AC vs. DC in Low Speed, High Torque application

Question

My club is building a 1/4 scale utility tractor (around 800 lbs) and for the past couple of years we have been using an electric hybrid drivetrain. Our system has been a 32 hp gas motor powering a DC generator which we convert to 3 phase AC to power our independent wheel motors with an operating voltage of 72V AC. My question is would we be better off using a DC motor considering we don't require high speeds but need very high torque for our pulling competition.

The motors we are currently using are rated at around 40 ft*lbs at 5,000 RPM and go through a 31:1 gear reduction which we thought was plenty but when they start seeing a high load, the current spikes and faults out our controllers.

The first year we ran this system the controllers faulted out because we didn't limit the amount of current the motors could have, we changed the system this year to have a current limit so the controllers wouldn't fault out and it basically shut our motors off under a high load, I guess the question isn't necessarily which is better but is one more efficient under high load to make the most out of the current we can generate.

RPM: Our final drives have a 31:1 ratio so anywhere between 3000 to 5000 RPM, HP our gas motor is 32 hp so we sized our current motors to be 15 hp peak per motor so we could equal the power capability of the gas motor, Budget is hard to gauge, we have a decent amount of funding but sometimes companies sponser us or give discounts, Torque: we initially thought 1200 ft*lbs per wheel was plenty but now it is looking like we would need more around 2000 ft lbs per wheel to be able to compete with the other teams at our competition. The pull lasts maybe a minute or two maximum so we are pulling peak current that whole time but we have long rest cycles between pulls and events so the motor doesn't see prolonged current draw. We have a .66 F Capacitor in our system to smooth the DC current we provide to our controllers.

Links: Motor Controller

Answer 1

What you have is a permanent-magnet synchronous motor. That is the same thing as a brushless DC motor. The motor may be too large for your controller or the controller may not be adjusted properly for the motor. If the motor and controller is suitable and adjusted properly, the problem could be that the engine-generator set is being loaded too heavily. Check to see if the DC voltage drops when the load is high.

The motor-controller combination that you have should perform as well as a brushed DC motor and controller or an induction motor and controller. One technology is not necessarily better than the others, but some models and designs of each may be better than the others. With any of them, the weakest link is the one that will be the limiting factor. A good design, with properly matched motor, controller and power source is essential. When the limit is reached, the controller should limit the current, not fault out.

Re comments and further consideration:

With the optimum design, a permanent-magnet synchronous motor (PMSM) should be capable of providing the most torque per amp of any motor technology. However, all designs are compromises involving various measures of performance and manufacturing cost. Selecting the best motor is a mater of evaluating the specifications of motors under consideration.

The performance of the motor must be evaluated based on a careful analysis of the requirements of the application. Tractor pulling is a contest in which a load is pulled as far as possible. As the load is pulled, the required force is increased by shifting the load weight from a wheel and axle to a skid thus increasing the friction between the load carrier and the ground. It should be defined whether the load increase is based on time or distance and whether or not there is a time limit.

It would probably be advantageous to operate the engine at maximum power for the entire pull. Since power is torque multiplied by speed, that would mean reducing speed and increasing torque during the pull. The most efficient means of doing that would likely be a continuously variable transmission (CVT) such as a hydrostatic transmission. A manual gearshift would be just as efficient, but probably not quite as effective. However the gearshift or both the gearshift and the CVT may be prohibited by contest rules.

A series connected, brushed DC motor inherently slows down and increases torque as its load increases. That characteristic may make it preferable to more efficient brushless types of motors for this application.

An induction motor can be controlled to operate with constant torque over the low end of its speed range and constant power with declining torque at higher speeds. With a current limiting controller, that would provide performance somewhat similar to the performance of a series DC motor.

The torque capability of a PMDC motor will be pretty much constant over its entire speed range with no capability to trade speed for torque. The torque capability is limited by the current available. In this application, the motor must decelerate to a stop pretty quickly when the current limit is reached. There may be a way to trade voltage for current in the supply and control systems. That could have the effect of trading speed for torque if the motor can operate at a higher current for the required time.

Before all else, it important to make sure that the selection of the fixed ratio of the drivetrain gearing provides as much torque as possible by reducing the speed as low as the application will tolerate.

Answer 2

The combination of permanent magnet synchronous motor (PMSM) and dedicated controller that you already have is the best option you can have (IMO). It is robust and reliable, better than brushed DC and its DC controller.

If you want lower speeds and higher torque you can change the transmission ratio, although I think that the problem may be that generator isn't sourcing enough power or the load is simply too heavy. In the last scenario you have to change both: generator and motors.

Answer 3

Swapping out your wheel motors sounds like a major job, one you wouldn't want to undertake unless you really believed it would give you an improvement.

Your system has so many components, it's not obvious what the limiting factor on torque could be. Unless you work on the actual factor that's causing you problems, your efforts could be wasted. It would be worth an investigation from the ground up, in order to find out what's going on.

However, your controllers faulting out rather than limiting gracefully is obviously your number 1 operational issue. Changing the type of motor won't (in fact shouldn't) change the electrical to mechanical parameters, other things like max speed staying equal. If changing the motors has the side effect of changing the controller for one that works properly, that's not really the best way of getting the controllers sorted.

Let's leave aside for the moment whether you have enough prime mover power, or wheel slip as a problem.

One way to get torque for free is trade it for speed. If you halve the top speed of your tractor through gear ratio, you will get twice as much torque for the same motor torque. Is there a minimum top speed in your class of competition? If you are faster than that, then you're wasting free torque.

Once you have the gearing right, can the motors you have deliver the torque?

Do you need higher continuous torque, or peak torque? Continuous torque requires constant power from the prime mover, and constant power dissipation from the motors. Peak torque could use transient energy storage for input, and could use motor thermal mass to deal with the temporary higher power dissipation. Motors can be transiently overloaded for torque by applying excess current if a) the current is limited to below the demagnetisation maximum (check specs) and b) they don't overheat during the transient.

To apply the extra current obviously needs a suitably programmed controller.

Other things being equal, it really doesn't matter whether it's a DC or AC motor. Newer high field BLDC motors tend to be more compact than typical brushed PM motors. BL tend to be more reliable than brushed. Brushed motors tend to have a closed construction and a wound rotor, making the cooling much less effective than some BLDC motors which put the windings on the stator and have an open frame construction.

If you already have BLDC motors there, think long and hard before you swap pout for something inferior. Get your controllers sorted. Remain open minded about what the next performance limiting thing is.

Answer 4

would we be better off using a DC motor considering we don't require high speeds but need very high torque for our pulling competition?

YES and NO

It all depends on Budget, Specs, max Torque, no load Speed and HP for sustained speed under full load. Did I say Budget?

https://www.tesla.com/blog/induction-versus-dc-brushless-motors?redirect=no

Geared DC motors are common in many styles including; the more reliable types of PMSM, cheaper ones like BDC motors such as automotive starters which are copper brushed (not carbon) for max current, torque at low duty cycle and may be cooled for longer durations or duty cycles in larger sizes.

So no complete answer is possible unless you have a spec for Torque, RPM, HP and Budget. Also include weight and size with batteries to power it. THen include active time duration and dwell time between uses for cooling and recharging battery C discharge rating and charge capacity in watt-seconds (J).

Can you supply any or all of this?

Perhaps you want a Tesla AC motor, LiPo modules with a different gear ratio. But then you might be able to afford it. Why does a Tesla car use an AC motor instead of a DC one?