Monday, December 7, 2009

Value Engineering the Stepper Motor PSU

The word "DESIGN" had been over rated & over used... Everyone in the advertisement design something... I prefer to use "SELECTION".  e.g. select a few different color clothing & you have a in-store fashion designer... do you "design" a 200hp engine for your car or you select? Do you select chopstick over fork & Knife or design... Can anyone claim the credit of designing a wheel? but we keep seeing this " we design of the wheel to xxxx, yyyy......."

Enough of my ranting, lets get back to the Stepper motor Selection.

Like everything else, we must know what we need before we can conduct a proper selection.

Stepper motor system consist of a stepper motor and a Stepper motor driver or amplifier.

Stepper motors takes in high current pulses which the motor driver made by chopping a DC current with respect to the instruction from the Brain. So we need to prepare a DC power supply for the Stepper motor system.

We starts with the Voltage rating,,

There is a straight forward & safe formular commonly used in the industry,
V=32 x SQRT(Coil Inductance in mH)
you can go higher but just watch out for the motor temperature.

EG,
My coil is 3.5mH
Vdc=60V

Secondary Voltage of the Transformer
Vsec=(60V+1.4V(voltage loss of rectifier))/sqrt(2)=43.4V
common transformer off the shelf are, 6V, 9V, 12V, 24V, 36V, 48V.
So, 48V is my choice. this will give
48xsqrt(2) -1.4=66Vdc#


Current rating
Most will follow the convensional wisedom and add up all the max.current rating of the motors & call that the PSU's current rating.... Nothing wrong when its a small system with total 6 Amp of current, but with a full size machine, the motor max rating of each motor can be 7~8 Amps... if there are 4 motors, the DC is 50V, that will require a 1400VA~1600VA about 2hp!!! That will be huge, heavy & expensive!

One must understand that we normally don't use up all the power the motor can do, i.e. the rated power... the motor will only produce "enough" power to move whatever they we designed to do, so it is all too common to see motor rated for 3kw only work 0.5kw with plenty of headroom.

With that in mind, lets do some "value engineering",
We know
we will never run the motor to the max limit because its a bad design practice. we know all the motors don't run at the same time...
Now we need to determine how much current the motor need to move the burden it was entrusted with. again there are more then one way to find the number,
By mathematics model, culculate the weight, inertia, moment... etc of the overall mechanism & find out the torque and hence the power required... I always keep this option last... even if I can, why should I go throught the whole trouble?
By experiment, build the machine, measure the torque required, this is always a good option, but I need to build the machine first....
Empirically, we can measure the current drawn by motors in similar machines.

I went for the empirical method, many reported to run on 300VA without risk of over heating and burning transformers, Even though they have a different voltage rating from mine, 300VA is the figure I need to know.

VA is a very close equivalent to Watt, VA is derive from multiplying the Voltage and the Current, this current is the resultant current and the current of W is the reactance current, but they are very close in number in this case.


Regulated or unregulated Power supply,
Again, common wisdom will pick regulated power supply, but really it is the worst choice for this application.

The nature of the stepper motor operation is such, when stop, they draw near zero current and when they move, they need current immediately or risk loosing steps,

Regulators are not famous for fast response, to ensure they meet the motor demand, they have to have the full power ready all the time, but the machines hardly use up the the max as we know by now, that extra power will turn into heat and wasted...

As for Unregulated power supply, they only give when the load ask and it does have a capacitor to act as a ripple filter & double as a reservoir.

SO since SMPS is normally regulated & the unregulated one are not good in drastic load demand, it is ruled out.

Transformers
There are few types of transformer core, The C-core, R-core, Toroidal & E-I core.
C-core & R-core are not as commonly available as the used to be and that leaves us with Toroidal and E-I core.
Toroidal Core is a more efficient transformer, but it is its efficiency that became its weakness in this application.
Toroidal response to suddent load demand extremely well, so well that it will burn itself when responding.
E-I core is less efficient magnetically, but because of this deficiency,  the hesitation acts as a buffer to protectects the transformers from burning its coil.

So, there you have it,
My best package selection is
Transformer :- 300VA E-I core transformer with a 48Vac secondary voltage
Rectifier :- Bridge rectifier with metal casing, rated over 100V & 25 amp
Capacitor :- 22,000uf @ 100V Capacitor based power supply.

This also turn out to be the cheapest & the least technological advance combination.

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