7i76 + 8i20 for BLDC motor control
18 Mar 2018 22:35 #107520
by andypugh
It comes down to how rigid things are. But I would say that 50 slots / 200 counts is about the minimum.
Replied by andypugh on topic 7i76 + 8i20 for BLDC motor control
What would be the minimum resolution I could get away with for a spindle encoder for say tapping if it had an index?
It comes down to how rigid things are. But I would say that 50 slots / 200 counts is about the minimum.
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27 Mar 2018 22:40 #108004
by blazini36
Replied by blazini36 on topic 7i76 + 8i20 for BLDC motor control
A bit off topic but you guys are pretty good so........
I'm trying to figure out a good capacitor value for the 8i20 with the 750w BLDC. I'll be running it off of straight rectified 120v AC so it's 160v DC. I have a bunch of 470uf caps I can run in parallel, a 1000uf cap or a 4700uf cap. I'd prefer to use a single 1000uf or the 4700uf because they have screw terminal and mounting options (plus I have access to as many 4700uf caps as I could ever want). they all have a voltage rating of 250v-450v. The only issue I know of with the higher capacitance is discharging them with the power off, I have a few 5w resistors for bleed down but they'll be wasting power while this thing is on. I'm just not sure how much capacitance is really necessary or what is too much.
I'm trying to figure out a good capacitor value for the 8i20 with the 750w BLDC. I'll be running it off of straight rectified 120v AC so it's 160v DC. I have a bunch of 470uf caps I can run in parallel, a 1000uf cap or a 4700uf cap. I'd prefer to use a single 1000uf or the 4700uf because they have screw terminal and mounting options (plus I have access to as many 4700uf caps as I could ever want). they all have a voltage rating of 250v-450v. The only issue I know of with the higher capacitance is discharging them with the power off, I have a few 5w resistors for bleed down but they'll be wasting power while this thing is on. I'm just not sure how much capacitance is really necessary or what is too much.
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28 Mar 2018 09:34 #108026
by Mike_Eitel
Replied by Mike_Eitel on topic 7i76 + 8i20 for BLDC motor control
Take a relais, connect on the ac side, if no ac power use contacts to unload the capacitors via your resistors. Only permanent power loss and heat is your relais.
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28 Mar 2018 09:42 #108027
by Muzzer
Replied by Muzzer on topic 7i76 + 8i20 for BLDC motor control
Why do you want to discharge them? Can't think of a good reason in normal operation. Different matter if you are continuing to play with the circuit, otherwise any residual charge simply means less inrush when you next power up. You won't significantly affect the life which is primarily down to the temperature of the caps.
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28 Mar 2018 13:26 #108031
by andypugh
You can find online calculators to help with this:
www.changpuak.ch/electronics/power_supply_design.php
(For direct-on-mains connection I think you just need to set the delta-volts-per-amp to 0. Load resistance can be got from R = V^2 / P )
Bear in mind that the servo won't be working at full power for very much of the time, so you should consider when doing the calcs.
One critical thing is the ripple current, and the ripple current rating of the capacitors. If your cap is something like this:
uk.rs-online.com/web/p/aluminium-capacitors/3819245/
Then it is rated 15A ripple current, which looks about OK given some allowance for the servo duty cycle.
It is interesting to note that the ripple current is higher with a bigger cap, partly because the average load voltage is so much higher.
On balance, I think I would use the 4700uF one.
Replied by andypugh on topic 7i76 + 8i20 for BLDC motor control
I'm trying to figure out a good capacitor value for the 8i20 with the 750w BLDC. I'll be running it off of straight rectified 120v AC so it's 160v DC. I have a bunch of 470uf caps I can run in parallel, a 1000uf cap or a 4700uf cap. I'd prefer to use a single 1000uf or the 4700uf because they have screw terminal and mounting options
You can find online calculators to help with this:
www.changpuak.ch/electronics/power_supply_design.php
(For direct-on-mains connection I think you just need to set the delta-volts-per-amp to 0. Load resistance can be got from R = V^2 / P )
Bear in mind that the servo won't be working at full power for very much of the time, so you should consider when doing the calcs.
One critical thing is the ripple current, and the ripple current rating of the capacitors. If your cap is something like this:
uk.rs-online.com/web/p/aluminium-capacitors/3819245/
Then it is rated 15A ripple current, which looks about OK given some allowance for the servo duty cycle.
It is interesting to note that the ripple current is higher with a bigger cap, partly because the average load voltage is so much higher.
On balance, I think I would use the 4700uF one.
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