Speed does not become zero even when potentiometer is made zero....

 

 

 

 

 

 

Some times, too much of gain in the Speed Amplifer makes the motor to rotate very slowly even at zero speed reference signal. The only method to prevent this from happening, if at all it is critical, is to reduce the gain without compromising on performance at higher speed levels. If the drive has a provision to apply a biasing signal of opposite polarity as that of the main speed reference signal, applying a bias may also solve this problem. This may add a small non-linearity in the overall performance.

Unstable or erratic Speed control.

Speed variation can be gradual over time or erratic and unpredictable. If the variation is gradual it could be arising from heating of some component, especially in the feedback circuit. Sudden or erratic variations can be caused by several reasons. Dry solder in reference path, leaky capacitors in Ramp circuit are just two of many reasons.

Motor starts only after reference voltage is increased substantially, but the control is okay once the motor is started...

This is a very peculiar situation. As the drive reference voltage is slowly increased, the motor does not repspond for quite some voltage. The motor starts running at a much higher speed. However, once started, the reference voltage has good control and the speed can be reduced almost till zerp without problem. This is caused by unequal supply voltages to the opertional amplifiers in RAMP circuit. Normally the voltages are +15 and -15 volts. If these voltages are unequal, e.g. +14.3 V and 16.5 volts, then the situation described above is arises. Given below is a typical arrangement of generating +ve and -ve voltages without using 3 pin voltage regulators. The selection of zener diode in one of the sections may reqire trial and error method to get exact equal voltages at the output of NPN and PNP transistors.

 

 

 

 

 

Download PDF version of these pages.

 

 

 

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?

 

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