Re: [CAD_CAM_EDM_DRO] Constant Current Drive - was: something else, can't remember.

Alan Rothenbush wrote:

>
> > Constant current works by using higher voltage (like 80V) on the
> > windings, and no resistors. The current ramps up faster thru the
> > motor windings because of the higher voltage. When the current
> > reaches a maximum, a monitoring circuit removes the voltage.
>
> This is not correct. What is being described above is a chopper drive,
> not a constant current drive.
>
> Let us step back a minute.
>
> Stepper motors are current devices. As such, the spec sheet for a stepper
> motor always has a current rating. It usually has a voltage rating as
> well but does not NEED one ( except in one special case ) .. only the
> current rating is of interest to us.
>
> This single current rating, assuming typical engineering, is usually two specs
> in one. The first is the current required for maximum power, the second is
> the maximum current without overheating.
>
> Let us take a theoretical stepper motor rated at 1 amp 1 volt.
>
> There are four basic drive circuits we can use to make this stepper move.
> In all cases, we want to supply 1 amp of current. More and it might
> overheat, less and we won't get all the power we paid for.
>
> The first circuit is the special case I mention above. Create a 1 volt
> power supply. Connect it to the motor ( through a suitable switching
> arrangement ) and the motor will turn. It will have as much holding
> power as any other circuit. And it will have the worst acceleration
> characteristics and lowest maximum speed of any of the drive circuits.
>
> The reason it will have the lowest speed is because the low voltage will cause
> the motor coil to "fill up" slowly. Since the coils fill slowly, the motor
> will turn slowly.
>
> This is the simplest possible drive circuit, but it will work and it will
> hold just fine.
>
> The next drive circuit is what you are using now, an RL drive. This circuit
> always uses a voltage higher than the "rated" motor voltage. Without anything
> else in the circuit, the motor would quickly overheat as a result of too
> much current. The resistor serves to limit the current to a safe value. The
> trick here is that "extra" current will flow when the stepper coil is first
> energized because an unenergized coil looks like a short circuit.

Wrong. The winding initially looks like an open circuit and will have full
supply voltage across it, which will start decreasing when the current ramps
up and causes more voltage drop across the resistor (time-const= L/R).

The resistor would be chosen so that only 1Amp can flow if left long enough.
There is no 'extra' current; only a faster ramp-up time.

> As the
> current in the coil rises, it looks less and less like a short circuit until
> eventually some equilibrium is reached, with the resistor serving to limit
> the current to the safe value.
>
> This circuit has the same holding power as all the others, but somewhat
> better acceleration and maximum speed than the circuit above because of the
> "extra" current that flows.
>
> The fourth circuit ( I can count .. # 3 will be described last ) is a chopper
> and is what was accurately described above. In such a circuit the current is
> turned on and off at a rapid rate such that the AVERAGE current is some
> appropriate value.
>
> No resistors are used, but a high voltage IS used. As such, the motor "fills
> up" with current very quickly. This "quick fill" results in excellent
> acceleration and the highest possible top speed. The holding torque is same
> as the first two because the average current is the same.

This is simulating constant-current after mechanical and inductive averaging.

> This is how all modern stepper drives are built.
>
> The third drive design is a constant current source, and is similar to the RL
> drive. In this case, the current limiting resistor is replaced by a bit
> of electronics. These electronics are designed so that some constant current
> will flow no matter what the conditions. Even if the motor is replaced by
> a short circuit, the same current will flow.
>
> Think of this circuit like a light dimmer with a person at the control
> trying to maintain a constant light level. When the sun comes up, the
> person turns the dimmer down. Clouds roll in, the dimmer gets turned
> up a bit. Night comes and the dimmer is turned up full tilt. With
> enough range and a fast enough person at the helm, the light stays
> constant no matter the conditions.
>
> The constant current source works the same way, keeping the current at
> precisely the same value, no matter the conditions.
>
> This is a wonderfully simple way to drive a stepper motor and has
> performance as good or better than the chopper drive with considerably less
> complexity. Again, holding torque is the same because the current is the
> same.
>
> The one problem with a CC source, and it's a big one, is the heat. The CC
> source does its magic by converting the "extra" power to heat, just as the
> R in the RL does. But the performance difference between the RL and CC drive
> is HUGE.
>
> Hope this greatly simplified description helps.