Grounding and three-phase ungrounded delta

[Chip Meyer]

Please excuse me, as machines that I design are typically DC or gas powered. I'm a relative newbie when it comes to AC.

I've been asked to design a scissor-lift truck that is powered by a customer supplied power source of 480 volt 3-phase ungrounded delta. From what I understand, this is three wires, potential across any two wires is 480 Volts. I will be using this power to run a 480 VAC motor, and also be sending 480 into a transformer to get 120 VAC for a control circuit. My problem lies with safety and grounding.

How should I ground this unit for safety? Should I have anything on the source side of the transformer? Should I ground the motor in some way? Do I need a ground on the 120 V side of the transformer, if so, just ground one leg to the frame of the machine?

Any help would be much appreciated. I've got a schematic and wiring diagram of an old unit on the way from the customer, but I'd like to double check your ideas versus the original design.

Thanks

 

[tom baker]

This topic is way beyond what we can answer here or you should attempt to design.
480V is very unforgiving.
This is an Industrial DIY question, IMO. I'm going to lock this post. If you disagree send a PM to the chief moderator, Charlie B

 

[charlie b]

The Forum was created as a vehicle for professionals in the electrical industry to give and receive assistance in understanding or performing their job-related tasks. We are not permitted to assist persons who are not electricians to perform their own electrical installation work. After an exchange of Private Messages with the OP, I have come to the conclusion that this is not a DIY question. It is a job-related task, and it is a design task assigned to an engineer.

Therefore, I am going to reopen this thread. That does not guarantee that any member will be able (or willing) to provide any helpful advice. The topic is beyond the scope of the NEC, and this is an NEC Forum.

I know, for example, that the NEC has rules that govern the grounding of portable electric generators. This question, however, relates to a portable machine that uses, not generates, electrical energy.

If any member has experience in this type of design, please feel free to offer your opinions.

 

[Onitram]

......... To build it for 480 delta!! will it have its own generator? this is though projet and it will need many devices to control, variable speed drives and the whole works, I was a machine builder some time ago, cant you just use dc motors on it? I mean by making the 3phase in to dc power..........

 

[scott thompson]

Ungrounded Low Voltage 3 Wire Delta systems do not have any SYSTEM CONDUCTOR Bonded to a Grounding Electrode System (to be referenced as "GES" in this message), whereas a Grounded A.C. System will have one system conductor bonded to a GES.

BTW: "Low Voltage Systems" reference = 600VAC L-L, and less

That said, an Ungrounded System _WILL_ have _BONDING_ Conductors, which are terminated to a GES on one end, and Metallic Equipment at the other end - like a typical Equipment Grounding Conductor would (to be referenced as "EGC").

Include EGCs and terminate them as usual on these circuits. Do the same with the control Transformer _Unless_ the Control Circuitry needs to be from a Grounded Power System. In this case, you have a few things to think about!

Keep in mind, when terminating the EGCs, no Bonding Jumpers to System Conductors are used, and in conjunction, no Grounded System Conductor is present in any Panelboard, Distribution Section or Service Section.
Termination of EGCs used for new Branch Circuits or Feeders should land on Equipment Grouning Bus Bars (if present), or bonded directly to _Known To Be Bonded_ Metallic Enclosures of Panelboards, or to the Grounding Bus Bars of Distribution & Service Sections.

Scott35

 

[Chip Meyer]

As this maching replaces an existing unit, I asked the customer to supply a schematic of the existing maching. From this I did find that they had an incoming ground line to the frame of the machine. They did not however have any ground on the 120 V control circuit side of the Transformer.

I plan to run the supplied ground to the frame of our machine, as well as a wire from the transformer on the 120 V side.

Note: this machine is very simple. It basically consists of a motor to pump hydraulics to the scissor lift cylinder and the drive motor, a transformer with fuses on input and output, starter (contactor) with overload heaters, and switches and solenoids for controls. Also there are various hydraulic valves to control functions.