Model Boat Mayhem - Forum

Please login or register.

Login with username, password and session length.
Pages: [1]   Go Down

Author Topic: 550 Motor  (Read 2067 times)

Pointy

  • Shipmate
  • *
  • Offline Offline
  • Posts: 12
  • Location: Sussex UK
550 Motor
« on: November 05, 2012, 08:38:53 AM »


I bought a 550 motor from Howes and it is rated at ...


Speed 16,500 RPM @ 12V.
Free load drain 0.21A @ 12V.


However in my bench tests simply hooking up to my power supply @ 12v the current was closer to 1amp. Using an optical sensor I was able to measure the RPM @ 11500. (In the boat I am getting about 5500 RPM & 1.25A @ 6V on the bench)


The motor is made by Johnson although Howes list it as Expo.


The question is are my tests wildly inaccurate or have they supplied my a different motor from the one advertised?


EDIT: Just found this page on Expo tools site, and the 555 specs look very similar to the ones on Howes site.


Regards,


Les
Logged

Subculture

  • Full Mayhemer
  • *****
  • Offline Offline
  • Posts: 3,910
  • Location: North London
    • Dive-in to Model submarines
Re: 550 Motor
« Reply #1 on: November 05, 2012, 10:28:32 AM »

Sounds like a different wind to me.

Curious as to why you say the current is 1.25A at 6 volts but 1A at 12 volts. That goes against physics, or is the drive arrangement in your boat rather inefficient?

Pointy

  • Shipmate
  • *
  • Offline Offline
  • Posts: 12
  • Location: Sussex UK
Re: 550 Motor
« Reply #2 on: November 05, 2012, 10:38:21 AM »

Curious as to why you say the current is 1.25A at 6 volts but 1A at 12 volts. That goes against physics, or is the drive arrangement in your boat rather inefficient?


DOH!  :embarrassed:

Actually that is the total current drawn by the all the electronics on the 6V battery. The motor itself was drawing 0.75a @ 6V no load, and about 0.90a @ 6V when connected to a propshaft on my test rig.


Regards,
Les
Logged

Subculture

  • Full Mayhemer
  • *****
  • Offline Offline
  • Posts: 3,910
  • Location: North London
    • Dive-in to Model submarines
Re: 550 Motor
« Reply #3 on: November 05, 2012, 11:22:54 AM »

I see. Well I would expect a motor to be within a few percent of the spec given, and yours sounds way off. So I would get in contact with Howes, and tell them of your findings.

andrewh

  • Full Mayhemer
  • *****
  • Offline Offline
  • Posts: 3,064
  • Location: Melbourne, Australia
Re: 550 Motor
« Reply #4 on: November 07, 2012, 01:41:56 PM »

Several things going on here - all at once.  Can I make a few observations?
 
Johnston make many thousand different types of DC motor.  They probably make several hundred that can correctly be described as 550 types
A 550 motor has (probably)  3 poles and an armature length of 55mm - that is all one can learn from it
Howes sell these economy motors in large quantities and try to make sure they are generally suitable for modellers
The nominal specifications :
Speed 16,500 RPM @ 12V.
Free load drain 0.21A @ 12V
.
Do not tell you very much apart from a suggestion that the kV is 1375 (but there is no mention of the load on this 12V speed figure so who knows?
Expo, I think are tool sellers and importers of motors (a bit like Como Drills)  so both the motor and howes can be correct
The 555 your refer to, Les is a 5-pole version of the same motor
The unloaded current is not a useful measure to identify the motor - it probably tells you more about the timing than the wind
So from the information available it is hardly possible to say if it is to spec or different - for a timed motor you could easily get the noload current you measured by running it in "reverse"
 
What seems to me to matter is - does it do the job you require in the boat? 
Is the speed what you require? 
Is the duration sufficient,
have you adjusted the prop to suit the performance you require
Was it good value for money?
hope this helps
andrew

 
Logged

Pointy

  • Shipmate
  • *
  • Offline Offline
  • Posts: 12
  • Location: Sussex UK
Re: 550 Motor
« Reply #5 on: November 07, 2012, 10:58:00 PM »

Thanks for the detailed reply Andrew.

I just assumed that hooking the motor up to 12v with nothing connected to the shaft and the RPM/Current should match the specs given.

Anyway I managed another test on the pond today with the Eagletree logger attached. The results are interesting and not quite what I was expecting. The motor is drawing about 7.5a at full speed and is turning at about 2500 rpm. After about 20 mins running the motor was at nearly 90c. The speed was much better than the first test, and all I had changed was the prop shaft grease. (Switched to silicone grease) For reference details of the boat here.

I am guessing that a 50mm prop is too big for the 550 motor, I might get a couple of smaller props to try. It will be interesting to see the relationship between current/rpm and speed with different size props.

Regards,

Les


Logged

Subculture

  • Full Mayhemer
  • *****
  • Offline Offline
  • Posts: 3,910
  • Location: North London
    • Dive-in to Model submarines
Re: 550 Motor
« Reply #6 on: November 08, 2012, 10:00:08 AM »

A 550 motor doing that RPM is far too much for a 55mm prop. You need a 500/550 size motor wound to run at about 4000-5000RPM unloaded, or gear the motor you have down at least 2:1.

At the moment you're just burning up a lot of energy in heat.

andrewh

  • Full Mayhemer
  • *****
  • Offline Offline
  • Posts: 3,064
  • Location: Melbourne, Australia
Re: 550 Motor
« Reply #7 on: November 08, 2012, 02:13:20 PM »

Hi, Les, Good information! :}
 
the Rpm is what tells us what is happening here.  A good speed to aim for (to avoid a distressed motor) is 80% of the calculated speed at  that voltage (this generally puts motors with a current draw which is maneagable and an efficiency which is reasonable)
 
IF the Kv is really 1375 and if you are running at 12V, then the no load speed is 16,500 rpm
80% is about 14000 and you are not getting that - probably don't want it either
 
So if you want to use this motor - fit a smaller prop.  If you want to use this prop fit a motor with a smaller kV and/or reduce the voltage.  Your vessel may be quite happy at 6V!
For information - if you want to rund the same prop - try a 555motor (5 poles in the same armature length) or even more poles.  But I still don't know what the boat is, so I have no direct way to help you
 
Subculture is exactly right
<<You need a 500/550 size motor wound to run at about 4000-5000RPM unloaded, or gear the motor you have down at least 2:1.>> 
but I might have said 545/555 motor!
There are lots of 540 types which would be suitable, but thay are not found in model shops - they are found in printers and lorry accessories and have lots of fine wire and low kVs
andrew

Logged

Pointy

  • Shipmate
  • *
  • Offline Offline
  • Posts: 12
  • Location: Sussex UK
Re: 550 Motor
« Reply #8 on: November 08, 2012, 05:34:39 PM »

Thanks again for the replies.

The boat is linked in my last post and details are in this thread

It is running on 6V and I don't want to change that. It is also using an Action P80S speed controller. According to the manual the original motor was a 755 max 9w.(that's all it says). Because of the mods to the coupling the original motor wont fit now, which is why I need a slightly smaller dia one. The 550 I have, size wise is perfect. The speed of the boat is good @ 2500 RPM in the water with the 50mm prop, it just needs to be more efficient I think.

While searching for 555 motors I found this, which led me to these. Not sure if any of these would be better.

Regards,

Les


Logged

malcolmfrary

  • Full Mayhemer
  • *****
  • Offline Offline
  • Posts: 5,568
  • Location: Blackpool, Lancs, UK
Re: 550 Motor
« Reply #9 on: November 08, 2012, 10:27:09 PM »

The first is fitted with a torque sleeve, which is supposed to do something fairly magical to the magnetic field, upping the motor torque.  The second could tick most of the right boxes as well (in the top line it mentions one use as shavers, maybe for Desperate Dan), but in either case a smaller prop would improve matters.  A general rule is prop diameter smaller than the can diameter.  This lets the motor spin more freely, and increases top speed at the expense of acceleration while increasing run time by reducing current drain.  The motor runs cooler as well.
Logged
"With the right tool, you can break anything" - Garfield
Pages: [1]   Go Up