Understanding a stepping motor nameplate
SteveD
Posts: 64
The nameplate on a stepping motor reads 3.2VDC, 3.25A.· I assumed this indicated 3.2VDC was the maximum input voltage the motor could withstand yet when I went to the manufacturers web site they had graphs showing torque curves at different voltages.· They indicated the input voltage was 24VDC and even 48VDC for this very same motor.· I obviously do not understand how to read the nameplate for stepping motors.· Could someone help me out?
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Thanks,
Steve ·
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Thanks,
Steve ·
Comments
Dave
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Dave Andreae
Parallax Tech Support·
This is one aspect of the problem.
The motor you have was rated at 3.2VDC and 3.25Amps for optimal performance. Ask your self if the motor really cares about these numbers or what are the destructive limits of the device.
They don't use 3.2VDC enamel wire to construct the device as enamel wire insulation is usually rated at a much higher insulation value. The thing that damages the device is heat that burns away the insulation and causes a short. They use the same wire in a 100VDC motor as a 3.2VDC motor is the diameter is useful.
So the real limiting factors are actual resistance and impedance of the coil, the voltage at which the insulation breaks down, and the temperature at which the insulation breaks down. These all go back to the wire size and the coil design.
A rough guess would be that 3.2 volts times 3.25amps equals 10.4 watts. So this is a likely limit of both how much power it can produce mechanically and how much heat it can physically tolerate at one instance.
Going backwards 10.4watts divided by 48 volts equals 217 millamps.
Seems to me that if the insulation is adequate and the amperage is roughly limited to 217 millamps, it would work at 48 volts just fine. After all, there are no brushes in a stepper motor and brushes are often the weakest link in other kinds of motor.
I suppose that 24VDC would do fine with a limit of 434 millamps for the same reasons.
It all converts to Watts.
The real dilemma might be what happens in a stall condition where the current isn't limited. I suspect smoke and sparks.
And regarding the impedance, it would be hard to calculate without testing under actual load., but may require you to add a margin of safety to your current limiting. Let's guess 10% lower.
You are not the only one confused. I see people freely swaping different rated stepper motors with minimal concern for what the lable says. It is not unusually for them to use a higher than rated voltage as a matter of convience.
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"If you want more fiber, eat the package.· Not enough?· Eat the manual."········
The main spec to keep your eye on is the current rating. What the voltage rating means is that, in a steady state (i.e. full coil saturation), if 3.2V were being applied, the coil would draw 3.25A. This means that the DC resistance of the coil is close to 1 ohm. But stepper motors are often not in a steady state. Coils are typically pulsing on and off rapidly and, with bipolar steppers, changing polarity. Because a coil is an inductor, the rated current does not begin to flow in it as soon as a voltage is applied; rather, it approaches the saturation current in a negative exponential fashion. This can cause a problem when coils are being switched rapidly, in that they will never reach their rated current; and this, in turn, limits the amount of torque the motor can produce at high speeds.
In order to overcome this limitation, many stepper drivers use a much higher voltage than the motor is "rated" for. With a higher voltage applied, the currents in the coils ramp up more quickly and reach their rated current much sooner, thus increasing the motor's torque at high speeds. But if the current increase under these conditions is allowed to continue unchecked (e.g. when the motor is being held in one position), the coils' rated current would quickly be exceeded, with likely damage to the motor. For this reason, such drivers include current sensors which detect when the rated current is reached in each winding, so it can turn off or "chop" the supply to that winding. Then, as the current diminishes, the winding can be turned back on, and so forth.
Another technique, which is less popular now than in years past, is the "L-4R" method. This involves putting a resistor in series with each coil, whose resistance is 4 times the resistance of the coil. This way you can drive the combined resistor-coil with a voltage that's 5 times as high as the "rated" voltage — even in a steady state — without damage to the motor. The reason that this is advantageous is that when the coil starts to be energized, it will be drawing very little current. So the voltage drop through the resistor is near zero, and the coil will see nearly all of that 5x supply voltage. Thus, the coil is forced to conduct more current sooner. But as it does so, the voltage drop in the series resistor increases, too, limiting the maximum current to the rated current. The reason this type of drive has fallen out of favor is its lack of efficiency and the consequent heat it produces. In your case, a 4-ohm resistor passing 3.25A would be dissipating more than 40 watts! That would require a huge heatsink.
I hope this explanation helps. Steppers can be tricky beasts when you want to wring the most performance out of them. Fortunately, the IC manufacturers have stepped up to make the job a little easier. Check the product offerings of both Allegro and ST Micro for examples.
-Phil