Comparing mechanical and electronic variable speed drives
Variable speed was accomplished by simple, yet complex, variable speed mechanical drives long before ac inverters or variable frequency drives (VFDs), vector drives, dc brushless controls, or integrated inverter/motor drives were available. Once semiconductor modules became reliable, retrofits of mechanical variable speed con- trols steadily increased.
But questions have continued to linger. Why should I change? Why should I replace what works? Is electronic control really that much better?
Mechanical variable speed controls, such as variable speed sheaves (Fig. 1), enclosed variable speed drives, and belt-boxes, provided reliable speed control of various processes. Hand turning a wheel, electronically moving a motor base, and creating more tension on the belts are all used to change the ratio of the operating diameters of connected sheaves. Not a bad way of controlling speed, but not exactly the easiest or most efficient.
Mechanical variable speed drives, which have the motor operating at full speed, can increase or decrease speed, but cannot provide the energy savings and efficiencies of today’s electronic controls. Mechanical drives are typically 70-80% efficient, while an ac inverter and motor combination can be 85-90% efficient in certain applications.
Ease of operation is the basic reason for changing to VFD. The simplicity of the change makes the use of a VFD for variable speed an excellent long-term solution. No belts to change, no oil to add to the sheaves, and broader speed ranges are some advantages. Not bad for a box with a bunch of wires.
A VFD can be easily installed in conjunction with an existing motor starter or as a replacement. Panel space or NEMA enclosed models could present a limitation, but inverters can be mounted separately. VFDs have been used for years in new installations, and for many facilities that are moving to more electronic controls than mechanical, the change to all electrical is becoming a less painful process.
The time to replace mechanical variable speed drives is up to the individual user. Factors such as increased maintenance costs, need for increased productivity, or cost-effective replacement if a mechanical system is damaged must be considered. Since most of the VFD expense is in the initial installation, cost savings are achieved through zero maintenance and higher efficiencies of the electronic system.
How VFDs work
VFDs accomplish speed control by changing the current frequency from the control to the motor. They have the ability to control both current frequency and voltage to make the ac motor operate over a broad speed range. Typically, speeds range from 4:1 for variable torque loads, such as fans and pumps, up to 1000:1 for machine tools and material handling applications.
Synchronous speed of an induction motor is determined by the number of poles in the motor and the frequency applied to it.
Speed = 120 X Frequency/# of Poles
AC induction motor designs are based on the number of magnetic poles in the motor. The number of poles determines the base speed of the motor. For example, a 1750-rpm (1800-rpm synchronous) motor is based on a 4-pole design with 60-Hz applied to the motor. If the frequency applied to the motor is changed, the operating speed changes.
Unfortunately, it is not entirely that easy. AC motors are also designed based on the amount of torque/horsepower they need to develop. This torque can be controlled as long as the ratio between voltage and frequency is maintained.
For example, a 5-hp ac motor rated for 460 V at 60 Hz can be operated from a VFD over a motor manufacturer-specified speed range. If this motor is slowed to half speed (30 Hz, or 900-rpm synchronous), the voltage-to-frequency ratio (V/Hz) must be maintained. The inverter must be able to provide 230 V to the motor in order to produce full torque capacity.
As loads and applications change, the need for additional features within the VFD increases. With built-in overloads and digital keypads (Fig. 2), the VFD can provide approved motor protection to meet many electrical codes and monitoring functions to replace other equipment in the system. The single-source control vs multiple “black boxes” can also be very cost effective.
Pitfalls to avoid
As with any change in process control, being aware of limitations or disadvantages up front makes the retrofit much smoother. One of the mistakes made during retrofit of mechanical systems is to overlook the advantage in torque production due to the speed ratios of the mechanical components.
A 5-hp motor operating a 4:1 V-belt drive at 450 rpm produces 58.33 ft-lb of torque when drawing full current. Replacing the system with a VFD may require sizing the system for the torque required at low speed. In order to operate an ac motor at 450 rpm, a 20-hp VFD and motor rated at 1750 rpm may be required if full torque capacity is needed. A 20-hp system, when rated at 1750 rpm, develops 3 ft-lb of torque/hp or 60 ft-lb. An alternative would be a 15-hp motor rated at 1200 rpm that develops 4.5 ft-lb of torque/hp. Usually, the issue is speed, not torque, and the system must be sized for the application.
Other precautions that must be taken when retrofitting a system are motor cooling and bearing damage. Inverter-duty motors have become prevalent in today’s marketplace. Although VFDs can operate noninverter-duty motors, the motors may not be designed to handle the speed ranges required for the application.
In ac motors, particularly those of a TEFC and TENV design, cooling capacity is directly related to either how fast the shaft turns the internal fan or the ability to radiate heat. Low speed, high torque operations can cause excessive heat and drastically cut the life span of the motor.
Bearing damage can also occur due to induced currents through the shaft of the motor and into the bearing housing. Many motors today are equipped with insulated bearings and are better able to survive these currents. The currents, created by the waveform from the inverter to the motor, can be reduced by placing filters in the line to the motor or through a choice of inverter design.
Integration of VFD systems has increased with the advent of inverter/motor integrated controls (Fig. 3). These controls have the inverter mounted either on or in the motor housing with all associated wiring self-contained. This approach can also reduce the initial cost of a system by reducing external wiring and eliminating the need for extra panel space for the VFD.
These systems can be greatly simplified with simple connections for speed, start/stop controls, and limited external peripheral equipment. Replacement of the system is also simplified; removal and installation of one unit is all that is required. Integrated inverter/motor control systems operate in a variety of applications, with a primary focus on material handling, pumping, and HVAC.
VFDs and integrated inverter/motor drives are a solution for new installations requiring variable speed control. Mechanical variable speed has been used before, but with the advantages and cost effectiveness of electronic controls, they are becoming the replacement choice. Whether for retrofits or breakdown conditions, the electronic version has a short payback and simplicity of operation. — Edited by Joseph L. Foszcz, Senior Editor, 630-320-7135, firstname.lastname@example.org
Comparing variable speed drives
20:1 speed range 4:1 speed range
Maintenance required: Maintenance required:
Clean fan and heat sink Lubricate and adjust belts
Quick speed change Adjust pulley diameters
with keypad and check speed with
The author is available to answer questions on applying and converting to variable frequency drives. He can be reached at 717-264-7161 x4434.
Electronic speed controls are more efficient, accurate, and cost effective than mechanical types.
Variable frequency drives can work with existing motors, but the application should be checked out.
Inverter motors, suited for variable speed applications, must have proper cooling fans and bearing protection.