Model Boat Mayhem - Forum

Technical, Techniques, Hints, and Tips => DC Motors (Brushed) and Speed Controllers => Topic started by: Angus64 on April 15, 2019, 12:53:15 AM

Title: blowing fuses
Post by: Angus64 on April 15, 2019, 12:53:15 AM
Hi boys. I am back with more questions, this time I am running two 300 watt 13 pole DC motors driving two 75 mm seven blade Raboesch c type propellers. They are pushing a 1.5 meter luxury yacht. The problem I am having is I am blowing 20 amp fuses to the ESC when the the boat has been operating for around half an hour, I plan to up the fuse to 30 amp, but I would like a bit of advice before I do that as the wiring in the boat is thirty amp copper wire. Anyone out there experienced that problem & could help would be great.
Title: Re: blowing fuses
Post by: grendel on April 15, 2019, 08:18:39 AM
well at 12v the 300W motors will each be capable of drawing 25 Amps so your 20A fuses are undersized, the way fuses work when they are overloaded is not as you would think, most fuses can take a short burst of higher current, but sustained higher current will cause them to blow.
fuses come in two types quick blow and slow blow, the quick blow will blow as soon as the current exceeds their rating, the slow blow will blow on a curve, so at 40A a 20A fuse will blow quickly, whereas at 25A they will only blow after a sustained length of time at the higher current.
so if you are running off 12V batteries, when the battery is fresh (12.7V), 300W is only 23.6Amps, as the battery voltage dips, the motors compensate by raising the current, as the current raises the slow blow fuses stand more of a chance of blowing, these currents are so marginally above the fuse rating that it is obviously taking you half hour to get to the point the fuse dies. if you are running on a lower voltage battery than 12V the currents will be higher.
upping the fuse to 25A should protect the wiring while stopping the problem.
Title: Re: blowing fuses
Post by: DaveM on April 15, 2019, 09:54:26 AM
AngusYou might find this table useful - it relates to silicon-covered flexible multistrand cable, as used by most car and boat modellers. Do keep all power wires as short as possible.DaveM
Title: Re: blowing fuses
Post by: NickelBelter on April 15, 2019, 11:11:07 AM
We are missing some information needed to help solve this case. 

Motor wattage is fairly useless as two 150w motors can have vastly different specifications with regards to stall current and RPM and voltage range, etc.  Similarly the length of the boat doesn't really affect the power consumption.  What we need is the prop diameter, which you have provided, and the max RPM of the motors with the pedal to the metal (War Emergency Power  %% )  75mm props should not exceed 4500RPM, according to Raboesch, and even that is pushing it.  3000 revs would be a perfectly suitable top speed.  If you are overdriving the props it's the same as burning rubber up and down the street in your car, and if your motor cannot easily overdrive them, it's like burning rubber uphill with a house trailer hooked to the rear bumper. 
Title: Re: blowing fuses
Post by: Angus64 on April 16, 2019, 07:00:14 AM
Hi. Thanks for the information, I will try a higher amperage fuse & see if that fixes the problem. I also  mentioned that I had fitted seven blade propellers which is incorrect, I fitted five blade propellers which work very well.The wiring is Ok according to the table that DaveM put up, & is kept as short as possible, so that should be ok  I will put up some pictures after I try the boat on the lake so you know what it looks like, I thought it came up pretty good. Designed & built from scratch & took me the winter & into the summer to complete. Thanks again for the info & parting with your knowledge, It's a great help.
Title: Re: blowing fuses
Post by: malcolmfrary on April 16, 2019, 10:20:50 AM
The purpose of the fuse is to protect the boat against anything thst a faulty ESC might do.  The ESC itself looks after the motor demand.  In a multi ESC setup, a fuse for each ESC makes sense, one faulty ESC doesn't stop the entire system (unless its the one carrying the BEC that provides the power for the radio).  While a fuse in the battery to single ESC does offer some protection, its main use, once the right value fuse has been fitted, is to provide a useful connection point for an ammeter, which is a good way to find out just what current is being used.

Title: Re: blowing fuses
Post by: Allnightin on April 16, 2019, 04:51:35 PM
What sort of maximum speed are you trying to get out of the model?
Title: Re: blowing fuses
Post by: Angus64 on April 17, 2019, 02:36:26 AM
When I first fitted out this boat I had fitted two separate systems two BECs two ESCs, But I am using a 10 channel Flysky radio with one throttle control & found  that the motors didn't start up together or run at the same speed, so I went back to one ESC which work quite ok. I am using a Pro boat 50 amp ESC at the moment but I intend to update the ESC to a Viper 75 amp which is what I use in my other two boats, They are as good as you can get if you are running big models with brushed motors. The motors are cooled with a fan blowing cold air into the motors & the boat is powered with two 12 volt 12 amp hour lead acid batteries. The motors don't run hot in the boat & they aren't hot when the fuses blow either. As for the speed I didn't have a speed in mind when I designed the boat but expected it to move reasonable fast which it does. I have a pictures of the boat I took during the first sea trials so I will put it up so you know what I am working with.
Title: Re: blowing fuses
Post by: Navy2000 on July 11, 2019, 05:29:03 PM

If you turn the shaft by hand and it seems snug that will also make the motor pull more amps as well. The shaft needs to turn as freely as possible.

Title: Re: blowing fuses
Post by: malcolmfrary on July 12, 2019, 11:12:40 AM
as the battery voltage dips, the motors compensate by raising the current,
Not with permanent magnet motors, unless they are overloaded and being forced into a semi-stall state.  Lower voltage means that there is less force available to push the current round, if the battery is low and unable to supply the current demanded, it drops its voltage still further.  The lower current is then what causes the motor to slow.  Field wound motors are..........different.
An overloaded motor being forced to slow sjould get hot before the fuse goes, but if it is an appropriate motor for the job, either something is binding on the way to the prop, or too much prop has been fitted.  It might need gearing to match it to the prop.
Pulling over 20A from a 12AH SLA is not a recipe for long battery life.  The ones that modellers can generally get are designed around the expectation of being used in a 10 or 20 hour discharge situation, so about 1.2A continuous, not much.  Lead acid can deliver enormous current briefly, notably until the car engine starts, but generally, those intended for continuous heavy discharge are built differently.  When they were the only game in town, their shortcomings were tolerated and worked round, but working alternatines are available.  Good ballast for tugs, beaten soundly by NiMH in all other respects in models.
Just an afterthought - a 3000rpm 13 pole motor will be commutating at about 650 times per second.  If this coincides with a harmonc of the pulse rate of the ESC, strange things might start to happen.