Good evening all

I'm following this instructable to build an arduino controlled AC motor winch
https://www.instructables.com/How-to-hack-an-Electric-Hoist-AC-motor/

I am trying to choose an appropriate resistor for the capacitor discharge, however my capacitor seems to be a 2-in-1 package
15uF & 6uF 450v

I am looking for advice on resistor values and is it safe to wire it in parallel as they have done so ?

Thank you in advance

 

Full disclosure: I don't quite understand what the purpose of the capacitor is (I assume it's either a start or run capacitor) nor why you need to specifically discharge it with a resistor.

It should be fine to put the two capacitors in parallel. This would simply add to the total capacitance, making it the sum of the two individual values. On the other hand, if your motor starts and runs fine with just one, I'd probably go with that and leave the second one disconnected. The motor should have a specified value for the capacitor, aim for that.

You only need one resistor in parallel with the capacitor(s) to achieve the discharge. I'd start with a high value such as 1 megohm (1 MΩ) and work down to 100K as needed to achieve the functioning you need. If it helps, the time constant for an RC circuit is 1/RC. So the voltage on the cap is given by V= V0•exp(-t/RC). This assumes all the discharge current is going through the resistor. If you're using the 15µF cap and a 1MΩ resistor, the time constant is 15sec^-1. So the voltage would drop from, say, 100V to 37V in 15 seconds. That sounds like a long time now that I say it, but I have no idea what degree of voltage drop you need versus time.

 

There is no need to place a discharge/bleed resistor across a motor capacitor (whether a start capacitor or run capacitor) since the motor winding will provide a low impedance discharge path.

 

Full disclosure: I don't quite understand what the purpose of the capacitor is (I assume it's either a start or run capacitor) nor why you need to specifically discharge it with a resistor.

It should be fine to put the two capacitors in parallel. This would simply add to the total capacitance, making it the sum of the two individual values. On the other hand, if your motor starts and runs fine with just one, I'd probably go with that and leave the second one disconnected. The motor should have a specified value for the capacitor, aim for that.

You only need one resistor in parallel with the capacitor(s) to achieve the discharge. I'd start with a high value such as 1 megohm (1 MΩ) and work down to 100K as needed to achieve the functioning you need. If it helps, the time constant for an RC circuit is 1/RC. So the voltage on the cap is given by V= V0•exp(-t/RC). This assumes all the discharge current is going through the resistor. If you're using the 15µF cap and a 1MΩ resistor, the time constant is 15sec^-1. So the voltage would drop from, say, 100V to 37V in 15 seconds. That sounds like a long time now that I say it, but I have no idea what degree of voltage drop you need versus time.

Interesting

Let me explain further, i believe its a start capacitor (2 in 1 package), I assume they are different values because the demand to raise the winch is higher than to lower it?

I am replacing the rocker switch with SSRs and whilst they work on the first run, once the capacitors and charged, the motor fails to start on the second attempt, hence why I feel I need a resistor across both caps

 

There is no need to place a discharge/bleed resistor across a motor capacitor (whether a start capacitor or run capacitor) since the motor winding will provide a low impedance discharge path.

The rocker switch seems to allow this to happen in normal operation but since I'm replacing it with SSRs (not relays) I'm assuming this stops the caps discharging correctly?

 

If like you said the motor is AC, there is nothing to discharge. Capacitors don't store AC they pass AC. They do however store DC.

 

As others have said the state of charge of the capacitor should not cause a problem with the motor. The capacitor can be charged to any voltage between plus or minus the peak value of the mains voltage waveform. This value will depend on the point in the waveform when the switch is opened.
This is the schematic of my hoist which looks the same as the one in your picture.

You can see that when the up / down switch is in the center off position there is no path through the windings to discharge the capacitor.
to discharge the capacitor. The capacitor produces a leading phase shift in the winding that it is in series with. (This controls the direction of rotation.) One leg of the supply is connected to the junction of the two windings. The capacitor is conncted between the other end of the windings when the switch is in either the forward or reverse positions. When the switch is in the forward position the the other leg of the mains is connected directly to one of the winding and to the other winding via the capacitor. in the reverse position the mains is connected directly to the end of the other winding. (So now the capacitor is in series with the winding that was connected directly to the mains in the forward direction.
The micro switch is an upper position limit switch.
I suggest that you do some voltage tests to see which of the solid state is not behaving as expected to understand the problem better.
Have you tried driving the inputs of the solid state relays directly with a DC supply rather than the outputs of the arduino ?

Les.

 

I think discharging the cap is a legitimate safety add-on. For the price of a resistor, ie. less than a nickel, you reduce the risk of a servicer to encounter a charged cap. It's inside an enclosure and it's not a huge capacitance value, so the risk is small but maybe worth a resistor?

 

As others have said the state of charge of the capacitor should not cause a problem with the motor. The capacitor can be charged to any voltage between plus or minus the peak value of the mains voltage waveform. This value will depend on the point in the waveform when the switch is opened.
This is the schematic of my hoist which looks the same as the one in your picture.
View attachment 288828
You can see that when the up / down switch is in the center off position there is no path through the windings to discharge the capacitor.
to discharge the capacitor. The capacitor produces a leading phase shift in the winding that it is in series with. (This controls the direction of rotation.) One leg of the supply is connected to the junction of the two windings. The capacitor is conncted between the other end of the windings when the switch is in either the forward or reverse positions. When the switch is in the forward position the the other leg of the mains is connected directly to one of the winding and to the other winding via the capacitor. in the reverse position the mains is connected directly to the end of the other winding. (So now the capacitor is in series with the winding that was connected directly to the mains in the forward direction.
The micro switch is an upper position limit switch.
I suggest that you do some voltage tests to see which of the solid state is not behaving as expected to understand the problem better.
Have you tried driving the inputs of the solid state relays directly with a DC supply rather than the outputs of the arduino ?

Les.

Wow I love this forum

I studied your suggestions and went with 2 X 500k resistors. Works perfectly!

 

I think discharging the cap is a legitimate safety add-on. For the price of a resistor, ie. less than a nickel, you reduce the risk of a servicer to encounter a charged cap. It's inside an enclosure and it's not a huge capacitance value, so the risk is small but maybe worth a resistor?

It was more for the fact that the motor wouldn't turn if the capacitor wasn't discharged so it was a total necessity. Now working perfectly with 500k resistors in parallel

 

That doesn't make a lot of sense to me, but great.

 

That doesn't make a lot of sense to me, but great.

The physical contacts in the rocker switch in the pendant discharged the capacitor, but when i changed to arduino control using SSRs, the capacitors didnt discharge! Hope that clears it up