Voltage sensing motor starters?

We have a lot of equipment in our facility that have multiple motors powered from one disconnect or power plug. The motors are 480 volt, and we like to monitor the 480 power to the motors to make sure that the power is enabled when we try to start the motor with the PLC. We install a 480v to 120v transformer and run a relay to send a signal to the PLC. We do this because the motor starter will still pull in if no 480 power is present.

For single motor setups this works great, as we put the transformer after the disconnect and fuses. On multiple motor setups however, we only monitor the power after the main fuses in the cabinet.

Has anyone used/seen/heard of a motor contactor that does not pull in if the line side 480 power is not present?

Never seen that build into a motor stater. But there are"Phase Monitors" to do that sort of thing.

http://www.google.com/#pq=ph+monito....,cf.osb&fp=21e6f9b0f7b4c3b3&biw=1268&bih=690

http://www.google.com/url?sa=t&rct=...3ej1Cw&usg=AFQjCNF6ZNY5TYveXRdI4N_8__uOUuj9dQ

Thanks, I did not know about those. They look like something worth looking into for our application.

I use them in every panel that controls a motor.

Helps to prevent single phasing a 3 phase motor.

We use the Time Mark brand and really like them. They're very easy to change since they mount in a standard relay base.

Why not just put auxiliary contacts on the disconnect. I have seen kg blocks with an extra pole for just this purpose. I believe square d makes one.

Rube said:

I use them in every panel that controls a motor.

Helps to prevent single phasing a 3 phase motor.

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A electroic overload works also.

The disconnect being on does not necessarily mean that 480 power is present in the cabinet. In our facility the 480 power is distributed via a bus duct system, which has its own fuses and disconnects. We want to actually verify there is 480 power going to the motor starter. Additionally, if you only monitor the disconnect handle and the motors are individually fused inside the cabinet, you will not know if any of the motor fuses fail.

I like the monitor relay idea because they are small and can replace our current transformer to relay setup in one package, as well as adding phase loss protection.

Paul351W said:

The disconnect being on does not necessarily mean that 480 power is present in the cabinet. In our facility the 480 power is distributed via a bus duct system, which has its own fuses and disconnects. We want to actually verify there is 480 power going to the motor starter. Additionally, if you only monitor the disconnect handle and the motors are individually fused inside the cabinet, you will not know if any of the motor fuses fail.

I like the monitor relay idea because they are small and can replace our current transformer to relay setup in one package, as well as adding phase loss protection.

Click to expand...

If your budget can swing it I would suggest the models with the live phase indicators. These will tell which phase is below the min voltage level. Alot of techs get confused with checking solidstate fuses.

I know im jumping in late and you have gotten alot of great advice so far.

We are installing these on our central hydraulic system along with a new control system. Have a look and see what you think.

http://http://www.automationdirect....l_Relays/Phase_Monitoring_Relays/PMRU-1C-480A

Clay B. said:

If your budget can swing it I would suggest the models with the live phase indicators. These will tell which phase is below the min voltage level. Alot of techs get confused with checking solidstate fuses.

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Do you have any examples of these type of devices? I haven't seen any with phase indicators in my searches so far.

Has anyone used/seen/heard of a motor contactor that does not pull in if the line side 480 power is not present?

Click to expand...

Paul, bluntly, yes, 95% of every motor starter that was installed in the U.S. 40 years ago!

That's right, the old NEMA motor starters (with included control transformer) could NOT pull in if no 480 volt power was present. Of course the reason was that the 480 also supplied the contactor control power, through the control transformer.

This is one useful function that has been lost with the widespread adoption of PLCs to control motors. The individual starter control transformer has gone out the window, because most PLC programmers do not want a bunch of individual power supplies which need isolated PLC outputs. But if you used isolated relay output modules, and individual control transformers on your starters, then your contactors WILL NOT pull in if there is no 480 volt power.

We gain some ground, but we lose some things along the way.

Why even use fuses for regular motor starter circuits ?
I understand that the shortcircuit breaking capacity is less for circuit breakers when 'higher voltages' such as 480V, 660V or 690V is used.
But when I check the catalogs, for 480V circuit breakers can be used up to approx 4 kW.

(edit: Siemens has a series of circuit breakers with "increased short circuit breaking capacity" in size S3. These can break 16kA at a rated current of 100A ~ 70kW motor @ 480V. I guess they are expensive though).

Motor circuitbreakers have these advantages
a. disconnects all phases.
b. allows an interlock via an auxiliary contact, hardwired or to the PLC.

Above 4kW or with 660V or 690V, you probably have no other way than to use fuses.
In that case, may I suggest to measure the current in all phases. It is a better safety than measuring the voltage at the source, as it does not check if a wire have come loose somewhere downstream. We do that for all vibrating equipment. Current transformers with integral switch are not expensive.