and set a torque value to that input (throttle position) and the motor runs at what ever speed that that torque pushes the motor to.

Speed control is like a cruise control in a car (this right?) and torque control would be like.. something else :) no good examples

So you may actually want to run your motorcycle on torque mode because you can ask for max torque whenever, where in speed mode you may never actually use max torque (its controller depended more). hope that makes sense. and but i never applied this to sepex... ill think some more, got work to do in the mean time
:)

[DaveAK]

Yeah, my Sevcon is setup for Torque control as it should be.

[podolefsky]

I agree with everything Nuts & Bolts said.

As a physicist...I'd want to know what load you are pulling at 5 mph. That will tell you the power you need in watts (plus some to make up for inefficiencies, figure about 10-20% loss). 24V * 200 A is the same as 48 V * 100 A...you can get the same output at the wheel by gearing it differently. In general, higher voltage is going to be more efficient because it will require less current (meaning less energy wasted by heating).

If this thing is going the same speed for a long time (like 1 min or more) then you'll be limited to your continuous ratings.

[DaveAK]

24V * 200 A is the same as 48 V * 100 A
Duh. This is what I've been looking for. I just couldn't get it in my head.

As for the rest of it. I have no idea of the load really, beyond as much as I can with the vehicle that I build. :D I think the largest loads will be for short durations, so I'm hoping not to run in to limitations of continuous ratings.

Basically I'm making it up as I go along. I've just roughed out a timeline and it looks like I wouldn't be able to get a test run until July/August, unless I get a serious influx of cash before then.