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
Best Answer
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.