DC motors

[Gordon WParticipant @gordonw]

I have two motors, both 24V DC, 250W, 14 Amps, 2500 rpm. These are permanent magnet 4 brush. Question is can these be connected in series to run on 48V ? It seems obvious that they can and will work ok but I'm losing confidence. Also can a cheap 48v speed controller be used ?

[Gordon WParticipant @gordonw]

[Norman BillinghamParticipant @normanbillingham91454]

I'm no expert on motors but I suspect not as a general rule. If the two motors are connected in series then the same current must flow in each of them. If they always have exactly the same load then that might work, but if the load on one changes then the current it draws must change and that must surely affect the other motor?

[john swift 1Participant @johnswift1]

It may be possible

but it depends on the motors being identical and the mechanical load on the motors being the same

 

my instinct is to use a 24V supply and a separate speed control for each motor

 

what are you wanting to do with the motors ?

If its some kind of electric vehicle is it possible to have the motors driving the same drive shaft ?

 

John

 

Edited By john swift 1 on 19/07/2018 11:36:58

[James Jenkins 1Participant @jamesjenkins1]

Just going to jump in here and say that having gone through "cheap" import speed controllers, I now use these ones:

**LINK**

and find them to be very good.

James

[Gordon WParticipant @gordonw]

Thanks- that was quick. It's for vehicle use, hopefully, They will be coupled together mechanically, both driving the same shaft via chain coupling, so should have same load and current draw. Motors are identical, off mini scooters, and are like new inside. The use of cheap controllers is noted, good advice I think.

[John HaineParticipant @johnhaine32865]

Since they are connected "in parallel" mechanically, they should also be in parallel electrically, not in series. If you then drive from a single 24V speed controller, they will see the same terminal voltage and want to run at the same speed, assuming they are more or less matched.

[Roger BParticipant @rogerb61624]

If the loads can be well balanced series connection could work. If you go to worst case and have one motor jammed and the other free the free motor will get most of the 48V.

Series / parallel switching of motors was (is?) used in full sized traction but these motors had series field windings. The permanent magnet motors you have are closer to shunt field motors.

Best regards

Roger

[Ron LadenParticipant @ronladen17547]

Gordon, I,m curious, how are you supplying 48 volts. Most 24 volt motors are run on 24 volt from 2 x 12 volt batteries wired in series, unless you are running 4 x 12 volt of course.

Ron

Edited By Ron Laden on 19/07/2018 12:42:00

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Gordon W on 19/07/2018 11:51:03:

Thanks- that was quick. It's for vehicle use, hopefully, They will be coupled together mechanically, both driving the same shaft via chain coupling, so should have same load and current draw.

.

Please forgive me for leaping in without any detailed knowledge of your proposed vehicle, Gordon

Have you considered the possibility of two half-shafts; each driven via a freewheel unit, by its own motor ?

… This produces a low cost drivetrain that roughly approximates a differential.

MichaelG.

[Gordon WParticipant @gordonw]

Michael- my original idea was 2 half-shafts. I decided on 48 v just to half the current and still get some power. The 48 v will be from 4x 12v lead acid batteries ( I have a mate who gets these trade) As usual with me cost is the deciding factor. I have two good motors, why buy another one? I must admit that buying 2 new motors and using half -shafts and using 48v is sensible. Thanks for the ideas.

[Ron LadenParticipant @ronladen17547]

Gordon,

I would wire the motors from the controller in parallel and run at 24 volts but thats just me, I dont like the idea of wiring them in series but you can of course. Why do you want to half the current..? its not as if its particularly high from the figures you have given. Also running at 24 volt saves the cost of 2 batteries when cost is a deciding factor. Also I dont know if this is a loco or another type of vehicle but depending on the 12 volt battery type 16 to 18 kgs each so a saving of over 30 kgs with 2 batteries instead of 4.

Sorry it sounds as if I am picking faults but I,m not its just that I cant see the benefit of going 4 batteries (cost/weight) – 48 volts and wiring in series. The motors you are using are the same type and model so wiring in parallel the motors will be fine they dont need to be identical, they will be close enough and will work ok.

Re cheap speed controllers, well I have one (Chinese) off Ebay for £15 (40amp) running 4 motors in parallel on the 0-4-0 I am building. It works but yes it has crossed my mind will it burn out or give up once it is working under load but at £15 it has to be worth a try. The link to the better controller on an earlier post is according to the spec "non reversible" by the way so if you need reverse on your vehicle the controller shown is not the one.

regards

Ron

 

Edited By Ron Laden on 19/07/2018 15:21:08

[duncan webster 1Participant @duncanwebster1]

As they are mechanically coupled they must rotate at same speed, so I can't see any problem running them in series, provided they are the same type of motor. As someone above said, plenty of full size electric locomotives had motors in series for starting, then switched to parallel for higher speed.

[Neil WyattModerator @neilwyatt]

If I recall correctly electric motors in parallel will tend to balance current to run at the same speed, with motors in series if one is stalled and the other is free the free one will overspeed.

[duncan webster 1Participant @duncanwebster1]

Posted by Neil Wyatt on 19/07/2018 19:47:23:

If I recall correctly electric motors in parallel will tend to balance current to run at the same speed, with motors in series if one is stalled and the other is free the free one will overspeed.

OP says they are coupled together mechanically, so they have to rotate at same speed

Edited By duncan webster on 20/07/2018 08:52:17

[Ron LadenParticipant @ronladen17547]

Though connected mechanicalIy, I still think if wired in series and depending on how one motor fails it could create problems for the good motor. In parallel the failure of one motor shouldnt compromise the good one unless of course the motor seized and then through the mechanical connection all would stop.

Of course there is no reason to assume that the motors will fail, they probably wont but its just an "if"

Just my thoughts.

[Ian S CParticipant @iansc]

Just make sure you use deep cycle batteries rather that auto starter batteries, the latter won't last, were as the former ones are the type used in DC battery banks, and mobility scooters.

Ian S C

[Tim StevensParticipant @timstevens64731]

You could, though, use each motor to drive one rear wheel, and vary the current sent to each. With equal current, you would get the same speed from each motor, but if the current is not the same both sides, the vehicle will turn (or at least pull) to the lower voltage side. This, carefully arranged, will act as power steering, and if your front axle is allowed plenty of freedom (further than the usual 45 degrees both ways) you will be able to turn the vehicle in its own length. This can be both useful and impressive. The bias in power delivery might be operated by turning the steering, making the system automatic.

Cheers, Tim

[Gordon WParticipant @gordonw]

Thanks for the ideas. This contraption will be a sort of "tug", used around the house, shifting the wheely bin etc. ( a 1 mile round trip ) Later if all is well to be a push bike pusher for road use. I think I will start with the two motors in parallel at 24V. Drive via. freewheel bearings or ratchet. At least I will some idea and at low cost.

[Ron LadenParticipant @ronladen17547]

A "wheelie bin tug" well that is different Gordon.

I assume it will be ride on..? I know you intend to mechanically connect the motors will it be 2WD or 4WD..?

Please keep us posted with progress it will be interesting.

Regards

Ron

 

Edited By Ron Laden on 22/07/2018 14:15:32

[Gordon WParticipant @gordonw]

2 wheel drive, ride-on if all goes well. Anybody remember the Good Life TV comedy ? They had a motor tiller towing a trailer, same sort of idea. Will keep posted on progress but something else has turned up today, doesn't it always?

[Ron LadenParticipant @ronladen17547]

Gordon, many moons ago when we had a 1/4 acre veg garden, I had a petrol engine garden tiller where you could remove the blades from the hexagon drive shaft and fit on a pair of pneumatic wheels. There was a trailer you could buy to fit on the back but I never got one, I did think of building one but never got around to it.

Edited By Ron Laden on 22/07/2018 16:19:30

[Gordon WParticipant @gordonw]

A recommendation- Most of the questions I asked, and the answers, are in Electric Motors for the Home Workshop, by Jim Cox, in The Workshop Practice series. I have this book and just re-read it again, I should have done it earlier.

[Author]

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