Q off Power
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Hi Guys,
Just looking at power supplies and I have a question regarding the specs of my stepper motors.
This is a Modified Tronxy X5SA Pro CoreXY.
Here is the spec's I've been given from the manufacture;
SL42STH40-1684A-23电机图.pdfWith reading the documents on figuring your power needs you multiply the rated current by rated voltage. 1.7a x 24v in my case = 40.8W
Then add an addition 50% for driver loss and acceleration = 61.2W per motor?
Surly this isn't correct 61w per motor.
There's very little online about these motors either.
That said, I have just found this. Although I can't read Chinese, I understand it mentioning something about 3vdc, which sounds more realistic?
42SHDC1067Z-25B RB477 电机外形图.pdfDizzwold.
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@Dizzwold Yes, 24V looks a bit ridiculous. I don't know if we can apply Ohms law to this situation but if we can then we have a rated current of 1.7Amps and a resistance of 1.2 ohms per phase which would imply a rated voltage of 2.04V per phase (V=IR) . Just guessing but maybe we double that (2 coils) so say 4.08 V which gives somewhere around 7 watts. I'd say that is a more realistic figure but others more knowledgable than I will no doubt correct me if I'm wrong.
Edit. Just for info as you mentioned that you were looking at power supplies, I'm running a DC UPS so I've been taking a fairly keen interest on power consumption in order to select appropriate batteries. I'm running a pair of 2 amp steppers on my CoreXY plus a 2.3 Amp Nema 23 on my Z axis and a 60 watt hot end heater with assorted fans etc and I print at 80mm/sec by default with 350mm/sec travel speed. Once the hot end is up to temperature, the total current draw is less than 2 Amps so less than 48 Watts using 24V. That's excluding the bed which is mains powered.
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@Dizzwold, the only thing relevant in the spec is the current. The voltage only determines how fast you achieve the rated current which determines how fast you can switch winding current. If you supplied your drivers with, for example 5V, you could reach full power (in a simplified way) at low RPM but as you increase motor speed you rapidly loose motor power because it takes a certain time for the winding current to come up. If you increase the supply voltage, your winding current, while still only the same as with the 5V supply, is achieved much faster. The net result is that you can increase motor RPM before motor torque starts to fall off.
Anyway, the power consumption is not based on the supply current but rather the voltage across the motor winding at the current that your drivers supply. Since current is not constant, the total power dissipation is rather difficult to calculate. As your driver switches from no current to full current it takes a certain amount of time to reach that full current. Since a higher supply voltage means that the required motor current can be achieved faster, you will increase overall power consumption as you go up in supply voltage but it is only a tiny increase because the motor can receive full current faster. Instead of the motor receiving full current for let's say 1/3 of the switching cycle with a 5V supply, it would achieve full current for 3/4 of the switch cycle with a 24V supply so power output is increased.Wow, that is actually much harder to explain than what I thought .... I will shut up now before I start rambling too much ....
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@deckingman said in Q off Power:
Once the hot end is up to temperature, the total current draw is less than 2 Amps so less than 48 Watts using 24V.
That’s fine to estimate the required capacity of a battery backup. For a power supply, however, you have to calculate the maximum current you might draw in the worst case. This worst case is difficult to determine, as we have to deal with strong spikes or pulses, not a continuous draw.
As a rule of thumb, I am fine with the rated current of all steppers and heaters (times voltage), not to forget the controller(s), illumination etc. Add to this 30% as a safety margin - and another 30% if you intend to buy a cheap Chinese power supply.
Sounds generous, but switching power supplies tend to die young if often run near their limits.
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@infiniteloop, just a PSA ..... If you buy a Meanwell brand name power supply that is common (ie 24V 350W), you pay less at Digikey.ca (and probably Digikey.com) than you would for what may or may not be a real Meanwell supply from China (Aliexpress). Of course you are still facing shipping costs unless you happen to need other stuff.
Oh, you say that Amazon has a good price on what the supplier claims to be a real Meanwell supply .... well yes but since Amazon does 'binning', you may get the real thing or you may get a cheap clone (been there, done that) -
@jens55 said in Q off Power:
Oh, you say that Amazon has a good price
Sorry, but I prefer to buy elsewhere
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@infiniteloop My point was that most people tend to overestimate the power consumption requirements. Of course the heater will use more during its initial warm up phase before PID kicks in but equally, the gantries will be stationary during that phase so the motors will only be using holding current. I doubt if my maximum power draw ever gets close to 100Watts. Of course, if the bed is heated using DC instead of mains, that's a completely different kettle of fish.
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@deckingman said in Q off Power:
My point was that most people tend to overestimate the power consumption requirements.
Sure. And in case your battery backup is permanently online, thus buffering all spikes, I really believe you could comfortably get along with a PSU of just a bit more than 2 amps. If mains power supply is reliable, a large capacitor (with a slow-charging circuit attached) could do the buffering as well, but common printers lack all of this, so the PSU has to match all demands, even if an extreme event only occurs once per year for a ms or less.
Maybe I'm too generous, but I once tried to precisely calculate the power needs of my printer and then provided a matching PSU (with just a narrow margin) - which failed after some months of operation. The larger replacement PSU (same brand, same product family) now works happily since then: 4 years, still counting …
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@infiniteloop You still miss the point. The DC UPS is just that - the (7Ah) batteries only cut in when mains power fails and will keep the machine running for over an hour. I never said that I use a 2Amp power supply. In fact I still use the much larger (Meanwell) DIN rail mounted supply that I used when the machine had 7 axes (CoreXYUVAB) and 6 extruders and a two zone hot end with two 80 Watt heaters. My point remains that most people over estimate the power requirements of their machines and waste money buying an over-specified PSU. For sure, leave some head room for future expansion, for sure use a quality PSU, but one that is capable of supplying 4,6,8 or 10 times the required power is a waste of money and can be dangerous because it might not trip if a fault on the machine develops.
