Modeling motor control

Technical Motors
1

I’m trying to calculate the torque of a motor based on the voltage, speed, and current.
I’m actually interested in the torque at the output of the transmission, not simply at the output of the motor.
This means I need to know the no-load current of the motor.
I was thinking no-load current was proportional to voltage. Apparently it’s not.

Here’s a graph:

This is measured with a Jaguar over CAN, though I’ve confirmed the approximate current trend with a Fluke 179. (I didn’t care to take 10,000 samples by hand)

Some curious things about this:
There is a sudden drop in current at 10v (about 16800 RPM on the motor shaft)
At this drop, the sound of the motor changes. Before, it is somewhat buzzing, and afterwards it is quiet and smooth.
After the sudden drop, current continues to decrease.
Speed remains proportional to Voltage, regardless of current.

Some more about my setup:
I’m using an rs550 motor with a Banebots 20:1 planetary transmission. This is attached via chain and sprockets (at a 2:1 ratio) to a small wooden wheel. (The wheel serves as a temporary until I get around to making mecanums for my half-size testing 'bot.)

I’m guessing this particular point is specific to this motor, but has to do with the speed of the motor (as opposed to the duty cycle of the Jaguar).

Have you ever seen this before?
What do you think is happening?

2

Here’s a drill motor, with no gearbox or pinion attached . I think it’s from a 6v Ryobi.

Unfortunately, there’s no encoder on it, so I’ll have to assume the speed is proportional to voltage.
The current curve seems a lot more regular; there’s less noise, and it looks like a continuous function [perhaps 1/x*log(x) ].

3

i think that the current has that weird curve is because there is not enough load for it to run at near no load, but there is not enough load for it to run consistently at multiple speeds, try adding some load to the motor and see if that does anything.

4

Why is the plot of current a series of horizontal lines?

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5

account for error i believe.

6

If he is setting the voltage and measuring current, I would have expected the error bars to be vertical.

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7

that is a good point, perhaps it is just a method for filling in an area, but i would think that would make it a solid color.

8

I suspect the horizontal lines are due to quantization error in the current measurement and exacerbated by a large number of data points.

9

Good point Joe. That would explain it.

@Marshal: can you confirm this? Is the resolution of your current measurements ~130ma ? Also, is there noticeable change in the sound of the motor as the peak of the current curve is reached and the current starts to decrease?

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10

Yes, that is the resolution. I can’t get any data between those increments.
(By the way, there’s 10,000 points of data on these graphs. They’re in power increments of 1/1000, with 10 samples at each increment)

The data in these graphs have not been processed, aside from scaling speed to account for the gearbox ratio and encoder placement.

If you’re talking about the second motor, yes there is a change in sound, but it’s a lot harder to hear because there’s no sudden change.

11

How should I create additional load in a repeatable way?

What would this tell me?

EDIT:
Are you saying the motor speed isn’t repeatable?
Based on the first graph, we saw that the speed is directly proportional to current, with fairly tight control. It’s only the current that is doing funny things.
I can tell you that at full voltage, it’s operating pretty close to no load, under peak efficiency. Below that speed, I have no data. That’s why I’m doing this test.

12

Marshal, I know you have read this thread, but for others who have not, it provides some additional background discussion and links to external info about no-load current vs voltage.

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13

I’m not sure why my data differs so vastly from those assumptions.

14

Perhaps because you are comparing apples to oranges.

Do you have an FP or a Globe motor, like the ones referenced, that you could run on your test setup ?

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15

I have a globe motor, but the current is too small to measure with a Jaguar.
I don’t have any Fisher Price motors (but plenty of different drill motors).
I’m going to try to get ahold of a CIM with a toughbox and encoder, but I may not get that for a few days.

16

If you run a CIM with a toughbox, what do you expect to compare that data to? There is no manufacturer’s data for this. Still apples and oranges.

I have a globe motor, but the current is too small to measure with a Jaguar.

If you run the CIM w/o the toughbox, the no-load current is too low to measure with any accuracy with the Jag, isn’t it?

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17

The current just needs to be greater than 1A to be measured.

The point of measuring a CIM with a toughbox is to see if it still has these strange current characteristics.

18

Per the datasheet, the accuracy of the Jag’s current measurement is +/-2 amps for currents less than 8A. The no-load current for the CIM is 2.7 amps.

The point of measuring a CIM with a toughbox is to see if it still has these strange current characteristics.

The point of measuring something for which you have manufacturer’s data is to validate your test method. It is possible that the funky data is a problem with your test method.

Just sayin’

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19

Would it be better to hook up (not really) a magnetic current monitor outside the wires to measure current that way? It might have a more accurate current reading, if you can find a model that can handle such low currents and ignore any interference from the motor.

20

I have a old current clamp I’m trying to get working.
I found most of my motors are too small to be measured by the Jaguar.
There also may be some issues with varying Jaguar accuracy as the PWM duty cycle changes.