Dynastarter Question 4

[UncleBrad]

So its a basic motor. Everything seems insulated. I put it together up to the negative brushes and I can connect a power wire to it and the casing and armature still is grounded. It is not till I connect the Positve brushes which have insulated seats that the whole system including the casing becomes a positive charge. Touch any part of it to a ground and you get a nasty spark.

I cleaned the commutator and still it carries current across the segments. Is that supposed to happen? Or would the short be somewhere else and perhaps multiple areas?

 

[waross]

A basic motor???
Permanent magnet?
Series?
Shunt?
An armature is almost a short circuit (very low resistance) until the motor starts spinning and starts generating aback EMF to reduce the current.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[UncleBrad]

Its a Bosch Dynastarter from the 70's. Four Brushes on the back plate and a Big Armature used to start engines.

 

[UncleBrad]

Is current supposed to transfer across the commutator segments? Wouldnt that short the negative brushes?

 

[MikeHalloran]

A typical engine starter normally draws a couple hundred amperes. Playing with loose wires will give you nasty sparks even if it's working right. Try connecting it with a contactor instead.

Don't let it run for more than a fraction of a second without a load; it will _really_ spool up. For that reason, tie it down, too.





Mike Halloran
Pembroke Pines, FL, USA

 

[UncleBrad]

If I put a hot wire to the commutator a current connects around the whole thing. Isnt the current supposed to be limited to the segments in the commutator touching the brushes...hence the segments??

 

[waross]

The commutator segments are connected together byvery low resistance windings. Normal multi meters are not very accurate measuring the resistance of starter fields and armatures.
What you are seeing, sparking and a very low resistance between com segments, is normal for both a good starter and a bad starter. Use a megger to check for ground faults. Use a "growler" to check for shorted windings in the commutator.
OR
Take the starter to a rewind shop.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ScottyUK]

Don't make direct connection to the commutator with wires. You will likely energise windings at the wrong angular position relative to the field, and will also damage the commutator. Use the brushes - that's what they are there for.


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[rhatcher]

UncleBrad, I was an auto mechanic during the late 70's. From that experience, I seem to remember that some engine starters only have one positive terminal. The negative terminal is the starter case.

If this is true for your starter, then there is only one large (>300 amp, >3/8") terminal that is connected to the positive brushes. The negative brushes are connected to the series field. The series field is then connected to the starter case internally.

If this is the case, then you should be able to make the following ohmic readings:
- lift all brushes: positive terminal, armature, and case are all isolated from each other.
- install positive brushes only: positive terminal is common to armature, case is isolated from armature and positive terminal.
- install negative brushes only: positive terminal is isolated from armature and case, armature and case are common.

If these tests confirm that you have a series connected, single terminal, starter motor, then you can run it by doing the following.

Connect the power supply between the positive terminal and the starter case. When you do this, the starter should run and there should be no spark between the case and ground (assuming the power supply ground equals the real ground.)

However, as MikeHalloran suggested, do not do this for long since the speed without load for a series motor is high enough to destroy the motor. Also, fasten the motor to something solid since the acceleration torgue of the starter will make it jump and roll when it is engaged.

Of course, my auto mechanic experience was a long time ago. At the time, I knew nothing about motors and very little about electricity. So, I hope this helps but, please forgive me if I am wrong.

 

[waross]

Not bad, ratcher. Actually pretty good.
But if that motor hasn't run by now, it may be time to chuck it. Some of UncleBrads's sparks should have resulted in a good motor spinning at least briefly by now.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[stardelta]

Dynostarts are not to be confused with traditional "series" type starter-motors.

Hope this link helps.

http://www.s109316956.websitehome.co.uk/vire7/How_To/Electrical/electrical.html

 

[rhatcher]

stardelta, nice job. The machine you linked to is indeed very different than what I was describing. It is also very different than anything I have considered before so I will not even speculate on what steps may help the OP solve his problem.

 

[UncleBrad]

Thank you all for the help.