# PM30 Conversion Video - Control Panel



## AdamDee (Mar 17, 2020)

The start of a PM30MV conversion. A look at electrical and some of the issues I’m having.

Toroidal Transformer Details:
primary winding (220V)
Secondary winding (60V)
Power: 900W
220V Fuses: ATQR15A (15 Amp Fuse)


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## mksj (Mar 17, 2020)

High inrush current from the transformer, I would suggest either switching to a 15A dual pole breaker with a D curve or trying a different brand of slow blow 15A fuses and/or consider 20A. I have a number of amps that have very large transformers with DC capacitor banks and use a thermistor to limit the current inrush. The lights still dim when turned on. Problem is they need to be sized correctly for your operating current.

Nice looking setup.


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## ahazi (Mar 17, 2020)

Many years ago on a larger power supply for a very high power (3-4 kilowatt) ham radio amplifier I faced the same problem. I solved it by adding a series resistor to limit the initial inrush current and a relay that shorts the resistor out after the initial high current drops.

Here is a diagram that uses DC relay.



I used a 220v AC relay so I did not need the diodes and the capacitor. You can probably use a 25 ohm resistor, use a small metal high wattage (25-50 watt) type resistor. You will get a delay of 2-4 seconds. 

16 amp fuse/breaker is way too big for a 900 watt transformer, it is an indication that something is wrong. Toroidal transformers are known to be able to pass high current for a short time.

Ariel


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## AdamDee (Mar 18, 2020)

Wow thank you for this detailed explanation! This might be the move. Have you heard of people using NTC thermistors in series with the primary coil to achieve this same kind of inrush protection?

ive got a 15A fuse on the 60VAC side, so I figured going to a 15A fuse on the 220VAC side wouldn’t be too risky.... but not 100% sure if my thinking is right on that one.


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## AdamDee (Mar 18, 2020)

mksj said:


> High inrush current from the transformer, I would suggest either switching to a 15A dual pole breaker with a D curve or trying a different brand of slow blow 15A fuses and/or consider 20A. I have a number of amps that have very large transformers with DC capacitor banks and use a thermistor to limit the current inrush. The lights still dim when turned on. Problem is they need to be sized correctly for your operating current.
> 
> Nice looking setup.


Thanks for the reply! I Was on Amerherm’s website yesterday, looking at sizing a thermistor. I struggled there because one of the inputs to their “thermistor selection calculator” was the capacitance of the circuit you’re trying to protect, and I’ve got no idea... I don’t have any capacitors on the circuit right now. I’ve got the 60VAC side disconnected from the drivers. It is an open circuit right now while I’m trying to get this all dialed in.


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## mksj (Mar 18, 2020)

Breakers protect the wiring, to a lesser degree the components, basically to mitigate short circuit situations with wire failure/short. Even small power (60-480W) DC supplies specify a minimum  of a 16A breaker, when fusing. If you look at something like VFDs cartridge fuses are often rated at 1.5-2X the rated input, a breaker is specified as 125-150% and that is with a soft start and an in-line resistor on power up. Alternative you could use a time delay relay that would short out an inline resistor as noted above, you can get programmable time relays and using something like 3 seconds. These may also be required if you are charging a large bank of capacitors after the transformer.

I would also look at specific fuses designed for high in-rush currents such as the LittleFuse KLDR, I still would recommend a 15-20A fuse with an internal transformer supply wiring of a minimum of 14AWG 90C rated.


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## ahazi (Mar 18, 2020)

AdamDee said:


> Have you heard of people using NTC thermistors in series with the primary coil to achieve this same kind of inrush protection?


Yes, but they have an inherent limitation of adding some resistance and power dissipation even in steady state. I don't like it in high current circuits as it increases the chance of failure and you have to make sure that the steady state power dissipation of the thermistor is taken care of (sometimes it is glued to the chassis/heatsink with non-conductive thermal epoxy to help dissipate the heat.)



AdamDee said:


> ive got a 15A fuse on the 60VAC side, so I figured going to a 15A fuse on the 220VAC side wouldn’t be too risky.... but not 100% sure if my thinking is right on that one.


15 amp x 230 volt = 3,450 Watt. Too big to protect the circuits/transformer in steady state. This is a good reasonable size for electrical panel breaker to protect #14 wires in the wall.

The circuit with the relay (use an AC relay, no diodes or capacitors needed) is simple and effective. It will kick in only after the capacitors on the other side are charged (or the transformer inrush current is reduced). It is a closed loop circuit, no timers to rely on or adjust.

Ariel


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## Be_Zero_Be (Mar 18, 2020)

I don't believe the transformer is the problem.
An inductor (i.e. transformer) does not have a high inrush current.
An inductor opposes a change in current.
This is why they are used in power supplies for filtering a DC voltage.
The initial impedance of an inductor is infinite and the initial current is zero.
Look at the current charge curve for an inductor.


The charge curve for a capacitor is the opposite of an inductor.
I would believe that the issue is something else.


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## macardoso (Mar 18, 2020)

ahazi said:


> Many years ago on a larger power supply for a very high power (3-4 kilowatt) ham radio amplifier I faced the same problem. I solved it by adding a series resistor to limit the initial inrush current and a relay that shorts the resistor out after the initial high current drops.



Interestingly enough, this is identical to how most AC motor drives and servo drives work. There is a "precharge resistor" which limits the inrush to the drive, mainly to protect the bus capacitors from hard cycling. Once the DC bus has charged, the resistor is bypassed by a relay. There is a real reason to avoid rapidly power cycling drives which is to prevent thermal damage to this resistor.


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## AdamDee (Mar 18, 2020)

Be_Zero_Be said:


> I don't believe the transformer is the problem.
> An inductor (i.e. transformer) does not have a high inrush current.
> An inductor opposes a change in current.
> This is why they are used in power supplies for filtering a DC voltage.
> ...


Thanks for this educational post! You're obviously much better with this stuff than I am. I can't speak for all transformers, but I have seen from many different sources that toroidal transformers specifically are bad for high inrush currents.


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## macardoso (Mar 18, 2020)

I got side tracked from the original post, but first off, nice wiring! Looks clean which is not a feat that many do well.

Take care with grounding and bonding. You may also wish to consider a perforated and grounded metal cage around that power supply. Toroidal transformers can be pretty noisy and I wouldn't want that right next to my 5V electronics. 

I'm a big fan of connectors at the panel wall. Bit of extra effort to add, but makes dealing with the control panel much easier once installed!


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## ahazi (Mar 18, 2020)

It is *usually* the large capacitors that get charged through a bridge rectifier from the secondary of the transformer. It easy to validate, just disconnect the secondary as unloaded transformer act as an inductor.

Ariel


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## AdamDee (Mar 18, 2020)

ahazi said:


> Many years ago on a larger power supply for a very high power (3-4 kilowatt) ham radio amplifier I faced the same problem. I solved it by adding a series resistor to limit the initial inrush current and a relay that shorts the resistor out after the initial high current drops.
> 
> Here is a diagram that uses DC relay.
> View attachment 317140
> ...


Ariel I've taken you advice and sourced an AC relay and resistor. If I understand your DC relay circuit right right, this is how I would wire up the AC equivalent? Photos attached.


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## mksj (Mar 18, 2020)

Transformers have very high inrush currents went first energized, independent of the output. An inductor once powered is used to smooth the power pulses drawn and helps decrease the THD.








						Transformer Inrush Current | Ametherm
					

Inrush Current Limiting thermistors can be used to protect a transformer at turn on for high inrush current.




					www.ametherm.com
				



Inrush Current » Transformer Inrush Current Protection
A transformer draws inrush current that can exceed saturation current at power up.
The Inrush Current affects the magnetic property of the core.
This happens even if the transformer has no load with its secondary open.
The magnitude of the inrush current depends on the point on the AC wave the transformer is switched on.
If turn-on  occurs when the AC voltage wave is at its peak value, there will be no inrush current drawn by the transformer. The magnitude of the current in this case will be at normal no load value.
If at turn-on, the AC wave is going through its zero value, then the current drawn will be very high and exceed the saturation current.


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## AdamDee (Mar 18, 2020)

AdamDee said:


> Ariel I've taken you advice and sourced an AC relay and resistor. If I understand your DC relay circuit right right, this is how I would wire up the AC equivalent? Photos attached.


Ahh Sorry! - I'm realizing my wiringisn't quite right.... I'll take another stab at this when I get home tonight!


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## AdamDee (Mar 18, 2020)

AdamDee said:


> realizing my wirin





mksj said:


> Transformers have very high inrush currents went first energized, independent of the output. An inductor once powered is used to smooth the power pulses drawn and helps decrease the THD.
> 
> 
> 
> ...


Very Informative!


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## AdamDee (Mar 18, 2020)

ahazi said:


> Many years ago on a larger power supply for a very high power (3-4 kilowatt) ham radio amplifier I faced the same problem. I solved it by adding a series resistor to limit the initial inrush current and a relay that shorts the resistor out after the initial high current drops.
> 
> Here is a diagram that uses DC relay.
> View attachment 317140
> ...


Sorry for the mixup.... realized my previous wiring was incorrect.... If I've followed your wiring diagram right, I'd wire it up something like this, no?


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## ahazi (Mar 18, 2020)

AdamDee said:


> Sorry for the mixup.... realized my previous wiring was incorrect.... If I've followed your wiring diagram right, I'd wire it up something like this, no?


Almost...

Pin "0" on the relay should go to "L2 120 VAC"

I assume that the relay that you are getting is N.O. (normally open) with 220 volt ac coil.

You will also get a reassuring delayed click with the relay.

Ariel


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## ahazi (Mar 18, 2020)

mksj said:


> Transformers have very high inrush currents went first energized, independent of the output. An inductor once powered is used to smooth the power pulses drawn and helps decrease the THD.
> 
> 
> 
> ...


This is 100% correct and especially true with big toroidal transformers. I just prsonally don't like thermistors and varistors.


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## AdamDee (Mar 18, 2020)

ahazi said:


> Almost...
> 
> Pin "0" on the relay should go to "L2 120 VAC"
> 
> ...


Like this? I've also added in the primary coil fuses...

Edit: Yes, it is N.O. 220VAC relay


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## AdamDee (Mar 18, 2020)

macardoso said:


> I got side tracked from the original post, but first off, nice wiring! Looks clean which is not a feat that many do well.
> 
> Take care with grounding and bonding. You may also wish to consider a perforated and grounded metal cage around that power supply. Toroidal transformers can be pretty noisy and I wouldn't want that right next to my 5V electronics.
> 
> I'm a big fan of connectors at the panel wall. Bit of extra effort to add, but makes dealing with the control panel much easier once installed!


Thanks for the advice! Noise is something I was wondering about - that is, wondering if I should be worried about. What's more is that the motor/driver cables I've got aren't shielded... hoping this doesn't bite me too hard.


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## ahazi (Mar 18, 2020)

AdamDee said:


> Like this? I've also added in the primary coil fuses...
> 
> Edit: Yes, it is N.O. 220VAC relay


Yes.

If you put the relay coil (pins 0 and 1) after the fuses, you will not hear a relay click if one of the fuses is blown or missing so it will serve as an audible indicator.


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## AdamDee (Mar 18, 2020)

ahazi said:


> Yes.
> 
> If you put the relay coil (pins 0 and 1) after the fuses, you will not hear a relay click if one of the fuses is blown or missing so it will serve as an audible indicator.


Ok! Thank you so much for your help! I'll be wiring this up tonight and testing. Stay tuned!


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## AdamDee (Mar 18, 2020)

Thanks very much for all your help, guys! Especially Ariel/ahazi   Everything works great. No more blowing fuses...... for now....

At some point in this video I call the secondary coil the "60 Amp" side - sorry. Correction: "60 Volt" side.

Next - moving on to bench testing Centriod Acorn/closed loop stepper motor bumping (hopefully).


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## ahazi (Mar 19, 2020)

I am glad it is working as I expected.

The reason that there is almost no delay is a proof that it wasn't the secondary that was loaded with capacitive load that takes time to charge through the 25 ohm resistor. It was the transformer inrush current that was very high when you turned it on around the "zero crossing" of the sine wave input (just as described in the posting from Mark and Be_Zero_Be.) It is a small delay (milliseconds) but enough to stop the circuit breaker from tripping. You can validate that it is working by measuring the voltage drop across the resistor, it should be 0.0 volt (be careful it is the high voltage side, don't touch it.)

I saw another posting about noise, not sure if it referred to acoustic or electrical noise. Electrical noise/magnetic radiation from toroid transformers is the lowest compared to traditional "E I core" transformers as all the magnetic field goes through the toroid core. Acoustic noise is dependent on vacuum impregnation of the coils and the transformer load and I would not be worried about it.

Good luck with  your project.

Ariel


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## macardoso (Mar 19, 2020)

AdamDee said:


> Thanks for the advice! Noise is something I was wondering about - that is, wondering if I should be worried about. What's more is that the motor/driver cables I've got aren't shielded... hoping this doesn't bite me too hard.



Many people don't have issues, but I have in the past. It is very difficult to troubleshoot noise problems, especially once things are built, so I typically go all out at the beginning of my builds.

Stepper motors aren't quite as noisy as say a motor on a VFD, but they still can throw a lot of noise. Unshielded cables act as antennas and radiate outwards. They shouldn't be run near any 5V wiring.

EDIT: I wouldn't worry too much about stuff outside your metal electrical enclosure, the noise won't penetrate to any significant amount.


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