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Variable Speed Drive Types
The precision and sometimes ability to operate in a particular mode can vary depending on drive type and specific operating conditions. The following discussion is general – specifications for a particular drive model or type should be carefully evaluated to verify its capabilities. Particular modes and performance level may be dependent on motor characteristics, drive train characteristics or even the addition of a feedback device.
Of course, there are drive types not discussed here – our goal is to address those drive types that are complimentary to Carotron’s primary areas of applications expertise, i.e. speed, torque and tension control.
Both AC Inverter and DC drives can have multiple modes of operation and control methods which can be selected in the basic drive due to self contained, internal, feedback devices and circuits, usually for voltage and current control. These same feedback signals may be used additionally to provide drive and motor protection.
Both drive types are available in “analog” and “digital” types. In general, analog types are adjusted by means of potentiometers or pots, and offer less “bells and whistles” than digital types. Digital types are usually microprocessor or digital signal processor (DSP) based and usually make use of a set of programming parameters to set operating characteristics and ratings. In most cases, an integral keypad will be used to access and edit these parameters. Additionally, software (free with Carotron models) allows use of a computer to access, edit and store the parameters.
The digital drives usually do have more capabilities, if properly implemented. These capabilities can include communications which allows networking in drive systems and interface to graphical controls such as touch screens or displays. They can also include extra programmable functions and circuits such as PID and Centerwind control with analog and relay inputs and outputs or IO.
How well these variable drives maintain motor set speed under variable conditions of loading is referred to as “regulation”. It is not a “given” that digital drives are more accurate or better performing than analog types though they usually offer better resolution in “setting” the operating level. The AC or DC digital drive may have a specified response and resolution or tolerance of output that is not achievable unless a particular motor and/or feedback device is being used.
1.) DC Drive & Motor Characteristics:
DC drive operation and mode control is more straightforward than in AC drives. This results from the motor characteristics. With the DC motor, in general, the speed is proportional to armature voltage and the torque produced is proportional to armature current. This relationship then makes it practical to measure armature voltage and current and easily judge the speed, direction of rotation and level of loading on the motor.
2.) AC Drive & Motor Characteristics:
With AC motors and drives, torque and speed control are more complex than in their DC counterparts. For rated torque and speed over the motor speed range, drive output must change in voltage and frequency level in a “constant volts-per-Hertz” relationship. For example, a 230VAC, 60Hz rated motor will achieve full rated speed at the full rated voltage and frequency. At 50% speed, both the voltage and frequency must be halved – 115VAC at 30 Hz. With AC inverters, torque is not directly proportional to motor current – in fact the motors can draw a significant level of “magnetizing” current without producing any torque.
AC Inverters includes several types of drives and control methods. They can use very similar hardware platforms and derive many of their capabilities from the “firmware programs” installed in them. More complex firmware combined with sophisticated feedback devices can give precise speed regulation and rated torque down to and at 0 RPM.
Inverters have a large advantage over DC drives and motors when used in “variable” torque applications. Fan and centrifugal pumps are ideal variable torque loads because their energy consumption varies by the cube of motor speed. For example, a fan or pump operated at ½ speed will consume only 1/8 the energy of full speed operation. This can result in tremendous $$$$ savings when applied to HVAC (heating, ventilation and air conditioning) and pumping applications.
3.) Open and Closed Loop Control
A “feedback” device in a drive related application refers to a “real time” signal generating device such as an encoder, tachometer, load cell, photo electric sensor, ultrasonic sensor, dancer pot, current sensor, etc. that provides a return or feedback signal to the drive system that is used to verify and improve or regulate the process or condition being controlled. This is known as “closed loop” operation.
AC and DC drives (with associated motors) can usually be operated without an external feedback device. Usually known as “encoderless” or “open loop” operation in AC drives and AFB or armature feedback in DC drives, these methods of control usually give lowest performance or regulation.
The feedback device can be specialized to return information on a particular aspect of the system operation. This could be velocity, torque, tension, position, level, etc. or combinations. For example, a specific type of encoder could provide both motor velocity and shaft position feedback. With most motor drives, a velocity feedback can be accepted and processed directly to improve speed regulation by compensating for design inefficiencies or losses in the motor, ambient and motor temperature change, AC line voltage changes and load change.
Additionally, the same feedback encoder (or other feedback device) signal can be used as a reference signal for another drive or process in a follower application. Isolation may be an issue to be addressed in this situation.
The function of a tachometer or encoder, i.e. whether it is a feedback or reference supplying device, can cause very diverse symptoms in the event of device failure. For example, loss or partial loss of a drive velocity feedback signal may be interpreted by the drive as “motor running too slow”. In this case, the drive could compensate by increasing motor speed or “run away” in an attempt to raise the feedback to an expected level. Loss of signal from the same device used as a source of speed reference could cause the follower drive and motor to slow or stop.
Be aware that some analog drives will directly accept velocity feedback from an encoder. The use of “encoder feedback” on these drives does not imply “digital” accuracy. In these drives, the encoder signal is converted to a voltage signal and then used in place of a tachometer feedback signal.
*Topics covered in this variable frequency drive tutorial: DC and AC variable frequency drives,variable speed motor drives, analog and digital type, motor characteristics, open and closed loop control
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