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Thread: Consequences of having P1000 VFD fed from a motor starter instead of a breaker.

  1. #11
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    Quote Originally Posted by GoldDigger View Post
    Certainly not. That makes the difference between verrrry large and infinite.
    But I would say that delta t is much shorter than 1 millisecond.

    Sent from my XT1585 using Tapatalk
    My point about i=Cdv/dt was in reference to Jraef's "The DC bus capacitors charge themselves instantly".
    Instantly would require dt to be zero and thus i would be infinite. We know that doesn't happen. That's all.

    In general VFDs have a precharge circuit as has already been mentioned*. I don't see why that would work any differently if the supply came from closing a breaker or closing a contactor.
    In fact, we did a good many VFD retrofits to replace fixed speed and used the existing starter. Worked fine. Obviously, I can't comment on code compliance/violation for this arrangement.

    *I have come across some VFDs (Alstom) that used SCRs (thyristors) at the front end to control the bucket capacitor bank charging current.
    Si hoc legere scis nimium eruditionis habes.

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    <Sigh...>

    Come on, you know that "instantly" was not meant to be taken literally. When I'm trying to help someone understand a new concept and I perceive they are going to be mostly just interested in the big picture and consequences, bogging the statement down with mathematic exactitudes is counter productive in my opinion. I wasn't trying to teach him how to build a VFD rectifier circuit, I was answering his question as to whether there was a risk in cycling power to a drive repeatedly. The calculation for rate of rise in the capacitor charging current is superfluous in this discussion.
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    Quote Originally Posted by Dzboyce View Post
    Jraef, thanks for that explanation. I normally use the Yaskawa P1000 drive. I have never noticed the 2,000 or so start limitation in their literature. However, I have seen this limitation stated in Hitachi VFD literature, and wondered why.

    So I understand, this limitation is not how many times the motor or load is powered on and off, It's how many times the drive is powered on and off?
    As GD said, yes. It's power up cycles, not motor operations. This is why there is a new trend in machine safely with VFDs to use what's called "Safe Torque Off", which means the drive has been tested and certified to safely remove torque producing energy from the motor during routine operational tasks without having to open a contactor or breaker ahead of the drive every time.

    The 2,000 number is not an exact figure, it's a generally accepted design concept; power up once per week for 7 years (which is the average lifespan of the capacitors) with a little fudge factor. I doubt Yaskawa builds their pre-charge circuit any better or worse than anyone else. It's a competitive world, but people who design crap don't last long, because sooner or later they run out of customers who have never bought from them or heard from someone who did and people who over design lose volume, which helps fund the research that keeps them in the game.
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    Quote Originally Posted by Jraef View Post
    <Sigh...>

    Come on, you know that "instantly" was not meant to be taken literally. When I'm trying to help someone understand a new concept and I perceive they are going to be mostly just interested in the big picture and consequences, bogging the statement down with mathematic exactitudes is counter productive in my opinion. I wasn't trying to teach him how to build a VFD rectifier circuit, I was answering his question as to whether there was a risk in cycling power to a drive repeatedly. The calculation for rate of rise in the capacitor charging current is superfluous in this discussion.
    Yes, I totally agree, it was superflous so no need to have brought it up in the first place.

    The question wasn't about cycling power to a drive repeatedly. It was whether a motor starter could be used.
    I don't see a problem with that. As I said in an earlier post, I've not only seen it, I've done it as well as the old song goes. The VFD doesn't care whether it gets its power from a breaker or a contactor.
    I agree that routinely stopping and starting it from there would not be a very good idea but could you anyway?. The VSD goes through a check cycle and (normally) won't start the motor just on applicatiion of power. It needs a go signal and all the health checks before it will run.
    Si hoc legere scis nimium eruditionis habes.

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    Ok true. The drive doesn't care if power comes from a breaker or a contactor.

    But a contactor implies a control circuit and that generally means it is going to be cycled on and off more than if it were just a breaker or fused switch. So the risk is not the device itself (other than a possible code violation), the risk is in allowing operators to cycle power too easily which generally results in too much. I just investigated a drive failure where they used a contactor that was INTENDED to be for E-Stop only. But the lazy operators slapped the E-Stop button every time they stopped the machine rather than hit the normal Stop button, resulting in the drive being power cycled 20 times per day. The only reason they used the E-Stop was because it was 6" closer to them than the normal Stop button ...
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    Quote Originally Posted by Jraef View Post
    Ok true. The drive doesn't care if power comes from a breaker or a contactor.

    But a contactor implies a control circuit and that generally means it is going to be cycled on and off more than if it were just a breaker or fused switch. So the risk is not the device itself (other than a possible code violation), the risk is in allowing operators to cycle power too easily which generally results in too much. I just investigated a drive failure where they used a contactor that was INTENDED to be for E-Stop only. But the lazy operators slapped the E-Stop button every time they stopped the machine rather than hit the normal Stop button, resulting in the drive being power cycled 20 times per day. The only reason they used the E-Stop was because it was 6" closer to them than the normal Stop button ...
    If you are doing a retrofit you ensure that the operator controls are linked to the drive, not the starter.
    Of course you have to consider the interface. That's just control system design.
    Si hoc legere scis nimium eruditionis habes.

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    Quote Originally Posted by Jraef View Post
    But the lazy operators slapped the E-Stop button every time they stopped the machine rather than hit the normal Stop button, resulting in the drive being power cycled 20 times per day. The only reason they used the E-Stop was because it was 6" closer to them than the normal Stop button ...
    Good argument for e-stops than can't be reset by the operator.

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    FWIW, the instruction manual says to refrain from switching the drive On and off more than once every 30 minutes.
    Tom
    TBLO

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    Quote Originally Posted by ptonsparky View Post
    FWIW, the instruction manual says to refrain from switching the drive On and off more than once every 30 minutes.
    During normal operation you shouldn't have to but it would a bit of a contraint during commissioning.
    Si hoc legere scis nimium eruditionis habes.

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    Quote Originally Posted by Besoeker View Post
    During normal operation you shouldn't have to but it would a bit of a contraint during commissioning.
    Gasp! Are you saying that it takes more than one try? My image has been destroyed.
    Tom
    TBLO

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