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Member
Engineering Fields - Power Engineering - New Member

Join Date: Jun 2009
Location: hydrabad
Posts: 8

RTD Induction Voltage for 11KV Synchronous Motors

12/27/2009 8:55 AM

Dear sir,

I am working in irrigation project in india we have 11 kv syncronous motors rating is 13.6MW while during trail runs of the motors we have been observed that the RTD'S(winding RTD'S) sensing induction voltage of nearly 145 volts(A.C) These RTD'S are connected with PLC.My consultant says that this voltage can damage the PLC modules.

we have taken a precautions, the junction box of RTD'S and RTD sheath also grounding has been done still the problem is same as mentioned above.

Kindlysuggest solution for this problem

happy new year to all

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Anonymous Poster
#1

Re: Induction voltage in RTD'S in 11KV MOTORS

12/28/2009 2:49 AM

Imstrument cables & power cables should be run seperatly from each other ....

The shield should be connected at one end only.

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Engineering Fields - Power Engineering - New Member

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Location: hydrabad
Posts: 8
#3
In reply to #1

Re: Induction voltage in RTD'S in 11KV MOTORS

12/28/2009 6:23 AM

instruments cables and power cables are leid separatly with distance of 600 mm

the shield are connected at one end only

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Power-User
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#2

Re: Induction voltage in RTD'S in 11KV MOTORS

12/28/2009 5:50 AM

When you wind a electrical motor and places RTD's inside this winding, it is important that the start and return pad of the RTD are in the same direction.

Do not make a circular pad with het RTD connection leads.

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Power-User
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#4

Re: Induction voltage in RTD'S in 11KV MOTORS

12/28/2009 7:31 AM
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Guru
Canada - Member - Specialized in power electronics

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#5

Re: Induction voltage in RTD'S in 11KV MOTORS

12/28/2009 9:12 AM

If you used a high impedance meter to measure this 145V, it might be only low energy coupling that will be negligible once connected to the low impedance input.

Connect a 1K resistor to ground (or a small light bulb) and measure it again. At these voltages, a few pF of capacitance can generate many volts in a 1Mohms meter.

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Guru
United States - Member - New Member Engineering Fields - Power Engineering - New Member

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#6
In reply to #5

Re: Induction voltage in RTD'S in 11KV MOTORS

12/28/2009 8:49 PM

Bingo, GA.

I often see people assuming they are seeing "real" voltages when in fact they only see it with a very high impedance digital multimeter. Using an older analog meter movement (D'Arsonval) which places a burden on the circuit will make that reading go away. The question then is, how does the PLC input read it? If you are using a specific RTD input module it probably takes that into design consideration, that's why you don't want to use a standard analog voltage input module for RTDs. If you don't have an RTD input module, or the one you have has high impedance (why it would I don't know), then you may need that resistor mentioned by marcot even in your PLC circuit.

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Guru
Canada - Member - Specialized in power electronics

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#7
In reply to #6

Re: Induction voltage in RTD'S in 11KV MOTORS

12/28/2009 9:55 PM

Thanks JRaef,

To read a RTD, the plc has to supply a current from a relatively low impedance source. The RTD usually has 100 ohms. While the analog input will have a high impedance, the source that supplied the reading current will have a relatively low impedance. This should bring the RTD voltage close to "ground".

If the coupling is only a few pF, everything should be fine when connecting to the PLC. BUT I would test first with a resistor load or the like as if it is a short to a coil, the result could be dangerous.

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