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randyjaco
09-26-2011, 11:16 AM
I have a Clausing drill press with a 2 speed 3 phase motor. On the press is a 2 speed drum switch. Everything works well on the drill press, but I have a constant problem with it tripping the TECO phase converter and giving me an OC-C error. This only occurs when I use low speed, never on high speed. It will run in low speed but often when I switch to OFF I will get the error. The phase converter is remoted, so it is becoming a PIA. I have to go to the phase converter, turn the phase converter completely off to reset it or I have to turn the remote on/off switch to the Off position and return to the P/C and push reset to reset the phase converter.

I presume that it is something in the drum switch, because it happens less often when I switch to OFF very quickly. The slower I switch to OFF the more it trips the phase converter.
Any suggestions as to how I can fix the problem?
TIA
Randy

EVguru
09-26-2011, 11:31 AM
From your description you have a Variable Frequency Drive/Inverter, not a phase converter.

Secondly, although some drives can tollerate it, you usually shouldn't have any switching and/or overload protection between the VFD output and the motor.

The current drawn by the motor will be different between the two speeds. You'd have to switch between two full load current settings in the VFD to give full protection.

Dawai
09-26-2011, 11:36 AM
Guessing the drum switch changes speeds on the motor connections?

Yes.. he's correct about no motor circuits being broken on a vfd.

OC.. over current , OV.. over voltage trip. Both say "danger will robinson" cause the transistors in the vfd just took a lick.

IF you leave the Drum switch in the motor circuit at the least do not switch it while the inverter is powered up.. AND the motor is drawing two current levels.. the inverter can only be programmed for one probably.. (I don't know everything).

J. R. Williams
09-26-2011, 12:14 PM
Are you switching the output line of the converter ? The unit should be switched on and off with the converter, not switching the output lines.
JRW

randyjaco
09-26-2011, 12:25 PM
Yes, basically I have a wall plug connected to a VFD that I can plug various machines into. I take it that that is a No No.
Could I safely do the wall plug with a RPC instead? I have both.

Randy

Dawai
09-26-2011, 01:51 PM
RPC is similar to a Normal 3phase source.. yes.. you can switch it on and off..
You should have current protection in the circuit somewhere for the motor tho.. contact heaters or a thermal overload. AND a drum switch.. not really designed for arc suppression to start and stop a motor.. OK?? it arcs each time you turn it on.. will for a while.. then the contacts are gone..

THE feedback circuitry in the inverter is what gets you.. some of it is really good.. I saw a hitachi VFD shut down when "spanish" worker beat a conduit in two shorting the wires out.. AMAZING.. to go from running waveform to SHUT DOWN in protection mode in milleseconds.. microseconds probably..

JoeFin
09-26-2011, 02:29 PM
Secondly, although some drives can tollerate it, you usually shouldn't have any switching and/or overload protection between the VFD output and the motor.
.

I know you have heard the argument before but I believe the NEC and OSHA still require the "Mechanical Air Gap" means of disconnection for motors with non-concealed moving parts. We used to provide a Motor Relay after the VFD to provide this and program in an auxiliary confirming contact from the relay to the drive.

From my recollection of the article the VFD can provide "Overload Protection" but doesn't meet the requirements for "Mechanical Means of Disconnection"

In this case the switch is more then likely not breaking all 3 phases at an equal time

J Tiers
09-26-2011, 10:16 PM
if the VFD is accepted by UL as an overload protector, it's acceptable as-such.

There is a provision for VFDs accepted by UL as disconnects, but I do not know of any such.



Yes.. he's correct about no motor circuits being broken on a vfd.

OC.. over current , OV.. over voltage trip. Both say "danger will robinson" cause the transistors in the vfd just took a lick.



Actually, the VFD can tolerate BREAKING the motor circuit just fine. It does that internally several thousand times per second, and has no problem with it. I design specialty motor-drive VFDs from time to time, there isn't a particular issue with transients, etc.. The IGBTs (or , as used to be, mosfets) turn off pretty fast in normal operation, and the motor current is simply taken by the anti-parallel commutation diodes.

For that matter, I have an inverter (really a single-phase VFD) that I use out in the shed. I have been turning motors off and on when connected to that for years. Some of the motors are universal, with commutators, associated arcing, etc. It has been going strong probably 12 years with that "terrible abuse" on a regular basis. Even if it quit tomorrow, it would not have been from switching motors.

The REAL problem with using a switch after the VFD is that it will almost certainly trip the overload on start-up, unless the VFD is oversized significantly. The surge when a motor is started by "dropping across the line" is often 6x the full load current, and the VFD is rarely rated for more than 1.5 or 2 times the motor current, unless deliberately over-sized.

The problem noted by the OP is likely more to do with the drum switch itself than with the fact of switching.

lakeside53
09-26-2011, 10:22 PM
Around here you either have a disconnect at the VFD input if it's with the machine, or disconnect at the machine if the vfd is not collocated (such as remote pump packages). This is to enable safe servicing, not as machine "on/off". A plug at the vfd input within sight of the machine suffices. Machine control is separate via VFD lv inputs.