Help me Understand Three Phase Soft Start Current Draw - Page 1
Forum section: electronics / general

author: Xplode
author_rank: Contributor
timestamp: January 31, 2016, 04:44:23 am
content: Hi guys,I'm new to the forum, so hopefully this type of question is ok (and in the right spot). Apologies if it's not.I am an electrician by day, and I actually like understanding why things work the way they do. Helps me be better at my job and certainly helps with troubleshooting.Today I was tasked with a motor upgrade on an industrial band-saw. The motor is a 3phase 460V 20HP 4 Pole AC Motor. We removed an older one that was beginning to underperform (suspect open rotor bars, unrelated). They are only .2A difference in Full Load ratings (old 24.4A, new 24.6A)The old motor was a 12 lead, and started with a traditional WYE-DELTA style starter, operating with a 5second delay before delta.The new one is only a 9 lead, and so we "upgraded" to a ABB Branded Soft Starter (PSE45-600-70). Here's where things get a bit funny.The Wye-Delta system always started the motor with no issues.The new Soft Start system can't get the motor started before the feeder trips a breaker. I've tried multiple settings for duration of ramp up, started at 30, 40, 50, 60, 70% voltage, and tried the various torque boost options. The drive has a kickstart option, but the bandmill is just entirely too much weight to get moving quickly for that to be of any help.I spent a half hour on the phone with the company rep, who used some ABB software to run sims on a pile of different settings and we couldn't come up with anything better than I had.Now the real question for you guys is this - The drive we selected only uses 4 SCRs to switch the A, and C phases, leaving B phase directly connected LINE to MOTOR. The switching action on the other 2 phases varies the voltage and current draw on those windings, but the B phase draws more current because of this configuration. I didn't have a power meter that could measure multiple phases simultaneously, so I don't have concrete numbers to work off of, but it seems awfully close to that magic 57.7% difference seen in open-delta transformer configurations. Am I on the right track here?The rep did say that the 6 SCR versions of the drive don't have that increased draw, and could potentially help solve our issue.Thoughts? Opinions? Links to educational material?

author: HAL-42b
author_rank: Frequent Contributor
timestamp: January 31, 2016, 05:02:43 am
content: So you are definitely sure the ABB guy is not trying to up-sell you.Are you also sure that the old motor was indeed WYE-DELTA and not something odd like Double-DELTA (Constant Power)? This is often found on machine tools. With 12 leads it is even possible to be Triple-Delta (Three speed - Constant HP).

author: Xplode
author_rank: Contributor
timestamp: January 31, 2016, 08:09:07 am
content: I am actually not 100% sure. I ripped out the starter, but I took a couple pics of the setup first so I will have to sit down in the morning and look it over.I am not overly familiar with the configurations your are describing. can you link to any relevant materials i could read up on to understand this setup? I was only aware of the Wye-Delta configuration for low voltage starting of motors (at least without resistor packs).

author: HAL-42b
author_rank: Frequent Contributor
timestamp: January 31, 2016, 08:43:19 am
content: Here is a typical double delta wiring for a machine toolProvides two speeds with constant power. This means there is a low speed with high torque and high speed with low torque. Both same kW.It is possible to provide 3 speeds by splitting the phases into 3, which incidentally means there would be 12 leads.Not done very often these days but used to be common before the inverters became mainstream.There are many other winding types, you need to know what you are dealing with. Most of the time there is a good reason they went to the extra trouble of choosing an oddball.

author: Fank1
author_rank: Regular Contributor
timestamp: January 31, 2016, 12:47:35 pm
content: What kind of breaker? Thermal only or inst. + Thermal?I have not worked with the 4 SCR version only the 6 SCR but at 4KV 1500 HP.You should understand that scr soft starters only limit the average current not the peak current.The old delta wye also limited the inst.You may have to get a thermal only breaker.

author: Xplode
author_rank: Contributor
timestamp: February 01, 2016, 01:32:57 am
content: Its a residential grade SquareD 100A Thermal breaker. I do not believe it has a magnetic component, so its not fault-tripping due to sudden inrush. In fact the motor gets most of the way through its startup before the breaker trips out. I don't have a tach, but I'd guess we're up to 90% of sync speed based on what i can hear from the saw.There's an odd system in place to power this thing that is bringing 208V 3ph power up to 460v via a Transformer. It is this feeder 100A 208V breaker that is tripping.It is no where near the correct way to doing it (since there is 600v available in the builder I would have used that and gone down OR ordered a 600v motor since the machine has taps to operate its controls from that voltage also).I haven't heard the "Double Delta" term, but now see what you meant but It doesn't actually provide two speeds - The speed is dictated by the frequency of the AC input (hence VFD theory). Speed = (120*Freq)/Poles. Also the motor was 230/460 rated, and operating at 460, so I don't believe it could have been operating in that sort of startup configuration anyway (would have to be wound for 460/920V). If it was being fed 208/230v then you can series the windings (ie connect in HV mode) to run at a reduced starting voltage, and then connect parallel to fully appreciate the power of the motor once it starts to spin up. I just don't think you can connect the motor in the high voltage configuration in any other way than the Wye-Delta approach (the other motor was not wound for higher than 460 in any configuration, so no single winding could safely accept 460v)Haven't had a chance to do much more reading on the topic, but I am hoping to tonight.EDIT: re-worked some thoughts