Joliet Technologies Blog

Modern variable speed drives (VSDs) are equipped with a multitude of features to provide programming flexibility, enhance efficiency and increase the accuracy of control. Let’s dissect some of these features, typified in this case by the A1000 series of industrial AC drives from Yaskawa (www.yaskawa.com).

Control methods: the A1000 provides up to seven different control methods to suit specific motors and applications. Methods range from basic scalar (volts/hertz, or V/f) control, which adjusts frequency and voltage output in direct proportion based on command reference, through open-loop vector control, to closed loop vector control. Vector control essentially “splits” the stator current into separate torque and field components, analogous to the separate armature and field components of a DC motor, and controls VSD output by regulating voltage magnitude, angle of displacement, and frequency. In open loop systems, modeling is used to calculate vectors and adjust output based on measured output current, while in closed loop configuration, sensors such as encoders or tachometers directly measure rotor position and speed and are able to control output even more tightly. In the A1000, V/f control can provide a typical speed control range of 40:1, while open-loop vector can achieve 200:1 turn-down. Due to the greater speed of direct measurement, closed-loop vector control can achieve turn-down of 1500:1. Note that all of these ranges are dependent on specific motor conditions and operating parameters and may vary somewhat per application.

Auto-tuning: this provides a means of fine-tuning the drive’s internal motor modeling data based on actual measurements of motor parameters during drive set-up. Auto-tuning can be static or rotational, depending on driven equipment accessibility. Drives such as the A1000 also provide for continuous auto-tuning during motor operation to adjust for changing motor conditions, such as increased resistance after motor heating. Note that auto-tuning is only effective when the drive is controlling a single motor, since measured parameters are skewed if multiple motors are connected to the same drive.

Motor and drive protection: like most drives, the A1000 provides electronic motor overload and over-current protection. Overload is based on a limit of 150% of rated heavy duty amperage for 60 seconds. Over- and under-voltage is monitored to protect the drive’s DC bus; power loss ride-through of up to a two-second duration is available; drive heatsink temperature is monitored via thermistor; and stall prevention during acceleration, deceleration, and run is available. Electronic ground fault monitoring is also provided.

Network communications: the A1000 supports several network protocols through the addition of optional network adapter cards, including Ethernet/IP, Modbus TCP/IP, DeviceNet, Profibus-DP, Profinet, CANopen, and Mechatrolink-II. Modbus RTU RS485 communication capability is standard, to allow 2-wire or 4-wire serial communication with basic network devices. Most industrial drive manufacturers provide similar communication options.

Input/output capability: a number of digital and analog inputs and outputs are provided as standard, with the flexibility of adding I/O with expansion modules. The I/O provide a means of accepting control inputs, for example a remote speed command (typically as a 0-10 volt or 4-20 ma signal), and sending output commands to monitoring and status reporting devices.

Input power conditioning: on ratings of 30 HP and larger, the A1000 is equipped with a DC link reactor as standard to protect the DC bus and reduce harmonics on the supply circuits caused by high-speed electronic switching in the drive. The reactor adds impedance to smooth bus ripple and slow the rate of rise of incoming voltage transients. AC line reactors are also available for additional harmonic reduction and better input electronics protection, at the expense of some voltage drop.

PC-based programming: various manufacturers provide software packages and PC communication interfaces for drive programming. While not required for basic drive set-up and operation, these software packages can enhance programming ease and flexibility, and are often necessary for custom programming. The A1000 includes DriveWizard software for drive configuration, and incorporates monitoring and trending functions.

While there are many more operating features in the A1000 than can be discussed here, I hope this overview has provided some insight into its capabilities. More can be found at www.yaskawa.com. As noted above, similar functionality is provided by other major drive manufacturers as well. Each may differ in terms of performance, controllability, and available options, but the choice often comes down to factors such as equipment standardization, operator familiarity, and price. Joliet Technologies can source drives from several manufacturers, so we’re sure to be able to meet your application’s requirements. Please contact us for additional information at info@joliettech.com, or respond in our Comments section. You can also visit our web site (www.joliettech.com) or call us at 815.725.9696. See you next week!

Regards,

Jay Baima
Joliet Technologies

Much has been discussed of the additional stresses that variable speed drives (VSD’s) can place on motor leads and insulation systems. However, in all but the most sensitive of installations, and assuming the use of properly rated motors, preventive or corrective measures are readily applied, especially during design and engineering stages. While each VSD and motor manufacturer has specific requirements for cabling design and installation practices, there are several elements common to all.

• Limiting motor lead length: by installing drives as close as practical to the motors they control, the magnitude of voltage overshoot is reduced, lessening the risk of over-voltage at the motor terminals (all other things being equal). There is a length, generally referred to as the critical length, beyond which the magnitude of reflected voltage pulses increases to the the extent that motor insulation systems can be affected. Critical length varies based on cable and drive characteristics and is calculated as:

The critical length is a consequence of the rate of travel of the pulse from the drive to the motor versus the rise time of subsequent pulses output by the drive. If the travel time is slower than 1/2 of the rise time, then the energy reflected from the motor terminals adds to the subsequent output pulse, increasing the overall voltage level. A rough guideline for cable propagation might be 500 feet per mu-sec, which with a typical pulse rise time of 0.1 mu-sec for IGBT output drives, results in a critical length of 25 feet. In practice, problems are rarely seen at this length unless the motor insulation system is very weak. However, it does point out the fact that with modern PWM drives, care must be taken when designing and locating equipment.

• Use of shielded cabling: Although in many cases individual non-shielded phase conductors can be used if installed in metallic raceway and properly routed, most manufacturers recommended shielded (screened) cable for effective reduction of electromagnetic interference (EMI). Motor leads are long enough to serve as an excellent antenna capable of radiating large amounts of EMI, which is produced as a function of the high-frequency switching of the drive inverter section. Empirical evidence shows that the lower the cable’s transfer impedance (a measure of the cable’s shielding capability) the greater the EMI reduction. This limits the noise propagated to adjacent sensitive equipment and, more importantly, any control conductors in the vicinity. Numerous cases exist of faulty sensor performance as a result of EMI-induced cross-talk from unshielded power conductors routed close to and/or parallel to low-amplitude signal control wiring.

Equally important is the proper termination of the cable shields. It is recommended that shields for motor leads be landed at both the motor AND drive end, and that 360-degree clamp connections be used to bond the full extent of the shields. Pig-tailing the shield and landing it on a bonding screw is not advised, since this increases the cable’s impedance to high-frequency currents and causes voltage on the cable to rise.

Many of these issues are complex in and of themselves and are suitable for future columns. In the meantime, please contact us for additional information at info@joliettech.com, or respond in our Comments section. You can also visit our web site (www.joliettech.com) or call us at 815.725.9696. Have a great week, and thanks for reading!

Regards,

Jay Baima
Joliet Technologies

At Joliet Technologies, one of  our strongest markets is the direct current (DC) motor and motor controls field. There remain a significant number of DC motor applications in the utilities, transportation, and manufacturing sectors, including among others the oil, pulp and paper, metals, and automotive industries. While many are familiar with typical alternating current (AC) variable frequency drives, DC drive applications are less common. However, the basic operating concepts share some common elements. Let’s examine DC drive basic operating principles in more detail. Some of the information which follows is excerpted from Siemens online training, which can be accessed here: Siemens Online Motors and Control Courses.

Most commonly, DC drives are used to regulate the speed of shunt wound or permanent magnet DC motors. In larger motor applications typical to industry shunt wound motors are used, and we will refer to those for purposes of this discussion. In shunt wound motors, the stator pole pieces are electromagnets wired in parallel with the armature (rotor) windings. Typically, voltage is supplied to the stator poles via a separate source of supply (referred to as a field exciter). This creates a magnetic field, called a shunt field, in the stator. When the drive supplies voltage to the armature windings, the resulting current produces a magnetic field in the armature as well. Simply put, it is the interaction of the shunt and armature fields which result in the torque needed to rotate the armature.

The heart of the DC drive is the converter section, which rectifies supplied AC voltage to produce variable DC voltage to supply the armature. Rectification is commonly accomplished via thyristor bridge, with two thyristors connected in series per phase, for a total of six thyristors. The thyristor is effectively a diode “switch” with a controllable gate; the gate (G) provides current to switch on the thyristor, which conducts as long as the anode (A) remains positive to the cathode (K):

Thyristor conduction (from Vidralta, 2007-09-10)

Gate current is applied to the incoming sine wave at a point along each half cycle (i.e. phase angle ?) specified by the drive controller.  The earlier in the cycle the gate fires, the longer the thyristor remains “on” (i.e. conducts), which results in a higher average voltage output by the rectifier. This can be seen in the following graphic; “A” being the incoming AC sine wave, “B”, “D”, and “F” the gates firing at progressively later times, and “C”, “E”, and “G” the resulting DC voltage level out:

Gate Firing and Output (from www.sciencelobby.com - 2012-05-02)

Voltage level is calculated as:As such, the highest average output from a 460VAC input would be a rectified 621 volts DC. This voltage is adjusted by controlling the firing timing (i.e. the gating angle) of the thyristors, and is then supplied to the armature to control motor speed. Note that drive parameters must be set to ensure that the output voltage does not exceed the rated nameplate voltage of the motor, typically 500VDC for a 460VAC supply.

The motor draws current from the drive proportional to the amount of torque needed to drive the load. At base speed and no load, very little armature current is needed, and so less voltage is output to the armature by the drive. For example, at no load the gating angle might be something like 45°, which translates to ~440VDC output. Once load is introduced, the voltage level in the armature must increase to maintain speed, and the thyristors are fired earlier to compensate – an angle of ~36° produces nameplate rated voltage of 500VDC.

DC drives have far more functionality than the basic operational concept above describes, but that functionality will need to wait for another column. In the meantime, please “fire” off any questions, thoughts, etc. (and don’t worry about the phase angle!) in our Comments section, or send them to info@joliettech.com. There is much more information on our web site as well, so stop by for a visit (www.joliettech.com) or call us at 815.725.9696. And please tune in next week for another issue. Have a great week!

Regards,

Jay Baima
Joliet Technologies