Speed Controller - error in Circuit

How to post pictures

Thanks Jason. First try so lets hope I don't mess it up

Effect of the Schottky Diode across the motor.

These DSO pictures show the effect of placing a Schottky diode across a 545 motor, taking ~1A, on a 7V supply.

The measurement is made across the power MOSFET being used as a switch. So the baseline is just above OV and occurs when the MOSFET is ON. When it switches OFF there is an inductive kick which is what we need to focus on. After the kick the voltage drop back to the motor generated EMF and the commutation spikes can be clearly seen.

Picture 1 - the schottky diode is fitted and the spike is limited to just over 7V

Picture 2 - As above but picture is expanded to show the clipping of the pulse with the diode more clearly

Picture 3 - schottky diode removed - the pulse is now 35V high

Picture 4 - expanded version of Picture 3. The clipping of the waveform at 35V can be seen as the body diode in the MOSFET begins to conduct.

This makes the case for the schottky diode even at low powers on a small motor. The energy going through the MOSFET during avalanche breakdown at 35V will cause the MOSFET to heat up by a surprising amount. Much better to let a diode push it back into the motor where it will do useful work.

Trevor

Very informative traces

Thanks for posting them, Trevor

MichaelG.

Given previous comments it is rather sad that some people do not have the curiosity to want to know how circuits work and how they are designed.

For those that are interested there are some notes on the quirks and foibles of power devices in my album 'Semiconductor Devices'.

A few spats on this forum wouldn't put me off publishing electronics designs and I did discuss the possibilities with a previous editor of ME/MEW, although in the event it didn't come to anything.

Andrew

... and for convenience; a direct link to Andrew's album.

No excuses, now

MichaelG.

Andrew,

I for one like to know how stuff works and on this point I have just spent the afternoon stripping apart my daughters old microwave into component parts. I may not always understand the logic of how something works, I'm refering to electronics here, but as a mechanical engineer I apply my knowledge of hydraulics and pneumatics to electronics and muddle through and if I can fathom it I ask.

Martin P

Edited By martin perman on 22/05/2015 17:48:36

I've found all this discussion very interesting, and as a result I've added a transistor driver stage to the FET I'm trying to control from an Arduino. The end result is to control one of those ex radar blower fans to drive a street organ. 24V at about 1.5A

I see the argument for not having a flywheel diode, but I don't want to blow the FET. Is it worth trying an RC snubber as used on AC loads? Start with a low R and increase it until the spikes get to say half the Vdss.

Hi Duncan,
The only argument that seems to make sense to me for NOT having a flywheel diode is to save a few pence. I suspect the motor you are using is series wound so it's characteristics will be very different to a permanent magnet motor. You do not say if you application is just switching the motor on and off or for PWM speed control.

Les.

It's variable speed using the Arduino pwm function, which I think runs at ~500hz. The motor runs in the same direction if you reverse the supply, so it isn't permanent magnet, don't know whether series or parallel field. Makers plate says

Airscrew Weyroc

type G.16,C4.113

24V DC

It was made in 1977, I bought it new about 9 months ago. If I block the outlet it seems to speed up a bit, but it's difficult to tell. I've tried contacting the makers, they never got back to me.

What I'm trying to do is measure the pressure in the windchest and adjust the motor speed to keep tye pressure reasonably constant. I emphasize 'reasonably', as I've seen one just driven from a lamp dimmer with no feedback and it seemed OK. I've got the pressure sensor working and the Arduino pwm responding , I'll have to invert it to suit the transistor drive, as that itself acts as an invert.

Hi Duncan,

Les is right you may save a few pence on the diode but you are getting no other benefit from not fitting the diode. The faster the PWM the greater the energy saving by fitting a diode ( and the cooler your FET will run).

Its worth checking if you can separate the armature from the field winding. If it was serial connected it would have poor speed stability so I would expect parallel wound. If that is the case constantly power the field winding and connect the armature to the controller for best results.

On the subject of the diode I have produced another trace (shown at two different timebase speeds so you can see both the big picture and the detail)

Sorry the current trace is a bit noisy but it is a homemade probe (unfinished project) however it shows the effect of the diode quite nicely.

The motor in this test is running at part speed on a 7V supply at 1A
Key:
Purple=drive into FET Gate which shows clearly the point at which the FET is turned off
Yellow=FET Drain voltage
Blue=current into motor(=FET+Diode current)

The important thing to notice is that current continues to flow INTO the motor after the FET has been switched off i.e. the motr is getting MORE power it is NOT being braked.

For clarity I have included the relevant part of the circuit showing the mointoring points.

Trevor

Hi Duncan and Trevor,
I have found that I have one of these motors which I bought for use a s blower on a small furnace. It is definitely series wound. (I confirmed this this by lifting one brush off the commutator while measuring the resistance between the motor terminals.)

Les.

Surprising Les, given that Duncans motor did not appear to slow down under load. However, your test seems conclusive.

Just goes to show you can't rely on a subjective assesment like the motor sound. Need to do a measurement.

Trevor

Blocking the output of a centrifugal fan puts LESS load on the motor as all it is doing is rotating a fixed quantity of air. You can notice this with a vacuum cleaner. If you block the inlet. (Or outlet if accessible.) the motor speeds up.

Les.

Thanks everyone, diode and transistor drive for the FET it is! I've learned a lot from the interchange. I had thought that the current flowing when FET off was because the motor was acting as a generator, but obviously not as that would entail current going the opposite direction

I've just checked the motor again, this time on full welly. If you block the output it definitely speeds up.

Jason, the idea of some kind of hubble bubble pipe bursting into life during quiet bits of music will take some time to fade! If I can't get the electronic one to work something like a weighted flap valve is the back up plan, the water idea certainly means very little pressure rise, just the bouyancy of the bit immersed changing. I reckon it would have to be passing all the time even at peak demand from the pipes to avoid bubbling, which would entail controlling the water level.

Turning off a tap against a lot of water flowing produces "water hammer", which is caused by a rapid increase in the pressure across the tap as it tries to suddenly bring the column of water behind it to rest. You can't suddenly stop a large mass that's moving at a decent rate. The current flowing in an inductor (such as a motor winding) behaves in a similar way, so if you try to turn it off suddenly, there will be a rapid rise in voltage (pressure). Trouble is, if you try to turn a power semiconductor switch off slowly to reduce or control the voltage transient, you will make it very hot, so the skill is to switch it as quickly as you reasonably can. You have to manage this kind of switching event very carefully and it forms the very basis of just about any switch mode power electronics circuit.

The PWM circuit in the article is rather like a water pipe (current in motor) with a fast tap (the FET) - but with no relief for the water hammer (the snubber is too small to do anything). It's subjecting the switch to the resulting voltage transient. "Avalanche breakdown" is where the switch breaks down and conducts, rather like a pressure relief valve. However, as the switch is passing both current and voltage simultaneously, it sees a high transient power dissipation which isn't good for it unless in moderation. Modern MOSFETs are inherently able (and optimised) to withstand some limited repetitive avalanche energy which is usually specified in the datasheet, whereas bipolar switching transistors which were the only commercially viable option 20 years ago don't like it. When they start to break down through overvoltage, there is a runaway effect called secondary breakdown that results in device failure.

I've been working full time in power electronics since the early 80s and I learnt very early on how the basic topologies work and why you need a flywheel diode in a buck converter. The last thing I want to see is enthusiasts trying out electronics for the first time and ending up with circuit failures. I think we owe it to them to give good advice and circuit examples that are simple but robust and well designed.

On a lighter note and in the spirit of enjoyment and enthusiasm, here's an interesting example of an hydraulic system that delivers water to a reservoir some height above the source. It's pretty much an hydraulic analogue of a "boost converter" which can boost a low voltage (eg 12V) up to a higher voltage (50V or more). It's the basis of many circuits such as power factor correctors (PFCs) used on the front of many VFDs for boosting mains voltages up to 400Vdc or more. Many of us on this forum are fairly intuitive and practical, so hopefully will find this kind of electrical / mechanical analogue interesting. You can't extend the mechanical model to explain all concepts in electrical circuits but it's a start. Once you begin looking into electronics, you find there's some pretty interesting stuff going on....

**LINK**

Murray

PS - here, the FET is valve 4 and the diode is valve 5.

Edited By Muzzer on 25/05/2015 17:47:16

When every penny counts, as on controllers for small lathes, strictly non-essential components tend to get missed off. In higher volumes, >100,000, even fractions of a penny can be important.

Andrew

So finally the issue 229 landed in my postbox also, and after reading a bit diagonally through that article I'm still a bit lost.

I'm sure someone with sufficient electronics knowledge would have no problem, but I'm not among these...

So please Neil, do you mean that the emitter of BC327 should point upwards to the BC337 (and the arrow changed naturally), or stays as drawn and just the arrow drawn correctly?

A corrected drawing in the next issue is good, but a clear description even better...

Regards. HansR.