controlling 250V DC/AC with solid state

[Simon]

Well I can't see a problem with a mosfet with resistor in series and another mosfet that has the source in common with the other one but the drain connected to the top of the resistor so that first the resistor mosfet turns on and then it changes for the full power resistor.

[oldway]

You could use 2 SSR of the serie 1-DC of CRYDOM for doing this, 01 of 7A (D2D07) for starting with resistor and 01 of 12A (d2d12) for working.

[Simon]

I don't know how high a value a resistor we could get away with to ensure that on direct powering it does not draw too much power, does the current to speed up from a low to high speed depend much on the speed you start at ? We could use say a resistor that will only let 5A through with full voltage across (start up) it that would drop to 2.5A once the speed stabilizes before the direct powering cuts in so only 125ish watts for any amount of time. 12A is cutting it a bit fine for constant running current. We could see up to 140V at full charge.

[IanB]

I don't know how high a value a resistor we could get away with to ensure that on direct powering it does not draw too much power, does the current to speed up from a low to high speed depend much on the speed you start at ? We could use say a resistor that will only let 5A through with full voltage across (start up) it that would drop to 2.5A once the speed stabilizes before the direct powering cuts in so only 125ish watts for any amount of time. 12A is cutting it a bit fine for constant running current. We could see up to 140V at full charge.

The start up current drawn by a typical DC motor depends on the load it sees when stationary.

With the railway traction motor I mentioned above, the load at start up is enormous--there is this giant heavy train to start moving. You may as well try to move a brick wall. The motor on start up will be stalled and unable to turn. You have to ramp the power up slowly to keep the motor from burning up.

With a free running blower motor the situation is very different. The power consumed by a blower varies approximately with the cube of the speed, so when the motor is stationary the opposing load is nearly zero. All the motor has to do is overcome inertia to speed the thing up. It will speed up very quickly. Now with a DC motor as soon as it starts turning it will generate back EMF that opposes the supply voltage and reduces the current draw. So if the motor speeds up quickly the start up current will reduce very quickly.

In short, you may not need any current limiting on start up. The best way to determine if this is the case is to put the thing on the bench and measure it under expected operating conditions. Uncertain predictions and guesswork will never serve you well in industrial engineering. Hard data and reliable calculations are what you need.

[oldway]

My Nilfisk 1000W 220V vacuum cleaner motor has a resistance of 12.8 Ohms
A 110V motor would have a resistance four times lower (for the same losses) , this would be 3.2 Ohms.
Inrush current would be 140V/3.2 = 43.75A.
This is far too high for the motor.
Nominal current is 9.1A at 110V (1KW)
15A inrush current seems to be a good choice, as motor start with no load.
With 140V dc, total resistance must be 9.34R.
Then current limiting resistor should be: 9.34 - 3.2 = 6.14 Ohms.
A 5 Ohms power resistor would do the job.
Inrush current: 140V/(5 + 3.2)Ohms = 17A
Seems to be OK.

[IanB]

Inrush current would be 140V/3.2 = 43.75A.
This is far too high for the motor.

I think this depends on how long the inrush current lasts? (What we really care about is the integral of power dissipation over time, not the peak current per se.) If the inrush current reduces to a lower value in a fraction of a second then it may not be an issue. We also may consider the resistance of the power supply wiring and the supply impedance. Voltage drops in the supply wiring may even prevent a current as high as 40 A from occurring.

[Zero999]

it is something at the back of my mind, it is a vacum cleaner motor, do they put starting resistors in vacum cleaners ? or is the DC a problem ?

No they don't put resistors in vacuum cleaners and the surge shouldn't be that much greater with DC than AC.

I forgot it's a vacuum cleaner motor. It will be fine, as long as the MOSFET is suitably rated to handle the surge and the circuit breaker doesn't cut out too quickly.

[oldway]

No they don't put resistors in vacuum cleaners and the surge shouldn't be that much greater with DC than AC

For sure it is.
In DC, surge is only limited by resistance who is very low to reduce losses.
In AC, current is limited by impedance, that's resistance AND inductance.
Inductance is high because we have the armature (more or less 1mH) in serie with two field coils.
As the air gap is large, this inductance normally do not saturate.
For this reason, surge in AC is readily low in comparison with surge in DC.
Vaccum cleaners are not intended to work with dc power grid.
They even often use a triac power control who don't work with dc voltage.

With surge overload, problem is not overheating, but bad comutation: you can have a "flash" between motor brushes, what can damage the motor.
Overload of more than four times the nominal current is not acceptable in a DC/AC universal motor.

[Zero999]

No they don't put resistors in vacuum cleaners and the surge shouldn't be that much greater with DC than AC

For sure it is.
In DC, surge is only limited by resistance who is very low to reduce losses.
In AC, current is limited by impedance, that's resistance AND inductance.
Inductance is high because we have the armature (more or less 1mH) in serie with two field coils.
As the air gap is large, this inductance normally do not saturate.
For this reason, surge in AC is readily low in comparison with surge in DC.
Vaccum cleaners are not intended to work with dc power grid.

You're right there will be a larger surge on DC than AC but not as much as you may think. There is magnetic coupling between the field and armature which forms a transformer with a shorted secondary when stationary.

Either way, measure the DC resistance and calculate the maximum surge from that - there's no way it can exceed that.

They even often use a triac power control who don't work with dc voltage.

The intention is to use the motor on its own so that isn't an issue.

With surge overload, problem is not overheating, but bad comutation: you can have a "flash" between motor brushes, what can damage the motor.
Overload of more than four times the nominal current is not acceptable in a DC/AC universal motor.

What do you mean? There will be arcing whether it's AC or DC.

[Zero999]

SCR's don't seem to be practical at DC but I could be wrong but could a mosfet do the job ? for DC it would work fine as as the DS channel acts like a resistor and will pass current both ways will it work in AC too ?

SCRs can be used with DC. The only trouble is getting them to turn off is tricky.

The most suitable method here would probably be to use two SCRs and a capacitor. You'd need optically couple the SCRs somehow for isolation.

http://www.sentex.ca/~mec1995/tutorial/triacs/triacs.html
http://www.onsemi.com/pub_link/Collateral/HBD855-D.PDF
http://www.most.gov.mm/techuni/media/EP02024_6.pdf

MOSFETs can be connected back-to-back for AC.

[IanB]

In DC, surge is only limited by resistance who is very low to reduce losses.

No, surge is limited by inductance. The motor is an inductance, that as you said, does not saturate. As with all inductors the current is zero when voltage is first applied and then rises as the magnetic field builds up. However, since the motor is unloaded it will start turning immediately and will speed up rapidly. In doing so the spinning motor will generate back EMF that will counter the build up of current. So the start up surge current will be far less than the current through an equivalent resistance.

[oldway]

You're right there will be a larger surge on DC than AC but not as much as you may think. There is magnetic coupling between the field and armature which forms a transformer with a shorted secondary when stationary.

Either way, measure the DC resistance and calculate the maximum surge from that - there's no way it can exceed that.

I don't agree. There is no shorted secondary when stationary. This is only the case with AC asynchroneous motors, not with DC/AC universal motors...

What do you mean? There will be arcing whether it's AC or DC.

You should read some technical papers about armature reaction to understand why there are comutation problems with high overloads.
That's what happens when you start a DC/AC universal motor in DC without current limiting resistor.
In AC, it does not happen because current is limited to a far lower value by inductance.
http://web.uettaxila.edu.pk/CMS/SP2012/etEMbs/notes%5Carmature%20reaction%20&%20commutation.pdf

[oldway]

SCRs can be used with DC. The only trouble is getting them to turn off is tricky.

That is a very big trouble
You also have to use fast turn-off thyristors...
Not good at all 

[oldway]

In DC, surge is only limited by resistance who is very low to reduce losses.

No, surge is limited by inductance. The motor is an inductance, that as you said, does not saturate. As with all inductors the current is zero when voltage is first applied and then rises as the magnetic field builds up. However, since the motor is unloaded it will start turning immediately and will speed up rapidly. In doing so the spinning motor will generate back EMF that will counter the build up of current. So the start up surge current will be far less than the current through an equivalent resistance.

I don't agree: time constant is very short compared with mecanical inertia of the motor so current reach the static value (u/r)well before motor can generate a back EMF.

[Simon]

Well i will have to measure the motor but I found a mosfet that will do err I think around 70A with a peak of 300A (not sure how long it can last for) I'll have to look at the datasheet when i get back into work tomorrow. I can measure a similar 240V motor to get an idea but I'll have to wait for the actual motor to arrive to be sure.

[Simon]

How about using the mosfet as the limiting resistor ? if the gate voltage ramps up over a period (say 1s) it will act as the resistor and as the full on drive, so one mosfet not 2 and no resistor.

[SeanB]

That can work, but be careful, as the instantaneous power dissipation of the die can very easily be exceeded. Though it might be rated for 300A peak pulse ( likely a limit from the bonding wires) the instantaneous power can limit the amount of power it can handle as a slowly switching device, it can have hot spotting and detonate.

[Simon]

I'll check the datasheet tomorrow see what power it can take, if the motor resistance is not too low and won't make the start current higher than 70A then we won't get huge spikes through the mosfet but we can use the mosfet to create some resistance to help ease the start. We're running off the batteries on the train kept topped up by alternators, I don't know how long the cabling is and how thick but no doubt it will also act as a starting resistor.

[calexanian]

Using a Bridge Rectifier is a simple solution if the forward voltage drop is not an issue. You can use any BJT or FET but insure the driving source is isolated so you are not shorting across anything.

[Simon]

Yes the problem is we are carrying up to 20A so no mean feat for a bridge and quite a lot of heat to dump in a case, also the AC option will be more likely when we are talking 240V this means we will need a mosfet that can handle even more voltage as the peak will be 340V on a good day so we need to look at mosfets for 5-600V which will start to get costly plus the cost of the bridge and heat sink, we could just revert to relays.

[Zero999]

You should read some technical papers about armature reaction to understand why there are comutation problems with high overloads.
That's what happens when you start a DC/AC universal motor in DC without current limiting resistor.
In AC, it does not happen because current is limited to a far lower value by inductance.
http://web.uettaxila.edu.pk/CMS/SP2012/etEMbs/notes%5Carmature%20reaction%20&%20commutation.pdf

All right, I don't have time to read the entire paper but assuming you're right about the increased arcing when overloaded, in this case the motor is driving a fan so the overload lasts for under a second, is it really a big problem? It think this is a case of test it and see what happens.

[SeanB]

It is on a train, not really a place for suck it and see. The reason a train car is 30 tons in weight is because the other cars are 30 tons in weight. Trains are not a place for gentle, you have to expect horrible spikes, surges, lightning and such, and that is for a light bulb.

[Simon]

The motor is a hand drier motor so no real mechanical load, as it happens I am using a vacum cleaner motor (off the shelf part and is the same type of motor) that is made to take some stick as it has to suck through a filter where as I have no such filter. the mechanical load of a fan increases I think to the square of the speed so the actual load during start-up is near non existent.

[Zero999]

I don't agree. There is no shorted secondary when stationary. This is only the case with AC asynchroneous motors, not with DC/AC universal motors...

I can tell you that's not true. The armature winding is magnetically coupled to the field. Connect 12VAC to the field coils and you'll measure a small AC voltage on the armature when it's stationary. If the armature is short circuited, it will vibrate a bit and will spin in either direction as it operates as an induction motor.

In the case of a vacuum cleaner, the armature is connected in series with the field so the stall current will be different, depending on the phasing.

You should read some technical papers about armature reaction to understand why there are commutation problems with high overloads.
That's what happens when you start a DC/AC universal motor in DC without current limiting resistor.
In AC, it does not happen because current is limited to a far lower value by inductance.
http://web.uettaxila.edu.pk/CMS/SP2012/etEMbs/notes%5Carmature%20reaction%20&%20commutation.pdf

That article concerns shunt wound machines, not series wound.

The motor is a hand drier motor so no real mechanical load, as it happens I am using a vacum cleaner motor (off the shelf part and is the same type of motor) that is made to take some stick as it has to suck through a filter where as I have no such filter. the mechanical load of a fan increases I think to the square of the speed so the actual load during start-up is near non existent.

I thought there's no point in discussing this so I thought I should test it.

I tried a standard 230VAC vacuum cleaner motor + bridge rectifier connected to the mains and it works perfectly. There's hardly any difference in arcing, whether it's run off AC or DC. The motor starts as usual (I didn't notice any massive arcs) and operates in exactly the same manner as it would do if run directly from the mains.

This is with an unfiltered DC supply. I don't have access to a smooth 230VDC supply otherwise I would test it. Unfiltered DC will have a lower peak voltage but 100% duty so the two effects could cancel one another out. Try powering it from a variac and add a large filter capacitor.

The motor is a giant hunk of iron and copper. I'd be more worried about the semiconductors which need to be rated to with stand the surge.

EDIT:
The bridge rectifier I used was GBPC808 which is rated for 800V and although the current rating probably wasn't high enough, it survived the test but I only ran it for a minute or so. A MOSFET may not have such a high surge capacity. If it's a problem, try the SCR method but again you're faced with the problem of getting it to turn off.
http://pdf.datasheetarchive.com/datasheetsmain/Datasheets-20/DSA-390489.pdf

[IanB]

the mechanical load of a fan increases I think to the square of the speed

Probably closer to the cube of the speed in fact. The developed head varies with the square of the speed and the flow varies directly with the speed. The power consumed is proportional to the head times the flow.

So yes, the load on start up consists only of the inertia of the rotating parts and the friction from the bearings and gearbox.