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Old 06-21-2018, 09:43 PM   #26
franken
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Re: Any electronics nerds interested in Arduino/Atmel?

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Well I don't know about PWM for AC, I think it is for DC motors only, but I may be mistaken. It will be interesting to find out how it goes.
Look into a triac for AC PWM... https://www.google.com/search?ei=6ko....0.okNKgaixWr8

As for BJTs vs FETs they use a bit different terminology but the basic difference is the former operates on base current, and the latter on gate voltage. FET tech typically uses less current and higher voltage.
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Old 06-27-2018, 11:44 AM   #27
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Re: Any electronics nerds interested in Arduino/Atmel?

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Well I don't know about PWM for AC, I think it is for DC motors only, but I may be mistaken. It will be interesting to find out how it goes.
It works for light bulbs, I'm just not sure about electric motors. I think if the PWM wave is fast enough (like 1000Hz at least) and the relay fast enough, it should work perfectly!

Since I have two phases I'd need two relays, though.
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Old 06-28-2018, 02:17 AM   #28
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Re: Any electronics nerds interested in Arduino/Atmel?

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It works for light bulbs, I'm just not sure about electric motors. I think if the PWM wave is fast enough (like 1000Hz at least) and the relay fast enough, it should work perfectly!

Since I have two phases I'd need two relays, though.
I'm not entirely sure what you want to achieve.

Do you have a AC (alternating current not air conditioning) motor that you want to soft-start? Are you trying to drive DC cooling fans with PWM for speed control? Is the fuel pump supposed to be variable output?
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Old 06-28-2018, 04:20 PM   #29
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Re: Any electronics nerds interested in Arduino/Atmel?

What is the purpose of soft-starting an AC motor? What does it achieve?
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Old 06-29-2018, 12:30 PM   #30
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Re: Any electronics nerds interested in Arduino/Atmel?

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What is the purpose of soft-starting an AC motor? What does it achieve?
Motors have high inductance, so when you power them on by just connecting it to line voltage, it is essentially a short for a small window of time. This causes a voltage drop in the system while the motor comes up to speed, and also means that the current in the motor is extremely high during this time.

Soft-starting drastically reduces the startup current and the current through the motor coils.
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Old 07-08-2018, 09:41 AM   #31
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Re: Any electronics nerds interested in Arduino/Atmel?

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Motors have high inductance, so when you power them on by just connecting it to line voltage, it is essentially a short for a small window of time. This causes a voltage drop in the system while the motor comes up to speed, and also means that the current in the motor is extremely high during this time.

Soft-starting drastically reduces the startup current and the current through the motor coils.
Although it sounds good, it is probably not a great idea to try soft starting a split-phase motor (cap start-cap run, or cap start-induction run) like is used on an air compressor. These motors pull a lot of amps on start up because they are high torque motors, and need to come up to speed quickly.
The type of motor usually used for soft-start is a 3-phase motor, and they are available with built-on or separate inverters/vfd module, so you can operate them from a single-phase power source
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Old 07-08-2018, 06:27 PM   #32
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Re: Any electronics nerds interested in Arduino/Atmel?

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Although it sounds good, it is probably not a great idea to try soft starting a split-phase motor (cap start-cap run, or cap start-induction run) like is used on an air compressor. These motors pull a lot of amps on start up because they are high torque motors, and need to come up to speed quickly.
The type of motor usually used for soft-start is a 3-phase motor, and they are available with built-on or separate inverters/vfd module, so you can operate them from a single-phase power source
I forgot to mention this.

Also, soft-starting reduces the average current, not the peak current through the coils.
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Old 07-10-2018, 10:35 AM   #33
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Re: Any electronics nerds interested in Arduino/Atmel?

Can someone explain cap-start? Does it store a bunch of surge energy in a cap for the initial inductive "kick" when the motor starts?

I decided not to monkey with my big compressor. I have a small desktop compressor I used for tires and air brushing that I will play with instead!

I built a switched outlet box with a 30A Craydon SSR in it to play with, we shall see. I searched and paid extra for a non-zero-cross relay. Normally they don't switch until the next time the AC crosses zero, which is OK as a switch but wouldn't work for AC slicing.

I've also been tinkering with a bunch of old retro displays and trying to interface them to modern electronics. Here are some of my results!

Little saga on the VFD display: it's a Japanese unit from Noritake and uses hand-coded timing loops to drive it that I could only get to work on the CPU they designed it for (which I'm not using because it's ancient). So for the $1.75 they cost, I dedicated an Atmel 328P (which it works with) just to controlling the VFD. It then exposes the standard LCD interface across the i2c bus so that the ESP32 can call it as if it were a local display. Works great, but was a bit of an investment.

I'm a software guy, so every one of these little wires is a victory ;-)
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Old 07-10-2018, 09:09 PM   #34
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Re: Any electronics nerds interested in Arduino/Atmel?

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Originally Posted by davepl View Post
Can someone explain cap-start? Does it store a bunch of surge energy in a cap for the initial inductive "kick" when the motor starts?

I decided not to monkey with my big compressor. I have a small desktop compressor I used for tires and air brushing that I will play with instead!

I built a switched outlet box with a 30A Craydon SSR in it to play with, we shall see. I searched and paid extra for a non-zero-cross relay. Normally they don't switch until the next time the AC crosses zero, which is OK as a switch but wouldn't work for AC slicing.

I've also been tinkering with a bunch of old retro displays and trying to interface them to modern electronics. Here are some of my results!

Little saga on the VFD display: it's a Japanese unit from Noritake and uses hand-coded timing loops to drive it that I could only get to work on the CPU they designed it for (which I'm not using because it's ancient). So for the $1.75 they cost, I dedicated an Atmel 328P (which it works with) just to controlling the VFD. It then exposes the standard LCD interface across the i2c bus so that the ESP32 can call it as if it were a local display. Works great, but was a bit of an investment.

I'm a software guy, so every one of these little wires is a victory ;-)
You have to have some way of setting up a rotating magnetic field in a single-phase motor. Otherwise, the field just alternates without rotating. In a cap-start motor, there are 2 windings, a run and a start. The run winding is across the ac line, the start winding is across the ac line with a capacitor in series with it. The capacitor causes a shift in that winding (along with the winding being wound slightly different than the run) so that the field will rotate, to start the motor. On many motors, once the motor comes up to about 75% or running speed, a centrifugal switch cuts out the start winding, This is a capacitor-start, induction run. For loads requiring more running torque, a capacitor start, capacitor run motor is used. These have two capacitors, one for starting and one for running. The run capacitor stays in series with the start coil all the time. Air compressors and table saws often use cap-start, cap run motors. Another type of capacitor motor for low-torque loads like fans omits the start capacitor and has just a run capacitor. These are called permanent split capacitor (PSC) motors and they are very efficient.
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Old 07-11-2018, 11:11 AM   #35
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Re: Any electronics nerds interested in Arduino/Atmel?

Aha! That must have been what my EE friend at the power company was talking about when he was trying to dissuade me from doing this on my big 240V compressor... he talked about the potential of a centrifugal switch.

I need to read up a little more on single-phase induction motors, as with brushes and no moving fields, they're a little mysterious!
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