Determining Field current of seperately excited Dc Motor

[mash98]

Good afternoon everyone I need to determine the rated field current of a seperately excited dc motor? while the field voltage mentioned on the name plate is 250 volts dc.
Below are the Motor's Nameplate Data.

Rotor rated voltage:600 dc
Base speed:500 Rpm
Rated current:1632 amperes
Rated Power 950 Kw
Rating continous
Required forced cooling:7000 CFM (supplied forced cooling 14800 CFM)

Regards

 

[Variac200]

I think for a rated field current in a separately excited dc motor is derived from the Voltage divided by the Impedance in the field winding.

 

[mash98]

Thank you but at what temperature? As the impedence increases with temperature.We may have different reading at different temperature so what is the standard temperature in determining field impedence?

 

[mikekilroy]

since the field is dc, the rated amperage of your field is simply 250vdc/R. It is usually assumes room temp (25C).

IF you want confirmation, just look up various dc field wound motors on line and you will rated amps is exactly rated volts dc/ resistance of winding.

Often there are 2 ratings: first is the above mentioned value that will make motor go 500rpm with rated armature voltage no load, second is MIN field amps - used to tell you where to set your adj speed drive if using field weakening for higher than base speed.

 

[mash98]

thank you for this information it sure has helped me in determining the rated field current of the above motor.Which I have calculated as 18.5 amperes while earlier it was set at 16 amperes and sure was one of the reasons for producing less than the rated torque.But I still want to be sure if it is really 25 degree celsius? Can anybody provide the reference?

 

[waross]

A sanity check is the voltage required to force the desired current through the field coil. Field coil rated voltages are generally standard voltage ratings. A 600 Volt motor may have a 600 Volt field but possibly less.
If you find that the voltage required is a couple of percent below an even number, the difference in voltage from an even number will be related to the increased resistance when the field warms up.
In one of the first plants I worked at we had a synchronous motor that was started by a power circuit breaker and had a manually adjusted field voltage. One of the last jobs of the night electrical crew was to start the motor in readiness for the production shift and adjust the field voltage/current. One of the first jobs of the day shift electrical crew was to re-adjust the field voltage/current as the increased resistance when the field warmed up would have caused the field current to drop a few percent.
If you are setting protection devices allow a margin for possible higher field currents until the field warms up.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[mash98]

dear Waross we have this motor having 950kw, armature voltage 600 dc, field voltage on nameplate is 250vdc, the motor is being run with Mentor II drive with field current controller the field current was set to 16 amperes by the time of its commissioning some 15 year ago.And recently we were having some tripping issues and I suspect this is due to low field current which is now 16 amperes and seems less than rated current. while I calculated 18.5 amperes by dividing field voltage with field resistance(measured when field temperature was 28 degree Celsius).Now before I set this new calculated current I want to know the reference/standard as it is usually being said that resistance should be measured with the field temperature around 20-25 degree Celsius in order to calculate field current.Can somebody provide any reference for the same?

 

[waross]

When the current is set at 18.5 Amps, what is the voltage?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[mash98]

It varies from 240 to 278 depending upon the field winding temperature. In the morning I have figure it out that 16 amperes were actually the rated current by the following procedure.
I set the current at 16 amperes and gradually increasing the motor rpm and measuring both field current and armature voltages simultaneously and at the moment where the armature voltages just touched its rated volts i.e. 600 volts and field just started to begin weakening I checked motor's rpm directly at its shaft it were just a bit higher than its base rpm (say around 1%).So it seems to me the correct field amperes and make sense.

 

[waross]

I would run the motor under normal load until the motor temperature stabilizes. I would then set the voltage at 250 Volts and read the Amps. If the field current at 250 Volts on a hot motor is reasonable I would use that current value. I may go 1% or 2% lower on the current if the motor is not heavily loaded and so not up to the maximum working temperature.
That's the best I can do without more information.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[mash98]

The motor is being run with Mentor II drive with a seperate field controller and we can only can give current value in the parmeters which adjusted voltage itself to maintain the set current till base speed. If the motor is running above base speed(which is usually the case) a sudden increase in load increases field current upto the set current limit which in turn decreases motor speed and increase its torque to meet the load requiremnt.Here I intent to go about 5% above rated field current & I beleive it would not give much harm to the motor as the operational speed is above base where the field current is around 70% of rated field current about more than 95% of the time in a cycle so an increase of 5% or so should not do any harm and we can meet sudden load increase demand with compromise on speed which is acceptable.