Our client currently is utilizing a Yaskawa P1000 VFD for a 125 HP pump motor. They are powering the VFD from a Size 5 motor starter. Are there any issues with having the P1000 VFD fed from a motor starter instead of a feeder breaker? Is there any adverse effect? When I asked manufacturer, they simply told me they don't want unnecessary impedance element in front of the VFD. But I am still trying to figure out if there is any consequence of having the starter feed the VFD that we should be aware of. Any help is highly appreciated.
Consequences of having P1000 VFD fed from a motor starter instead of a breaker.
- Thread starter Aru
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Every VFD must have what is called a "pre-charge circuit". The DC bus capacitors charge themselves instantly and will pull current at a rate commensurate with the AVAILABLE FAULT CURRENT at the rectifier terminals if allowed to. That rate of rise of the cpacitor charging current can damage the rectifier components and the caps themselves, even the transistors down stream. So to prevent that from happening, the pre-charge circuit uses some sort of method of limiting that capacitor charging current for the first few cycles whenever a VFD is powered up. most of the time, this is a current limiting resistor in series with the DC bus, then once the caps are charged, that resistor is removed from the circuit with a shunting contact (relay or contactor depending on size). In really small VFDs, this is sometimes done with an NTC (Negative Temperature Coefficient) thermistor, a type of resistor that has a high resistance when cold and the resistance drops as temperature rises. So it limits current for a second, then is almost non-existent (saves from having to add the relay). In some mid-size drives (which varies by mfr) the rectifier uses SCRs instead of diodes so the SCRs are ramped into full conduction which limits the current just like a soft starter, but that means adding a ramp control and firing board and SCRs are more expensive than diodes. So economically, they go back to resistors at some point.
If your VFD uses the resistor or thermistor method, those components have a finite life, usually around 2,000 operations. If you only power a drive up and down for maintenance or utility line loss situations, you never get to 2k operations in a normal lifetime of a VFD. But if you power up and down once per shift, 2 shifts per day, you get to 2k in less than 3 years. Once the pre-charge resistor burns out, the drive doesn't power up or if the relay/contactor welds, the rest of the components fail very quickly.
If your VFD uses the SCR method, you may never see a problem. The tricky part is going to be getting Yaskawa to tell you what they use on YOUR specific VFD, 125HP is right in the bread basket where some mfrs use that. But if they won't tell you, sometimes you can tell by looking at a recommended spare parts list. If it lists an SCR or rectifier firing board as a component, that means is isn't using a resistor.
If your VFD uses the resistor or thermistor method, those components have a finite life, usually around 2,000 operations. If you only power a drive up and down for maintenance or utility line loss situations, you never get to 2k operations in a normal lifetime of a VFD. But if you power up and down once per shift, 2 shifts per day, you get to 2k in less than 3 years. Once the pre-charge resistor burns out, the drive doesn't power up or if the relay/contactor welds, the rest of the components fail very quickly.
If your VFD uses the SCR method, you may never see a problem. The tricky part is going to be getting Yaskawa to tell you what they use on YOUR specific VFD, 125HP is right in the bread basket where some mfrs use that. But if they won't tell you, sometimes you can tell by looking at a recommended spare parts list. If it lists an SCR or rectifier firing board as a component, that means is isn't using a resistor.
ptonsparky
Tom
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That is the way our POCOs control load during peak demands. VFD or not. So far, I have not had failures due to that.
IDK how rapid cycling of power would affect the VFD, but I would try to limit that.
I used to be able to use the POCO equipment to directly interface with the vfd, but policies have changed.
IDK how rapid cycling of power would affect the VFD, but I would try to limit that.
I used to be able to use the POCO equipment to directly interface with the vfd, but policies have changed.
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Did a little sluething for you, mostly just to satisfy my curiosity. From Yaskawa's spare parts list for a 139A rated P1000 drive...
Separate issue; If it is a combination motor starter with an MCP breaker in something like an MCC, you probably cannot use it anyway. Most factory built combination motors starters will use a "Mag-Only" breaker because there is a motor thermal overload relay taking care of the thermal current. But that combination is only allowed when feeding a motor DIRECTLY. The VFD is required to have Branch Circuit Protection ahead of it, and an MCP is not allowed to be considered as the BCP other than directly feeding a motor. If by chance your starter has fuses or uses a thermal-Mag breaker, then that's OK, just not an MCP.
Looks like it is a resistor type pre-charge circuit and a diode front end, so using a contactor ahead of the VFD is going to cost you in terms of service life of the drive. I would recommend against it.
Separate issue; If it is a combination motor starter with an MCP breaker in something like an MCC, you probably cannot use it anyway. Most factory built combination motors starters will use a "Mag-Only" breaker because there is a motor thermal overload relay taking care of the thermal current. But that combination is only allowed when feeding a motor DIRECTLY. The VFD is required to have Branch Circuit Protection ahead of it, and an MCP is not allowed to be considered as the BCP other than directly feeding a motor. If by chance your starter has fuses or uses a thermal-Mag breaker, then that's OK, just not an MCP.
Dzboyce
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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?
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?
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Yes.
i=CdV/dt
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And when you close a contactor at other than the voltage zero crossing, dV/dt can be verrrrry large.
Even at zero crossing, the first cycle dV/dT can be very large compared to the dV/dT near the peak of the sine wave (which will be the prime charging current once the capacitor is precharged.)
But the dt is not zero.And when you close a contactor at other than the voltage zero crossing, dV/dt can be verrrrry large.
Even at zero crossing, the first cycle dV/dT can be very large compared to the dV/dT near the peak of the sine wave (which will be the prime charging current once the capacitor is precharged.)
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Certainly not. That makes the difference between verrrry large and infinite.
But I would say that delta t is much shorter than 1 millisecond.
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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.
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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.
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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- San Francisco Bay Area, CA, USA
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- Electrical Engineer
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.
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.
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- Electrical Engineer
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 ...
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.
zbang
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Good argument for e-stops than can't be reset by the operator.
ptonsparky
Tom
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- EC - retired
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.
ptonsparky
Tom
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- EC - retired
Gasp! Are you saying that it takes more than one try? My image has been destroyed.
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