I'm currently dealing with an issue involving a piece of equipment in our shop. It's a 240V 3-phase rated device that was originally installed on an electrical system using an isolating delta-delta transformer to step the voltage down from 600V to 240V three-phase. During its ten-year tenure in our facility, we encountered no significant problems. However, after selling it to another company last year, they've chosen to employ an autotransformer with X0/H0 terminals for the same voltage conversion. Since its installation, more than 20 out of the existing 45 servo drives have experienced malfunctions requiring repair or replacement. My suspicion is that these issues stem from the XO/HO terminals of the autotransformer not being grounded. I believe this grounding should be achieved by either connecting a neutral from the primary side or grounding the transformer itself. Presently, the XOHO configuration is floating, and I suspect this might be an incorrect installation. However, the electrical engineer overseeing the setup argues that this is the intended configuration. So, my question is, do I need a neutral connection, or at the very least, does the autotransformer need to be grounded to prevent these issues?
Do I need to connect the neutral and ground my three-phase autotransformer if the autotransformer is with the neutral option?
- Thread starter JohannW
- Start date
synchro
Senior Member
- Location
- Chicago, IL
- Occupation
- EE
First of all, I think an autotransformer is not the best for this application. To transform 600V to 240V three-phase, the output windings of the autotransformer would need to supply 60% of the input voltage, which is then subtracted from the input voltage to get 240V L-L. With that large voltage ratio, an autotransformer will provide little or no advantage.
And for sensitive equipment, it provides very little isolation from any voltage spikes, noise, etc. on the input.
In 210.9 "Circuits Derived from Autotransformers" says:
""Branch circuits shall not be derived from autotransformers unless the circuit supplied has a grounded conductor that is electrically connected to a grounded conductor of the system supplying the autotransformer.
...
Exception No. 2: In industrial occupancies, where conditions of maintenance and supervision
ensure that only qualified persons service the installation, autotransformers shall be permitted
to supply nominal 600-volt loads from nominal 480-volt systems, and 480-volt loads from
nominal 600-volt systems, without the connection to a similar grounded conductor."
There is not an exception for transforming between 600V and 240V. But I don't know what the Canadian code provides.
I believe the lack of a connection from neutral to the H0/X0 could very well have caused the failures that occurred.
Asymmetric L-L loads could have caused some voltage imbalances. But I think what might be most important is when the autotransformer is energized, because if one of the legs on the iron core saturates then there would be a very low impedance between L-X0/H0 on that leg. That could then pull the X0/H0 voltage towards the 600V line voltage if it's not connected to neutral, which could then overstress the equipment.
The neutral should not be bonded to equipment ground in an autotransformer.
And for sensitive equipment, it provides very little isolation from any voltage spikes, noise, etc. on the input.
In 210.9 "Circuits Derived from Autotransformers" says:
""Branch circuits shall not be derived from autotransformers unless the circuit supplied has a grounded conductor that is electrically connected to a grounded conductor of the system supplying the autotransformer.
...
Exception No. 2: In industrial occupancies, where conditions of maintenance and supervision
ensure that only qualified persons service the installation, autotransformers shall be permitted
to supply nominal 600-volt loads from nominal 480-volt systems, and 480-volt loads from
nominal 600-volt systems, without the connection to a similar grounded conductor."
There is not an exception for transforming between 600V and 240V. But I don't know what the Canadian code provides.
I believe the lack of a connection from neutral to the H0/X0 could very well have caused the failures that occurred.
Asymmetric L-L loads could have caused some voltage imbalances. But I think what might be most important is when the autotransformer is energized, because if one of the legs on the iron core saturates then there would be a very low impedance between L-X0/H0 on that leg. That could then pull the X0/H0 voltage towards the 600V line voltage if it's not connected to neutral, which could then overstress the equipment.
The neutral should not be bonded to equipment ground in an autotransformer.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
If you connect the H0/X0 point to ground you will be affecting the 600V system also.
Leaving a Wye-Wye connection floating is no different than using an ungrounded Delta-Delta connection.
If the new owner brought in new equipment with filters connected to ground, you can expect failures if the system is ungrounded.
I agree you need a true isolation transformer, maybe a 600V to 240Y/138.
Leaving a Wye-Wye connection floating is no different than using an ungrounded Delta-Delta connection.
If the new owner brought in new equipment with filters connected to ground, you can expect failures if the system is ungrounded.
I agree you need a true isolation transformer, maybe a 600V to 240Y/138.