Over-Volting a DC Motor

For a PMDC motor, the torque constant KT (in Nm/A) equals the back-EMF constant KE (in V/radians/s). 18 oz-in/A = 0.127 Nm/A. 15 V/kRPM = 0.143 V/rad/s. I can only speculate on the reasons they don't match exactly; maybe friction introduces an error in your readings or you are close to saturation when measuring KT. In my opinion they are close enough to show that your measurements are reliable.

Erik

 

I measured the KE (rpm) with the prop in air and the load reduced the rpm some but I am glad they almost agree. The resistance measurement with a locked rotor is suspect with my long wires and my voltmeter on the wrong side of the ammeter.

Based on some calculated curves, it looks like the rated power input of 1.3 kWatts is going to operate the motor way past the peak efficiency at 36 volts. I am expecting to measure an output power of around 950 watts or 73% efficiency. At 48 volts and 1.3kWatts in, I expect about 1050 watts output power or 80% efficiency.

 

Thanks Gentleman for all of this effort into such a deep investigation of this concept. It seems that the initial thoughts were disappointing, and then after more study, there seems to be some sense in trying to move forward. Though, I am not an electrical engineer, I am absorbing a good deal of what has been said and demonstrated. Some of the Layman's answers were quite helpful, and the motor that I have is in fact the Motorguide 109 lb Thrust unit (Without Speed Control Board). Any testing with this model would be extremely helpful.

After considering the response here referencing Propeller choices or alterations, I feel that this opens an entirely new discussion, likely inappropriate in this thread. Well, after saying that, I'm going to ask anyway...Keeping a propeller size below, let's say, 12" diameter, would there be more efficiency in a faster turning prop as long as you do not overload the motor? As a gross example, a very slowly turning prop allows water to flow around the blades, but water cannot turn this corner at high speed for simple inertia reasons alone. Hence, is this faster 48 volt motor going to become more prop efficient with a carefully selected Prop? The Pop Tip is doing the most work in most Props. I recall in my last boat that after a lot of grounding in gravel bottom, that I lost about 1/4" of diameter of the very pointed tips...about .06 Square Inches. I was surprised at the significant regained performance with a new propeller.

Anyway, please keep this alive and any help in this subject will be well appreciated.

Regards,
John

 

You have to match the prop to the motor and hull, since you don't want to overload the motor at the speed you will be traveling. If you have a hull that is easy to drive, then the original prop or something close may well work. You need to look at the prop pitch and the expected speed and see what that does for you. A slightly smaller prop with less pitch may well be called for. However, within the speed range for both the prop and the hull, there isn't going to be much change in prop efficiency. The original trolling motor prop is most likely not matched very well to you needs anyway. These small motors are more about very low speed thrust, and not so much about real longer distance propulsion.

 

Remember the rule of thumb, to go twice as fast you need 4 times the power.
Why not just add a second trolling motor one on each hull at 36 volt and pitch the props up?

 

The right answer is to use a 48 volt battery pack and 48 volt motor controller and run the motor controller throttled at no more than 36v. The amount of speed gain over 36v very small unless your hull is way over 20' long.

The attached plot is where I overvolted a 24v motor to 36 volts on a 12' light weight v hull boat. It's a little hard to follow the plot. Basically, it took triple the electrical power to go from 24v (4.5 mph) to 36 volts(5.4 mph).

This is a function of the boat's drag and no mater how much we tweek the props or optimize the motors, we are pushing up against a wall.

If you had a pair of these motors and a very long light hull, you might get a few more mph at 48v. I did a rough measurement of the 3 bladed machete prop and it is around 4" to 4.5" of pitch. Some of my other trolling motors props are as low as 3" of pitch!

When I get more test results, I will post here.

Please post some information oun your hull's shape, size, and weight so the
experts here can provide some drag vs. speed estimates.

 

Current is the limiting factor in motors. Too much current generates heat that slowly (or quickly if it's high enough) destroys the insulation. You can increase the voltage a lot before arcing becomes a problem. Brushed motors are also rpm limited by the commutator.

An example is the brushed motor we used to set the electric boat speed record. It started life as a 48 volt hydraulic pump motor. We ran it at 133 volts nominal (around 120 volts at 700 amps) and 850 amps peak current. The motor ran well for the short period it took to go through the kilometer long trap twice at almost 100 mph. The commutator needed re-cut after a season of test runs and the record run, but the motor was otherwise fine.

The graph below shows the voltage and current for one 22 volt lithium polymer battery pack. The boat used 7 of these packs in parallel with six of these parallel sets in series. The rpm is propeller rpm. the motor ran the propeller through a speed increasing quick change gear box. For the run shown the ratio was 1.75 to 1.

Today's batteries are much more capable and could power a bigger motor. We were seriously limited by the requirement to run a brushed motor, and to run 144 volts. Since we didn't know how the voltage would be measured, we were conservative.

Lohring Miller

 

Moving Forward

Thanks Lohring for your conclusive summary.

Thanks, also for all of you others that offered their advice. I believe that I have enough information here to attempt what I want to do. Some here were concerned over the hull drag, indicating that this was going to be a futile effort. I believe that I can push this craft to higher speeds because of it being a Catamaran with hulls that have a 16 to 1 Length to Beam ratio, and I am not building up too much of a wake at present. I will monitor my amperage (Wattage) and see just how far I can push it, and find out where my max efficiency is. Perhaps I can still play a little with prop pitch.
Regards,
John