Over-Volting a DC Motor

Hello Friends,
I am trying to upgrade to a larger trolling motor to power my Electric powered boat. My intention is to take a 36 volt Trolling motor and because I want just 4 12 volt batteries for weight consideration, and because I believe that added RPM on the stock propeller will give me the speed boost that I am trying to achieve. It seems that those Electric scooter guys have pushed this envelope quite a bit with over-volting...many of them smoking motors. I feel that the non-direct connection to water instead of fixed ground, without any slippage, along with the intense cooling is what will be my buffer to make this work.

To do this, I want to run the DC motor on 48 volts. I will assume that the RPMs will increase by possibly 25%. I do understand that I will need to keep the peak wattage at the same as it would be if running at 36 volts. Motor cooling is at it's best, as it is running underwater. Without getting into Thrust bearing restraints, or other mechanical issues, I would like to have the input of those knowledgeable in this area.

My questions are:
Is this a real possibility?

If so, can you push the wattage a little, as I have been told that Amps are the killer, not the Volts?

Is there a limit as to how high in voltage that I can go?

Is it in fact better to run a DC motor at a higher voltage with less amps in general?

Are the Brushes the weakest link, etc.?

Any other input would be appreciated.

JV

 

Watts and heat are the primary killers, not volts or amps. I've seen motors that reliably run at 2x the rated watts for very short bursts.

A temperature sensor inside the motor might help.

 

Are you using a different controller? The motor controller might blow-up if given 48v.
The motor itself would be fine.

 

The motor will run fast and the controller may blow up.

Doesnt sound like a good idea.....

Best to ask the tech rep. At the trolling motor company

 

The motor can be harmed only by heat, which is the product of current and voltage (Watts), multiplied by the duty factor.

Assume that the motor can operate continuously on 36 volts, drawing 10 amps and generating 360 watts.
At 48 volts the current rises to 13.3 amps, so 48x13.3= 638 watts. To prevent the motor from burning out, the duty factor must be reduced to 360:638= 56% !!
That means the motor should be switched off 44% of the time.

Whether or not the controller survives depends on the quality of its output stage. The increase in current alone is not significant, but at 48 volts there are spikes of nearly 100 volts that may prove fatal if the output stage is only rated at 80 volts.

Is it a good idea? No, it is not....

 

Any chance you could wrap the motor housing with copper tubing to pump cooling water thru?

 

Thanks guys for the input.
To help clarify what I'm trying to do, let me explain that I will set an amperage limiter in the Pulse-Modulated Speed control that I will use to hold the wattage the same at 48 volts......30 amps x 36 volts = 1080 watts / 48 volts = 22.5 amps. I believe that the craft can go faster without much more drag, and that the limited pitch of the plastic propeller is holding me back. I might experiment with an aluminum prop that I might be able to tweak some extra pitch into. I will have current monitoring instruments in the dash to carefully monitor the motor's load.

My hopes are to increase RPMs for the prop pitch and allow for just 4 batteries. Is the above scenario acceptable?

All things being equal, is it OK to run any DC motor at an over-voltage provided you keep the wattage the same, and if so, do you believe that only upping the voltage by 25% is an acceptable practice and could the motor be run continuously that way?

My worries are over the Motor Brushes and Commutator. Am I right in thinking that 1080 watts running at a slow RPM/36 volts would develop the same heat as 1080 watts at a faster RPM/48 volts? My thinking tells me that the less amperage at higher voltage would be easier on the system, or is it just down to total watts period? Remember, this trolling motor is totally submerged in water at all times for good cooling.
I also feel that the sophisticated Pulse Modulated controls make things easier on motors. Is that true?

Again, All help with this is appreciated.

PS: The trolling motor people are of no help at all. They won't even give you the RPMs of the motor.

Regards,
JV

 

One more note.
The motor will run at full output only part of the time. 1/2 or more time is spent low trolling while fishing.
JV

 

I would guess business competition means the manufacturer has taken any cooling advantages into account and sized all motor components accordingly, so an X sized water cooled motor is built lighter than the same sized air cooled motor.

Probably different brands and quality of construction would have a big impact on how they handled any overload.

 

How are four batteries lighter than three? You will be better off by calculating the power storage requirements for the time you expect to use the motor and find the lightest set of batteries.

 

Hey Gonzo,
Good point. My boat is a catamaran, and I need to divide the weight between hulls. I am prepared to go to 6 batteries if needed, but the purpose of this exchange is to see if thinking "Outside the Box" has any merit at all. As mentioned before, those Scooter guys are pushing this envelope and in some instances, are getting away with it. I welcome those that will attempt to answer my questions, as I am not an electrical engineer, and have them ponder what what I am trying to do in theory at least. If I am all wet, then I will drop this, but I would like an education around this subject. In other words...If this - Then This, etc. Sometimes, you enter into more deep discussion than than just yes or no. I am looking for maybe's, or if you try this....you, under these circumstances might get this.
You can see that I don't easily give up, but I have hopes. Please educate me.
JV

 

Trolling Motors

I have upped the voltage in trolling motors before and I am upgrading a 36 volt unit now. I have two 24 volt models that I run on 36v with a scooter motor controller. My observation is that once you get past the rated voltage, you need to reduce the prop drag to be effective. If you don't the current will go up fast in the motor, you will get too much heat and make diminishing increases in power.

I am an Electrical Engineer but learning more about hydrodynamics as I go. I was under the impression that the prop would 'unload' as you speed up but this doesn't occur with the trolling motors much.

More voltage = more current and a loss of efficiency.

I was up against the hull speed on my row boat in this plot. The X axis is speed (GPS MPH) and the Y axis is electrical power. The pink line is the standard 'power prop' and the green line is using a high pitched prop. Increasing the pitch got me faster with lower voltage but would draw much more current.


In the past, I have used a standard trolling motor prop with the diameter cut down. These motors are designed to push a heavy boat slowly through the water. A light boat should not need as much prop area so I thought. I am still working on this.

This is work in progress and I will post more as I go.

 

Sorry,

I didn't answer your questions.

My questions are:
Is this a real possibility?

Yes. You can do it.

If so, can you push the wattage a little, as I have been told that Amps are the killer, not the Volts?

The wires/windings will get hotter and water coling is already designed into the trolling motor. Excessive power, voltage, amps, and rpm are all the 'killer'.

Is there a limit as to how high in voltage that I can go?


Brush arcing and overheating will eventually take it's toll. Continious use at 48v/36v = 133% voltage will eventually overheat the brushes. I wouldn't push it over that. Also consider the safety of high voltages on the water.


Is it in fact better to run a DC motor at a higher voltage with less amps in general?

In general, it's best to stay within the motor's design limits. Spinning faster might cause you to operate above the rotor's natural frequency causing excessive vibrations. The commutator will usually come apart at high speed first.

Are the Brushes the weakest link, etc.?


The entire motor is the weakest link if designed properly. Almost all dc motors can handle short bursts of over power. 48 volts for a minute or two should be safe. If you are using a 36 volt motor rated at 40 amps, try to keep the current below 40 amps and it might hold together longer.

I added a plot which shows my speed vs. voltage for my 425lb row boat. I also show electrical power vs. voltage on the secondary axis. The reference is 40 volts*36 volts for 100% power.

The speed is proportional to the square root of the electrical power and the electrical power is proportional to the square of the voltage. This results in the power being proportional to the cube of speed as expected for a displacement condition. The motor efficiency is going down with high current so I am actually doing worse than the theoretical speed.

I will need way more power to get over 6 mph with my 12' hull or find a way to reduce the drag. Adding a 'fairing' to the motor vertical shaft will help some. Reducing weight will also help as well. It will take some really extreme measures to ever achieve much more that 6 mph with a trolling motor on my 12' boat. If you have a longer lighter and slicker hull, you might go a little faster.

 

This thread reminds me of plugging a shop vac into an outlet that was incorrectly wired for 220 volts. I was amazed how well the crappy old vac was working, but after about 5 seconds the vac imploded and I then figured out why it had worked so well. I suppose it was the mechanical equivalent of a human meth rush.

 

If he limits the current to 22.5 amps max from a peak battery voltage of 48V, the wattage dissipated in the motor will remain the same as it was with 36V and 36 amps. The pulse controller is maintaining the RMS current to 22.5 amps by narrowing the individual pulse width so there should be no problems. Insulation of the motor windings should handle the increase easily.

There may be some practical issues being overlooked but mathematically it works fine. Making the pulses narrower does increase the frequency spectrum and that may decrease the actual power to the prop, or maybe not.