shaded pole motor control

If you want to redefine what "magnetic amplifier" means, go ahead. That just isn't what it means to the rest of the world. (Not that the rest of the world cares too much about mag amps at this point.)

Cheers

Phil Hobbs

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Amplifiers amplify, which means using a small signal to control a bigger signal. A magnetic amplifier does the amplification magnetically. A regular transformer, operating as such, is not a magamp.

John

There are lots of what are referred to as "magamps" used in switch mode power supplies as secondary regulators:

I did a Spice hysteretic model of one of their magamp toroids.

Metglas make (made?) a range of suitable cores, too.

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"For a successful technology, reality must take precedence 
over public relations, for nature cannot be fooled."
                                       (Richard Feynman)

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It is not so much as saying old is good, but that i find it disingenuous to represent it as a new circuit / discovery.

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The range of speeds is zero to about 90-95%. The speed isn't stable as shaded pole motors have very poor regulation versus load and there is no feedback. However, you could add a tacho feedback and get a very stable speed up to the torque capability of the motor for that speed.

Interesting comments about mag amps. My original design dates back to my B.Sc project in 1973 where I wanted to stabilize a voltage of a cathode ray oscilloscope as part of a bigger project. The object was to provide a stable voltage to the CRO despite fluctuating mains voltages. The original design was to have been a saturable reactor but I accidentally found I could control the voltage dropped across the primary by varying the current in the secondary.

I call it variable inductance controller as the inductance of a primary winding is very high with an open circuit secondary but very low (leakage inductance) with it shorted. Also, very little power seems to be dissipated in the transformer indicating it is operating as an inductor rather than a resistor. What it is doing is transforming the resistive value on the secondary by the square of the turns ratio.

Today I use them as ceiling vent fan controllers with a thermistor (15k at

25C) connected collector to base of the darlington transistor base and a variable resistor (10k) connected base to emitter. That makes a very simple automatic room temperature controller for the summer. The very high Hfe of the darlington (1,000-3,000) removes most of the effect of increasing secondary voltage as the temperature drops and keeps the control point almost constant.

Peter

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Have you accidentally created a method to dim incandescent's without all the RFI that TRIAC's create ?:-) ...Jim Thompson

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OK, I can see this working well for nearly off and nearly full on, but when you are at say 1/2 power, doesn't the darlington on the secondary side dissipate a lot of power?

It seems to me it is the equivalent of putting a resistor in series with the load, its just that the resistor value is transformed by the turns ratio and the rectifier makes it DC instead of AC, but you are still basically putting a resistor (not an inductor) in series with the load...

no?

Mark

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Yeah, I think you are right the 'beauty' from my point of view is that you are now regulating a lower voltage DC signal.

George H.

From a previous post this arrangement doesn't replace triacs with their associated RFI etc because triacs work as pulse width modulators (PWM) and dissipate very little power. The darlington will have a maximum dissipation (at

1/2 power) of about 1/4 of the power rating of the motor it is driving so it will need a heat sink.

What this circuit does, as you say Mark, is to create a variable resistor. However, it isn't quite as simple as that as because as the resistance increases at some point the inductance of the secondary will take over. So considering the operation more carefully I think this circuit can be more accurately represented as a variable resistor in parallel with the large inductance of the primary winding. The power factor will therefore be variable between 1 (low resistance - full power) and zero (entirely inductive - off). A shaded pole motor will be an inductance with a series resistance and will have a fixed power factor.

To be honest I've never really given the theory of operation too much thought as it just worked from the first time I tried it. I might have a go at producing a phasor diagram in Excel. Peter greenpaper usualsymbolhere btinternet.com

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From a previous post this arrangement doesn't replace triacs with their associated RFI etc because triacs work as pulse width modulators (PWM) and dissipate very little power. The darlington will have a maximum dissipation (at

1/2 power) of about 1/4 of the power rating of the motor it is driving so it will need a heat sink.

What this circuit does, as you say Mark, is to create a variable resistor. However, it isn't quite as simple as that as because as the resistance increases at some point the inductance of the secondary will take over. So considering the operation more carefully I think this circuit can be more accurately represented as a variable resistor in parallel with the large inductance of the primary winding. The power factor will therefore be variable between 1 (low resistance - full power) and zero (entirely inductive - off). A shaded pole motor will be an inductance with a series resistance and will have a fixed power factor.

To be honest I've never really given the theory of operation too much thought as it just worked from the first time I tried it. I might have a go at producing a phasor diagram in Excel. Peter greenpaper usualsymbolhere btinternet.com

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What was this circuit called 40-75 years ago?

I Googled around for "motor control transformer short secondary" but didn't come up with anything relevant.

Thanks!

Michael

I've never seen this circuit before I 'discovered' it - I won't say I invented it as it is so fundamental someone must have done this before. However, I've never seen it in any magazine (Wireless World / Practical Electronics) or book on electronics control going back to the late 60s and I can't see anything on Google even remotely like it.

One thing to be very careful of if using this circuit is NOT to have any capacitors across the load (motor). This could form a series resonant circuit with the primary winding inductance and I totally wrecked one transformer this way before I realized what was going on. As protection you should have no more than a 1 amp fuse for a 100VA of transformer rating. If the fuse blows investigate the problem.

Peter

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