Home > MacroLetter > 7.1 - Autumn 2001 > Selecting Motors & Controls

Selecting Motors and Controls

With advances in both microprocessors and power semiconductors components, equipment designers have various forms of digital AC and DC motor control solutions to select from.  The fundamental task in selecting the proper motor and control for an application is to match the following:

• Torque-speed capability of the motor to torque-speed requirements of the load: fans, pumps and web handling applications. 
  
• Process control of a complete system: primary nip controlling a secondary nip
  
• Initial investment
  
• Savings in operating costs: energy, material, productivity, etc.

The ability to accurately and responsively control web tension is very complex.  The ability to control motor speed (therefore motor torque) at the advertised specification of 0.01% regulation over a 1000:1 speed range can be accomplished with a closed loop AC flux vector motor and controller.  This ability to precisely control torque (therefore motor speed) on a winder spindle is a feature a standard DC solution cannot achieve.  The other advantage on smaller horsepower motors (5 hp to 60 hp) is that the cost difference for AC flux vector versus a DC solution is negligible.

Where torque control, speed regulation and speed range are not as critical, it may be best to provide a DC solution.  An extruder application is a good example.    A digital controller with a standard brush DC motor with tachometer feedback will achieve speed regulation of 0.1% to 1.0% for a speed range of 100:1.  With a DC solution, the operator will be able to control the output and melt temperature.  On larger horsepower motors (60 hp and above) a DC solution is very cost effective, though the general maintenance of a DC motor is more expensive. 

Approximately two-thirds of all electricity generated is used to run motors.  Reducing your operating cost on variable torque load applications (fans and pumps) can make the difference between profits and losses.  Generally in these types of applications the required speed regulation and speed range is not as critical.  For these cases a standard AC motor with an open loop variable frequency drive (VFD) will achieve 1.0% to 3.0% speed regulation within a speed range of 10:1.  A VFD solution costs $1,500 to $2,500 less than an AC flux vector or DC solution.  

Refer to Example 1 for an illustration of the advantages of a VFD solution.

Example 1:  Energy cost comparison between a VFD versus a damper in an air ring blower application

Air Ring Blower AC Motor= 35 hp (26.1 kW) 
Number operating hours per month = 730 hours
Electricity Cost (kWh) = $0.08
Weighted horsepower for blower with damper = 32.6 hp (24.3 kW)
Weighted horsepower for blower with FVD = 13.95 hp (10.4 kW)

Operating cost = (# of kWh) x (# of hours) x (electricity cost)
Operating cost for blower with damper = $1,419 per month
Operating cost for blower with VFD = $607 per month

Total savings for blower with VFD = $812 per month

A comparison of the different systems discussed above is illustrated in Table 1.

Table 1: Comparison of AC and DC Systems for Different Applications