How the PWM can save you more current when operated correctly. 

 

Thrust to Amps Ratio

This Blog deals with the actual current to thrust ratio for a 30lb electric trolling motor and will show why a PWM can save current and increase your time on the water. The following chart is based on a 30lb motor mounted on a KAYAK. The run time for each measurement was sufficient to establish the Amps-MPH average values. The Amps were tested with a meter, the MPH with a GPS and the battery used was a 12Vdc 75Ah lead acid.

Motor Amp vs MPH chart
Control Amps MPH Calculated MPH @ 30Amps full power

1 7.2 1.6 A/MPH = 7.2/1.6 =4.5Amp per MPH =30/4.5= 6.67MPH

2 9.3 2.0 A/MPH= 9.3/2.0 =4.65Amp per MPH 30/4.65= 6.45MPH

3 12.5 2.4 A/MPH=12.4/2.4=5.2Amp per MPH 30/5.2= 5.76 MPH

4 15.3 2.8 A/MPH=15.3/2.8=5.46Amp per MPH 30/5.46= 5.49 MPH

5 29.8 3.5 A/MPH=30/3.5=8.57Amp per MPH 30/8.57= 3.5 MPH

The chart shows a clear picture of what happens as the current is increased in your motor. In position (1) the motor draws 7.2 Amps and yields a speed of 1.6 MPH, the math shows that you get 1MPH for every 4.5 Amps you burn, or you should get a top speed of 30Amps/4.5Amps = 6.67 MPH when your motor draws 30 Amps.

So far so good.

The other positions (2), (3), and (4), show a very similar linear trend in Amps per MPH. The idea that you get a 1 to 1 relationship between Amps – Thrust is totally destroyed in the last 2 positions (4),(5) MPH.

Position (4) shows that you get 2.8 MPH drawing 15.3 Amps. If there is a 1 to 1 relationship, then if you double the Amps from 15 to 30Amps your speed should double also from 2.8 MPH to 5.6 MPH.

If the 1 to 1 relationship were to actually exist then when you add the Amps and MPH from position (4) you would get a higher MPH.

At 15.3 Amps you get 2.8 MPH and, adding 1 MPH to get 3.8 MPH requires only 5.46 Amps, that totals (15.3A+5.46A = 20.70 Amps), you have 30.00 Amps – 20.70 = 9.24 amps left.

Lets add 1 more MPH and that requires we add another 5.46 Amps.

The MPH goes from 3.8 to 4.8 MPH and the Amps used goes from 20.70 + 5.46 = 26.16Amps. You now have 30.00-26.16 = 3.84 Amps left. If we draw the full 30Amps we should be at (3.85/5.46 = .705 MPH ), add 4.8Mph to .705 MPH and you get 5.5MPH.

The tests show that in position (5) you are only doing 3.5MPH not 5.5MPH, like the 1 to 1 ratio indicated. So what happened to to all the extra Amps you had. The truth is they are wasted as heat in the armature of your motor.

They contribute to slow deterioration and fatigue of your motor, they are slowly wearing out your motor and you are going no-where fast. The power wasted in your motor will never translate to thrust output from your prop.

But wait! You say, I bought a 30lb thrust motor and it draws 30 Amps, why don’t I get my full 30lbs of thrust.

You do get your full 30lbs of thrust, it just happens before you draw 30Amps of current from your battery.

It all comes down to this: the characteristics of a Shunt Wound DC Motor has what I like to call the 80% / 80% factor. The motor is very efficient at the 50% current level, 15Amps yields about 80% of your thrust.

If you have a 30lb thrust motor then you get about 24lbs of thrust when you draw 15Amps of current.

• 30lbs thrust x .8 = 24lbs of thrust. Now you have 15Amps left and 6lbs of thrust left.
• Take ½ the current left (7.5Amps).
• Now add the 15Amps to the 7.5Amps for a total of 22.5Amps.
• Now take 80% of the remaining 6lbs of thrust (6 x.8)=and you get 4.8 lbs of thrust.
• Add 24lbs of thrust to the 4.8lbs of thrust you get 28.8lbs of thrust.
• You now have 7.5Amps remaining and 30lbs-28.8lbs = 1.2lbs thrust left.

Here you have a choice, waste 7.5 Amps of current to get 1.2lb of thrust, or back off the throttle and save the 7.5Amps for a longer stay on the water, you decide.

Now you know why you need a PWM for your motor. One you set your motor to position #5 and the PWM will give you 0-100% power in precise settings and two you will save about 25% of your current if you just back off the throttle a bit. The readout of you LCD controller or the read out of the voltage/current meter installed in your battery case will help you monitor your settings. We have given you two very important tools to operate your trolling motor. The PWM and the voltage/current readings. Kind of like having a speed odometer and a gas gauge all in one. So now you know “How the PWM can save you more current when operated correctly”.

Thanks and happy fishing, kayaking, or just casual boating, but do it with a WTP PWM.

http://www.wirelesstrollingpro.com/

My next blog will deal with the performance and cost of PWM's used in trolling motors vs the low cost high current units found on Ebay. If you are a DIY'er you will love this one.