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Old 06-06-2015, 00:44   #1
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Reversible DC motor

I have a transaxle steering arrangement which drives the rudder stock via a modified truck transmission via a chain.

An autopilot which drives a reversible DC electric motor would be the simplest to install via a chain to a second sprocket.

Anyone got a name, model or example of such motor?

Thanks


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Old 06-06-2015, 01:51   #2
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Re: Reversible DC motor

Aren't DC motors bi-directional by reversing the polarity? My windlass motor has poles for up/down (forward/reverse). Any ideas of power needed?
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Old 06-06-2015, 02:09   #3
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Re: Reversible DC motor

This recent thread may help

Raymarine autohelm type 1 hydraulic pump/drive repair
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Old 06-06-2015, 04:13   #4
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Re: Reversible DC motor

Well, I knew that some motors are reversible but not my forte. Not sure how to calculate power requirements for the purpose. Thanks for the link but the OP of that thread obviously shares some of my blank spots. 😃


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Old 06-06-2015, 04:53   #5
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Re: Reversible DC motor

Most likely you will need 3 things. The motor obviously and a gear reduction unit with sprocket to fit the chain. Most DC motors turn too fast to use directly as an autopilot. So a gear reduction will change the speed to something useful.

Then the third thing you will need is a shaft brake. Typically brakes can be found that either hold when power is applied or when power is not applied. You will likely need one that holds when power is applied. Most autopilots have a brake output circuit for this. When the pilot is not turning the rudder it will apply the brake to hold the rudder in position. The people that make gear reduction units also usually make brakes that bolt in line with the gear reduction. The brake usually goes between the motor and the gear reduction unit. When the autopilot is powered off the brake will release and you should be able to turn the wheel manually.

Some autopilots use a manual transmission that is operated by a cable or lever at the helm. Alpha makes some pilots that are like this. You may want to look into this as it is simpler than the electric brake. And it can reduce the resistance when manually steering.

Any 2 terminal DC motor will work with an autopilot. The best kind would be a permanent magnet motor. You have to make sure the current drawn by the motor is less than the maximum rating of the auto pilot. It does not require a huge motor. The size of the motor mostly controls how fast the rudder can turn from lock to lock. You don't want too strong a motor else it may break things as the rudder hits the stops. Depending on the efficiency of the gear reduction unit a typical motor for a 20M boat might be a 12V 20A motor (about 1/4 horsepower). The time for turning lock to lock might be 15-30 seconds for a typical autopilot. This is not super critical but you don't want it to be 2 minutes else the pilot may not be able to keep up with adverse conditions. Neither should it be 5 seconds because a motor that big will likely break things.

You may have to experiment a little but it requires more mechanical skills than electrical. The motor is super simple, it has just 2 wires.
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Old 06-06-2015, 05:08   #6
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Re: Reversible DC motor

That is helpful. I can understand the brake component.

So are there DC motors that are low RPM such that a gear is not required? Understand the mechanism and can see how to backwards-engineer the gearing. Not sure how to calculate power need but can do bit of trial and error I guess.


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Old 06-06-2015, 10:29   #7
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Re: Reversible DC motor

https://www.easycalculation.com/phys...horsepower.php
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Old 06-06-2015, 10:58   #8
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Re: Reversible DC motor

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Originally Posted by stillbuilding View Post
That is helpful. I can understand the brake component.

So are there DC motors that are low RPM such that a gear is not required? Understand the mechanism and can see how to backwards-engineer the gearing. Not sure how to calculate power need but can do bit of trial and error I guess.


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There is very little demand for slow turning low horsepower DC motors. It will not be easy to find such a thing. Most auto pilots fall into 2 groups.

One group connects the pilot directly to the wheel. Usually a belt is used and the diameter ratio between the motor sprocket and the sprocket on the wheel solves the gearing problem.

Another method is to attach as movable rod to the rudder quadrant (or tiller) and move the rudder directly. These have either worm gears or they use a hydraulic pump to push a cylinder back and forth.

All of these systems solve the gearing problem.
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Old 06-06-2015, 10:58   #9
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Re: Reversible DC motor

Ah, but what HP motor does the system require? I can recall seeing quite heavy motors used on other yachts but can see your caution in going too powerful. Maybe just try 1/8 HP and work up.


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Old 06-06-2015, 12:53   #10
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Re: Reversible DC motor

most non belt drive electric autopilots use linear actuators. They make the whole system simple, just connect them on an arm bolted to the rudder shaft. Sounds like you're further down another track You might be able to find a geared motor off a truck or car (the motors on car seats might be too small). I reckon there might be some bigger motors on trucks to tilt cabs or operate heavy devices. An electric scooter motor might work. This company Dc Quiet 12v 24v Dc Motor With Planetary Gear,Dc Motor - Buy Quiet 12v 24v Dc Motor With Planetary Gear,Dc Quiet 12v 24v Dc Motor With Planetary Gear,Dc Motor Product on Alibaba.com
might be able to help.
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Old 06-06-2015, 12:56   #11
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Re: Reversible DC motor

They typical 13M yacht autopilot motor is quite small. Probably 1/8 HP. A 20M yacht might need twice that or a bit more. Certainly not more than 1/2 Hp.
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Old 06-06-2015, 14:03   #12
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Re: Reversible DC motor

Look for a split field motor with a planetary reduction gear. I have seen gate operators using high torque assemblies like this.
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Old 06-06-2015, 15:00   #13
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Re: Reversible DC motor

Most autopilot computers are not designed for split field motors. They want a motor with just 2 wires.
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