1/50 hp shunt motor bridge circuit 2

[richard29e5]

I bought a bridge rectifier for this small motor armature and field. The rectifier is quite oversized at 600v and 60A. When connected to the standard 120v ac house source, the rectified voltage was 118v dc rather than 1.4X120V or 168 v like I expected. Motor and field voltage is 115v.
I tried to put a 1000 uf capacitor across the rectified leads and the voltage then went to 162v dc, so I didn't use the capacitor and just tried connecting armature and field to the one 118v dc source and all works fine. Question is . . . . why is the output 118vdc rather than 168vdc?

 

[itsmoked]

Your meter is not a "true rms" meter and so it is misinterpreting the pulsating DC you are actually trying to measure. When you add the cap you change from pulsating DC to DC which your meter is happy with. You must still then subtract 2V for the bridge rectifiers and then you must account for what your local immediate line voltage actually is at the moment you are checking. (It often changes by several volts though out the day.)

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[richard29e5]

It Smoked . . . so the true dc voltage is 162v. I wonder if that is cooking my 115vdc armature and shunt? The output on the motor is supposed to be 58 rpm and it is running at like 72 rpm. I would be happy with the motor running like it is if it doesn't damage the motor? Otherwise I would have to put the capacitor back in the circuit and use a dropping resistor at 25 watts or so (which I have). Opinions?

 

[itsmoked]

Yes once you have bridge rectified it.

Most small motors are universal. Meaning you can use DC or AC on them. Specifically if they have brushes. In this case the 120Vac would probably be fine by itself.

If it is truely a DC only motor then yes you need to reduce the voltage some. Of course this is all based on a bunch of assumptions on my part because I don't know what your motor will be doing. Is it going to run constantly or only occasionally? Is it for a work bench thing or in your wife's sewing machine, etc.

If it is going to be a bench thing then get your hands on a Variac transformer and "dial a voltage" with little heat or loss.

If this is too big(two grapefruit) then I would find transformer to hook up as a "buck" type auto transformer. In this manner the transformer does not have to "transform" all the power only the difference between what you have and what you want. This means it will be a small transformer.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[swgrmfg]

The expected DC output voltage from a full wave, single phase bridge is 108VDC calculated by: 1.414*.636*120V

 

[richard29e5]

swgrmbg . . . I thought dc output from full wave bridge rectifier was 1.414*120V . . . from 120v ac source.
When you state 1.414*.636*120 . . . .what is the .636 for?

 

[ScottyUK]

richard,

Your equation assumes that the DC has been smoothed to a zero ripple condition by an infinitely large capacitor bank. The 1000uF cap when burdened only by the input resistance of the multimeter you're using approaches this condition, and the result of the equation you're using will be close to the measured value. The error between the theoretical value and the measured value will increase as the load current increases because the DC will have AC ripple superimposed on it because the finite size of the smoothing capacitor does not hold enough charge to hold the voltage at a constant level while delivering charge to the load or storing charge from the source. swgrmfg is quoting the average value of the DC from a fullwave rectifier with no smoothing whatsoever. The 0.636 factor arises from the mathematical derivation, integrating the voltage with respect to time over a half cycle.

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I don't suffer from insanity. I enjoy it...

 

[richard29e5]

ScottyUK
When I first hooked up the large bridge rectifier and connected it to the motor armature and field, my voltmeter read 118v. I then put the 1000uf capacitor in the circuit but did not connect the motor leads. It read 162volts . . . which is too high for a 115v motor. I was told on this forum that when I was reading the 118v with just the bridge rectifier that my meter was not sensing the true voltage, and that it probably was actually seeing 162v?
Scotty, what you and swgmfg are saying is that the motor was seeing 118v or 108v.
I tried it with some resistance in series (240 ohm at 25 watt) and the meter was reading like 85v.
I have the capacitor and various dropping resistors. What do you suggest? Does the capacitor have to be grounded in the circuit.
The other thing I can tell you is when I had the motor connected only to the bridge rectifier and voltage was 118v, the gear motor was running at about 72rpm vs base of 58rpm. At the lower 85v, the motor was running at a tick less than 58rpm.

 

[ScottyUK]

In all probability the 162V was somewhere near accurate because the capacitor is charging up to the peak of the rectified DC and there is no load other than your meter to discharge it. So the meter effectively reads the peak voltage.

If you take the capacitor out, the voltage will be as swgrmfg said, save for a small error caused because real-world diodes have a small but finite forward volt-drop. So instead of 108V average DC you might have 106V average.

You are seeing the old problem of an averaging meter which is scaled in RMS, or perhaps even a true RMS reading meter being used in an application where an average reading instrument is required. Most college level electrical principles books have a decent chapter on the mathematics behind the different measurements - it might be worth having a look in your local bookstore or in the library.

The following link might be useful - I haven't read it in detail, but it seems ok on a brief skim through.

http://www.tpi-thevalueleader.com/rms.html

One possibility for your overspeed condition is that you are running with a slightly weak field, at about 90% of rated current. This, counter-intuitively, causes the motor to run faster. It is a technique known as field weakening. The motor will also be have a rated speed quoted at rated power. The speed will rise off-load.

If you can provide some smoothing capacitance, the DC will have less ripple. This is a good thing. Instinctively I doubt 1000uF will be enough to produce a smooth DC, but it is a start. From the smoothed DC supply you could then use one resistor in series with the field to set the field current to its rated value, and a separate resistor in series with the armature to set the speed to your desired value. This should give better results than simply dropping the combined field and armature voltage through one resistor.

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

I don't suffer from insanity. I enjoy it...

 

[benta]

Why make it so difficult?
Rectified 120 VAC is pulsating DC with an effective value of 120 V - bridge drop, so 118 V sounds right. (This is based on a circuit with NO capacitor).
There is absolutely no need to convert back and forth with Pi and 0.636 factos etc.
I'm sure the motor will run happily on pulsating DC.

Benta.

 

[richard29e5]

I just used the bridge rectifier diode on this . . . no capacitor or dropping resistors . . . and the motor works great with no overheating, etc.
Thanks for all the help.