Voltage drop with voltage spike on opposite poles of single phase.

[Fred B]

Went on service call for lights dimming in a smallish woodworking shop after installing a large planer 5hp 230v single phase, (I didn't do the install), planer has a 2 stage motor, when starting it will drop current to 108.1 volts to neutral either pole, quickly returning to a balanced 122.2v. While I was metering came across a different curiosity. When the worker ran a chop saw, single pole 120v, voltage dropped to 119.2 while opposite pole raised to 124.4v on mains (planer not running). All of shop seems to be wired on mwbc. Shop only provide with 200A at entry. Not sure if I have 2 unrelated things here or both symptoms of the same issue. Could the voltage drop with corresponding spike be related to the mwbc, an issue with the new chop saw, or not an issue at all? Neutral ground voltage. 0.1V doesn't change with either event. None of the circuits are marked so not sure which one is the circuit for chop saw, also a larger load, still 120V, (unverified) of a dust collector was operating at same time as the chop saw, reportedly on a different circuit (unverified). They were shutting up the shop for the day so unable to test further, will be going back when they reopen on Monday to measure amperage and trace to identify circuits.
Any suggestions as to a direction to look for the problem of the chop saw, or is it a non-issue, maybe loose neutral connection at saw circuit? I think I might also check the other wiring connections for loose fittings?
Not sure if the large voltage drop associated with the planer is a normal condition with such a large device and the 2 stage starting capacitors? The spec sheet says it's designed is that it could be a 3 phase or single phase, looking at the wiring diagram it looks like it just a connection difference on the motor controller, not familiar with 3 phase equipment to know for sure, (not like I'm going to alter any of that). They also want a run time meter put on the planer, for scheduled maintenance on the planer.
Thanks.

 

[winnie]

Current flow always causes voltage drop, and a 5hp single phase motor will draw lots of current to start.

First thing to do is evaluate how long the service conductors are and what gauge they are, and if you can get the info the Kva and impedance of the service transformer. Then calculate if the voltage drop you measured matches what you should expect.

It is very possible that nothing is broken, simply that the service is undersized for the connected loads.

Same with the large 120V load. There might be a bad neutral connection, but very likely the neutral is simply too small.

Jon

 

[Fred B]

Did voltage drop calculations (don't have transformer info from utility) and came up with that it shouldn't be more than about 2.7 volts thus the approximate 28 volt drop definitely seems excessive. Would a "soft start" device be an appropriate addition to fix the voltage drop?
The other thing having trouble getting my head around is the small drop on L1 with concurrent equivalent spike on L2 with mfg rated 12.6A saw load

 

[ActionDave]

It sounds like you have a bad power feed to the woodworking shop. Is it fed directly from the utility or from a main building somewhere else on the property?