Jim McKee (deceased)
Alabama Electric Motor Service
The ability to vary the speed of our prime movers has always been desirable. Electric motors are no exception. Many designs and types of motors have been developed over the years to fill this need for variable speed. The most widely used and successful designs include the direct current motor, eddy current coupling/ motor combination, three-phase shunt commutator motor (brush shifting or Schrage motor), and various mechanically variable devices such as adjustable pitch pulleys and cone/ wheel combinations.
EASA service centers often find it desirable or necessary to replace one of these vintage variable speed devices. There are many reasons for replacing older drives; they include improved efficiency, unavailability of repair parts and high maintenance costs.
A properly sized Variable Frequency Drive (VFD) /three-phase induction motor combination is often the right choice for the new drive.
Take close look at original drive
For the replacement motor and VFD to be successful, a thorough understanding of the application and the characteristics of the original drive is critical.
A review of the old drive’s characteristics is a good place to start. The DC shunt wound motor provides constant torque from zero to base speed, and constant horsepower from base speed to maximum speed. If the drive controller will allow, the DC motor can produce torque, even at zero speed, of 200 percent or greater for brief periods of time. This can be very important in certain applications; for example, an extruder. With proper cooling provisions, a DC machine may be run at slow speeds continuously.
Eddy current couplings are usually driven by a conventional squirrel-cage induction motor. Often the motor and coupling are built as an integral unit. Output torque is a function of the applied coupling coil current, and slip, or speed difference between the output shaft and the induction motor.
There are many reasons for replacing older drives; they include improved efficiency, unavailability of repair parts and high maintenance costs. A properly sized Variable Frequency Drive (VFD) / three-phase induction motor combination is often the right choice for the new drive.
Output speed is highly dependent on the load. Eddy current couplings usually have an integral tachometer and must be speed regulated to avoid wide speed changes with relatively small load changes. Maximum torque is determined by the drive motor size and is available at any output speed.
Brush shifting motors are rated at constant torque over their speed range with horsepower output varying in direct proportion to the speed. A typical nameplate may list three horsepower ratings for minimum, synchronous and maximum speeds. They exhibit speed vs. load characteristics similar to squirrel-cage motors except percent slip at full load is higher, particularly at low speeds.
Sometimes, mechanically variable speed devices present the most difficult challenge when selecting a suitable VFD replacement. These devices are generally considered to deliver constant horsepower. Therefore, as the output speed is reduced the available torque goes up. They behave like variable ratio gear boxes. Most of the time, the prime mover is an induction motor. Therefore, once the speed is set, speed vs. load characteristics are the same as the induction motor.
There are some other variable speed devices not mentioned here, but by now you have the picture of what we’re trying to do. Make sure you understand the capabilities of the existing device before trying to replace it.
Understand the requirements of the application
Of equal importance is understanding the requirements of the application. Find out as much as possible about the application. What are the speed ranges required? Applications which require continuous operation at lower speeds (less than 50% of nameplate) probably will require auxiliary motor cooling or the use of a motor that is rated for inverter duty.
Determine the load characteristics. Centrifugal fans and pumps, for example, are variable torque loads. As the speed increases, required torque increases and vice versa. Constant torque loads are those where torque is relatively independent of speed. Some examples include conveyors, positive displacement pumps, extruders and packaging machinery. Certain loads exhibit constant horsepower requirements, most notably those in the metal working industry. Many times the highest torque is required at the lowest speed. Determine the starting and stopping requirements. Some loads are high inertia and require considerable energy to start. Examples include large diameter or heavy fans, blowers, and flywheels such as those on punch presses. Find out if controlled deceleration is required. This may indicate the need for additional braking resistors or other means for the VFD to stop the load. Does the application require reversing or jogging?
Finally, the capabilities of the new VFD and motor need to be examined. General purpose VFDs, sometimes called Volts/Hertz drives, combined with standard EPAct efficient motors are usually capable of constant torque speed ranges from 4:1 up to 10:1. Usable speed range, for example, for a 1750-rpm rated motor may be from 437 to 1750 rpm. This same combination may have a variable torque speed range of 20:1. Sensorless vector VFDs may extend this constant torque speed range. Flux vector drives with encoder feedback can develop full torque at zero speed.
Manufacturers of VFDs provide hp or kW ratings as a matter of convenience, but to ensure that the drive motor is capable of delivering full torque, make sure the drive has a continuous current rating equal to or greater that the full load current rating of the motor.
Once all the “evidence” has been gathered about the old drive and the application, decisions regarding the size and type of VFD and motor can now be made. Look at the worst case situation for speeds and torque. Meeting the speed requirement is usually the easiest. All VFDs are capable of operating at frequencies above 60 Hz that will allow the motor to run above nameplate speed. Be sure, however, that the motor is suitable for operation at the higher speed. Maximum safe speed rating is available from the motor manufacturer. Operation above 60 Hz results in constant horsepower being available since the drive can no longer increase the voltage as frequency increases.
Sometimes meeting the torque requirement may result in a larger horsepower motor and VFD than the original motor. This will most often happen when replacing a mechanically variable speed drive or where high inertia loads must be started.
Examples help explain the process
A few typical examples follow which may help one to understand the process required for a successful drive replacement. First, let’s consider replacing an eddy current coupling used to control the speed of a centrifugal pump in a wastewater treatment plant. While this is a common application for the eddy current coupling, it also is an example of more drive than is needed since this pump is a classic variable torque load. Make a list of what is known about this application. Speed range can be determined from the coupling nameplate as well as horsepower and current rating. Since this is a variable torque load, it is a good fit for a standard motor and VFD with a variable torque rating. The horsepower of the new motor should be the same as the original.
Consider replacement of a DC motor and drive on an extruder as a second example. An extruder is considered a constant torque load. Required minimum and maximum speeds can be determined from past operating practice and a few calculations of gearbox ratios. (Usually only the extruder screw speed is known.) This may seem to be any easy example, but be careful. Extruder screws are notorious for “freezing up” when they are stopped. Break away and slow speed torque requirements can be very high (200 to 300%) when trying to restart. Consider over-sizing the drive and motor as well as discussions with your customer about options such as allowing more heat to build up in the extruder before trying to restart. Select a VFD and motor that will develop at least 150% rated torque at zero speed. This may be a good application to discuss with your VFD supplier before proceeding.
Consider replacing punch press drive
Finally, consider replacing the drive on a punch press. Even though this is another example of an eddy current coupling, the application itself is much more demanding. We now have a large flywheel and a big mass of moving machinery. Again, look at what we know. Speed requirements come from the coupling nameplate. The most obvious difference is the flywheel, a very high inertia load, which must be started. In this case, the eddy current coupling has a big advantage since it can deliver lots of torque at low speeds to get the flywheel started. If a standard inverter
and motor is used, it probably will need to be oversized to start the flywheel. The other big problem may not be so apparent. Once the flywheel is up to speed and the press is engaged we now have a big mass going up and down. On the up stroke the motor and drive are loaded. On the down stroke the motor is being pulled by the falling mass and now goes into the “regeneration” mode. Most VFDs when equipped with the proper dynamic braking resistors will handle this cyclic load/regen condition. Some trial and error on resistor sizing may be required.
These examples certainly do not cover all of the possible situations that you may encounter. But the process of matching capabilities of the new VFD to the existing application is the same regardless of the situation. Determine the capabilities of the old drive and the requirements of the existing application and be sure the new VFD and motor possess all the capabilities that are required.
Related Reference and Training Materials
Print