MOT's sharing voltage  - Page 1
Forum section: electronics / projects

author: davelectronic
author_rank: Frequent Contributor
timestamp: November 28, 2023, 01:32:06 pm
content: I have recently acquired three MOT transformers, I have used them in pairs in series in the past with good results in PSU projects. What I've come up against is the transformers sharing the AC voltage equally. I have two large frame transformers, and one smaller frame transformer. They are all from 800 watt 240 Volts microwave ovens. The physically smaller transformer will share voltage with either of the bigger frame transformers, but the two larger frame transformers won't share voltage very evenly. The two large frame transformers, one will run at 104 Volts AC the other 152 Volts AC the primary windings are in series. I wanted to use the two larger transformers, as they can hold the most secondary windings for a higher secondary voltage. Testing each large transformer, individually, one draws 4 Amps AC the other large transformer draws 8 Amps AC. But the smaller transformer runs stand alone at 8 Amps AC and will run equally with either large transformer. But the two larger transformers won't share evenly. So I wandered if there is a way to get them to share the voltage more evenly. Thoughts appreciated, thanks for reading.

author: NiHaoMike
author_rank: Super Contributor
timestamp: November 28, 2023, 01:46:21 pm
content: They'll balance out once you apply load. Also consider using a SSR to only switch on the primary when needed, for example in an amateur radio transmitter, it only needs to be switched on while transmitting.

author: CaptDon
author_rank: Super Contributor
timestamp: November 28, 2023, 03:21:06 pm
content: What are you doing with the secondaries? Since one side of the secondary is grounded to the transformer frame I guess you are running the secondaries in parallel? Perhaps a single diode half bridge from each transformer to a common D.C. high voltage filter? Why running the primaries in series? Just to keep the transformers out of saturation? Give us some more info about how you are using these transformers.

author: mikerj
author_rank: Super Contributor
timestamp: November 28, 2023, 04:08:17 pm
content: Quote from: CaptDon on November 28, 2023, 03:21:06 pmOP has strongly implied that the secondaries will be rewound. That said they are not the best candidates for PSUs etc. due to the very high magnetising currents which create heat and often noise.

author: CaptDon
author_rank: Super Contributor
timestamp: November 28, 2023, 05:54:55 pm
content: I wonder if the O.P. has knocked the pair of magnetic shunts out of the transformers yet? That will certainly change things!! I have a M.O.T. creating high voltage for a ham radio amplifier. I removed the shunts and added about 20 turns to the primary to get down out of saturation. I also needed to remove some secondary turns to drop the voltage after rectification and filtering to about 2200vdc. I was very lucky with this particular transformer as the start of secondary winding came from a well insulated area physically and I was able to remove the connection to the transformer body and bring it to a full wave bridge. The one drawback (of course) is that the secondary has a good deal of resistance, perhaps 500 ohms, and therefore has very poor regulation between full load and no load. I have a huge filter bank and the poor regulation is hardly noticeable in SSB mode but it sucks in CW mode If I am sending code at a slow rate in the novice portions of the bands. The tube is an 8930 which is a 4CX250B with a BIG plate radiator. It makes an easy 400 watts of output CW continuous key down with 10 watts of drive, the PEP on sideband is closer to around 600 watts.

author: davelectronic
author_rank: Frequent Contributor
timestamp: November 28, 2023, 06:15:02 pm
content: Thank you for your replies, yes a will rewind the secondary windings with AWG 12 cable. There for a 15 Amp to 20 Amp power supply. I like the idea of using a SSR to turn on when demand for power is needed. And yes I do knock out the shunts. I have an AC DC clamp meter that can do high current reading. When I measure the idle current of the primary windings in series, it's only reading 0.260 mA of current. Considering what they run at as stand alone transformers, two transformers in series run quite cool. So they are running at half the rated voltage, and the rewound secondarys will be run in series to keep the voltage up, due to sag under load. I just wandered why the two larger frame transformers won't equally share the input voltage, approx 240 AC UK mains. I couldn't run them as single stand alone transformers, like you say, that's flat out and generates silly amounts of heat, and need forced air cooling. But two transformers in series is exceptable, although you do have to get as many secondary turns on the transformers as possible. There is a significant secondary voltage drop when the transformers are loaded. I just couldn't understand why they won't share the supply voltage.

author: Gyro
author_rank: Super Contributor
timestamp: November 28, 2023, 07:24:56 pm
content: Make sure you don't use zero crossing type SSRs, they will cause the transformers to saturate and draw huge current spikes at every turn-on. Ideally you would use random switching SSRs, monitor the mains waveforn and time them to switch at somewhere approaching mains peak. Using random switching SSRs on their own would at least prevent saturation oneveryturn-on.

author: davelectronic
author_rank: Frequent Contributor
timestamp: November 28, 2023, 08:17:08 pm
content: Yes, I would have to do some homework on demand switch on first, like inrush current, and time delay that might happen. So although they are at different voltages with no load, under load the transformers will share equally ? Is that right. I know the two big frame transformers, one primary draws 8 Amps stand alone, and the other draws 4 Amps stand alone. Is this why they won't share when they are idle. I measure each primary windings resistance, one is 2.9 ohms the other is 2.0 ohms resistance. The higher resistance primary is the one that draws 152 Volts AC and the 2.0 ohm resistance primary on the other transformer runs at 104 Volts AC.

author: NiHaoMike
author_rank: Super Contributor
timestamp: November 28, 2023, 11:57:31 pm
content: What output voltage are you going for? Seems like a switching supply would make more sense nowadays, especially if you want a multiple of 12V which can be obtained using PC or server PSUs.

author: davelectronic
author_rank: Frequent Contributor
timestamp: November 29, 2023, 08:38:58 pm
content: The voltage I'm looking for, final regulation is 12.00 Volts to 13.80 Volts. It's only for powering radio equipment. I wouldn't know where to start with a switch mode PSU project, I think it might be beyond my ability. Mind you, there was a time I thought building a linear PSU wasn't doable, but I've managed it. So it might not be impossible. I like the MOT as it's something for nothing, but two MOT transformers do weigh a lot, but I do have very sturdy steel cases available.

author: langwadt
author_rank: Super Contributor
timestamp: November 29, 2023, 08:49:36 pm
content: Quote from: davelectronic on November 29, 2023, 08:38:58 pmsell them as scrap and buy something like this,https://www.ebay.com/itm/164018856168

author: CaptDon
author_rank: Super Contributor
timestamp: November 29, 2023, 11:40:50 pm
content: Your pair of M.O.T.'s wound with 12 ga. on the secondaries will have a fault current of 30+ amps and could probably push 20 amps through a proper voltage regulation circuit continuously. If done correctly you could power a 100 watt 12vdc operated ham transmitter easily to full capacity. With a hefty diode bridge with heatsink and a big variac on the input side you could have a 20+ amp battery charger that could probably do a 50 amp boost during cranking. Each of those M.O.T.'s could contribute about 500 watts to the total output if everything downstream is up to the task. M.O.T.'s are often rigged up with rewound secondaries for arc welding use!!

author: davelectronic
author_rank: Frequent Contributor
timestamp: November 30, 2023, 11:25:54 pm
content: There's no scrap money in MOT transformers, the windings are aluminium. When you say fault current, do you mean short circuit current. I've had to climb down from 12 AWG cable, as can't get enough turns on the core to get the voltage up. So using 14 AWG wire. I use silicon sheathed cable, it's very easy to wind a core with it, and it supposedly rated temperature is 200°C not that it would ever see that. So it's looking like 15 Amps continuous, with possibly short burst of 20 Amps. Nothing I run goes over 100 watts anyway. As I'm in a tower block, I have to be mindful of running RF power. Shame the transformer cores can't hold enough 12 AWG cable. It can if I use the MOT as a single unit, but idling at 8 Amps is ridiculous. So that's why I run a pair of them in series, which run cooler, but at a lower final voltage.

author: davelectronic
author_rank: Frequent Contributor
timestamp: December 02, 2023, 07:32:32 pm
content: I thought I would leave a conclusion. So with two MOT's rewound secondary windings with 14 AWG cable (silicone insulation) The pair powered up idle, don't share the 240 Volt AC supply evenly. One transformer runs at 105 Volts AC the other transformer runs at 152 Volts AC. Both transformers where in series configuration for the primary input, and the secondary output. The secondary voltage was 19.80 Volts AC, then I connected up four 12 Volt 50 watt halogen lamps for a load. The load was run for 30 minutes, the voltage under load was 16.80 Volts AC, and the load was drawing 19.70 Amps AC. With this load, the primary current was 1.7 Amps AC as measured with my clamp meter. I did think it might pop the halogen lamps, but it didn't. It was at the maximum of the clamp meters limit on current. The primary windings barley got warm, the secondary windings got to approximately 52 °C Which for a continuous 30 minutes, I didn't think was bad. I won't be drawing 20 Amps when there put to use, 15 Amps will be the limit. So the transformers are something for nothing, oh, only the cost of the cable to rewind the secondary windings. The only downside is two MOT transformers weigh a lot compared to the equivalent toroidal transformer. But if the weight is not a problem, I think they where worth rewinding. It's a shame 12 AWG wouldn't allow enough secondary turns to get the voltage up. You really shouldn't run MOT's singly, as they draw to much primary current, and get far to hot, unless you use forced air cooling. Although I wouldn't run them as stand alone transformers, as it's just to wasteful of power. But running a pair in series works out ok. Thanks for all your replies.

author: johansen
author_rank: Super Contributor
timestamp: December 03, 2023, 01:39:58 am
content: Dont run mystery transformers in series series. One will run more saturated than the other.Primaries in parallel or secondaries, but not both in series. Both in parallel can work if they have exactly the same turn ratio, which can be a pita to add turns to the primary of an MOT.

author: davelectronic
author_rank: Frequent Contributor
timestamp: December 03, 2023, 01:46:55 pm
content: They run fine in series, even if they do run a different voltage. The temperature of both transformers is the same, even after an hour of drawing 20 Amps. I did run them for a longer time, they got to 67°C with just passive cooling. I'm not going to run them at 20 Amps in the power supply there intended for. I need 8 Amps to 12 Amps DC, for the radio equipment in use FM draws 8 Amps and SSB 12 Amps, well that's the measured current of the radio equipment in use. Standby is a few hundred milliamps. As I said you can't really run them alone or in parallel, they waste to much power, and get far to hot in use. You could with a howling gale of air passing over them, but that's pointless wasting that much energy.

author: Terry Bites
author_rank: Super Contributor
timestamp: December 04, 2023, 06:22:49 pm
content: I'll call the undertaker now....

author: davelectronic
author_rank: Frequent Contributor
timestamp: December 05, 2023, 10:00:09 pm
content: No need lol, I've used MOT's before. But they shared the voltage in series fairly evenly. These two transformers won't share evenly. The high voltage windings are gone, replaced with silicone insulation cable. Run them on AC with a 200 watt load approximately, they seem fine, no over heating or other problems. I was surprised the voltage drop with a 200 watt load was so low, about 2.9 Volts or less. Considering there running at half there rated voltage, approximately ish, I was expecting a bigger secondary voltage drop.

author: johansen
author_rank: Super Contributor
timestamp: December 06, 2023, 02:45:00 am
content: Quote from: davelectronic on December 05, 2023, 10:00:09 pmah, half the voltage. that explains the miss match.because all the E's are on one side and the I's on the other, you can have a variable air gap from one transformer to another. if you plug each one in separately to 120vac or 240vac depending on your situation, you will likely see significant differences in one transformer to another in the no load amps. 4-8 amps is typical for american 120v large microwave ovens

author: davelectronic
author_rank: Frequent Contributor
timestamp: December 06, 2023, 12:44:02 pm
content: Yes this is true, on 240 Volts UK mains AC, one transformer draws 8 Amps, the other draws 4 Amps. But running them at full rated voltage wastes to much power, and the heat is just to high. It's not easy to add primary windings to the transformers, so the next best thing is to run them in series, both primary and secondary windings. The secondary windings have been rewound with 14 AWG silicone cable. I know the maximum current is going to be around 15 Amps continuous, but I have pushed them to 20 Amps. The temperature got up to 67° C after 1 hour. One transformer is running at 152 Volts AC the other is Running at 104 Volts AC. Idle primary current is 260 mA, and with a 20 Amp load the primary current is 1.7 Amps, which is quite high. But I only need 8 Amps to 12 Amps with a 50% duty cycle. So the transformers are usable, with the added burden of the extra physical weight. But it would be a static power supply, so the weight isn't to much of a problem. I'm using a 7812 1 Amp Voltage regulator, and 6 x TIP36C BJT PNP transistors for the current. I've built similar circuits like this before, with good results.

author: NiHaoMike
author_rank: Super Contributor
timestamp: December 06, 2023, 01:40:45 pm
content: Might it be possible to cut the input voltage with thyristors the way Carver did back in the day?

author: CaptDon
author_rank: Super Contributor
timestamp: December 06, 2023, 02:32:30 pm
content: The O.P. probably could reduce the primary voltage with a control circuit and run the primaries in parallel, but he needs the physical space of two transformers to get enough turns of heavy gauge wire on the cores to produce the needed secondary voltage and running the primaries in series was an easy answer. As to why two 'identical' transformers show such imbalance all I can think of is the silicon content of one core is different from the other. Perhaps the transformers were made to the same spec and part number by two different contract manufacturers and the steel core mix is of a different blend. The O.P. mentioned 'aluminum wire' on the transformers? We scrapped dozens of 'not worth repair' microwaves at my father's repair shop and I don't recall aluminum wire on a M.O.T. ever. I modified many of those transformers for friends and they were all wound with copper wire.

author: davelectronic
author_rank: Frequent Contributor
timestamp: December 06, 2023, 11:10:05 pm
content: I wouldn't know how to utilise thyristors to reduce the input voltage, I did think something like a triac dimmer type switch. But it's an inductive load, and there designed for resistive loads. All retail microwave oven transformers are using aluminium wire now. I have come across the odd copper wound variant, but its rare, there in the older microwave ovens. Or maybe commercial ovens. The secondary windings are aluminium, I lightly rubbed the insulation off an inconspicuous area of one transformer, and indeed it's aluminium wire. It's cheap for them to produce compared to copper windings. I'm only using them as it's something different in a power supply. I do have a lot of toroidal transformer power supply projects in various states of progress, some nearly finished, some just started.

author: johansen
author_rank: Super Contributor
timestamp: December 09, 2023, 08:22:39 pm
content: Thyristers to reduce the voltage is a dumb idea. And you need a lot of inductance to make them work with a rectifier and capacitor filter. For less losses and less copper and iron than the inductor will cost, you can add more turns to the primary to bring the transformer losses down.

author: NiHaoMike
author_rank: Super Contributor
timestamp: December 10, 2023, 04:19:21 am
content: Quote from: johansen on December 09, 2023, 08:22:39 pmCarver didn't use a separate inductor, I assume the transformer has enough leakage inductance for that to work.

author: CaptDon
author_rank: Super Contributor
timestamp: December 10, 2023, 02:11:46 pm
content: Indeed, the older carver M400 type amplifiers had a very conventional power transformer driving a very conventional rectifier and big filter bank, nothing special there. Part of the magic was that the transformer for 120VAC use was wound so that around 90VAC would give the proper voltages at light load and 120VAC would produce fully loaded full output power so the triac was always seeing 80% and up of conduction cycle. I think the triac was also of the 4-quadrant variety. I worked on a few M400 units, they were very rugged in fact. My main Carver repairs are for the PM-2.0T with the 1KHz SMPS. That transformer looks like a 75VA 60Hz control transformer but it will do over a kilowatt at 1KHz all day long. Sadly the control circuit for the PM-2.0T and CBA-1000 series isn't tolerant of poor A.C. input power and things go bad from excessive switching current when the line voltage is low and from excessive voltage spikes across the switch transistors when the line voltage is high. These amplifiers tended to self destruct when used on 'county fair' generator circuits. If you do the math, with all things being equal a 75VA 60Hz core size transformer should support 1250VA at 1KHz but in reality with the additional losses about 1000VA is more realistic. Could be why the '1250 watt' home consumer PM-1250 and PM-2.0T were down rated to 1000 watts for the commercial versions sold to Clair Brothers as the model CBA-1000 with three variants of that model.

author: johansen
author_rank: Super Contributor
timestamp: December 10, 2023, 03:48:38 pm
content: Quote from: NiHaoMike on December 10, 2023, 04:19:21 amThe endless "always an exception" comments are relatively useless.

author: CaptDon
author_rank: Super Contributor
timestamp: December 10, 2023, 06:59:43 pm
content: Indeed, since the O.P. already found a solution.