# How to get 345Vac for field winding rectfier of a DC motor



## Signode (Oct 9, 2016)

Dear Friends;

We have a 90kw DC motor which field winding supply is 310dc 4.5A. To get 310vdc, i need 345vac power supply source to connect with a bridge rectifier. For a permanent solution, we will get a transformer but for a test run of this motor with machine (if it can fulfill our requirement or not), we need a temporarily source of 345vac supply. I have an idea (sorry if it is a stupid idea) to use a frequency inverter's (5kw spare inverter is available) output supply to achieve 310vdc after a bridge rectifier. Is it possible and has anyone done such experiment? 
The DC drive which is already working at said position don't have field supply output. 
If this is not possible then any other solution?


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## inetdog (Apr 13, 2016)

How did you calculate that you need 345V AC to get 310V DC? Are you assuming that the motor current through the rectifier will correspond to the average of the absolute value of the AC?

Sent from my XT1585 using Tapatalk


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## Southeast Power (Jan 18, 2009)

The OP had my interest with the 90kw motor


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## Tonedeaf (Nov 26, 2012)

http://www.variac.com/staco_480_1ph.htm

use an autotranformer


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## John Valdes (May 17, 2007)

Get a DC drive.


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## B-Nabs (Jun 4, 2014)

In Canada 347V L-N is a common utilization voltage (part of a 347Y600 3 phase system). Not that that does you any good. Just saying.


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## JRaef (Mar 23, 2009)

inetdog said:


> How did you calculate that you need 345V AC to get 310V DC? Are you assuming that the motor current through the rectifier will correspond to the average of the absolute value of the AC?
> 
> Sent from my XT1585 using Tapatalk


I agree, your math is not correct.

310VDC from a rectifier will require an AC RMS voltage of 438VAC. 310VDC motors were designed to be run from a 440VAC input (when that existed). When that standard went away, older DC drives would use SCR voltage control on the input to rectify higher AC voltages to the proper DC levels, modern DC drives will accept a much higher input AC voltage and use PWM to control the output voltage to the motor now.


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## Mike_kilroy (Sep 2, 2016)

220vac bridge rectifier to 100mfd capacitor will give you 310 vdc for your test.

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## just the cowboy (Sep 4, 2013)

Be careful. A DC drive will use field lose as part of its control. A DC motor that loses its field voltage will run away and could self destruct.


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## Signode (Oct 9, 2016)

Dear Mr. JRaef;

I am sorry I could not understand about my mistake. I calculated using below mentioned formula (neglected diode voltage drop;

Vdc = Vac X 1.414 X 0.636
310 = 345 X 1.414 X 0.636

We did experiment several time in our lab and we always use this formula to get required DC voltage.


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## JRaef (Mar 23, 2009)

Signode said:


> Dear Mr. JRaef;
> 
> I am sorry I could not understand about my mistake. I calculated using below mentioned formula (neglected diode voltage drop;
> 
> ...


You are using the unfiltered formula for average voltage of pulsating DC from a full wave bridge rectifier. That's not what DC motors are going to want, that will give you a severely pulsing output. If you just put a DC bus cap in there, you end up with the peak V.


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## Mike_kilroy (Sep 2, 2016)

JRaef said:


> You are using the unfiltered formula for average voltage of pulsating DC from a full wave bridge rectifier. That's not what DC motors are going to want, that will give you a severely pulsing output. If you just put a DC bus cap in there, you end up with the peak V.


But isn't that how most SCR DC drives make the field supply today? I do not think there is an capacitor input supply used on most of these? My Bardac DC drives certainly don't have that; we can get the avg DC of a phase for our fields... 

I agree of course the voltage will be humping along at 120hz, but the L in the field will smooth out the current - and isn't THAT what makes the magnetic field anyway? Isn't this why most DC drives with field supplies today just rectify the line? Either 1/2 wave or full wave then dump that right into the field coil?

I have not thought about your vs OP method of figuring avg DC, so won't comment on that difference. 

Again, if he wants about 310vdc he can get it easy enough for testing by simply full wave rectifying 220Vac line and add a capacitor to the output. Instant 310Vdc for his little field.


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## JRaef (Mar 23, 2009)

Mike_kilroy said:


> But isn't that how most SCR DC drives make the field supply today? I do not think there is an capacitor input supply used on most of these? My Bardac DC drives certainly don't have that; we can get the avg DC of a phase for our fields...
> 
> I agree of course the voltage will be humping along at 120hz, but the L in the field will smooth out the current - and isn't THAT what makes the magnetic field anyway? Isn't this why most DC drives with field supplies today just rectify the line? Either 1/2 wave or full wave then dump that right into the field coil?
> 
> ...


Right, and minus the cap, the average voltage on a full wave single phase rectifier is peak x .636 (1/2 wave it is peak x .351 I think). 

So if the L of the field is acting as the filter instead of the cap on cheap small DC drives, I suppose that might be valid. If that's the case though, then I would think you DON'T need to boost up to 345VAC to get the average voltage at 310VDC because the L of the field is going to make the pulsating field average out anyway?

I'm not a DC drive designer, that ship was already sailing when I got in the business, I never bothered to spend much time on it, I'm an AC drive guy.

PS: Looking at a few internal schematics on single phase DC drives, I see you are right, no caps on the field supply portion. Also no step-up transformers either...


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## Mike_kilroy (Sep 2, 2016)

We mostly in agreement 

I am more the drive designer side. With no cap, the L of the field smooths out the current and does nothing for the voltage, so your formula is still correct for the DC voltage; no pump up at all. I think most of the good DC drives, three phase also, still use a simple full wave rectifier off 2 input wires. No caps. Actually our good fancy big and small 3 phase ones use SCRs instead of diodes in the full wave bridge so we can reduce the voltage that way for field weakening and fine tuning the below base speed field current.


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## Signode (Oct 9, 2016)

So lets back to my original question. 
How do I get 345V just to test the motor with load? If successful then I will arrange a transformer 345V.
One reply is to connect a capacitor after a bridge while 220VAC connect at bridge input. Other idea?


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## just the cowboy (Sep 4, 2013)

*Get a new drive !!!!!!* As I said in a prior post

"Be careful. A DC drive will use field lose as part of its control. A DC motor that loses its field voltage will run away and could self destruct." 

You talk about TEMP for test, THEN say get transformer later. Do not do it this way it is a safety hazard!!!!


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