How do I run a Dynamo as a motor?

[DaveT]

Can anyone out there answer a query that i have?

I have a couple of spare 6v dynamos that I’m not sure if they work or not.
 
I believe that there is a method to connect the dynamo so that it runs like a motor.
 
But as my 1935 Ruby is a positive earth how would I wire the terminals to the battery to ensure that I don’t reverse the polarity 
 

Dave in grey but dry Cumbria

[austin]

By chance I have a video of me doing exactly this, should help

[DaveT]

By chance I have a video of me doing exactly this, should help

Thanks for the video clip they say a “picture is better than a thousand words”  so it now becomes very clear what I have to do. 

But, in my case would I Earth the dynamo casing to the positive and join the two terminals together to the negative to prevent reversing the polarity of my dynamos?

Dave

[DaveT]

By chance I have a video of me doing exactly this, should help

Thanks for the video clip they say a “picture is better than a thousand words”  so it now becomes very clear what I have to do. 

But, in my case would I Earth the dynamo casing to the positive and join the two terminals together to the negative to prevent reversing the polarity of my dynamos?

Dave

I've run both my spare Dynamos and they rotated in the right direction. 

I then removed the drive gear and connected them to a mains powered electric drill and ran at full speed. 

I then put a voltmeter across the terminals and the reading on one was 7 volts and the other was zero. Would I expect to get more volts or am I doing something stupid?

Without connecting the dynamo to the car setting everthing up and running the car how do I determine if the spare units are knackered?

Dave

[Mark Dymond]

Indeed what is the collected wisdom on testing and setting up the dynamo, off the car. We have rebuilt ours but dont get get 6 volts when spinning it up in the lathe. Looking at the gearing the generator spins at greater than engine speed, should 2000 RPM give full volts at no load? What speed shoul I use to set the third brush.

[Dave Wortley]

Dave,
I set up a rig with electric drill and piece of strong hose connected to the shaft of the dynamo after first removing the drive gear. I tried 5 dynamos which I had rebuilt years previously. I was initially very disappointed with the voltage readings I was getting. These varied from about 1v to 4v if I remember correctly. In disgust I put the rig and dynamos back in the cupboard and got on with other things for a few months. I then tried them all again later and found that if I let the drill rig run for a few minutes there was suddenly a big increase in torque which twisted and destroyed the hose drive and also showed that the voltage generated was certainly over 6v in all 5 dynamos once I used a more substantial hose. I will have to refer to some notes I made at the time and re-post later.
Cheers,
Dave.

[Adrian Payne]

For the dynamo to produce any output there must be some residual magnetism in the field coil cores. Some times it takes a while for this magnetism to generate enough power to properly energise the field coils. Also because the starting voltage is very low any oil or contamination on the commutator will prevent the field getting energised.
Adrian.

[Dave Wortley]

Dave,
Here are the results of dynamo voltages. First test followed by second test:-
1929 chummy 4 field coil dynamo 0.24v and 12v. C35A with C35M brush gear 0.1v and 8v.
C35M 0.9v and 12v. C35A 0.1v and 8v. C35M 1v and 7.9v. C35M 0.4v and 8v.
As Adrian says these dynamos needed a good few mins running before the field coils would function.
They had been in the cupboard for many years.
All 6 dynamos work as motors.

[DaveT]

Thanks Dave for the detailed info it now gives me hope that with a bit more patience and lengthy high speed running I might get the results I had hoped that I'd see.

[Dave Mann]

To bench test a dynamo I use a piece of 1/2" square bar with one end turned to 1/2" diameter to fit my electric drill and the appropriate socket to fit the drive gear nut on the other end. The dynamo is securely clamped in a vice with and you need to hold the drill very firmly otherwise it could end up on the floor. You may need a jubilee clip on the drive shaft to stop the socket migrating towards the drill and if it's a reversible drill make sure it drives the dynamo clockwise looking on the drive gear end. At about 2500 rpm I find a standard dynamo in good condition generates between 15 and 20 volts depending on how much grunt the drill has.