Why do trolling motors burn out?

Discussion in 'Electric Propulsion' started by kroberts, Apr 11, 2017.

  1. kroberts
    Joined: Mar 2009
    Posts: 318
    Likes: 12, Points: 18, Legacy Rep: 210
    Location: Chicago area

    kroberts Senior Member

    Hi,

    I fish a lot of stock ponds. A lot of times you get underwater vegetation growing pretty thick.

    I've gone through 2 trolling motors in 2 years. In both cases the boat was doing fine and then the motor burned out before I knew there was a problem. In both cases, the prop was wrapped up in weeds.

    With a gas outboard the engine will bog down and stop, but all you need to do is clean the prop and start it up again.

    Why is the electric system so fragile? And why can't they make some sort of circuit breaker that turns it off before it turns to junk?

    I'm not completely ignorant about electric motors. I've wound my own brushless motors for RC hobbies, and I've rewound a 5 hp motor for a compressor, and I've replaced brushes etc on various electric motors which are not submerged. I know a little bit about speed controllers for BLDC motors (which seems to be a rebranding of PMAC so hobbyists can think they have something new)

    I know that in order for the motor to burn up it has to overheat. I know that in order to overheat it has to draw lots of current. Is it that the prop stalls the motor but draws no more current, and the motor depends somehow on the spinning prop to cool, or is it that the stall causes the motor to draw more current than normal?

    The way I understand series-wound motors a stall would cause the current to jump, and the torque to jump too. It seems to me a simple circuit breaker should protect you here.
     
  2. CDK
    Joined: Aug 2007
    Posts: 3,324
    Likes: 145, Points: 63, Legacy Rep: 1819
    Location: Adriatic sea

    CDK retired engineer

    It takes more than just a circuit breaker to protect a motor. The point where the breaker trips must not be a fixed current because sometimes you want to exceed that current for a short time to accelerate, cope with waves or avoid an obstacle.

    A microprocessor with current, temperature and rpm sensing, with a digital map of the safe operating area is required. It would seriously increase the price or lower the profit of the manufacturer....
     
  3. kroberts
    Joined: Mar 2009
    Posts: 318
    Likes: 12, Points: 18, Legacy Rep: 210
    Location: Chicago area

    kroberts Senior Member

    For sure the part about lowering profits would be true, as it is I've been buying a new trolling motor every year.

    This map of safe operating areas and such, is this common for all motors of a size or is it unique to each motor? I could scrape together a raspberry pi and some sensors.
     
  4. CDK
    Joined: Aug 2007
    Posts: 3,324
    Likes: 145, Points: 63, Legacy Rep: 1819
    Location: Adriatic sea

    CDK retired engineer

    The motor supplier must be able to provide the necessary data, like a graph showing rpm and current under various loads. Much depends on the thermal properties of your motor. The heat is generated in the windings and subsequently dissipated and/or transported to the outside world. If that path conducts heat well, the motor will survive a certain amount of overload. How much it can handle also depends on the ambient temperature.

    There is a cheap but reliable current sensor ACS-712, suitable for Arduino and Raspberry.
     
  5. kroberts
    Joined: Mar 2009
    Posts: 318
    Likes: 12, Points: 18, Legacy Rep: 210
    Location: Chicago area

    kroberts Senior Member

    IMO any RC model motor manufacturer builds this stuff into the controller. That argues for the additional cost right there. There are boat and aircraft motors significantly larger than what I'm thinking about, so that part isn't a problem.

    IMO this is a built-in guaranteed profit for trolling motor manufacturers. They have no interest in keeping their motors running.
     
  6. rxcomposite
    Joined: Jan 2005
    Posts: 1,925
    Likes: 151, Points: 63, Legacy Rep: 1110
    Location: Philippines

    rxcomposite Senior Member

    My vacuum cleaner has a thermal cutout switch in series with the motor. It would shut down whenever the vac overheats due to prolong use or clogged bag but will cut in when it cools. I have replaced it once and never had any problem. Most electrical motor I have opened has a thermal switch, even the lowly electric fan has a fuse resistor.

    It is very cheap and is packaged in a metal can like a TO-3 transistor case. No soldering needed. Watch out for the current, voltage and temp ratings.
     
  7. alan craig
    Joined: Jul 2012
    Posts: 220
    Likes: 28, Points: 38, Legacy Rep: 14
    Location: s.e. england

    alan craig Senior Member

    kroberts, I replied to your other thread before reading this and now realise I have told you stuff you already know.
    I didn't know trolling motors burn out so easily, they look very rugged inside. Possibly they are very inefficient and run hot anyway. I do know that they will attempt to keep running at the same speed if the load increases and this inevitably means more current = more heating effect. I don't know of any small brushed controllers with current limiting.

    The thermal cutout switch sounds like a good idea. You could even have one on a separate circuit which operates a buzzer, say, to give warning of high temperature without actually cutting the motor.
     
  8. Barry
    Joined: Mar 2002
    Posts: 1,105
    Likes: 66, Points: 48, Legacy Rep: 158

    Barry Senior Member

    Or possibly

    At normal operating loads assume your motor draws 360 watts. This is a fixed draw and the fixed load.

    A watt is a volt times amp.

    Without getting too involved, say your fully charged battery is 13 volts,
    At 13 volts your motor draws 360 watts divided by 13 = 27.7 amps

    And assume that your motor is designed to dissipate whatever heat this will put into the motor at this load

    Now over time the battery discharges to 11 amps. Now the current draw is 360 watts divided by 11 volts or 32.7 amps.

    So just in normal operating conditions, with a discharging battery, the current draw has gone up 18%, More current, more heat that the motor might not be able to dissipate well.

    And if you load up the prop with more load, weeds etc, the current input will go up even further with even more heat that it can not get rid of.

    I understand the concept of the thermal cutout but am wondering how you might get this into an area where it might recognize the heat.( as an aftermarket add on as compared to being installed at the factory) Ie it and the motor is in cold water so the cutout would have to be insulated from the water somewhat.
     
  9. Caroute Motor
    Joined: Jun 2016
    Posts: 54
    Likes: 1, Points: 8
    Location: CN

    Caroute Motor Junior Member

    Different copper wires can withstand different currents. When your voltage is too high or the propeller stuck, the current may rise instantaneously. It will burn your motor.

    The motor insde heat is large, electric motor' copper insulation coating will become dark and thin after long-term usage. After a certain limit, the motor may burn up. Current brush electric trolling motor may burn up for continus using about 300 hours.
     
  10. kroberts
    Joined: Mar 2009
    Posts: 318
    Likes: 12, Points: 18, Legacy Rep: 210
    Location: Chicago area

    kroberts Senior Member

    All of my burnouts happened with a fairly full battery, and I had something wrapped around the prop. Weeds on both, fishing line also on one. So it got into the gap between the prop and motor housing.

    Alan Craig, the motors I'm thinking of using on the paddle wheel are brushless model aircraft motors, controlled by an external motor controller which sometimes costs almost as much as the motor. They can be very powerful. I've seen one site which supplies motors as high as 19kW. That's something like 25 hp. This is WAY more than I want to use, only an example for others who may be interested.
     
  11. rxcomposite
    Joined: Jan 2005
    Posts: 1,925
    Likes: 151, Points: 63, Legacy Rep: 1110
    Location: Philippines

    rxcomposite Senior Member

    The motor controller controls the maximum amount of current and the time needed. When the motor starts, it needs a lot of current for a short period of time until it reaches momentum where the running/operating current goes down. The same goes for the stalled condition when the motor freezes due to tangled weed or fishlines.

    If the motor controller was bought separately, it is probably not matched to the motor. It is supposed to limit the time and current for starting and limit the current to a safe level in the stalled condition. Controllers with programmable starting and limit current are expensive.

    Another factor is the internal resistance of the battery. If the battery is big, has a high cranking amperes, it can short your controller. Unfortunately, high internal resistance battery has low reserve power.
     

  12. W9GFO
    Joined: Dec 2014
    Posts: 198
    Likes: 13, Points: 18, Legacy Rep: 39
    Location: Olalla, WA

    W9GFO Senior Member

    A typical trolling motor is not great at getting rid of excess heat. The housing which holds the magnets are well cooled by the surrounding water but the rotor has no path for heat conduction aside from the small bit of shaft that the plastic prop attaches to. If the motor is frequently fouled by weeds then it will be under greater than normal load. You can purchase an aftermarket solid aluminum spinner to place on the shaft that will greatly increase the surface area for the rotor's heat to be conducted to the water. I don't know that it would prevent the motor from burning up, but it should delay it somewhat.
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.