Questions about AC

Athosworld · Oct 7, 2022

My relay hums when powered with 6vAC, is this normal?
My dc motor kinda works in AC but it only vibrates.
Why does that 6vAC sparks more than dc?

rjenkinsgb · Oct 7, 2022

AC voltage passes through zero twice each cycle, so the relay is turning on and off continuously, even if it holds in.
The current will also be lower due to the inductance of the coil.

Relays or solenoids designed for AC are made slightly differently than for DC, different winding for the same voltage and with a "shading coil", which has current induced from the main coil and maintains the magnetic field through the AC voltage zeros. (Or some may have a rectifier and work on DC).

Shading coil - Wikipedia

en.wikipedia.org

The motor is trying to reverse every half cycle of AC as the voltage reverses, so it will not move far and will be taking high current as it's effectively stalled each time it has to restart.

Brushed AC motors are built as "Universal motor" style, with wound field coils rather than permanent magnet field.
The field reverses with the AC reversals as well, so the motor turns the same way regardless of the instantaneous AC power polarity.

Universal motor - Wikipedia

en.wikipedia.org

MaxHeadRoom78 · Oct 8, 2022

Universal motors also run on DC, hence "Universal"
If your motor is PM field then it Will vibrate instead of run, when AC powered..

Athosworld · Oct 8, 2022

rjenkinsgb said:
AC voltage passes through zero twice each cycle, so the relay is turning on and off continuously, even if it holds in.
The current will also be lower due to the inductance of the coil.

Relays or solenoids designed for AC are made slightly differently than for DC, different winding for the same voltage and with a "shading coil", which has current induced from the main coil and maintains the magnetic field through the AC voltage zeros. (Or some may have a rectifier and work on DC).

Shading coil - Wikipedia

en.wikipedia.org

The motor is trying to reverse every half cycle of AC as the voltage reverses, so it will not move far and will be taking high current as it's effectively stalled each time it has to restart.

Brushed AC motors are built as "Universal motor" style, with wound field coils rather than permanent magnet field.
The field reverses with the AC reversals as well, so the motor turns the same way regardless of the instantaneous AC power polarity.

Universal motor - Wikipedia

en.wikipedia.org

Thanks, do you know a reliable way to convert those 6vAC into dc? Maybe with a FULL bridge rectifier.

rjenkinsgb · Oct 8, 2022

Athosworld said:
Thanks, do you know a reliable way to convert those 6vAC into dc? Maybe with a FULL bridge rectifier.

Yes, a bridge rectifier and smoothing capacitor. The DC output will be roughly 1.4 * the AC input voltage, less the drop in the bridge rec.
That is typically 1.2 - 1.4V at light load as there are two diodes in series feeding the output, with either polarity input.

Without a capacitor, the average voltage will be near the AC voltage (less the diode drops), but it's pulsing twice each AC cycle rather that a constant or near-constant voltage.

Also note that transformers give a rather higher voltage at light load than the rated voltage, which is give for full load.
Typically, the smaller they are, the worse the regulation between full load and no load.
(If a datasheet is available for a transformer, it should give the regulation value, which is the percentage of voltage increase when off load).

MaxHeadRoom78 · Oct 8, 2022

Just to add, you do not need a cap on the output of a bridge, if it is purely to supply the DC motor alone.

Athosworld · Oct 14, 2022

rjenkinsgb said:
Yes, a bridge rectifier and smoothing capacitor. The DC output will be roughly 1.4 * the AC input voltage, less the drop in the bridge rec.
That is typically 1.2 - 1.4V at light load as there are two diodes in series feeding the output, with either polarity input.

Without a capacitor, the average voltage will be near the AC voltage (less the diode drops), but it's pulsing twice each AC cycle rather that a constant or near-constant voltage.

Also note that transformers give a rather higher voltage at light load than the rated voltage, which is give for full load.
Typically, the smaller they are, the worse the regulation between full load and no load.
(If a datasheet is available for a transformer, it should give the regulation value, which is the percentage of voltage increase when off load).

Is there any way to store AC like a capacitor?

danadak · Oct 14, 2022

Yes in the form of heat, or a mechanical battery like pumping water into an
elevated tank and getting electricity back thru a generator driven by flow
back out of tank.

But nothing handy like a part.

If you convert to DC then a capacitor for storage a possible solution.

There are other physical processes that can be used.....

Regards, Dana.

rjenkinsgb · Oct 14, 2022

Athosworld said:
Is there any way to store AC like a capacitor?

Not as "AC", or not with any kind of electronic component, not like a capacitor or battery can store DC.

That's why such as uninterruptable power supplies convert to DC for storage and back to AC for power out, using rectifiers and built-up "inverter" circuits - power transistors switching the input to a transformer to produce the alternating voltage output.

Tony Stewart · Oct 15, 2022

Modern DC motors use an ESC, (electronic speed controller) to vary the voltage and polarity for bi-directional control with a two half FET bridges that can be controlled for direction on top side pair and by duty cycle on the low side.

This is better than a diode full bridge which just stores the peak voltage into a small battery or large capacitor while the load decays that capacitor between peaks. A switch causes an abrupt acceleration with up to ~ 10x the current due to the low coil resistance.

The DC motor also requires coil commutation either by armature switches or brushless switching (BLDC) to be done by FETs in the ESC.

Athosworld · Oct 15, 2022

rjenkinsgb said:
Yes, a bridge rectifier and smoothing capacitor. The DC output will be roughly 1.4 * the AC input voltage, less the drop in the bridge rec.
That is typically 1.2 - 1.4V at light load as there are two diodes in series feeding the output, with either polarity input.

Without a capacitor, the average voltage will be near the AC voltage (less the diode drops), but it's pulsing twice each AC cycle rather that a constant or near-constant voltage.

Also note that transformers give a rather higher voltage at light load than the rated voltage, which is give for full load.
Typically, the smaller they are, the worse the regulation between full load and no load.
(If a datasheet is available for a transformer, it should give the regulation value, which is the percentage of voltage increase when off load).

Can you share the schematic?

rjenkinsgb · Oct 15, 2022

Athosworld · Oct 15, 2022

rjenkinsgb said:

Thanks, it worked, i can tell because the AC led flickering is gone

crutschow · Oct 16, 2022

Athosworld said:
Is there any way to store AC like a capacitor?

In theory you could store it with a super-conducting inductor and capacitor in a resonant circuit, but it would not be practical or economical to do, which is why it's never done.

Athosworld · Oct 16, 2022

rjenkinsgb said:
AC voltage passes through zero twice each cycle, so the relay is turning on and off continuously, even if it holds in.
The current will also be lower due to the inductance of the coil.

Relays or solenoids designed for AC are made slightly differently than for DC, different winding for the same voltage and with a "shading coil", which has current induced from the main coil and maintains the magnetic field through the AC voltage zeros. (Or some may have a rectifier and work on DC).

Shading coil - Wikipedia

en.wikipedia.org

The motor is trying to reverse every half cycle of AC as the voltage reverses, so it will not move far and will be taking high current as it's effectively stalled each time it has to restart.

Brushed AC motors are built as "Universal motor" style, with wound field coils rather than permanent magnet field.
The field reverses with the AC reversals as well, so the motor turns the same way regardless of the instantaneous AC power polarity.

Universal motor - Wikipedia

en.wikipedia.org

Will a 127 vAC transformer work on 6 vAC?
Just for experimenting, i connected a light bulb and it worked but it was way dimmer.

rjenkinsgb · Oct 16, 2022

Athosworld said:
Will a 127 vAC transformer work on 6 vAC?
Just for experimenting, i connected a light bulb and it worked but it was way dimmer.

I'm not sure what you mean - how did you connect them?

Athosworld · Oct 17, 2022

rjenkinsgb said:
I'm not sure what you mean - how did you connect them?

I mean, can i get a 127 vac transformer work on 6 vac? If I connected it?

MaxHeadRoom78 · Oct 17, 2022

Don't expect much out from it!

For The Popcorn · Oct 17, 2022

It's important to know which wires are the primary (input) and which are the secondary (output).

If your transformer converts 120VAC to 6VAC, the ratio is 6/120 = 0.05 to 1.

If you connect 6 volts to the primary, the output will be 6 × 0.05 = 0.3 volts.

If you connect 6 volts to the secondary, the primary will be at 120 volts: 6 × 1/(0.05). Use caution. This could be deadly (particularly with the knowledge you have demonstrated). This would supply very little current but more than enough to kill you.

If you connect 120 volts to secondary [Please don't do this] the output voltage will be 120 × 1/(0.05) = 2400 volts. Until the transform explodes or bursts into flames.

Athosworld · Oct 17, 2022

For The Popcorn said:
It's important to know which wires are the primary (input) and which are the secondary (output).

If your transformer converts 120VAC to 6VAC, the ratio is 6/120 = 0.05 to 1.

If you connect 6 volts to the primary, the output will be 6 × 0.05 = 0.3 volts.

If you connect 6 volts to the secondary, the primary will be at 120 volts: 6 × 1/(0.05). Use caution. This could be deadly (particularly with the knowledge you have demonstrated). This would supply very little current but more than enough to kill you.

If you connect 120 volts to secondary [Please don't do this] the output voltage will be 120 × 1/(0.05) = 2400 volts. Until the transform explodes or bursts into flames.

Yikes, better have caution, i connected it to the primary.

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