Can someone please explain "chopper drive"

[stuart76]

Just wondering if anyone out there could explain in simple terms what a chopper drive is and what you do or dont need for it?

I have a "4xcnc10b" driver board from dlttc.com, im running 4 x 1.8v-3amps steppers (6wire) with a 12v - 25amp power supply. Im told i need 16 x 50watt-3.5ohm resistors (between board and motors). Ive read in other forums saying chopper drives are cheaper and easier but i have no knowledge of what they are. Any enlightenment on this matter would be much appreciated for myself and hopefully a few others. thanks

[drk]

I simple way to understand a chopper drive is to compare it to a car on the highway,
you could drive from a to b at a constant speed of 60, being comfortable to yourself and others, or could spurt out with bursts of excitement by zipping to 120, slowing down and again zipping ahead.
You will at the end travel the same path and would have taken lesser time.

A chopper drive for a motor is no different in concept.
It simply provides bursts of voltage much higher than that of your stepper ( mostly 10-15 times), during that time , the stepper motor can rotate much faster than what it could at its rated value.

Ofcourse , The reason we cant run it at a continuous high voltage is cause the high current will burn the motor out.

So by providing the higher voltage in pulses, it is ensured that the overall current buildup is kept at the rated current the coils can handle.

[bunalmis]

Choppers are regulate the load currents.

Load current of coil easly can be set the desired current.

How does work?

Chopper completely cuts the coil voltage if coils currents exceed the referance value.

Chopper apply full voltage to coil if coils currents lower then the referance value.

Therefore coil current swing near the referans current.

[stuart76]

do i still need to use resistors between the board and the motors if using a chopper drive?

[bunalmis]

You dont need series resistor for coils. But chopping freuency and motor inductance must be match. Else motor currents have big ripple and this current ripples heats the motor. Also you may hear unwanted acustic noise.

[gar]

050611-0905 EST USA

stuart76:

No you do not need resistors with the chopper driver.

Here is the theory:

Any device has a thermal limit. For many electrical components this is around 100 deg C. (by around I include 150 deg C.) The temperature of a component is a funtion of the power put into the device and its ability to dissipate this power and the ambient conditions the device is within. A very crude rough approximation is that the temperature rise will double if the power input is doubled. Temperature rise is the difference between the internal temperature with no input power and when power is applied.

In a stepping motor much of the power dissipated within the motor is from electrical current flowing thru the resistance of the windings. This power is the RMS current thru the resistance squared times the resistance. Or it is the integral of the instantaneous power divided by the integration time. For a non-chopper driver with constant excitation to one coil this is the DC current squared times the resistance. A constant DC current has an average value equal to its peak value and this is also its RMS value. In a chopper driver the average current will be less than the RMS current. Thus, a DC ammeter measuring the average current will not tell you the RMS current, but it will serve as an estimator.

In a series L-R circuit (inductance (coil) in series with resistance) the current changes slowly when a voltage is applied to the circuit. This is because of a fundamental characteristic of an inductor that its current can not change instantaneously. The current vs time curve has an exponential characteristic. The time constant of this curve is define as L/R in seconds for L in Henrys and resistance in Ohms. The lower the resistance the longer the time constant. The internal resistance of the motor coil is the lowest resistance in the circuit. By adding external resistance we can shorten the time constant and therefore increase stepping rate. Torque is a function of current thru the motor coil. By adding external resistance we lower the current for a given source voltage. Therefore when adding external resistance it is necessary to raise the source voltage to get the desired current. This external resistance disspates power that serves no useful purpose.

In a chopper circuit the external resistance is eliminated (eliminating its wasted power) and instead a fast acting switch is turned on and off to modulate the average current (real goal is RMS current) to the motor. Even though the L/R time constant is longer than with the external resistor, by sourcing the series LR circuit from a high voltage we can get high speed.

Either with or without the external resistance we source from a high voltage to get faster response, but with the chopper drive less power is wasted and you do not have to buy expensive resistors.

.

[Chunky]

I simple way to understand a chopper drive is to compare it to a car on the highway,
you could drive from a to b at a constant speed of 60, being comfortable to yourself and others, or could spurt out with bursts of excitement by zipping to 120, slowing down and again zipping ahead.
You will at the end travel the same path and would have taken the same time.

So-- keeping with your car analogy-- what's the benefit?

[drk]

let me correct that,

at the end, you would have taken less time and not the same:

that can be equated to, going the same no. of steps with the same motor in lesser time

[Chunky]

let me correct that,

at the end, you would have taken less time and not the same:

that can be equated to, going the same no. of steps with the same motor in lesser time

Namaste. I'm sure there is a reason, but why is it better to go from slow to fast rather than just fast constantly? Do the intermittent slow downs preserve the motor?

[ger21]

I think you're getting off track here.

A chopper drive let's you use a much higher voltage than the motors rated voltage. This higher voltage allows the current to flow through the motor faster, which gives you the ability to spin the motor faster, with more torque. The drive also keeps the current at a fixed level, so as no to damage the motor. A chopper drive is much more eficient, and can usually use a power supply with only 1/4 the current of a non chopper drive.

A non - chopper drive can also be run at higher voltages, but usually needs resistors to limit the current. These resistors waste power, produce a lot of heat, and can be quite expensive.

It has nothing to do with running fast at times and slow at others. The only thing is that a chopper drive will usually be capable of spinning motors faster.

[pminmo]

Chopper Drive as you refer to it is a method of current limiting. This is necessary when a power supply voltage higher than the rating of the motor is used. Current limiting resistors can also be used inplace of a chopper. Current limiting resistors will disipate heat and be large in wattage value if the power supply is significanly higher that the motor rating.

Phil

[Chunky]

Are all choppers boards bipolar?
Are all bipolar boards choppers?