DC COMPOUND MOTOR

I agree with everything that "GW" has stated, however, there is one condition that has not been addressed. If the motor is not near field saturation and it's speed is being limited by the external load, it is possible that the motor will increase speed with the increase in field current (due to the extra torque produced by the increase in field current). If it is your intention the increase the field current then you must pay attention to the increase in the motor winding temperature which may be disastrous for the motor.

Hope that this helps.

IA.

DC motor operates under two modes namely constant torque and constant power.In the constant torque mode speed variation is achieved by varying the armature voltage.The rated speed of 1750 rpm is at rated field and armature voltage.Constant torque mode is below the rated speed and constant power is above base speed(Field weakening range)The max speed is decided by the commutation current limit and the strength of mechanical parts.Assuming that you are holding the armature at rated voltage and increase the field current the speed would drop.The power rating of the field coil will determine how much you can increase. Normally all dc motors have class F or superior (Now a Days Class H) so temp will not be a limiting factor but yes power rating will be

If Class F insulation was used, you can believe the manufacturer used all of the capacity to reduce copper! There is no room, unless it was specified as Class F insulation with say a class B rise, and that incurs a large $ premium for the motor.

Class F temp at design is limited to 145Deg C where as max temp permissible is 155 Deg. Earlier it was 140Deg with a 15Deg cushion for hot spot which now a days is limited to 10Deg.So this can be availed if required though safety margin will be nil if entire margin is used.But with a 10Deg cushion I feel 1 amp increase by way of temp. reflection should hardly matter as it is not likely to result in 10Deg rise.

Power (heat) increases with the square of the current. Now lets assume the ambient is 45 deg C, and the rise is 100 C. Assume the excitation voltage is 240volts. Then the power dissipation is 3x240=720 watts. If we increase the current by 1 amp, the excitation voltage increases to 320 volts, the power increases to 1280 watts, and the thermal temp increases to 45 + 100 x 4/3^2= 277 C. The 1 Amp is very significant! This is also the average temp of the winding and does not include the hot spot rise of the coils. The internal temperature may be much higher.

GOT A DC COMPOUND MOTOR, RATED FILED CURRENT OF 3 Amp, @ RATED SPEED OF 1750 rpm. WHAT MIGHT HAPPEN TO THE MOTOR CHATACTERISTIC -SPEED , TROQUE, ETC - IF THE FIELD CURRENT IS INCREASED TO 4 Amp. Thanks for your corporation. Hope you all had a happy new year 2008. All what you said are correct, i wanted to clear all misunderstanding with other colleagues at work. Actually this motor is on on ship to shore crane. 8 dc motor connected in 4 parallel lines with two motors in series. When one of my colleague increased the field current, to avoid over current on these motors, which is not correct according to the motor name plate. Last time i noticed that the speed is not the same and is less than 1750 rpm which is true, and the motor is getting more heat from power loss. Do feel free to contact me for any help. Thanks Dharmarajen.