390 DC Motor

[enginepeter]

My Black Prince build is under powered so I have started to look at motors and gearboxes,

I thought I had a 380 motor fitted to a 4:1 gearbox (as described on the Invoice - no longer have the box). When I started to look at motors and measurements I realised they are 390's.

The 'no load' revs are just under 25,000 rpm and the stall amps around 27amps

Comments on the forum suggest Igarashi as being the solution but I would like to compare the specs. If small higher powered motors will not work I will then have to look at remodelling the gearbox.

Not found any specification on the Internet of this - or even a similar motor.

Does anyone know the stall torque of the 390??

Thanks

Peter

[silversurfer1947]

I got this off a for sale description on Amazon
Stall Torque : 133.1 mN-m, Stall Current : 14.5A

[enginepeter]

Thanks for the Info but I wonder if this the same motor as I measured a stall current of 27 amps? I have to admit I am not conversant with motors, amps etc and am feeling my way! Peter

[silversurfer1947]

If 390 motors are anything like the 540 and 550 motors used in rock crawlers, the torque on a motor varies according to the number of turns, the more turns the higher the torque, but lower speed and vice versa. I am guessing that 390 motors can differ in their performance. I am no expert. Incidentally, this is the motor that I foundhttps://www.amazon.in/HRS-RS-390-High-Torque-Mo ... B01NBU69S7 Again, I am guessing but I think the figures quoted are for running at 12v.

[enginepeter]

Yes, the 390 I have has a no load rpm of 25,000 at 7.2 volts so I am guessing the torque figure quoted is not similar to mine. I guess I will have to try and measure my motors output some how!

Thanks for replying. Peter

[tankme]

Motors can be a bit tricky to figure out when the person selling them doesn't list all the specs. Richard is correct that motors can vary greatly depending on how they are wound. A 390 isn't necessarily the same 390 just by appearance. You could see if Erik at Taigen has the torque specs of those 390 motors you are using. If you are looking for more torque you might look at different motors like 480s as they are shorter than 390s with a bigger can. You would have to change the mounting holes from what I remember about them and they may have a different sized pinion gear. I'm not sure how much space is in the Black Prince lower hull, but you may have to go with some sort of an ETO/Modellspiegel type setup to get more torque. I'm assuming more torque is what you want since the BP is not a fast tank.

[enginepeter]

Yes, I am after delivering more power to the tracks and don't have a lot of room. It might be that I have to build a different gearbox, within the space constraints that I have. Thanks for the suggestion to contact Eric at Taigen. Peter

[Herr Dr. Professor]

Thank you, Silversurfer and Tankme, for the explanation you give here. It would be fine to have a tutorial on motors and perhaps even reviews. That is, an accessible-to-the-greenhorn explanation of the effect of rotor diameter, number of windings, voltage and amperage on motors. And it would be great to have reviews indicating which available motors are faster, which have more torque, which fit which mounting dimensions. Available gearboxes could also be discussed as to what reduction ratio they produce.
O.K. I woke up from a dream now.

[enginepeter]

Thank you Herr Dr Professor I agree about more info on motors. Ideally different motor/ gearbox combinations should be tested and that output quoted. For example is a 4:1 gearbox really 10% more efficient than a 5:1 as it has 1 less set of gears meshing or are the gears in a different gear box better made so they are more efficient?

It is a minefield and currently I am stuck in it and don't know a safe way out!

[tankme]

Sometimes it also comes down to trial and error. I will tend to error on the side of a bigger motor than needed as you can always slow a motor down, but it's much harder to speed it up safely (IE without killing the brushes in it). A lot of times a 6V, 12V, or 18V brushed motor can be run at higher voltages. It usually means they produce more heat internally which is why the brushes can become an issue. Usually the higher the speed the lower the torque and the higher the torque the lower the speed, but overvolting will result in higher speed and torque numbers at the expense of heat.

I have also experimented with brushless motors which are great, but they require brushless ESCs. Some of the tank boards can use external ESCs which would allow someone to play with brushless options, but I have found most of the off the shelf car brushless ESCs have too many issues with deadbands or cannot be reversed without first going to a neutral stick for a set time period. Erik suggested I try a drone ESC for brushless control, but I haven't tried it yet. Brushless is a whole other rabbit hole...