TECO 2HP VFD or motor tripping the contactor O/L

Jarrod · Apr 15, 2019

Yes, the contactor is on the input side. The vfd is rated for 2 hp (7.5A) and the motor is only a 1.5 hp (5.0 FLA). The setup is on my lathe. When I run the gearhead at the max speed (1500 rpm) it trips the contactor O/L after about a minute or less and I'm not even cutting anything! Running at 60 Hz I derated the vfd thermal protection for the smaller motor, but this is baffling me. I should not be drawing more than the vfd's input rating, using a smaller motor, should I?

I'm running out of ideas.

Any thoughts are appreciated.

Thanks.

atomarc · Apr 15, 2019

You won't like my thought. Dump the mag before the drive, set the drive up to the motors FLA and make chips! Problem solved.

Stuart

JST · Apr 15, 2019

What is the reason for the contactor?

I would probably put in a disconnect and whatever fuse or breaker the VFD requires instead. I see no particular reason for a contactor, since most VFDs can be programmed to either require a start signal after a power loss, or not.

A protector beore the VFD is of little or no use, since the VFD controls the current, causing the protector before it to not see the motor current, and many VFDs are rated to be motor protectors anyhow.

The VFD may pull current at a bad power factor, which increases the input current, but that will to some degree depend on the source. Is the source single or three phase?

Jraef · Apr 16, 2019

+1 on getting rid of the contactor and overload relay. First off, if it’s a bi-metal OL, it’s a known issue that the bi-metal strips heat up from the harmonics and nuisance trip meaning they trip at less than the rated current if you are only looking at fundamental (60Hz) current. Small VFDs with no reactor or DC bus choke can have have as much as 80-100% harmonic current distortion, meaning there can be as much distortion current flowing as there is fundamental current. Bi-metal OLs react to that.

In addition, using a contactor ahead of a VFD can cause undue stress on the part of the DC bus called the “pre-charge circuit”, a current limiting resistor used to keep the capacitors from pulling charging current so fast that it damages themselves or other components. If you burn out that resistor, the drive becomes a door stop.

Jarrod · Apr 16, 2019

Noted. Contactor et al will be replaced with a simple disconnect.

The manual suggests it as an option. I thought it was just an easy way to turn the supply voltage on and off, which it is, but its causing problems.

TDegenhart · Apr 16, 2019

If the contactor is in fact a starter with bimetal overloads and you are feeding single phase, the OL is probably tripping because of phase unbalance. This is capability that solder pots don't have.

Do your stopping and starting using the controls built into the VFD.

Tom

atomarc · Apr 16, 2019

I'll say this..my shop is bulging with frequency drives and they stay powered all the time...can't see any sense is cutting the power to them when I shut the door and turn off the lights.

Stuart

Jraef · Apr 17, 2019

Jarrod said:
Noted. Contactor et al will be replaced with a simple disconnect.

The manual suggests it as an option. I thought it was just an easy way to turn the supply voltage on and off, which it is, but its causing problems.

The manual is showing you that IF you need to use a contactor, it should be on the line side, not the load side*. Some machine safety regulations require that the VFD be automatically disconnected from line power when a Safety Relay detects an issue. More cap[able drives have what's called a "Safe Torque Off" (STO) feature added to them that is an approved Safety device that reliably kills power to the FIRING CIRCUIT of the transistors instead, which avoids that stressing of the pre-charge circuit. But low cost drives don't tend to have that qualification because it's expensive for the manufacturer to attain, so you would be forced to use a Safety Contactor, and again, the manual is just telling you WHERE to wire it if you must. If you are not using a Safety Relay system for your machine, you don't want to use the contactor.

*Many of the translated Asian manuals don't properly go into the details of this issue, they just show you a little picture of a contactor, you are supposed to know what that means because in countries OTHER than the US and Canada, the Machine safety directives are laws, not "suggestions" like they are here.

JST · Apr 18, 2019

We get thunderstorms. Have one now, in fact. I prefer if possible to disconnect electronics when there is a chance of high voltage spikes on the line. Of course during working time that is not possible, but.....

Vontactors are also not the same as disconnects, not so rated in general. So you would end up needing a disconnect even with a contactor, in general.

Jarrod · Apr 18, 2019

TDegenhart said:
If the contactor is in fact a starter with bimetal overloads and you are feeding single phase, the OL is probably tripping because of phase unbalance. This is capability that solder pots don't have.

Do your stopping and starting using the controls built into the VFD.

Tom

Thanks Tom,

All the controlling of the motor is done using the vfd's control board, with a 3-wire start/stop and fwd/rev configuration. The contactor was simply (or so I thought) a way to turn the line power on or off.

My wood lathe was OEM supplied with a vfd and only a power cord, which is only plugged in when the machine is in use. I wanted to have something requiring less bending of the knees.

SAF · Apr 18, 2019

Jarrod,

If you can remove the overload relay from the motor starter, you should still be able to use the contactor as you intended. Show us what your working with.

SAF Ω

Technical Ted · Apr 18, 2019

Is it an older motor? Is it rated to run off a VFD? Does the motor get very hot after running a while. Have you taken an amp readings on BOTH the primary and secondary of the VFD? Here's the reason for these questions... I bought a used SB lathe with an original 2HP 3-phase motor. I believe the lathe was made in the 70's. The motor ran very hot at 60Hz even when not pushing it at all. Voltages were fine. Checking current draw on the primary was OK, but the secondary was WAY HIGH! I talked to the tech where I got the Teco drive (FactoryMation) and he said that can happen on some older motors. He wasn't sure if it was due to harmonics or what, but said "it happens sometimes".

I purchases a Siemens vector rated 3-phase motor and my problems when away. Runs very cool even at lower Hz.

Just food for thought...

YMMV,
Ted

Jarrod · Apr 21, 2019

SAF said:
Jarrod,

If you can remove the overload relay from the motor starter, you should still be able to use the contactor as you intended. Show us what your working with.

SAF Ω

Fantastic idea! Worked like a charm and got me back up and running!. The motor is a new Leeson 1.5 hp, which is running cool and clean. The vfd is also derated to limit the current to the smaller hp.

Thank you!

Jarrod · Apr 21, 2019

Technical Ted said:
Is it an older motor? Is it rated to run off a VFD? Does the motor get very hot after running a while. Have you taken an amp readings on BOTH the primary and secondary of the VFD? Here's the reason for these questions... I bought a used SB lathe with an original 2HP 3-phase motor. I believe the lathe was made in the 70's. The motor ran very hot at 60Hz even when not pushing it at all. Voltages were fine. Checking current draw on the primary was OK, but the secondary was WAY HIGH! I talked to the tech where I got the Teco drive (FactoryMation) and he said that can happen on some older motors. He wasn't sure if it was due to harmonics or what, but said "it happens sometimes".

I purchases a Siemens vector rated 3-phase motor and my problems when away. Runs very cool even at lower Hz.

Just food for thought...

YMMV,
Ted

Thank Ted. I did check the current and voltages on both sides of the drive. That's what was driving me crazy! It didn't make any sense. The motor is brand new albeit tampered with to change the mounting. It runs relatively quiet and very cool. I've pulled the O/L and am not having any more issues. We will see how she goes on a long (relative) production run.

Regards,

J.

Jarrod · Apr 21, 2019

Pictures

Here is what I'm working with...

The Lathe is a Standard Modern 11x20.

Thanks for all the chatter!

J.

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