Inefficiency of saving motor-generator sets

Oct. 1, 2000
Replacing all or part of an old motor-generator set with a solid-state dc drive makes sense for a number of reasons. Here is a closer look at some of the obvious, and some not-so-obvious, advantages of solid-state control.

Maybe you feel, as do a lot of others, that there is no pressing reason to replace your venerable motor-generator (M-G) sets with more up-to-date solid-state controllers. After all, they may have been faithfully performing for years with only minimal maintenance requirements. But, if you check out the advantages of conversion, you may be in for a shock. In typical applications, delaying the move to solidstate d-c drives is not only costing you a lot of money, but it probably is also affecting production quality and efficiency.

Why convert?

One obvious advantage of solid-state, dc controllers is that they save on maintenance costs. They eliminate the need to periodically lubricate and replace bearings and replace brushes. Also, you can forget about periodically rewinding and overhauling the dc generator.

Solid-state controllers are inherently reliable and are unaffected by many ambient conditions that shorten the life of M-G sets. Also, solid-state drives are modular, which means that replacing a unit is as simple as taking out one unit and replacing it with another. Should a drive-related problem occur, their modularity, along with the self-diagnostics available with solidstate controllers, will get you up and running with a minimum of downtime.

Meeting various application needs is only a matter of reprogramming controller software rather than extensive hardware modification. Also, a communication link between controllers is often available to network multiple drives in a coordinated system.

Speed regulation of solid-state drives using digital tachometer feedback is about 100 times better than using an analog tachometer feedback system as typically used with motor-generator sets — 0.01% for a digital system versus 1% for analog. This means that converting to a digital controller system will increase machine precision, possibly increase productivity, and reduce scrap.

Economics of conversion

Perhaps the most compelling reason to convert to solid-state dc drives is the opportunity for dramatic energy savings and reduced operating costs. For example, a motor- generator set’s two rotating elements operate with an efficiency of only 72% to 81%. But, solid-state drives, when used in conjunction with a line transformer, operate with 95% to 97% rectifier-transformer efficiency. By substituting a solid-state, dc drive for a motor-generator set, and using the existing dc motor, drive efficiency can be improved from 18% to 33%.

When an energized solid-state rectifier operates at no load (when the equipment is stopped), the typical loss rate is 0.6% to 0.7% of the power supply’s power rating. But, a spinning motor-generator set experiences a no-load loss of between 10% and 12% of its full-load rating due to friction, windage and excitation losses.

Example: A solid-state drive replaces a motor-generator set, but the dc motor is retained. The dc motor is rated at 100 hp with an efficiency of 88%. The machine operates 10 hr/day, 6 days/week and over 3,000 hr/year. The cost of electricity is $0.06/kWhr. Excluding power factor penalties, operating cost is:

Co = Operating cost, dollars
Ce = Electricity cost, $/kW-hr
P = Power rating of motor, hp
T = Time of operation, hr
μm = Motor efficiency, %
μc = Converter efficiency, %

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The average motor-generator set efficiency is 73%, versus 96% for the solid-state dc drive (based on typical run/stop and adjustable speed duty cycle). Therefore:

Operating cost using the motor-generator set = $15,259 ÷ 0.73 = $20,902

Operating cost using a solid-state drive = $15,259 40.96 = $15,895

Operating cost savings for the retrofit = $5,007*

Approximate cost to recondition a 100- hp motor-generator set = $5,000

First year out-of-pocket savings = $10,007*

Conversion considerations

Obviously, there are many benefits to retrofitting motor-generator sets with solid-state dc drives. But, to ensure a successful conversion, consider these questions.

Is regeneration required? Motor-generator sets are inherently regenerative, and solid-state dc drives can be if the feature is specified. Regenerative capability is often used for high-cyclic applications for controlled slow down and those where the load can overhaul the motor and require holdback torque. Typical applications requiring regenerative drives include unwinders, winders, cranes, hoists, elevators and press feeders.

It is easy to determine whether or not a drive is regenerating. Simply install a dc zero-center ammeter and observe the direction of current flow. If current flow is both plus and minus, power is being both motored and regenerated.

Since a solid-state dc drive is not inherently regenerative, be sure to specify regenerative capability when retrofitting, if the application calls for it.

Does the controller armature and field excitation match that of the dc motor? Probably not. The standard field supply for a solid-state controller is 150 V whereas motors powered by M-G sets usually have armatures and fields rated for 230-V or 240-V. Thus, a dc controller is required that matches the armature rating and has an optional field exciter rated to match the motor field voltage and current. Some manufacturers offer quick and inexpensive kits to supply the required field voltage and current.

What is the plant power line voltage and frequency? Many prime movers in M-G sets operate at 380 Vac, 460 Vac, 575 Vac, or some other voltage, Figure 1.

Usually, a solid-state dc drive with six SCRs is needed to supply the 240 Vdc required by many existing motors. Such a controller typically will require an input of 230 Vdc. If the line voltage is something other than 230 Vdc, an isolation transformer will be required, Figure 2.

Is an armature choke necessary for the dc motor to commutate properly on SCR power? A dc controller’s six SCR bridge configuration normally yields a waveform smooth enough to provide satisfactory operation of the motor-generator set’s dc motor (1950 design, or later) without an armature choke.

But, if excessive heating or excessive arcing and sparking at the brushes occurs on start up, an armature choke should be installed. A good rule-of-thumb for sizing a dc choke is to choose one with twice the motor inductance and match its current rating to the full-load armature current of the motor.

The choke should be installed in series between the dc output of the controller and the motor. Also, changing from solid brushes to split carbon brushes will improve commutation on six SCR power.

Is the operator station adaptable? Unfortunately, the existing operator’s station probably will have to be replaced or modified to obtain proper speed-potentiometer resistance. Check with the manufacturer to assure proper values of resistance and wattage.

What other special features are necessary? Check the application for other required features such as dynamic braking, reversing, interlocking contacts, or remote reference signals for proper machine interfacing.

Is it economical to keep the existing motor? If keeping the existing dc motor requires adding costly components, it may be more economical to replace the entire motor-generator system, including the motor. For example, the cost of a special field supply, plus an isolation transformer, plus a choke may exceed the cost of a new dc motor. Typically, a small motor-generator set can be economically replaced by a low-cost single-phase input dc controller and motor.

Ray Hemeyer is senior product specialist, dc drives, at Reliance Electric Co., Cleveland.

* If the SCR drive operates frequently at low speeds, and power-factor penalties are applied, actual dollar savings may be less than calculated.

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