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# How do I limit voltage?

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#### conmgt

##### Newbie level 6
how to limit voltage

Hello,
First, I am new to this forum and am not savvy in electronics but I am an electrician.
I have a commercial grade wood planer that is 2 years old. There is a large flat metal table that moves up and down by way of 4 spiral shafts chain driven by a 24VDC motor made by Elra. The machine itself is fed by 240VAC 3 phase. There is a small transformer for the DC motor which has an output of 30VAC +/- 4V. Attached to it is a simple PC board with a rectifier (4 posts) and a 50V 10,000mF capacitor. That's it.
The problem is that at one point the 30VAC into the PC board produced 50+ VDC and burned out that 24VDC motor...$400! Now the output is down to 30VDC again. The question is, how did this happen? More important, how do I prevent it? Can I add something between the rectifier and the motor to prevent anything over 30VDC from passing through? I'm thinking some sort of a diode. Should I limit the VDC to 24? Let's keep it in laymans terms. Thanks #### mister_rf ##### Advanced Member level 5 t8l250v There's a rectifier circuit known as a bridge rectifier and a capacitor for filtering the output voltage. But that smoothing significantly increases the average DC voltage to almost the peak value (1.4 × RMS value). This can explain up to a 45-50 Vdc measured for a no load situation. I suppose this voltage will drop out to 30-35Vdc if the motor is connected. In order to design a voltage stabiliser you need to know the maximum current required by the motor or the rated power. Anyway I assume this was designed to work like that and the motor failure was related to a different problem. BojanR ### conmgt Points: 2 Helpful Answer Positive Rating ### BojanR Points: 2 Helpful Answer Positive Rating #### johnyaya ##### Full Member level 2 limit voltage The 240Vac to 24Vdc power supply that the machine uses is one of the simplest made. As the mains voltage varies so will the DC voltage output. As mister_rf already stated, the output voltage will also vary with motor load. Here are a couple approaches you can take: Add a crowbar circuit to your existing power supply. This will effectively short out your power supply when the voltage goes too high, thus popping a fuse/breaker but protecting your motor. Another approach is to look into replacing the inexpensive power supply with a switching power supply that will always generate the 24Vdc required regardless of the mains fluctuations. These power supplies will also incorperate over-voltage and over-current protection to protect your high dollar motor. The problem then becomes finding out what the maximum current the motor can safely pull, then you can select an appropriate power supply. ### conmgt Points: 2 Helpful Answer Positive Rating #### conmgt ##### Newbie level 6 limit input voltage First , thanks for your replies. The output of 50VDC was occuring while I was reading 245VAC into the transformer and 34VDC leaving it and going into the rectifier. I confused on where the additional voltage was coming from. As far as current draw, there is a fuse that says T8L250V...8A? I've e-mailed the motor manufacturer for specs. #### johnyaya ##### Full Member level 2 how to limit output voltage You stated: 245VAC into the transformer and 34VDC leaving it 245Vac is high for 3 phase in the USA. It should be closer to 208Vac from phase to phase. There is no DC component on the output side of the transformer, did you mean 34Vac? A transformer has Vac in (primary side) and Vac out (secondary side). The rectifier and capacitor converts the Vac to Vdc. Vdc = 1.4 * Vac, without a load. This is because the meter measures Vac in RMS and the Vdc that you measure is related to the peak Vac due to the rectifier and cap. Here's a power supply that may work for you (depending upon what the motor manufacturer provides) and would allow the motor to run on 120Vac (L1 to N), 208Vac (L1 to L2, 3 phase), or 240Vac (L1 to L2, single phase): https://www.jameco.com/webapp/wcs/s...01&catalogId=10001&pa=137657&productId=137657 It costs less than a motor but it will require clean air though to keep it cool, which may be a problem considering how it will be used. You can also place a crowbar circuit across the output of your existing power supply that would short it out if the voltage got too high and therefor pop the fuse. A quick search with google did not turn up any commercially available units that you could easily add on though. ### conmgt Points: 2 Helpful Answer Positive Rating #### conmgt ##### Newbie level 6 how to limit voltage output yep...I meant 34VAC. That power supply you mentioned looks to be perfect and dust won't be a problem since there is a sealed compartment for all of the electronics plus this motor is used for 10-15 seconds at a time maybe 4 times a day. I wouldn't expect it to generate much heat. When I first checked for DC output 3 weeks ago I got the 50V reading which agrees with the 1.4 rule with no load. Today, its 245VAC to 34VAC to a 30VDC output with again nothing connected to the DC output. I'm confused. Is it defective? The manufacturer of the machine wants to sell me a new exact replacement power supply for$180 based on the 50V I measured 3 weeks ago. I'm not sure what I'm going to do. Maybe I'll make the crowbar myself and use the old power supply.

My building has a 2 phase service and I'm running the machines on a static phase changer. When I finally get the chance, I'll switch over to 120/208Y. I've installed a 1000A main panel and the utility company set up a new pole with the transformers.

So is the fuse 8A?

#### johnyaya

##### Full Member level 2
t8l250v fuse

When I first checked for DC output 3 weeks ago I got the 50V reading which agrees with the 1.4 rule with no load. Today, its 245VAC to 34VAC to a 30VDC output with again nothing connected to the DC output. I'm confused. Is it defective?

Sounds that way (I'm assuming that there is still no load). One thing that can happen is that the large electrolytic capacitor starts to fail and then your DVM will read a lower DC voltage, but isn't the lower value (30 Vdc) the correct value? I'm also assuming that the transformer is a single-phase transformer. Does this power supply only provide power to the motor? Something I just noticed is that you are measuring 50Vdc from an electrolytic cap rated for 50Vdc. This is not good. The normal design technique would be to use a cap rated for 50V when it is expected to operate at 30V, so the cap you have in your power supply may already be damaged from running at too high of voltage.

I'm not sure what I'm going to do. Maybe I'll make the crowbar myself and use the old power supply.

I lean towards just replacing the power supply. You suspect a problem with the one you have. You can also find a better power supply that costs less than the original. Using a defective power supply will only cause grief. You are already out $400 for a new motor. I would go the extra$100 and get a good power supply as well. Of course this opinion is based on a couple of pictures and a little bit of text.

I'm running the machines on a static phase changer

You need to be very careful with the static phase convertors. Some require a motor load to operate properly. Some static convertors leave two legs at 180 deg and then generate the third. In any event you do not end up with a good 3 phase power source. The rotary phase convertors are the best. I have a 3-phase convertor that completely regenerates the 3 phases (kinda like a UPS does), but it cannot handle startup surges like the rotary convertor. It appears that you may be running the equipment with too high of voltage. You have measured 245Vac on the primary side of the transformer, but under normal 3-phase power it would be closer to 208Vac. With 208Vac on the primary side of the transformer the secondary side would be 29Vac and the power supply output would be closer to 40Vdc. I think that the static phase convertor might have caused your problems. A replacement power supply similar to the one I listed will give you good DC voltage regulation (the original power supply doesn't give you any load regulation), and operate under a wider input voltage (up to 264Vac for the one I referenced).

### conmgt

Points: 2

#### conmgt

##### Newbie level 6
how do you limit voltage

As far as 30VDC being the correct value...well...considering that the the motor says 24VDC on it, it's probably a little high if anything. Certainly 50V is not good. I can't say for sure if the motor was lost to high voltage. All I know is that it has no continuity and that after I disconnected it's leads and checked for power coming to it (7 days after it stopped working) I read 50V.

The transformer is tapped for 30VAC for a solenoid which switches the feed to the motor.

I'll buy the switched power supply if I can get one by Friday.

At the building mains I read 240V and all of my machines were shipped ready for 240 vs 208 and can easily be changed once I switch over my service. Every tranformer in my machines have seperate taps for 208, 240, and 400. This one was connected at 240.

Of the four wires feeding the phase converter, the two from phase A go straight to L1 and L2, the two from phase B are used to create L3 in the converter.

I haven't figured out how to "quote" on a forum yet so I apologize for not doing so.

#### johnyaya

##### Full Member level 2
voltage limit

The transformer is tapped for 30VAC for a solenoid which switches the feed to the motor.

It sounds like you may need to keep the transformer unless the solenoid will also run off 24Vdc. Is the noid controlled by a mechanical switch or is it controlled by other electronics?

all of my machines were shipped ready for 240 vs 208

Good.

Of the four wires feeding the phase converter...

Do you have any other equipment connected to the phase convertor? If so, could operating the other machine(s) change the phase convertor output so that you see a different output from the power supply? Have you checked voltages between phases on the output of the phase converter with and without load?

### conmgt

Points: 2

#### conmgt

##### Newbie level 6
limit voltage

I ordered the power supply you suggested. Jameco had a great price. Unfortunately I'm 3,000mi away and took a hit on shipping. I need this machine running by Friday. It was too late in the day for shipping from any other time zone.

Anyway, interesting point about the 'noid. I'm at home now but I'll check it tomorrow. It would be nice to not have to need both supplies and I'll check the converter output under different loads too.

Can converters be run in parallel? I'm pretty much running it to its limits on occasion. I have two that came with the property and really can't divide the 9 loads evenly between them because there's no pattern as to which loads are going to run simultaneously.

Maybe I should determine which of the three 240 VAC feeds come directly from the 2ph service and put the power supply on them.

#### johnyaya

##### Full Member level 2
how to limit the voltage

Can converters be run in parallel?

Don't know. I would guess not though. You could match 2 of the 3 phases, but I would be worried about the psuedo 3rd phase. I guess I'm chicken 'cause I wouldn't try it unless the manufacturer said it would work. Let me know how it goes if you decide to try it.

#### johnyaya

##### Full Member level 2
how do i limit voltage

3S1 is a height cutoff switch (too high), 3S2 (too low).
3S3 is a push button for incremental raising of the table (up only)...very important.
3S4's are for manual up and down.

This makes sense. Switch 3S4 appears to be a center-off type switch which isn't clear in the schematic.

From what I know about the whole system now, I still think that part of the problem is related to the pseudo 3rd phase from the static 3ph convertor. Have you had a chance to check to see if the unit is wired such that L3 is the pseudo 3rd phase? If so, swapping L3 and L2 to get away from using the pseudo phase on anything but the large 3ph motor (1M1) may solve your problems such that you do not need the new 24Vdc power supply. I have seen too many of the static 3ph convertors that are designed only for running a motor, such that the 3rd phase is REALLY bad unless there is the proper load attached. I am concerned that you have measured 50Vdc across the output of the power supply, and thus across the 50V electrolytic cap. The cap can be replaced for much less than a power supply though.

Shouldn't it have overvoltage protection?

Well... it would be nice, but if you had a real 3ph service you may never have had a problem either. The manufacturer could have easily built overvoltage protection into the power supply.

OK, I guess I need something in between the DC out of the original supply and the motor. Any suggestions?

There are a couple of ways to do this. The 9.5Vac tap turns out to be close to 12Vdc. You could get a 12Vdc power supply in addition to the 24Vdc supply (2 power supplies), or you can insert something in series with the 24Vdc output, using 3S4 (1+2). That something could be a large resistor (150 W, resistance not yet determined), a very high power zener diode (not cheap), or specific light bulb (not a good option either). Adding a second power supply (12Vdc) for low-speed operation is the better option of the two. Switch 3S4 (1+2) would be used to select which power supply output to use. I'll help you figure out the wiring if that is what you choose to do.

#### conmgt

##### Newbie level 6
3s4 schematic

Well folks, I just got back from picking up the new motor and a new original power supply for \$480 total. I'll be working on it tonight.

Your right about 3S4 and I'll definitely check to see which is the wild leg and make sure to keep the power supply off of it. I'm also going to try FvM's thought on using the 400VAC tap and see how fast the table moves up and down.

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