controlling 250V DC/AC with solid state

[Simon]

well it's a 20Krpm motor so it will laugh at the inertia of a light sheet metal impeller

[Simon]

the motor does achieve full speed very quickly, there is quite a jolt of the motor on start up from the recoil, if it's not held firmly down it will shoot across the benh - yes voice of experience speaking.

[IanB]

well it's a 20Krpm motor so it will laugh at the inertia of a light sheet metal impeller

The motor itself has inertia, which is why it shoots across the bench...

[Simon]

well yes that too, what I meant is that apart from the armature weight it has very little weight to sin up the only real loading comes from it moving air so it really takes off on power up, it's definitely at full speed within a second

[oldway]

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.

That you measure a small voltage on the armature when you feed the fields with ac proves that the armature is NOT a  short circuited secondary.

That article concerns shunt wound machines, not series wound.

Armature reaction happens as well with both. DC traction motors (series) have armature reaction compensation.

About starting universal series motor with DC without current limiting resistor:
My Nilfisk 1000W 220V vacuum cleaner motor has a resistance of 12.8 Ohms, inductance is 46mH.
Assuming a cos phi= 0.8, nominal current should be 5.68A. (measured current = 5.3A)
Impedance (50Hz) = 19.3 Ohms.
Time constant (L/R) = 3.59 ms
Inrush current with 220Vac: 11.4A (two times nominal current)
Inrush current with 220Vdc without current limiting resistor: 17.2A  (three times nominal current)
Inrush current with 280Vdc without current limiting resistor: 21.87A (3.85 x nominal current)

[Zero999]

That you measure a small voltage on the armature when you feed the fields with ac proves that the armature is NOT a  short circuited secondary.

It proves the armature acts as a secondary winding. If it's put in series with the field coils, the current induced in the armature will either be added or subtracted to the surge current, depending on the phasing. To find out which, you'd need to test the phasing of the motor, which I didn't do.

About starting universal series motor with DC without current limiting resistor:
My Nilfisk 1000W 220V vacuum cleaner motor has a resistance of 12.8 Ohms, inductance is 46mH.
Assuming a cos phi= 0.8, nominal current should be 5.68A. (measured current = 5.3A)
Impedance (50Hz) = 19.3 Ohms.
Time constant (L/R) = 3.59 ms
Inrush current with 220Vac: 11.4A (two times nominal current)
Inrush current with 220Vdc without current limiting resistor: 17.2A  (three times nominal current)
Inrush current with 280Vdc without current limiting resistor: 21.87A (3.85 x nominal current)

That occurs for under a second so is not going to overheat the motor, which has a huge thermal time constant. During the test I conducted both the and 5A top fuse in the plug survived, so as long as the semiconductors are rated to stand the surge, it won't be a problem.

[Simon]

I found a IRFP4768Pbf it will take up to 93A continuos at 25C, 66A at 100C and will withstand 370A pulses the width of which are "limited by the junction temperature" which is limited to 175C

[oldway]

@Hero999:
Please read what i wrote:

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.

High overload can produce a flash at the collector (this can damage the motor) and wearing the collector and brushes...No concern at all with overheating.
You seems to prove that you know nothing about dc motors...

it proves the armature acts as a secondary winding. If it's put in series with the field coils, the current induced in the armature will either be added or subtracted to the surge current, depending on the phasing. To find out which, you'd need to test the phasing of the motor, which I didn't do.

The surge current is the same in the armature and in the field coils as they are conected in series.

[oldway]

I found a IRFP4768Pbf it will take up to 93A continuos at 25C, 66A at 100C and will withstand 370A pulses the width of which are "limited by the junction temperature" which is limited to 175C

You can also use a PWM drive of the Mosfet instead of a series resistor.
So you provide a "soft start" of the motor applying reduced voltage at start up.
As the motor is highly inductive, you don't need any additional inductor, only a fast free wheeling diode.
NB: series current limiting resistor is a solution to be applied only when you can't use the PWM solution. (relay or SSR)
NB2: to be "bulletproof" in case of output short, i would add a little inductance (no way to protect a MOSFET against short if there is no inductance, they are too fast) and a cycle by cycle peak current limiting circuit.
NB3: PWM has other advantages:
1) there is no current surge at all, you can choose a lower current MOSFET.
2) it's safer for the motor.
3) you can limit the output voltage at 110V by PWM. The max voltage of 140Vdc seems too high for a 110Vdc motor.

[Simon]

surely with PWM you will still get the huge spike, it won't last as long but it will be there. We;d need a lot of "tank" capacitor to help average the current out and not blow a circuit breaker.

[Zero999]

High overload can produce a flash at the collector (this can damage the motor) and wearing the collector and brushes...No concern at all with overheating.
You seems to prove that you know nothing about dc motors...

No, as i said before, I tested it and it works fine. It doesn't matter what the article says, practical results indicate in reality it doesn't make any difference. The arcing on the commutator is virtually the same irrespective of whether the supply voltage is AC or DC. The surge lasts for under a second which is not long enough to cause any damage to the brushes or commutator. I'd also expect the starting torque will be higher on DC, causing more rapid acceleration, thus the surge will not last as long on DC than it would for AC but I didn't test this.

I'd suspect over the long term, there would be less heating with DC than AC as there won't be eddy or hysteresis losses in the field coils but I didn't run it for long enough to see if there was any difference.

The surge current is the same in the armature and in the field coils as they are conected in series.

I didn't say the current in the armature and field coils was different, of course it will be the same.

What I mean is the motor behaves like a transformer with the field as the primary and armature as the secondary. Connect the primary and secondary coils in series on a transformer and the current will depend on the phasing. Try this with a 240V to 12V mains transformer and you'll see, the current depends on the phasing of the primary to secondary winding. Again, I didn't test this with the motor so don't know what the phasing is.

I found a IRFP4768Pbf it will take up to 93A continuos at 25C, 66A at 100C and will withstand 370A pulses the width of which are "limited by the junction temperature" which is limited to 175C

No there's no point in messing around with PWM or a current limit resistor, providing the MOSFET can handle the surge it will be fine. In my test I used a 3A bridge rectifier to power a 750W motor for a minute so I'm pretty sure you'll be able to use such a beefy MOSFET to drive your much larger motor.

I thought you were intending to use the same switch for both 230VAC and 120VAC? If so you will need a 400V device.

The question then is whether you use a bridge rectifier or connect two MOSFETs back-to-back.

[oldway]

surely with PWM you will still get the huge spike, it won't last as long but it will be there. We;d need a lot of "tank" capacitor to help average the current out and not blow a circuit breaker.

Why would you get a "huge spike"?
You can drive the MOSFET with very short pulses, and the average voltage will be very low, a few volts.(inductance of the motor is "averaging the voltage" applied on the resistance of the motor)
Current is then limited by motor resistance. (3 Ohms for example).
With a min average voltage of 6V (for example) and a 3 Ohms motor resistance, you will have a 2A start up current.
Is that an "huge spike"?
You must than have an soft start increasing average voltage up to 110Vdc.

If you have to feed the motor with both dc an ac voltage, i would use a relay in ac feeding and a relay + MOSFET (with PWM drive) in dc feeding.
Relay in dc feeding may only be actueted without current, with MOSFET in off state. (actueted, i means changing of state) (NB: i am not very fluent in english)
Also make a contact logic for protection so ac and dc relays cann't never be both on.

[Zero999]

surely with PWM you will still get the huge spike, it won't last as long but it will be there. We;d need a lot of "tank" capacitor to help average the current out and not blow a circuit breaker.

Why do you need a capacitor? The motor has enough inertia that it doesn't make any difference.

The only capacitors and inductors you need are for RF suppression. The motor I tested already had a capacitor (150nF if i remember correctly) connected in parallel with it for EMI suppression but no inductor. I didn't conduct any EMC tests but as this is on a train, I suspect it may need to meed more rigorous EMC standards than a domestic vacuum cleaner.

[oldway]

@Hero999: To deny measured and calculated values and to substitue them by "I feel that", this is not serious at all.
Before saying anything, you should inform yourself a little better on what you say.

Tests should be made of a scientific manner respecting the actual conditions.
You did absolutely no surge current measurement, you have no 110V battery to test such a 110V motor.
For testing, you must have a dc source of low internal resistance, as low as a battery.

In addition, considerations such as "I did two tests and the motor was not damaged" are completely ridiculous.
A collector aged, clogs, the air humidity can change, etc. ....

You're an amateur handyman and this is a circuit that must be installed on a train, this is not the place for low level crafts.

We must never accept a project where we did not respect the rules of technology.

[Zero999]

@Hero999: To deny measured and calculated values and to substitue them by "I feel that", this is not serious at all.
Before saying anything, you should inform yourself a little better on what you say.

Tests should be made of a scientific manner respecting the actual conditions.
You did absolutely no surge current measurement, you have no 110V battery to test such a 110V motor.
For testing, you must have a dc source of low internal resistance, as low as a battery.

In addition, considerations such as "I did two tests and the motor was not damaged" are completely ridiculous.
A collector aged, clogs, the air humidity can change, etc. ....

You're an amateur handyman and this is a circuit that must be installed on a train, this is not the place for low level crafts.

We must never accept a project where we did not respect the rules of technology.

What you say makes absolutely no sense. The surge current last for under 1 second so can't damage the motor in such a short period of time. There's a reason why it's called a universal motor - it can be run off either AC or DC, go and look it up.

I agree, testing is important but you haven't even tested it, just done some rough calculations which didn't take any of the things you've mentioned such as environmental conditions into account. You're right, more rigorous tests are needed so I suggest Simon should go and conduct some himself.

[Simon]

surely with PWM you will still get the huge spike, it won't last as long but it will be there. We;d need a lot of "tank" capacitor to help average the current out and not blow a circuit breaker.

Why do you need a capacitor? The motor has enough inertia that it doesn't make any difference.

I meant a power supply capacitor so that the PWM does not generate repeating high current spikes, oldway keeps banging on about the average voltage. A fast reacting circuit breaker will not give a rats ass about the average voltage but the spikes that are being drawn off the supply. If a tank capacitor helps feed the modfet it will help take the spiky load off the power supply line, we do have to get this through emc tests.

[Simon]

I agree, testing is important but you haven't even tested it, just done some rough calculations which didn't take any of the things you've mentioned such as environmental conditions into account. You're right, more rigorous tests are needed so I suggest Simon should go and conduct some himself.

pft I won't pass that on to my employer, i know what they think of R&D: nothing, our customer is even more ignorant and i think our electronics subcontractor is yet to learn ac versus dc power control as they usually deal with digital control boards.

[oldway]

What you say makes absolutely no sense. The surge current last for under 1 second so can't damage the motor in such a short period of time. There's a reason why it's called a universal motor - it can be run off either AC or DC, go and look it up.

I agree, testing is important but you haven't even tested it, just done some rough calculations which didn't take any of the things you've mentioned such as environmental conditions into account. You're right, more rigorous tests are needed so I suggest Simon should go and conduct some himself.

A flashover is an arc generated around the commutator and eventually causing a short circuit in the positive and negative brushes and damaging the commutator and the bruh holder. 

This arc can be generated by ionisation of the air by comutation sparking anomalies ocurring during high overloads.

I gave you a link over armature reaction to explain what occurs during such overloads, but you did not read it.
http://web.uettaxila.edu.pk/CMS/SP2012/etEMbs/notes%5Carmature%20reaction%20&%20commutation.pdf

Armature reaction move the field neutral line and this results in very severe comutation sparking anomalies.
How many time do you think it is necessary to generate an arc ?
Have you ever seen a flashover in a dc motor or dc generator ?
Remember that whe have to start a 110V motor with dc voltage as high as 140V.

[Simon]

well the motor in use is actually rated for 120V and that has to be designed to deal with at least +/-10% as is the specified variation on the mains so that takes it to 132V not far off the 137v they are giving as a maximum (110V * 1.25 which is the max voltage of lead acid batteries)

[oldway]

I meant a power supply capacitor so that the PWM does not generate repeating high current spikes, oldway keeps banging on about the average voltage. A fast reacting circuit breaker will not give a rats ass about the average voltage but the spikes that are being drawn off the supply. If a tank capacitor helps feed the modfet it will help take the spiky load off the power supply line, we do have to get this through emc tests.

There are no repeating high current spikes in PWM control of an inductive load.
Equivalent schematics of a running dc/ac universal motor is an inductance in series with a resistance and in series with a counter EMF.
The inductance of the motor is "filtering" the current and, if the PWM frequency is high enough, the ripple will be very low...it will be almost dc current.

Current spikes in the MOSFET have the same value as the dc current in the motor.
Somes very short high current spikes could occur if you don't use a fast diode as free wheeling diode.

[Simon]

you still have to start the damned thing !!! that first PWM cycle will still draw an instantaneous 70A to start !

[oldway]

you still have to start the damned thing !!! that first PWM cycle will still draw an instantaneous 70A to start !

Not at all !
http://eprimes.wordpress.com/2012/04/25/fundamentals-of-non-isolated-buck-converter/
The only difference is that whe don't have a capacitor C.
NB: Equivalent schematics at starting of a dc/ac universal motor is an inductance in series with a resistance.

[IanB]

I meant a power supply capacitor so that the PWM does not generate repeating high current spikes

No, because the motor coils have significant inductance. Inductances resist change in current. This means that the first instant you apply a voltage to an inductor the current will be zero. If you PWM an inductor it will act as a current smoothing device.

[Simon]

ok but what will the scope trace of the current draw look like ?

[Zero999]

A flashover is an arc generated around the commutator and eventually causing a short circuit in the positive and negative brushes and damaging the commutator and the bruh holder. 

This arc can be generated by ionisation of the air by comutation sparking anomalies ocurring during high overloads.

I gave you a link over armature reaction to explain what occurs during such overloads, but you did not read it.
http://web.uettaxila.edu.pk/CMS/SP2012/etEMbs/notes%5Carmature%20reaction%20&%20commutation.pdf

Armature reaction move the field neutral line and this results in very severe comutation sparking anomalies.
How many time do you think it is necessary to generate an arc ?
Have you ever seen a flashover in a dc motor or dc generator ?
Remember that whe have to start a 110V motor with dc voltage as high as 140V.

I have read that paper now several times.

Yes I do understand how over loading a parallel wound DC motor can cause commutation problems and eventually magic smoke and fire, long before the windings overheat. The flux generated by the armature opposes that of the field winding, thus shifting the magnetic neutral angle away from the geometric neutral. As a result, the brushes start to interrupt the current flowing in the armature causing arcs to be generated due the back-EMF. If this gets bad enough, a chain of arcs will occur between all commutator contacts, forming a short circuit across the armature. In a generator this would result in catastrophic failure, whilst in a motor, one would hope the circuit breaker would trip and prevent a fire.

I don't believe this is will happen when a vacuum motor starts on a DC current with no inrush protection. There are two reasons for this:

#1 You're right that the surge will be greater on DC than AC but because this is a series wound machine, any increase in armature flux is mirrored by the field, until either the field or armature saturates. The armature flux will not weaken the field flux because the current in the field will increase along with the current in the armature.

#2 The situation in the paper you've linked to describes a motor or generator which is already spinning when subject to overload but isn't the case here. In order for the magnetic neutral angle to be shifted, the armature has to be spinning first. If it's stationary as in the instant the motor is started, the magnetic neutral angle will be equal to the geometric magnetic neutral angle. Once the motor is spinning, the armature will be generating a back EMF and the current will dramatically decline, which will happen more quickly on DC than AC as the starting torque will be higher.

I've only managed to get the arcing you've described by removing the fan and reconfiguring the motor as a parallel wound machine. In my experiment I removed the fan, connected the field to a 12VAC transformer secondary and the armature to the output of a variac. Interestingly the arcing seemed to be worse when I ran it from AC than DC and running it backwards made it worse still, presumably because the brushes are already pre-aligned to give proper commutation in one direction only. This was awhile ago. I used an old 650W Panasonic vacuum cleaner motor with a 12V 50W toroidal mains transformer powering the field.

Practical tests show running an unmodified motor off 230V unfiltered DC doesn't produce any more arcing than normal. I suspect the inductance of the field coils would help to smooth the mains ripple so the fact it's not filtered may not be as significant as I first imagined. If I can find my hall effect sensor, I'll measure the current ripple on an oscilloscope, failing that I'll just measure the inductance and do a calculation to estimate it.

ok but what will the scope trace of the current draw look like ?

Sawtooth.
http://ee.lamar.edu/EELABS/ELEN2107/Lab6.pdf

pft I won't pass that on to my employer, i know what they think of R&D: nothing, our customer is even more ignorant and i think our electronics subcontractor is yet to learn ac versus dc power control as they usually deal with digital control boards.

Seriously?

There must be plenty of 12V SLA batteries around, you can connect up in series to the vacuum cleaner motor and MOSFET, obviously use a fuse with a 150V DC rating an AC fuse won't be able to break the same DC current, an arc could form. It should take long to find out if anything bad is going to happen when the motor is started. It's very important this kind of thing is tested. Remember all that buggering around with the sensors? You don't want a repeat performance or worse, magic smoke and fire.