Vector Control Competes with Electric Servos


General comments

Regarding the trend of embedding a motion controller into the more sophisticated flux vector control products and systems, this is analogous to combining a motion controller and a servo drive into one package being done by several servo system manufacturers.

Although a significant number manufacturers now supply flux vector control drives down to 1 hp size (0.75 kW)-or even 0.5 hp-for positioning applications, their real competitive range with servo control is at larger sizes.

Inertia considerations

Baldor Electric Co. (Fort Smith, Ark.)
For an ideal package, the load and motor inertia should match (i.e., a 1:1 ratio). This is obtained by changing gear ratios, pulley diameters, or pitch of ballscrews. Sometimes this may not be possible. Typical ratios of load to motor inertias can be:

1:1 to 3:1 - for robotic applications
4:1 to 7:1 - for machine tool type applications
8:1 to 10:1 - for other factory automation applications.

This guideline originated with brush-type dc drives, which were adjusted via potentiometers. However, even in today's digital world very large mismatches (i.e., large load, small motor) are not ideal. Optimal ratios minimize power dissipation; and mismatches of even 4:1 result in power dissipation that's almost 5 times larger than in the optimal case. Mismatches of 10:1 result in power dissipation 25 times larger than in the optimal case.

Yaskawa Electric America (Waukegan, Ill.)
System inertia for a vector control system should be close to a 1:1 ratio (matching motor inertia to load inertia). This ratio is also ideal for servo applications; however, there are applications that can be greater than 5:1. In these applications, the bandwidth of the servo system can be adjusted or tuned, whereas the vector control drive maybe limited in its speed and torque bandwidth.

Cost perspectives

When both servo and vector control systems compete for an application, an economic breakpoint occurs based on the size or rating of the system. Here are some experiences of various manufacturers.

At Rockwell Automation/Reliance (Cleveland, O.), 2 hp (1.5 kW) and above is the start of vector control systems' cost benefit versus servos using a brushless dc motor. In addition, servo products, due to their high switching frequencies (up to 20 kHz), can generate higher levels of radiated RFI/EMI noise that can impact the installation cost of a system, explains Mr. Anderson.

A slightly lower breakpoint is noted at Rockwell Automation/A-B (Mequon, Wis.). 'Vector drives become competitive at sizes as low as 1 hp, dependent on application need,' says Mr. Sinner.

Indramat (Hoffman Estates, Ill.), sees the cost 'transition point' around 10 kW. PM servos look more cost-effective at or below this point (about 5% less in the 5-10 kW range). Around 15 kW, the advantage swings to vector control by 5%. PM servos get rapidly more costly in larger sizes.

Yaskawa Electric America 's (Waukegan, Ill.) experience shows servo control as cost efficient over VC below 3 hp, competing for the same applications. Up to 10 hp, VC can be very competitive if system dynamics are well defined. As this is not always the case, the servo may be oversized to hold performance as system changes occur over time, resulting in higher cost, explains Mr. Koehler. 'Above 10 hp, servos typically become more costly.'

Warner Electric (Bristol, Conn.) sees a price junction at around 5 hp. Examples: A $3,500 list price on a 5 hp vector drive, induction motor, and encoder package with 15 lb-ft (20 Nm) continuous torque rating compares to a similarly rated servo at $4,100. A 2 hp (6 lb-ft continuous torque) VC drive with $2,900 list price loses out to a like-rated servo unit at $2,200.

Kollmorgen (Radford, Va.): 'For applications under 1-2 kW, the solution is nearly always PM servo,' says Kollmorgen's Mr. England. In the 2-10 kW range, the solution usually remains PM servo, but some applications can swing to VC. For lower bandwidth usage over 10 kW a vector drive can often be the solution.

Control Techniques (Chanhassen, Minn.): In the majority of cases where vector is a viable alternative to servo, price becomes a determining factor. The obvious savings are in motors above 1.5 to 30 hp (average 25% cost reduction). From 30 to 200 hp the savings are inherently greater. Cost savings can also be realized with hardware reduction (gearboxes) in higher inertia applications.

Miscellaneous topics

Rockwell Automation/Reliance Electric
Vector control uses encoder (or other) feedback to maintain optimal torque production by keeping 90 deg alignment between the motor's torque and magnetizing current components. This feedback allows the motion controller to measure rotor speed and control position. Typically, the vector motor encoder feedback can be daisy-chained to the motion controller, or in other cases an encoder-repeater option card may be needed. If not, a dual-output encoder will suffice.

Torque linearity of a vector drive will be within 3% of the commanded value. A motor at operating temperature (hot motor) performs slightly better due to assumptions made within the flux vector control algorithm.

Typical servo applications :

Typical vector control applications :

Rockwell Automation/Allen-Bradley
Mr. Sinner likewise notes that servo drive responsiveness is overkill in many cases. Previously, a servo method was used because it was the only platform able to do positioning. Based on customer demand, 'simple' positioning capability has been added to A-B vector drives. 'One example is speed profiling that allows users to program multiple speed/position points, instead of simply following a speed reference,' comments Mr. Sinner. He sees the demand continuing for positioning features in vector products. 'The challenge is balancing those demands against the complexity they bring to the product.'

A further electrical characteristic in a servo drive/motor: When operating above base speed the drive must provide flux current to oppose the PM field. This forces the drive to control both flux and torque currents, much like an induction motor drive.

Since both technologies can now provide similar performance capabilities the application focus shifts to the feature set provided in the drive. For example, servo drives typically have highly responsive speed/torque regulators designed to take advantage of the low inertia characteristics of servo motors. Induction motor drives typically have traded pure response for other application features, such as mixed speed/torque modes.

Two penalties come with a 'vector (induction) servos': you throw away some extra heat in the motor because energy is expended to create secondary poles; and extra capacity is thereby needed in the drive-say an extra kW, maybe 5% more power to expend in the drive and the motor, explains Mr. Erickson. Five percent is generally insignificant, but in some applications with tight power requirements this may change a decision.

Regarding high-resolution feedback: In the past (10 years ago), resolution was dictated by bits of resolver resolution or counts per rev in encoders. The feedback device is no longer the limiting factor; now it's manufacturing tolerances, temperature variance, etc.

Yaskawa Electric
When comparing vector control to servo control capabilities with respect to positioning applications, the following control parameters need to be reviewed: speed range, system inertia, speed and torque control bandwidth, analog input scan times, and encoder resolution. Typical vector control speed ranges are 1,000:1, whereas servo control is 5,000:1.

Baldor Electric Co .
Vector drives also provide tight speed regulation from virtually zero rpm up to 3-5 times base speed, and constant power range to about 3-3.5 times the base speed, according to Mr. Mazurkiewicz.

To provide the most reliable and efficient package for the application, the vector motor used in the application should have:

1. High-efficient winding design, including an efficient lamination design and high-temperature insulation materials; and

2. Electrical windings to include protection against high dv/dt rates (short rise-time pulses) and voltage reflections that degrade the electrical winding life.

Advantages of dc servos include proven reliability and well-known, mature technology. In positioning applications, and in comparison to vectors, dc servos come in smaller sizes and provide lower inertia, which translates to faster acceleration. Positional accuracy depends upon encoder line count used.

These technologies must also be compared with brushless servo drives, available as either dc brushless or ac brushless (the feedback device determines whether it is dc or ac). The feedback device may be either Hall sensors, encoders, or resolvers.

With Hall sensor feedback, two of the three motor windings are energized at any given time. And within each time period, a dc level of power is applied, which is directly proportional to the desired operating motor speed. Thus, the term 'brushless dc.'' Encoder feedback is used when position data are required in the application. Encoders are available with Hall outputs. Again, the Hall signals are used for commutation of the brushless motor.

With resolver feedback, a sinusoidal waveform is applied on the motor windings; thus, the term 'ac brushless.'' Advantage of this technology is that, for the same torque (compared to brushless dc), the ac brushless will require less current. Therefore a smaller drive may often be used in the application. This becomes possible since the motor has a three-phase sinusoidal winding, being powered by a three-phase sinusoidal current waveform.

Advantages of brushless technology include higher speed capability, higher torques in a smaller package, much lower inertia (thus much faster acceleration capability), and long reliable life. These drives provide good low-speed operation down to zero speed. Brushless controls are available with autotuning capability.

Warner Electric
Vector systems allow direct control of the torque of an AC induction motor. The ac induction motor offers many advantages over dc motors in that the peak torque is not limited by brush commutation, or as in the case of brushless dc motors the magnet limitations of demagnetization and temperature. The ac induction motor is in many cases a standard motor designed in high volume and easily procured (of course it will need a feedback encoder).

An ac induction motor and flux vector drive have a torque/inertia ratio, without special tuning and oversizing the drive, such that the motor rotor can be accelerated from rest to full speed in about 100 msec. This limits the drive to less demanding applications.

Standard induction motors can run up to their breakdown torque levels, typically 300% of continuous torque, but the flux vector drive is usually limited to 150% of rated current/torque.


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