Speed drops when load increases....







Some times, even when the load is well within the limit of the rating of the drive and that of the motor, is is observed that speed drops when the load is increased. There can be following reasons for this.

1.    The drive is working under Voltage Feedback Control and that IR compensation ( i.e. the compensating action to compensate for the Resistive Drop of voltage in the Armature Resistance ) is not adjusted properly. IR compensation is not to be applied when using Tachogenerator Feedback arrangement.

2.    If only four thyristors are firing out of six, an increase in load can push the drive in current limit it it is set very critically without any margin left. When four thyristors fire, average current in the motor is less but the peaks are higher. These higher peaks cause the drive to go in Current Limit. A quick check on Oscilloscope will reveal this immediately.


More on IR compensation...

IR stands for I into R. i.e. Armature Current multiplied by Resistance of the Armature winding. In a dc motor the speed N is given by the formula  N= (V- IR )/Flux.  V is the applied voltage, Flux is the magnetic filed in the air gap between stator and the rotor. Considering the magnitude of IR compared to that of Voltage applied, one can ignore it. IR is not more than 5 % of full armature voltage. Speed of the motor therefore, can be simply assumed to be proportional to the applied voltage. To get somewhat closer to the real fact, one can generate a signal proportional to I x R by sensing actual armature current and use to compensate the error in N by subtracting IR from V. Thus, a feedback signal which is proportional to V-IR rather than only V is more accurate and able to control the speed more closely. Excessive compensation of IR can cause overcompensation and may tend to increase the speed as the load increases. This is not desired.  






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Block Diagram



Ramp Circuit


Speed Amplifier


Current Amplifier


 Firing Circuit


Power Circuit


Types of DC drives 



 Checking Thyristor


Voltage feedback Vs Tacho feedback


Current Limit


Constant HP & constant Torque


Field Weakening


Using CRO 



Motor runs at full speed


Fuses Blow


Hunting in speed


Motor gives jerks


Belts vibrate


Speed control not satisfactory 



Speed drops on load


Motor overheats


Sparking on commutator


DC drives and Power Factor


Your comments and suggestions 


Drive stops all of a sudden 


Motor not able to drive the load


DC to DC isolation


 Motor or Drive?