dc variable speed drives, abb 500 horsepower control cabinet

DC Variable Speed Drives

The first electric motors were designed and built for operation from direct current power.The following should be considered when choosing DC variable speed drives for production:

  • Wide speed range
  • Good speed regulation
  • Compact size and light weight
  • Ease of control
  • Low maintenance
  • Low cost

dc variable speed drive, 500 horsepower ABB variable speed drive

In order to determine if DC variable speed drives have the capability to provide the above characteristics, the DC drive has to be analyzed as two elements that make up the package. These two elements are the electric motor and the control.General purpose DC motors used on nearly all packaged drives have a very simple performance characteristic; the motor shaft turns at a speed directly proportional to the voltage applied to the armature.The Armature amperage of a DC motor is almost directly proportional to output torque regardless of speed. This characteristic indicates that a small fixed amount of current is required to turn the motor even when there is no output torque. This is due to the friction of the bearings, electrical losses in the motor materials and load imposed by the air in the motor winding.

DC motor speed is primarily determined by the applied armature voltage and DC motor torque is controlled by armature current.Understanding these concepts of DC motors provides the key to understanding the performance of DC variable speed drives. DC variable speed drives use a control module. This control module rectifies alternating current power and converts it to direct current for DC motor control. The control module also controls the DC output voltage and amperage in response to various control and feedback signals. The feedback signals then regulate the DC motor performance via speed, and torque.

The regulating function of DC variable speed drives provide an electronic circuit that monitors a number of inputs and sums these signals to produce an error signal. This error signal is processed and transformed into precisely timed pulses that are applied to the gates of the Silicon Controlled Rectifier(SCR) in the power bridge, thereby regulating the power output to the DC motor.In order to control DC motor speed accurately it is necessary to provide DC variable speed drives with a feedback signal related to motor speed. The standard method of doing this in a simple control is by monitoring the armature voltage and feeding it back into the regulator for comparison with the input set point signal.When armature voltage becomes high, relative to the set point, established by the speed potentiometer setting, an error is detected and the output voltage from the power bridge is reduced to lower the motor’s speed back to the “set point”. Similarly when the armature voltage drops an error of opposite polarity is sensed and the control output voltage is automatically increased in an attempt to re-establish the desired speed. The “Armature Voltage Feedback System” which is standard in most packaged drives is generally called a “Voltage Regulated Drive”.

A second and more accurate method of obtaining the motor speed feedback information is called “Tachometer Feedback”. In this case the speed feedback signal is obtained from a motor mounted tachometer. The output of this tachometer is directly related to the speed of the motor. Using Tachometer Feedback gives a DC variable speed drive improved regulation characteristics. When “tach feedback” is used the drive is referred to as a “Speed Regulated Drive”. Most DC drives are capable of being modified to accept tachometer signals for operation in the tachometer feedback mode.In some newer high performance “digital drives” the feedback can come from a motor mounted encoder that feeds back voltage pulses at a rate related to motor speed. These (counts) are processed digitally being compared to the “set point” and error signals are produced to regulate the armature voltage and speed.

In addition to the normal external adjustment such as the speed pot, there are anumber of common internal adjustments that are used on simple small analog type DC variable speed drives.The following is a description of the functions that these individual adjustments serve and theirtypical use.

Adjustments Of DC Variable Speed Drives & Motors

DC Variable Speed DrivesIn most cases when DC variable speed drives are initially installed, the speed potentiometer can be turneddown to its lowest point and the output voltage from the drive will go to zero, then the motor will stop.There are many situations where this is not desirable, because there are some machinesthat need to be kept running at a minimum speed and accelerated up to operating speed asnecessary. There is also a possibility that an operator may use the speed potentiometer tostop the motor to work on the machine. This can be a dangerous situation since the motor hasonly been brought to a stop by zeroing the input signal voltage. A more desirable situation iswhen the motor is stopped by opening the circuit to the motor or power to the control usingthe on/off switch. By adjusting the minimum speed up to some point where the motorcontinues to run even with the speed potentiometer set to its lowest point, the operator mustshut the control off to stop the motor. This adds safety into the system. The typicalminimum speed adjustment is from 0 to 30% of motor base speed.

The maximum speed adjustment sets the maximum speed attainable either by raising the inputsignal to its maximum point or turning the potentiometer to the maximum point. On a typicalDC motor the rated speed of the motor might 1750 RPM but the control might be capable ofrunning it up to 1850 or 1900 RPM. In some cases it’s desirable to limit the motor (and machinespeed) to something less than would be available at this maximum setting. The maximumadjustment allows this to be done. By turning the internal potentiometer to a lower point themaximum output voltage from the control is limited. This limits the maximum speed availablefrom the motor.

Digital DC variable speed drives are capable of constantly monitoring the DC motor current. The current drawn by the armature of theDC motor is related to the torque that is required by the load. Since this monitoring and controlis available an adjustment is provided in the drive that limits the output current to amaximum value.This function can be used to set a threshold point that will cause the motor to stall ratherthan putting out an excessive amount of torque. This capability gives the motor/drive combination the ability to prevent damage that might otherwise occur if higher values oftorque were available. This is handy on machines that might become jammed or otherwisestalled. It can also be used where the drive is operating a device such as the center winderwhere the important thing becomes torque rather than the speed. In this case the current limitis set and the speed goes up or down to hold the tension 0f the material being wound. Thecurrent limit is normally factory set at 150% of the motor’s rated current. This allows the motorto produce enough torque to start and accelerate the load and yet will not let the current(and torque) exceed 150% of its rated value when running. The range of adjustment istypically from 0 to 200% of the motor rated current.

IR compensation is a method used to adjust for the droop in a motor’s speed due to armature resistance. As mentioned previously, IR compensation is positive feedback that causes thecontrol output voltage to rise slightly with increasing output current. This will help stabilize the motor’s speedfrom a no load to full load condition. If the motor happens to be driving a load where the torque isconstant or nearly so, then this adjustment is usually unnecessary. However, if the motor is driving a loadwith a widely fluctuating torque requirement, and speed regulation is critical, then IR compensationcan be adjusted to stabilize the speed from the light load to full load condition. One caution isthat when IR compensation is adjusted too high it results in an increasing speed characteristic. This meansthat as the load is applied the motor is actually going to be forced to run faster. When thishappens it increases the voltage and current to the motor which in turn increases the motor speed further. If thisadjustment is set too high an unstable “hunting” or oscillating condition occurs that is undesirable.

The Acceleration Time adjustment performs the function that is indicated by its name. It willextend or shorten the amount of time for the motor to go from zero speed up to the set speed. It alsoregulates the time it takes to change speeds from one setting (say 50%) to another setting(perhaps 100%). So this setting has the ability to moderate the acceleration rate on the DC variable frequency drive.If an acceleration time that’s too rapid is called for, “accelerationtime” will be overridden by the current limit. Acceleration will only occur at a rate that is allowed by theamount of current the control passes through to the motor. On most small DC variable speed drives, the acceleration time is not linear. What this means is that a change of 50 RPM may occur more rapidly when the motor is at low speed than it does when the motor is approaching the set pointspeed. This is important to know but usually not critical on simple applications where these drives are used.This is an adjustment that allows loads to be slowed over an extended period of time. Forexample, if power is removed from the motor and the load stops in 3 seconds, then the deceleration timeadjustment would allow an increased time and “power down” the load over a period of 4, 5, 6 ormore seconds.

To learn more about DC variable speed drives or for DC variable speed drive repair and replacement quotes, contact Precision Electric, Inc.

 

 

Information References:

http://www.fincor.net