Over-Volting a DC Motor

Why will increasing the input voltage to 48 volts increase the motor and propeller speed if the propeller is the unchanged and the current is pulsed so the power consumed remain the same as with the input voltage at 36 volts and constant current?

 

If you throttle the controller so the average dc voltage is 36, the motor should draw about the same current if you applied 36vdc.

I'm not sure if the this trolling motor has a bulit-in controller. Mine did and I bypassed it to connect directly to the brushes. If you dont break the connection to the controller, I bet it would smoke! Mine had only 50volt caps in it.

You will need much less prop drag to stay below 22.5 amps at 48 volts. An ammeter in plain sight would be useful.

 

If you want different speed operation but same output power you can just swap the propeller.
Propeller is "just like a gearbox". Now you are planning on using a controller to chop higher voltage and resulting higher current to limit the output power. Well controller is "just like a gearbox".

You should explain the whole scenario including current and target speeds and the boat in as much detail as possible - pictures from under the water hull would help.

If your current prop is optimized in your current ca. 1000w / 36V setup going up in RPM will not happen unless you go up in power. The water can seem like something that slips etc. but it does absorb the power just like wheels on tarmac would. If it wasn't like that everyone would just gear up their motors and get more for nothing.

 

It's not the power input into the motor that cause heating, it's the power losses. Power losses in a DC motor are mainly from the winding resistance, P=R*I^2. It's the current that will generate heat, not the applied voltage. The no-load speed for an electrical DC motor is propotional to the voltage. The torque is proportional to the current. Power = speed * torque. The conclusion is that you can take more power out of a DC motor if you increase the speed.

If you want to increase the maximum torque from an electrical motor you need to increase the motor size. The winding design in the motor decides which motor speed you get at a certain voltage. The winding desing also determines the winding resistance. If you design a certain motor for a higher voltage at a given speed, the winding resistance will increase. These factors will counter each other, so whatever voltage you design the motor for, the losses will be the same if the design speed is the same.

Generally, DC motor manufacturer are good at optimizing the motor design. Your trolling motor is optimized to have the desired speed and power output at 36 V. If the manufacturer had designed it for 48 V and the same speed, the motor would be exactly the same except for the winding design. The motor manufacturer has for some reason decided that the speed you now get at 36 V is the optimal speed with the propeller size you have. My guess is that you will exceed the maximum propeller tip speed if you increase speed (by increasing voltage).

If you use a switch mode motor controller designed for 48V+ you can work from 48 V batteries anyway and simply not apply full voltage over the motor.

I hope this didn't come out too confused.

Erik

 

I'm sure Erik know this but to avoid confusion it may be worth noting that a motor only produces more power at an increased speed if the torque does not decrease faster than the speed increases. Increasing the speed of a motor by reducing the torque of the load on the motor may or may not increase power depending on the shape of the torque vs speed curve and the point on the curve the motor is operating at.

The current draw of a motor depends on the resistance of the windings and on the back EMF which increases as motor speed increases.

 

Let's try it with a fixed torque of 1.

HP = Torque x rpm / 5250

HP1 = 1 x 1000 / 5250 = .19 HP
HP2 = 1 x 2000 / 5250 = .38 HP

But I suspect you meant something more like "more torque is often needed to produce an increase in rpm".

 

My mistake. Thanks for catching it.

I should have said " a motor only produces more power at an increased speed if the torque does not decrease faster than then speed increases". I'll correct it above.

 

I say you just run 6 batteries.

It sounds like you want more power, talking about more voltage, bigger propellers, but if you put bigger props, it will just spin slower. No free lunch there.
If you run higher voltage, you risk low effeciency, and failure.

It sounds like you need bigger trolling motors.

 

The voltage drop across an electric motor has two components, the winding resistance x the current draw, plus what is called the "back EMF". Back EMF is dependent on motor speed. You want the current to be the same so that the motor doesn't overheat. For that reason you have to run the motor at a higher speed so that the higher "back EMF" component can increase and keep the current in the winding the same. This gives you a higher operating voltage, but a similar current draw, same torque, higher rpm and therefore more power. So long as you don't sling the windings out or overspeed it so much that the brushes get eaten, it can work ok. As noted before, you need a prop with lower pitch than it had originally, so the motor can run faster and not have too high a current draw.

 

Heat is determined by the power (watts) dissipated in the windings, not current (amps). Ie, same current, increased voltage and you will risk overheating. You could try to increase motor efficiency (power in vs shaft power out), but you are probably past that point.

 

Sorry but you don't understand the concept of "back EMF", but the resistance losses in an electric motor are directly related to the current flow. There are magnetic losses, but in the stator windings these aren't as large as the resistance losses.

The amount of heat dissipated in the windings is equal to the current flow squared times the resistance of the windings. Think about it, the windings have a fixed (essentially) resistance. Heat generated across a fixed resistance is equal to the current squared, times the resistance. Simple as that. The higher voltage is required to overcome the higher back EMF (or voltage) created higher speed, but that is not what generates the heat in the windings and the resistance losses.

Another way to look at it is what happens if you put a higher load on a DC motor and drag down the speed. The current increases as the speed decreases. The increased load results in higher current and more losses, and if you overload it too much it overheats and you let the smoke out...

 

Yellowjacket - I am an amateur and what you type makes sense. But what boggles my mind is that wouldn't what you suggest mean that efficiency goes up with RPM with no ill effects?
To get 2kw instead of 1kw from one motor with roughly the same amount of heat to dissipate you could just gear it for higher RPM operation (if less torque is required to keep the current same) and use higher voltage. Am I missing something here.

 

Kerosene,
It is true that you double the output power while keeping the losses (almost) the same if you double the speed, as long as you work with relatively low speeds. As Yellowjacket has pointed out, there are more factors than the current that cause losses, although heat generated by current dominates. When the speed gets too high, you get trouble with bearings and brushes.

When designing an electric drive for whatever there is always the question if you should use a smaller size motor working at higher speed plus reduction gears, or a larger motor with higher torque that doesn't require a reduction gear. With the systems I work with, my preference is usually to work with large high torque motors and no reduction gear, but it depends on the application.

One thing I've learnt on this forum is that there is a limit to the propeller tip speed trough the water, so you cannot just increase the propeller speed and decrease pitch. If you try with any of the propeller design software you can find online, you will see that the program refuses to go beyond a certain propeller diameter for a given rotational speed. I'm sorry I didn't bother to understand the physical reason, or what will happen if you increase tip speed beyond this point.

It's likely that the propeller is already working at maximum propeller tip speed. If that is the case, increasing speed is a bad idea. Better to check that aspect first.

Erik

 

As Erik noted above you generally can't just increase speed because there are losses involved in switching the fields and these are called "magnetic losses" and what they do is reduce the amount of force that you get for a given amount of current. Electric motors are current machines. Current creates the magnetic flux that creates the forces. If you have greater magnetic losses you need more current to make the same amount of torque, so that is how the magnetic losses create electrical losses since the higher current results in higher resistance losses.

Moreover, you just can't increase speed to the moon since you will eventually sling out the guts of the armature, and also those pesky magnetic losses keep going up, but in general, higher speed electric motors are smaller for a given amount of power output, and run at higher voltages.

Also as Erik noted, you end up with brush arcing, wear in the bearings and brushes and cooling issues if the machine gets too small and too fast, so there is no free lunch.

We are working with some advanced high speed electrical machines that are running at over 50,000 rpm, and there are very real advantages in the size of that machine. Remember that in a generator how fast the rotor cuts across the magnetic field determines how much power you can create, so if you make the machine turn at high speeds, you can actually make a much smaller alternator and within the limits of the amount of heat that you can carry away from the windings make something that will work electrically that weighs a lot less.

As Erik also noted, the problem with marine applications of higher speed motors is that they have to be geared to take advantage of that higher speed since there are optimum speeds that the prop wants and needs to turn.

Everything is a compromise. The folks designing a trolling motor are looking to make something that doesn't cost too much, is simple, reliable and works on a common voltage, and match the prop to the motor to get the performance they want. If you have a hull that is a lot easier to push then you might be able to up the voltage and run with the same prop and get more speed at not much of a hit in reliability. If your boat is larger and heavier and is harder to push, then you need to put a different prop on the motor to reduce the torque load on the motor and keep it from overheating and burning up. By measuring the current in the installation at a given voltage you can compare that to the motor ratings and see what the motor can do. If the current is too high then you would need to put on a lower pitch prop. If it is low, you can up the voltage and get more speed.

 

Another consideration, at least for commercial manufacturers, is safety. Contact with a high voltage / high current source can be detrimental to human health. In the automotive world 42 volts is considered safe without special precautions to prevent human contact. If the voltage is much higher then added precautions are used.