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Old 11-05-2021, 07:13 AM   #1
rockfordpi
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Default Why do lester Chargers use 660vac Capacitors

I just replaced a bad 3 mfd capacitor in a Lester 36v accucharge transformer type Charger. In doing so I noticed the factory capacitor was rated a 660vac.

I understand what the capacitor does related to the charge process. I am just curious as to why they used a capacitor rated at 660v max?
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Old 11-05-2021, 08:47 AM   #2
Volt_Ampere
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Default Re: Why do lester Chargers use 660vac Capacitors

It's probably overkill on the voltage rating. I doubt that voltages anywhere near that exist in these chargers running off of 110V. Switching voltages way higher than 110V do exist though. Higher voltage rating is fine for capacitors - and there may be a reason for it. Shouldn't be an issue for a capacitor of that size. I would replace it with same rated cap.
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Old 11-05-2021, 11:53 AM   #3
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Default Re: Why do lester Chargers use 660vac Capacitors

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Originally Posted by rockfordpi View Post
I just replaced a bad 3 mfd capacitor in a Lester 36v accucharge transformer type Charger. In doing so I noticed the factory capacitor was rated a 660vac.

I understand what the capacitor does related to the charge process. I am just curious as to why they used a capacitor rated at 660v max?
That is probably the next or second higher standard voltage rating above the 480VAC applied to the boost capacitor during normal operation of the ferroresonant transformer circuit.
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Old 11-05-2021, 11:56 AM   #4
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Default Re: Why do lester Chargers use 660vac Capacitors

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That is probably the next or second higher standard voltage rating above the 480VAC applied to the boost capacitor during normal operation of the ferroresonant transformer circuit.
That makes sense! I have not dealt with any chargers that use that circuit. My more modern ones are all based on switching regulators without a big transformer.
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Old 11-05-2021, 12:14 PM   #5
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Default Re: Why do lester Chargers use 660vac Capacitors

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Originally Posted by Volt_Ampere View Post
That makes sense! I have not dealt with any chargers that use that circuit. My more modern ones are all based on switching regulators without a big transformer.
The theory of operation for that circuit is a riddle, wrapped in a mystery, inside an enigma, but it works.
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Old 11-06-2021, 03:17 AM   #6
rockfordpi
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Default Re: Why do lester Chargers use 660vac Capacitors

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That makes sense! I have not dealt with any chargers that use that circuit. My more modern ones are all based on switching regulators without a big transformer.
So was there a technical reason 660v was selected/used/needed or was that voltage simply based on convenience and/or cost?
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Old 11-06-2021, 11:15 AM   #7
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Default Re: Why do lester Chargers use 660vac Capacitors

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So was there a technical reason 660v was selected/used/needed or was that voltage simply based on convenience and/or cost?
All of the above.

Technically the capacitor's farad value has the match the transformer's inductance and the capacitor's voltage rating has to exceed the applied voltage, which is roughly 500VAC.

Custom built capacitors are expensive and most likely have a long lead time, so using them is inconvenient. Therefore supply chain managers prefer to use off the shelf items since they cost less and are more readily available.

Check the voltage applied to the capacitor, increase it by a third or more and select the next higher Motor-Run capacitor with the correct µF value.

The voltage applied to the capacitor on the schematic I posted is 480VAC and that value plus a third is ~640VAC, so a 660VAC rated capacitor is most likely is the next higher standard voltage.

370VAC capacitors are more common because there are a lot of HVAC motors that run on 220VAC and fewer AC motors running on 440VAC, but there are some in commercial applications, so 660VAC capacitors can be found at HVAC places the do work for commercial customers.
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Old 11-07-2021, 08:36 AM   #8
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Default Re: Why do lester Chargers use 660vac Capacitors

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All of the above.

Technically the capacitor's farad value has the match the transformer's inductance and the capacitor's voltage rating has to exceed the applied voltage, which is roughly 500VAC.

Custom built capacitors are expensive and most likely have a long lead time, so using them is inconvenient. Therefore supply chain managers prefer to use off the shelf items since they cost less and are more readily available.

Check the voltage applied to the capacitor, increase it by a third or more and select the next higher Motor-Run capacitor with the correct µF value.

The voltage applied to the capacitor on the schematic I posted is 480VAC and that value plus a third is ~640VAC, so a 660VAC rated capacitor is most likely is the next higher standard voltage.

370VAC capacitors are more common because there are a lot of HVAC motors that run on 220VAC and fewer AC motors running on 440VAC, but there are some in commercial applications, so 660VAC capacitors can be found at HVAC places the do work for commercial customers.
JB,

I am having trouble understanding where the 480VAC referenced in your example is coming from?

My understanding (or lack thereof) of the charger operation (which may be the problem) is:

120v applied to primary Stepped down to 2x charge voltage in in center tapped secondary each half of secondary Rectified to DC charge voltage through the diodes, capacitor used to smooth peaks and valley of AC.

This may be an academic question that I simply lack the background to understand. If so, as a practical matter, it is not critical as I always replace the capacitor with one with same specs as I take out anyway.

However, even when I know how/what to do, I am just wired (no pun intended) with a need to understand WHY, LOL.

After looking at your diagram further, is that a third transformer winding in there stepping the voltage up to 480v? If so, I am going to stop there with the "Why's", LOL.
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Old 11-07-2021, 02:27 PM   #9
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Default Re: Why do lester Chargers use 660vac Capacitors

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JB,

I am having trouble understanding where the 480VAC referenced in your example is coming from?

My understanding (or lack thereof) of the charger operation (which may be the problem) is:

120v applied to primary Stepped down to 2x charge voltage in in center tapped secondary each half of secondary Rectified to DC charge voltage through the diodes, capacitor used to smooth peaks and valley of AC.

This may be an academic question that I simply lack the background to understand. If so, as a practical matter, it is not critical as I always replace the capacitor with one with same specs as I take out anyway.

However, even when I know how/what to do, I am just wired (no pun intended) with a need to understand WHY, LOL.

After looking at your diagram further, is that a third transformer winding in there stepping the voltage up to 480v? If so, I am going to stop there with the "Why's", LOL.
Yes, the secondary winding the capacitor is connected to, steps the 120VAC input voltage to the primary winding up to 480VAC.

For inquisitive minds: Without the capacitor connected, the nominal voltage across the two secondaries connected as a single center tapped winding is 60VAC. With it connected, the output voltage is boosted to 90VAC.

However, when a 36V battery pack is connected, the pack tries to draw high amps, saturating the transformer core and limiting the output to about 18A, or so, and the output voltage equals whatever the battery pack's on-charge voltage is for that amp flow and SoC.

As the battery pack charges and its on-charge voltage increases, the transformer's saturation point changes and the number of amps decrease down to a minimum value and stays there as the on-charge voltage continues to rise.

If the charger has a control board (It has no influence on the charging process other than turning the charger on and off), it will terminate the charge cycle when the pack's on-charge voltage climbs to 45V.
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Old 11-08-2021, 12:28 PM   #10
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Default Re: Why do lester Chargers use 660vac Capacitors

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Yes, the secondary winding the capacitor is connected to, steps the 120VAC input voltage to the primary winding up to 480VAC.

For inquisitive minds: Without the capacitor connected, the nominal voltage across the two secondaries connected as a single center tapped winding is 60VAC. With it connected, the output voltage is boosted to 90VAC.

However, when a 36V battery pack is connected, the pack tries to draw high amps, saturating the transformer core and limiting the output to about 18A, or so, and the output voltage equals whatever the battery pack's on-charge voltage is for that amp flow and SoC.

As the battery pack charges and its on-charge voltage increases, the transformer's saturation point changes and the number of amps decrease down to a minimum value and stays there as the on-charge voltage continues to rise.

If the charger has a control board (It has no influence on the charging process other than turning the charger on and off), it will terminate the charge cycle when the pack's on-charge voltage climbs to 45V.
JB,

THANKS! Now I actually sort of understand what is going on (though I will not even pretend to comprehend the math behind it LOL).

So, based on your info, I did a science project last night (some of us live dull lives). I took an old Lester charger someone discarded as bad transformer. Stripped back everything but the transformer (no timer/diodes/fuse/capacitor)

I put 120v on primary. Without Cap, the cap coil read 516v. I have 30v between the 2 white wires, 30v between the 2 black wires, and 35v between each black and white pairs.

If I am understanding what you wrote, and the Lester troubleshooting guide, this transformer was actually not bad?

Still probably not worth fixing due to cost of timer board, but it was a good exercise for inquiring minds (assuming I am interpreting the results correctly).
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