PowerPro Input Power

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BuckeyeDennis
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Re: PowerPro Input Power

Post by BuckeyeDennis »

I see that the scope’s blue current input is now AC-coupled, whereas before both channels were DC-coupled. That may be introducing the relative phase shift on the display.

Regardless, the rough calibration seems to indicate that the current measurements are generally valid. So I still can’t figure out why the peak inrush current was only 15A. It didn’t look as if the current probe was saturating, either. I’m probably assuming something about the input circuitry that doesn’t match reality.

Here’s a thought. It’s possible that we’re only powering up a small electronics power supply when the switch is first closed. In that case, the controller would power up the big motor power supply when it was ready to run the motor. That would actually be pretty smart, as any contact bounce on the power switch would be settled out long before the big current draw happened.

A good next experiment would be to trigger the scope when the motor powers up. I’d bet that a trigger threshold of 20A would work for that purpose.
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Re: PowerPro Input Power

Post by RFGuy »

BuckeyeDennis wrote: Tue Jul 20, 2021 2:26 pm Regardless, the rough calibration seems to indicate that the current measurements are generally valid. So I still can’t figure out why the peak inrush current was only 15A. It didn’t look as if the current probe was saturating, either. I’m probably assuming something about the input circuitry that doesn’t match reality.

Here’s a thought. It’s possible that we’re only powering up a small electronics power supply when the switch is first closed. In that case, the controller would power up the big motor power supply when it was ready to run the motor. That would actually be pretty smart, as any contact bounce on the power switch would be settled out long before the big current draw happened.
I am still trying to understand the logic of this thread and what is intended with the measurements here. What presumptions are you guys making about the in-rush current of the PowerPro headstock? I can't comment on the PowerPro specifically, but most "good" designs for SMPS have circuitry to limit the inrush current on startup, often by means of a startup ramp circuit. So, just because the PowerPro is rated 1.75HP doesn't necessarily mean its in-rush current is around 15A on initial startup, but rather is being limited to a certain level. Also, just to be clear, isn't this just the controller and power supply being measured at startup, i.e. the motor is not spinning correct?
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edma194
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Re: PowerPro Input Power

Post by edma194 »

The motor is not spinning at power up. And it is only spinning at power down due to an emergency, a mistake, or loss of power.
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DLB
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Re: PowerPro Input Power

Post by DLB »

BuckeyeDennis wrote: Tue Jul 20, 2021 2:26 pm I see that the scope’s blue current input is now AC-coupled, whereas before both channels were DC-coupled. That may be introducing the relative phase shift on the display...
Good catch Dennis, thank you. I'm using the high amp range on the probe and at these low currents I was seeing about -5mV of offset, so I switched to AC coupling then forgot all about it. But I had saved a trace with DC coupling and you are right.

- David
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BuckeyeDennis
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Re: PowerPro Input Power

Post by BuckeyeDennis »

RFGuy wrote: Tue Jul 20, 2021 2:40 pm
BuckeyeDennis wrote: Tue Jul 20, 2021 2:26 pm Regardless, the rough calibration seems to indicate that the current measurements are generally valid. So I still can’t figure out why the peak inrush current was only 15A. It didn’t look as if the current probe was saturating, either. I’m probably assuming something about the input circuitry that doesn’t match reality.

Here’s a thought. It’s possible that we’re only powering up a small electronics power supply when the switch is first closed. In that case, the controller would power up the big motor power supply when it was ready to run the motor. That would actually be pretty smart, as any contact bounce on the power switch would be settled out long before the big current draw happened.
I am still trying to understand the logic of this thread and what is intended with the measurements here. What presumptions are you guys making about the in-rush current of the PowerPro headstock? I can't comment on the PowerPro specifically, but most "good" designs for SMPS have circuitry to limit the inrush current on startup, often by means of a startup ramp circuit. So, just because the PowerPro is rated 1.75HP doesn't necessarily mean its in-rush current is around 15A on initial startup, but rather is being limited to a certain level. Also, just to be clear, isn't this just the controller and power supply being measured at startup, i.e. the motor is not spinning correct?
I can’t speak for DLB, but I’m just curious to see what kind of power-supply circuitry the PowerPro has. There used to be a lot of reports of RF interference from the motor drive, and I still see reports of the drives tripping GFCI’s, so the power-input waveforms might shed some light on these issues.

The last time I designed a motor drive was about 10 years ago, and those were industrial drives. Back then, industrial drives generally still had “dumb” input rectifiers, although consumer equipment such as PC’s typically had PFC front ends that would allow the controls to limit the inrush current without a lot of extra hardware. The industrial motor drives that I designed at the time, the biggest being 10hp, had simple precharge circuits that would charge the main capacitor bank through a power resistor, prior to closing the main power contactor and operating the motor.

What was confusing me was that DLB’s waveforms showed no sign of the switching activities of a PFC front end. And I calculated an input impedance that was much too high, by maybe an order of magnitude, for a 1.75 hp motor drive. Which led to my current hypothesis that so far, DLB has only measured the power-up of a small non-PFC power supply for the control electronics.
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Re: PowerPro Input Power

Post by DLB »

RFGuy wrote: Tue Jul 20, 2021 2:40 pm I am still trying to understand the logic of this thread and what is intended with the measurements here. What presumptions are you guys making about the in-rush current of the PowerPro headstock?
I started the thread mainly to continue a side discussion from another thread. There was already a complaint that the dialog had strayed from the subject. And I thought the PowerPro and PFC Rectifier discussions were detracting from what I thought was an important and relevant point about unswitched neutral conventional headstocks. Dennis had also mentioned the possibility that PowerPro switch contacts should be wired in parallel and I thought it worthwhile to explore that. Anything else for me is just a chance to learn a bit about how the PowerPro works.

The following traces are with the motor spinning for a 3450 RPM headstock speed. The first two are steady state at 3450, the others are during startup. The current trace is much noisier at the higher current levels. For these I was using the current probe set to 1mV/10mA. On the first trace the scope measured trace 2 at 248 mV RMS (2.48 A RMS) while the DMM indicated 2.04 Amps.
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I set the trigger level to catch this, it occurs fairly early in the spin-up of the motor. I'd have a hard time explaining this as anything but software.

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This one is triggering near the time the motor reaches commanded speed. Setting to the trigger at a higher level, 600mV for example, results in no trigger at this speed. Note the gradual increase of Amps in each cycle.

- David
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Re: PowerPro Input Power

Post by RFGuy »

I haven't designed motor drives, so I will defer to Dennis here. However, I do have some experience with power management circuits, so from my experience what I see in the latest plots from David is what looks like about a 22.5kHz switching harmonic riding on top of the input current waveform (it isn't noise). I wonder if this 22.5kHz frequency is fixed or varies with speed or load? Keep in mind this is a switched reluctance motor, so both the voltage and current waveforms going to the presumably 3 phases (could be more) of the motor are constructed (see link below). In order to optimize the torque, a custom waveform for voltage and current is created to drive each phase of the stator. It would be interesting to see these waveforms, but may be difficult/impossible to measure them directly on a PowerPro. So, it is not just some switch turning on each phase of the motor, i.e. it is more complicated than that. It is still interesting to observe from the input waveforms to investigate, but you may not gain much insight without knowing what is going on between the motor drive (SMPS) and the motor itself.

Sorry, I wasn't trying to be obtuse, but sometimes a post like this one catches me off-guard. It just helps me to know what the goal is or what you intend to investigate. I don't know if any of my comments are helpful here or not. If the latter, just move on with this thread without me. I just find discussions about the PowerPro and the circuitry inside interesting so I wanted to understand better what was being presented...

It would be interesting to see the current waveforms under some load at 10,000 RPM since there has been quite a bit of discussion regarding the PowerPro overheating during extended high speed sessions.
sw_reluc_motor.pdf
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P.S. Here is a good primer on SRM's:
https://chargedevs.com/features/a-close ... ce-motors/

Also, I found a write-up on Striatech's technology in e-Drive magazine from 2017, but I had to pull it out of the web archive. Most of it was an overview with limited technical details, but I found the line below to be interesting. It confirms that they have designed it so that they minimize transient currents, e.g. on power-on/off, etc. Based on this one sentence, I would expect that in-rush current should be tightly controlled for the PowerPro by the microcontroller and SMPS.

"A Striatech drive has the ability to turn off and on with no power spike, providing a low-voltage start and no wear on the motor when turning it off or on."
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edma194
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Re: PowerPro Input Power

Post by edma194 »

When power is removed the controller stays running for a few seconds, detects the loss of power and displays the low voltage message. Unless the controller or power supply fails it may be going through an orderly shutdown process every time, even if the switch is thrown or power lost when the motor is running.
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Re: PowerPro Input Power

Post by DLB »

I think 'noise' is a relatively common description applicable to several aspects of switching power supplies. Where I worked we had a number of different 'specialty engineering' teams, so we had our very own 'RF guys.' :D IIRC this phenomenon comes under the heading of Conducted Emissions. Our equipment was carefully tested to ensure emissions were below a defined threshold in multiple frequency spectrums, and for susceptibility to said emissions. But I was only referring to difficulty in reliably triggering the scope on either channel in 'normal' mode. In hindsight, I'm the only one seeing the scope in normal mode so I'm just whining. ;) But it would be difficult, at best, to reliably trigger the scope at a specific point due to these emissions.

I did two steady state 10K RPM checks for info. I'm planning to pop the hood and look at the motor drive (to the extent practical, two current probes would be better) soon. I don't recall any access to the terminals for the motor drive, and suspect I will be limited to current waveforms with existing equipment. The DMM read roughly 8A at 10K RPM. Current probe set to 1mV/10mA. I didn't try to capture any spin-up information at 10K because I think the 3450 traces tell that story.
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- David
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Re: PowerPro Input Power

Post by DLB »

I found the power lines to the motor to be inaccessible without removing the lower belt and tilting the motor. For these measurements only the motor is spinning, without the mechanical load presented by the headstock. So for perspective I repeated the max RPM measurements, it is noteworthy that amps measured with the DMM changed from ~8A spinning the headstock to ~2.25A without the load. First trace is input voltage and current at 10K unloaded:

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There are six power lines between the power supply and motor, along with several signal lines. Because of the absence of a load, I don't know how representative these are. The first sample trace:

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I haven't come up with an idea to present much meaning from the current traces with only one current probe. In the absence of other ideas I decided I could try to run the motor at 3600 RPM to synchronize with line voltage. I calculated a commanded RPM of 6000 at the upper spindles (not turning) would equal 3600 at the motor. There was a bit of drift that was correctable with the RPM control. At 6001 RPM commanded, the current waveform was well synchronized with line:

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So if all my math to that point was correct there are eight repetitions of the current drive waveform per rotation. Note that the pulse just right of center on the scope is significantly different than the others. With the scope in normal mode, that timing relationship was constant. I found that interesting, but don't know why it would be that way. Possibly to correct speed?

- David
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