Motors and thermal protection
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I have an old Delta DP-220 drill press that is sort of okay for light work, but if you work it moderately, it will turn off, the switch will go to an intermediate position, and you cannot turn it back on until it sits for a few minutes. At that point, you flip the switch to off and can then move it to the on position. I’d like to fix this.
I’m guessing this is thermal protection. Can someone help me understand how thermal protection works? What components are involved and how do you diagnose things? I don’t understand how the switch senses temp in the motor unless there’s a separate sensor and conductors to bring a signal into the switch box. Or, is the “switch” looking at some aspect of current vs. voltage? Honestly, I’ve not even looked inside to see if there’s more in the switch box other than a switch.
Can someone point me in the right direction to learn more about this? I’m a physicist (or was), so I know basic things and am happy with a technical answer, but I don’t know the technology here.
If it matters: This drill press is probably 70-80 years old.
Typically, there is a small sensor embedded in the windings somewhere. When it senses a particular temperature it trips out, as you have noticed. It has an internal (to the sensor) timer which resets when the time is up. This timer could be as simple as waiting for the something to cool down, or it could be electrical in nature. I’ve never seen one of these which could be altered since it is probably just a small encapsulated device.
Usually if you repeatedly trip one of these sensors, it is for good reason. My suggestion would to be to replace the motor with something heavier.
Photos
This is a single phase ac motor that uses either a start or a start-run capacitor. It is that black lump on the top of the motor housing. The purpose of the capacitor (cap) is to change the timing of the AC phase between the windings and the armature. Caps are designed to pass AC and block DC current. As capacitors get old (over years) they start to pass DC and eventually will fully short. So instead of the winding seeing AC, they are seeing AC riding on a DC current, and the windings are not designed to pass DC and will heat up (act like the grid in a toaster).
Before you condemn the motor, find a friend that has electrical knowledge and have him test the capacitor. Capacitors cost typically less than $25. You will be hard pressed to find a motor of the quality you have now for less than $125. Don’t buy a replacement cap on Ebay or Amazon. Use an electronic supply house (Digikey, Newark Electronics) as there are many counterfeits out there. Stick to a name you have heard of (Sony, Panasonic, etc,). The voltage rating must exceed 220 and the Farads should be the same as you have now and have a temp rating of 110C.
Hej Mike,
Being a tad daft, I have to ask: where does the DC come from? Is it a consequence of the failing capacitor, or is AC mains by default biased (if that’s the term for arithmetic mean not being zero)?
Sven-Olof Jansson
London, UK; Boston, MASven-Olof, I was wondering the same. I’m definitely going to evaluate the cap because, with just a little investigation, I see that Mike is pointing me in a good direction to diagnose the drill press. I’ve thought a little about mechanism, and what comes to mind is this, but it is me guessing. If the cap is failing in the sense of allowing a leakage current, i.e., if it could pass DC, then I would expect the effect of the capacitor would be to produce a current that is the sum of a portion with a 90 degree phase shift (as all good capacitors do) plus an AC current with zero degree shift. The purpose of the cap is to shift the phase in one of the motor windings, so what you’d end up with is a bit of in-phase current where you only want 90-degree shifted current, and the result is a motor that is fighting itself, in effect. In short, the “DC current” is really an AC current, in phase, and arising from the failing cap’s ability to leak DC current….even though there’s no DC current. Wild guess on my part.
I have a bit of work to do, but then will test the capacitor.
By the way- no, no “bias” on the mains. In the US, power is transmitted and distributed with 3 phases. By the time you reach the pole outside a residence, you are dealing with just one of those phases. It is brought into a transformer with proper winding ratios so that the total voltage across the side of the transformer feeding the residence is 240V. There is a tap in the middle of that side which is bonded to earth. You now have three conductors to drop to the residence one at 0V, one at +120V, and the other at -120V. At the residence, the conductor that was grounded at the pole is grounded again at the residence. You now have two ungrounded conductors and one grounded conductor in the panel. You can either grab the two ungrounded and have 240V or you can grab the grounded and one of the ungrounded and have either +120V or -120V relative to the grounded conductor. If you didn’t ground at the service entrance, you’d have a bias because of impedance between the pole and the residence and, if you look inside the residence some distance away from the panel, there will be a voltage drop across the neutral (outlet vs. panel) just because it has some resistance. So, again you’ll see a “bias”….a couple of volts relative to ground, but only when there is current on the neutral / grounded conductor. This isn’t DC, though and I don’t think I’ve ever heard anyone call it a bias. (Okay, the electricians can jump on me now and tell me where I butchered it 🙂
Thanks for coming back Ed!
Clearly I draw too much out of the “formal” definition of DC bias as being [non-random] deviation from the expected mean of a periodic function (I see a sine-wave in front of me: expecting mean to be zero ± random variation, but observing a mean consistently deviating in one direction from the expected). Suppose it goes to show that one should be careful with extrapolating.
Your description on US power is really helpful. Replacing the gas hob in Boston with an electric one, probably wouldn’t be quite as complicated as I imagined; once again found guilty of extrapolation, this time from the Swedish power system, where there is 400V (16 – 24 amp) 3-phase all the way to the residence’s fuses, at which point the fuse will decide what comes out of the socket: 400V 3-phase for appliances like the hob, 220V 3-phase for some heaters, and 220 V single phase for the rest. A ground fault breaker sits at the centre of the net.
Logic – though mine at time falters – seems to say that as there is no DC in the mains and none created by a faulty capacitor, then the DC has to be that “in phase”.
Looking forward to be quashed by the electricians.
Sven-Olof Jansson
London, UK; Boston, MAI am a bit rusty on motor theory but ill give it a go, learned so much here its the least I can do. 🙂
I’m not really familiar on how its done in the USA but a basic 1 phase motor has a capacitor in it to create a phaseshift.
If you look at the schematic it has the two main wires comming into the motor connected to a coil. There is another connection to a capacitor that feed into another coil that is physiclly at a different angle around the rotor (moving part). What bassicly happens is that the capacitor will delay the current going to the second coil by storing it. A capacitor is like a bucket with a hole in it.
So if you look at my really bad paint drawing 😉 you get your normal coil in the green and than the second coil in red.
There are a type of magnet in the rotor that will try to allign it self with the magnetic field created by the coils causing it to rotate. Since one of the coils is shifted in time the magnetic field will shift position causing the rotor to keep moving. Kinda like a dog chasing its tail.If the capacitor is broken it cant make the phase shift so there isnt any delay, no delay means no rotation.
The motor will still draw power but wont rotate, the power has to go somewhere so it will turn in to heat. The fact that normally there is a cooling fan attached to the rotor it will overheat even quicker cause the fan isnt spinning.I’m gonna agree with the others that probably the cheapest and most logical fix is gonna be to change the capacitor. Try finding someone that has a good knowledge of electric motors to help you out. Maybe ask him to measure the windings of the motor while your at it, isolation does degrade over the years and can cause shorts in the motor.
If your still wondering about the thermal protection, motors these days are usually equipt with Clixons or thermistors. A clixon is basicly a bi-metal switch that will switch once the motor is to hot and resets it self once its cooled of again. An thermistor is a heatsensitve resistor that can be used to measure the temperatuur more accurate.
Another cause of the trips can be a motor protection device (dont know the propper term in english) like a 3RV20 from Siemens. These device are like the circuit breakers in your house but they can be set to the specific motor current. They also have a pretty ingenius mechanical solution to make sure current is being drawn equally from all phases. I do doubt that that thing is in such an “experienced” machine.
Just to make it clear, if you dont know what your doing, dont mess with electricity it can hurt/kill you.
Thanks, Joost. When you say “measure the windings of the motor,” what is being measured? Inductance? DC resistance across the coil? DC resistance between coil and ground / chassis?
Im not sure what its called in English but in Dutch its called meggeren. You use a special device to measure the insulation resistance of the wires. The device puts a high voltage on the wiring and than uses that to measure what the resistance is. Here its usually done with 500V.
Dont forget to disconnect the capacitor if you dont want a firework show.
In house wiring, it is called “megging” a circuit and is done with a device called a “megger.” A friend is a master electrician and I might be able to interest him in the project.
So, the capacitor is 144 uF, but the range on the can is 107-129 uF. I’m surprised that it would fail _high._ It takes more than 5 seconds, almost 10, for the meter to report a result. It shows a moving bar (“working”) and nF for the scale, then autoranges up to the uF scale, then reports 144 uF. I see someone saying that leaky capacitors test slowly and end up being overcharged by the tester and thus can read high (even though the normal failure is to show low capacitance).
Btw, this cap was soldered on, no removable lugs. There was barely enough room to reach in with a soldering iron and it took some shenanigans to get something in there to make sure it was discharged.
I replaced the cap with a Cornell Dubilier 108-130 MFD 125V cap. It seemed better, but then tripped. There aren’t other caps that I see, so it sounds like a failure in the motor itself. I don’t see any point in trying to save a 70 year old motor, but am open to suggestions.
I need to decide if this machine is worth saving. I’d like to, but once the shutoff is solved, I need to address the runout, and the plan for that is to send the quill and spindle to a guy who will rehab them for $150. Looks like a motor is $150. So, that’s $300 to rehab a 14″ floor-stand drill press.
I may have finally figured out this motor overload problem. The motor is a 1/3 HP single phase motor with a Square-D starter switch. I opened the starter switch box and found a table inside that lists full load amps vs. the required heater size. The installed heater is 2x too small. So, I think a previous owner rewired from 240V down to 120V and did not bother to replace the heater in the thermal overload protection. I’ve ordered the proper heater and we’ll see if that fixes it. This drill press was rescued from the trash. I wonder if this is why it was there? The runout is poor, but good enough for a lot of woodworking. I don’t use this press very much, but it is nice to have, especially if it just cost me $15 for a heater and about the same for the capacitor.
(If you are wondering about what a heater is…..The motor needs to be protected from drawing too much current. The overload circuit feeds the motor after passing the current through a little heater coil. More current means more heat. The coil sits next to a device that will trip if it becomes too warm, shutting off the motor when it is drawing too much current. All of this is located in the starter switch, not the motor. People call these thermal overload protectors, but it really doesn’t have to do with sensing heat in the motor. It is sensing current drawn by the motor. The heater is the little coil in the photo.)
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