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Speed Control of a Brushed Starter Motor

08/07/2017 9:00 PM

A friend has an old cutaway flathead used for car shows. The starter was rewound to run on 110 v. But when it is plugged into 110 VAC, there is a lot of sparking at the brushes and the starter gets warm. Doesn't surprise me; running a DC motor with AC is not exactly kosher. And the friend wants to have speed control too. I'm thinking a variable DC power supply or maybe PWM of a 110VDC supply. Any cheap and easy ideas? Maybe a light dimmer followed by a bridge rectifier?

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#1

Re: Speed Control of a Brushed Starter Motor

08/07/2017 9:39 PM

There are fairly economical DC speed controls that take 120VAC input. Grainger is one possible source, along with many local electrical suppliers.

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#2
In reply to #1

Re: Speed Control of a Brushed Starter Motor

08/07/2017 9:48 PM

Thanks, Tornado. Looks like we can get one for $100 or less at Grainger.

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#3

Re: Speed Control of a Brushed Starter Motor

08/07/2017 10:14 PM

Some DC motors have permanent magnet fields and cannot run on AC. Others have a wound field and can run on AC or DC (universal motors). Your starter motor could be of either type:

http://what-when-how.com/automobile/starter-motors-and-circuits-automobile/

Rewiring a motor for higher voltage (and less current) would entail replacing the wiring with many more turns of finer gauge wire. (We can only guess how it was rewired.)

You should find out how much current this motor is drawing at different voltages. I would maybe borrow a variable DC power supply and ammeter and make some measurements.

If you use a light dimmer and full wave bridge, make sure that the dimmer can handle inductive loads (some cannot), and that the dimmer and bridge can handle the motor current.

I hope this helps.

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#9
In reply to #3

Re: Speed Control of a Brushed Starter Motor

08/08/2017 9:46 AM

Thanks for that link. It has lots of good info. I haven't seen the starter yet, but my friend says it has lots of "fine wire" in it. My friend wants to retain the starter in the display and be able to run it at slow speed so one can see all the moving parts of the flathead. I'll try to get a pic. Sounds like a real find.

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#13
In reply to #9

Re: Speed Control of a Brushed Starter Motor

08/08/2017 10:45 AM

Sounds like an interesting project. Rixter covered most of my concerns in his reply but I still have two concerns. The starter is designed for short bursts of operation. As such the very high amount of self heating is not considered in its design. Having this display constantly move by this motor might overheat the winding. My other concern is how the starter engages the flywheel. This starter may use a Bendix drive. The Bendix drive engages the flywheel only once the starter is spinning and uses the mechanical energy of a spring along with the ICE ignition forces to disengage. Spinning this rewired starter at a slow speed may make the region of engage or disengage a little dicey.

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#19
In reply to #3

Re: Speed Control of a Brushed Starter Motor

08/08/2017 10:48 PM

Many of the cases where grounded (-ve polarity) - be careful you don't electrify the case.

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#4

Re: Speed Control of a Brushed Starter Motor

08/07/2017 10:16 PM

For what purpose do you need such a motor rewound to run on a far higher voltage?

As for speed control, yes standard PWM will work to a point but if it was rewound and kept in its original series would design the PWM control is going to work more as a variable torque control unless you add some sort of speed feedback circuit.

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#6
In reply to #4

Re: Speed Control of a Brushed Starter Motor

08/08/2017 3:23 AM

One possibility is to use that motor to turn the <...cutaway...> engine over for demonstration purposes at the <...car shows...>.

It might make sense to use a slower external motor to drive the engine off its crankshaft instead, and forget about the rewound starter motor, perhaps?

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#10
In reply to #4

Re: Speed Control of a Brushed Starter Motor

08/08/2017 9:52 AM

We can only guess what the original designer of the display was thinking. Maybe he didn't want to depend on a battery, but just plug in wherever the display was.

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#5

Re: Speed Control of a Brushed Starter Motor

08/07/2017 10:23 PM

No surprise, that motor is not rated for continuous use....I would just go with a 1hp universal motor...gear it down to the speed I want...

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#7
In reply to #5

Re: Speed Control of a Brushed Starter Motor

08/08/2017 3:24 AM

Ditto.

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#8

Re: Speed Control of a Brushed Starter Motor

08/08/2017 8:53 AM

It is obvious you have not yet found out what currents/voltages you need to get the speeds you want or to unstick/breakaway initially. However, if 200A @ 12V was original, about 20A at 120V is likely. AC to DC supplies may not have the useful short-time "overload" capacity of batteries or may be damaged by motor start loads.

The following diagram is a modification of a scheme which was used for telephone exchange battery output control.

Closing switch 1 connects

the motor at low voltage. Closing switch 2 and up applies increasing voltage, diodes becoming reverse biased in succession. Parts of the battery are charged separately, so that using the "taps" does not result in uneven discharge.

Using taps may avoid starting/ballast resistors to get the voltage you need.

Because the battery is only switched in steps, current surge is reduced & the contactors only have to handle 12/24V according to what redundant auto/golf cart parts you can find.

The diodes have to be rated for the maximum current, but because only one works at a time, several can be put on one heat-sink.

If multiple AC to DC supplies in series are used in place of some or all of the batteries, do not forget that the first & following supply turned-off/on gets reverse biased (which may damage it). The solution is to put a reverse biased diode across the output of each DC supply of adequate rating & heat-sink - such a scheme may be an alternative way to vary the DC voltage and motor speed, by turning on AC supplies in succession.

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#11
In reply to #8

Re: Speed Control of a Brushed Starter Motor

08/08/2017 9:57 AM

Fascinating info re telephone battery. I worked in telephony for years, converting old CO's from mechanical steppers to electronic switches. We threw away dumpsters of old relays. Wish I had kept a few. Would have made great table lamps!

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#12
In reply to #8

Re: Speed Control of a Brushed Starter Motor

08/08/2017 9:59 AM

Since the flathead is all cutaway with no compression, the torque to turn it over will be a lot less than the original motor needed. I'll do some instrumentation to get numbers.

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#15
In reply to #12

Re: Speed Control of a Brushed Starter Motor

08/08/2017 3:08 PM

I thought "no-compression" was likely. Any possibility of using old dynamo as belt-drive motor?? You would need about 100:1 reduction.

Thinking about it, starter motor cranking at about 100 rev/min at rated voltage must mean only maybe 12V on "120V" motor for 10 rev/min.

I should have written that auto alternators have some robust diode rectifiers built-in, they get blast cooled by the alternator fan - but that can be replaced by fans scavenged from old computers or a well-finned natural flow or drainpipe chimney aided heatsink.

Guess checking resistance of "120V" motor and thinking if applying 12V would draw much current as a start. Start with low voltage from variable bench DC supply - at no-load, overspeed is a risk.

Is there any sign that motor was rewound with a shunt, or part shunt & series, field?

DC series motors for jet engine start had some shunt field to limit overspeed since the engine disconnected by a centrifugal clutch.

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#17
In reply to #15

Re: Speed Control of a Brushed Starter Motor

08/08/2017 3:21 PM

I haven't seen it yet, but I think most old starters were series wound. I'll get some pics posted.

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#16
In reply to #12

Re: Speed Control of a Brushed Starter Motor

08/08/2017 3:08 PM

I suspect that 1/4 HP @ 30 RPM would be a great plenty to spin the engine given if it spins too fast it hard to follow all the moving components through their respective cycles.

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#14

Re: Speed Control of a Brushed Starter Motor

08/08/2017 12:58 PM

Consider replacing the generator with an AC motor that looks similar and driving the crankshaft with that. Just a thought.

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#18

Re: Speed Control of a Brushed Starter Motor

08/08/2017 5:45 PM

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