pulse width modulation dc motor

[AMAECHI]

i am working on a project to pwer a 5kVA dc motor with pulse width modulation(PWM). i need help urgently pls.thnx.

 

[Styx]

5KVA? take it since you mention VA and not watts that the main power comes from the AC mains network?

Also this a brushed DC machine?

if so then your best bet is a thyristor chopper to vary a DC-link (speed ~ voltage)

PWM is imployed in the thyristor rectifier.


Such a method is the most convinient.


However, the other way is straight rectification to a DC-link, then PWM a H-bridge to vary the voltage applied the machine

IF you can clarify what cheme best meets yr needs then I am sure help can be provided

 

[hjl4]

I am also confused here.

5 thousand volt /amp??

He does say DC.

One major project on the way here.

But yes, Styx pretty much summed it up.

Either way........ Expensive.

 

[Russlk]

What is the motor voltage? What kind of speed control is needed?

 

[Styx]

I am also confused here.

5 thousand volt /amp??

He does say DC.

One major project on the way here.

But yes, Styx pretty much summed it up.

Either way........ Expensive.

he says a DC machine, but 5KVA is a fair amount of power so he must be getting it from the mains and then rectifying it to DC to drive the machine.

 

[Oznog]

What is the power source? What's the voltage like? What's loading this motor?

 

[hjl4]

Were all speculating here, so maybe we'd better wait and see if AMAECHI
will give us more details. Until then I prefer to stay out of this one.

BTW, anybody know what kind of machine a 5000 Volt, dc motor could be running? Seems rather large.

 

[hjl4]

Actually, ignore my previous reply.

Here is what I have found.


3000Hp DC DRIVE

The drive system will consist of two - 2400 ADC / 750 VDC six pulse drive cabinet sections. The power converter SCR heat sink bridges are manufactured by Joliet Technologies and are fitted with Siemens High Horsepower command modules (6RA70). Siemens control and firing technology is applied for superior operation of SCR gating, current, speed and monitoring of motor drive. Each of the 2400 ADC sections work together by load sharing the high current demand. The drives will be designated as Master / Slave. By supplying each section with a single isolation transformer with two outputs Delta & Wye, we can minimize the harmonic distortions caused by SCR type converter drive systems. The overload current limit rateing is 150% for 1 min. every 10 min.

The following features are included:

Two - two door, modified Nema 12 cabinets, Mild steel construction measuring (90"x78"x30"). Paint is factory standard, ANSI 61 Gray.
Two - 2500 Amp. Molded Case Switches mounted w/ through the door manual switch handle (pad lockable).
Two - 2400 ADC 1-Quadrant SCR bridge assemblies.
Two Sets - SCR AC line fusing.
Two - cabinet air cooling and filtration packages.
Two - Siemens High HP command modules.
One - 85 ADC/single phase, Siemens field module and transformer.
Customer supplied, encoder feedback speed control as per customer drawings.
One main motor blower circuit 20 HP @ 600V Max.
Three digital meters for the following: Armature Amps, Armature Volts and Field Amps. Meters are mounted in the Master drive cabinet door.
Siemens door mounted programming and fault indication keypad.
Door mounted Hand/Off/Auto selector switch.
Copper pad bus areas for incoming and out going AC/DC power connections.
Note: All above electrical construction is assembled to meet UL and IEEE standards.

(price good thru December 2005) Price $88,725.00

OPTIONAL DC LINK T-REACTOR

DC T-Reactor will be connected in the DC link between the two converters of differing phase to limit circulating currents due to instantaneous voltage differences. The reactor will be built in a separate Nema 1 enclosure.

Specification:

2 Reactors @ .5335 milli. Henrys / 2000 Amp
1 - Nema 1 enclosure
4000 Amp. Center tap
Copper windings, 150 Deg. C rise, 220 Deg. C Insulation
(price good thru December 2005) Price $6,615.00

OPTIONAL ISOLATION TRANSFORMER

5000 KVA with the following specifications:

Primary Voltage 25 KV, 60 Hertz, 3 phase Delta, 5000 KVA
Secondary Voltage 600V Delta, 2500 KVA and 600/347 Wye, 2500 KVA
Aluminum windings, 150 Deg. C rise, 220 Deg. C Insulation
K-Factor 4
Nema 1 enclosure floor mount
(price good thru December 2005) Price $77,595.00

 

[Oznog]

Good god... you have a monster there. What on earth do you want to do with it?? Pull a train?

So you have a 750VDC, 2.4KA supply and a motor but no idea how to connect them?

Usually these motors and fields are PWM controlled. This voltage is generally too high for MOSFET switches so most likely we'd be looking at IGBTs.

But can the power source itself be throttled up and down by adjusting the phase angle of the SCRs? That might make a lot more sense for an AC-to-DC supply.

I'm going to go out on a limb and say this is likely beyond the experience of anyone here. And we don't know the best way to manage armature vs field for that particular motor at the type of load you've might be using. I suggest you talk to the mfg of the equipment on how to drive it.

Even if we know the exact components and schematics of the way to do this, making 2.4KA interconnects and thermally mounting devices at this power level is a major, major design and mfg task. Normal PCB traces can't be used to handle current of this magnitude either. Even 1/0 cable, which is huge, has a capacity under 300 amps.

 

[Styx]

I'm going to go out on a limb and say this is likely beyond the experience of anyone here. And we don't know the best way to manage armature vs field for that particular motor at the type of load you've might be using. I suggest you talk to the mfg of the equipment on how to drive it.

*cough* not me, thats my job *cough*

Plus anyway where has this 5000V come from?

He said 5kVA ? The fact he is talking in VA and not WATTS is implying that he is taking power from an AC network

So for a unity power facor (it wont be) that is 5kW which is tiny.

Even if it was 5000V, that is only 1A of AC current.

For the UK mains
3ph power: => AC current of 4.1A

single phase power => AC current of 21A

I dont see the problem, this is small fry. I am more concerned with the use of a brushed DC


As it has already been stated we need more info

 

[Oznog]

*cough* not me, thats my job *cough*

Ah, he's in luck then!

He said 5kVA ? The fact he is talking in VA and not WATTS is implying that he is taking power from an AC network

So for a unity power facor (it wont be) that is 5kW which is tiny.

Even if it was 5000V, that is only 1A of AC current.

For the UK mains
3ph power: => AC current of 4.1A

single phase power => AC current of 21A

I dont see the problem, this is small fry. I am more concerned with the use of a brushed DC

He said "5000 KVA", not "5KVA". As in 5MVA! This is consistent with the scale of a 3K HP motor and a 2.4KA/750VDC power supply. So I don't think it's a typo. The scale's 1e+3 bigger than you think.

 

[Nigel Goodwin]

He said "5000 KVA", not "5KVA". As in 5MVA!

Where did you get that figure from?, the question clearly states:

5kVA dc motor

 

[Styx]

*cough* not me, thats my job *cough*

Ah, he's in luck then!

I'll try but I am not going to give him an inverter just like that

He said 5kVA ? The fact he is talking in VA and not WATTS is implying that he is taking power from an AC network

So for a unity power facor (it wont be) that is 5kW which is tiny.

Even if it was 5000V, that is only 1A of AC current.

For the UK mains
3ph power: => AC current of 4.1A

single phase power => AC current of 21A

I dont see the problem, this is small fry. I am more concerned with the use of a brushed DC

He said "5000 KVA", not "5KVA". As in 5MVA! This is consistent with the scale of a 3K HP motor and a 2.4KA/750VDC power supply. So I don't think it's a typo. The scale's 1e+3 bigger than you think.

please go back and re-read his post.

 

[JimB]

For the UK mains
3ph power: => AC current of 4.1A

single phase power => AC current of 21A

The single phase case I can understand, but how did you arrive at 4.1A for the three phase /?
With a phase voltage of 415v and a PF of 1, I get 7Amp.
Am I missing something?

JimB

 

[hjl4]

I'm sure most of you know this.

KVA: Kilovolt amperes (1000-volt amps).

kVA is obtained simply by Amps x Volts divided by 1000.

 

[Oznog]

please go back and re-read his post.

Ah I was working on the details that were actually posted by hjl4, I thought that was the hardware in question. It's apparently not related to anything, I'm a bit confused what posting this was about.

 

[Styx]

For the UK mains
3ph power: => AC current of 4.1A

single phase power => AC current of 21A

The single phase case I can understand, but how did you arrive at 4.1A for the three phase /?
With a phase voltage of 415v and a PF of 1, I get 7Amp.
Am I missing something?

JimB

Phase voltage in the UK is 230V (actually is 240V but increased tolerances, but maths dune in accordance with the 16th Edition)

This gives a Line voltage of 400V (415V for 240V phase)

For a single phase system giving 5KVA for a unity PF = 5kW
5KW = 230*I => I = 21.7A

For a 3ph system again working with phase voltage

5kW = 3 * Vph * Iph
5000 = 3 * 230 * Iph => Iph = 7.2A

So yup you are correct, I took line voltage by mistake that 1st post

 

[Styx]

I'm sure most of you know this.

KVA: Kilovolt amperes (1000-volt amps).

kVA is obtained simply by Amps x Volts divided by 1000.

We all know that.
What I dont understand is where you suddenly pulled the 5000V figure from

Were all speculating here, so maybe we'd better wait and see if AMAECHI
will give us more details. Until then I prefer to stay out of this one.

BTW, anybody know what kind of machine a 5000 Volt, dc motor could be running? Seems rather large.

since then kicked off the discrepency between Oznog and the thread.

 

[JimB]

For a 3ph system again working with phase voltage

5kW = 3 * Vph * Iph
5000 = 3 * 230 * Iph => Iph = 7.2A

So yup you are correct, I took line voltage by mistake that 1st post

OK I see the way you were thinking.
I usually think it as W = sqrt(3) x Vline x Iline x Cos(phi).

Your way has an extra sqrt(3), but you use the phase voltage rather than the line voltage so it cancels out.

JimB

 

[Styx]

For a 3ph system again working with phase voltage

5kW = 3 * Vph * Iph
5000 = 3 * 230 * Iph => Iph = 7.2A

So yup you are correct, I took line voltage by mistake that 1st post

OK I see the way you were thinking.
I usually think it as W = sqrt(3) x Vline x Iline x Cos(phi).

Your way has an extra sqrt(3), but you use the phase voltage rather than the line voltage so it cancels out.

JimB

yer, its only cause for the last 4years I have been working in the Aerospace industry and for some reason they work on phase voltage and not line voltage.

I was always use to working on line voltage for 3ph stuff, so when I started this job and they kept going 115V 3ph my rough calcs in my head were always way off (since using that figure for line voltage, where actual lune voltage is 200V)

Once you get use to working with phase voltage you just dont bother with the sqrt(3) to calc power