DC motor drives are used in a variety of commercial and industrial applications, including rolling mills, printing presses, and elevators. When working as intended, these drives use energy more efficiently and provide better process controllability than their electromechanical counterparts. In terms of power quality performance, DC drives present a unique set of challenges, making the technology one of the most difficult to protect against electrical variations. Despite these challenges, utility personnel and consultants have achieved success in protecting customer processes containing DC drives against electrical disturbances. The key is to know what solution to apply and when to apply it.
The primary considerations for using DC drives center around those applications where there is a need for either high torque or precision speed control. In addition, designers and system integrators working with mature processes are more comfortable and familiar with using DC motors. If you add the fact that installation costs are lower, you can see why these engineers continue to recommend the technology over some of the new AC-drive alternatives.
In order to specify power-conditioning solutions for DC drives, it is important to understand that there are different types of DC drives available. In this article, I'll focus on the most common — the SCR current source drive — and I'll include some pertinent discussion on regenerative braking.
The primary components of the silicon controlled rectifier (SCR) drive are a front-end converter and a DC field-winding source. Both of these sources must be able to ride-through electrical disturbances during normal, as well as regenerative, operation.
Controlled rectifiers (or thyristors) provide a variable DC output voltage to the armature circuit by varying the phase angle at which the thyristors are fired, relative to the applied AC line voltage (see Fig. 1). Adjusting the phase angle, and subsequently increasing or decreasing the duty cycle of the rectifier changes the DC motor speed, and the mechanical arrangement of the commutator and brushes provides the unidirectional rotor current.
For increased flexibility in controlling the DC motor, a separately controlled DC source excites the field winding. This DC source can be generated by a separate, completely variable SCR bridge, or it can be set to a fixed level by the user.
Alternately, a diode bridge can completely fix and supply the field winding. In all cases where the goal is to provide better sag and momentary interruption ride-through, the field excitation must be made robust enough to withstand power variations.
During a DC drive's normal operation, an SCR fires every 1/6 of a cycle in a sequence determined by the phase rotation of the AC supply. Voltage sags can create havoc with the phase-lock-loop (PLL) timing circuitry. Firing the timing circuitry requires a solid zero crossing, and the circuitry looks for this crossing to occur at regular intervals. The phase-angle jumps that occur during voltage sags will impact the regular intervals of the zero crossings and may cause drives to lose synchronization with the source. The PLL circuit can usually hold the synchronization for a short time. Too often, however, the drive experiences confusion in the timing circuit, which can cause the drive to trip (because of overcurrent) or blow one of the input or SCR protection fuses.
In some applications, such as the steel and paper industries, regenerative braking and speed reversal are frequently required. Such applications dictate the need for what is referred to as “four-quadrant” control of the drive. Using four-quadrant control allows the drive to do one of two things: draw power from the utility to operate the motor, or inject power back into the utility to slow down or “brake” the motor (see Fig. 1, on page 36).
Reversing the roles of the two converters accomplishes speed reversal. During regenerative braking, the active converter operates as an inverter, and the DC motor acts as a generator, supplying power to the electrical system. If a voltage sag occurs during this inversion operation, commutation failure could occur, resulting in blown fuses and process interruptions.
Power Quality Solutions
SCR-controlled, DC-drive solutions are some of the most challenging in terms of cost and feasibility. However, with a good understanding of how the drive operates and what the process requires as far as torque and speed, any process that employs DC drives can be assessed for potential ride-through opportunities.
By using a systematic approach, engineers can evaluate the various ride-through solutions within five categories. These include retrofit solutions, control power solutions, programming solutions, embedded solutions, and DC drive replacement with AC technology.
Retrofit solutions for sag and momentary interruption mitigation include modifications intended to improve the voltage quality supplied at DC-drive input terminals. Examples include the installation of power conditioning devices, improvements to either nominal steady-state voltages or existing voltage balances, and installation of add-on circuitry or controls intended to keep drives from shutting down. With retrofit solutions, it's important to ensure that the DC source controlling the motor field winding is conditioned along with the primary drive input.
Improvements to nominal steady-state voltage levels
The first line of defense against low rms voltage variations (primarily voltage sags) is to make sure that a DC drive has a nominal operating voltage that is “at or slightly above” the drive's steady-state nameplate rating. This helps minimize the number of nuisance trips the DC-drive system experiences.
Actual field problems with DC-drive nuisance tripping have been partially resolved by bringing the nominal voltage at the drive terminals to a level that is slightly above the equipment nameplate rating.
Many DC drives are typically set with a sensitivity threshold at 87% of the nameplate voltage, and they will trip if the voltage drops below this value for more than a few cycles. The drive will use either peak detection or rms detection circuitry to monitor the input voltage and determine when to shut down or trip. Because the trip threshold is a fixed value (e.g., 400V for a 460V-rated drive), the chances of the drive not tripping during moderate voltage sags is better with higher nominal voltages. The primary means of accomplishing steady-state voltage improvement is to either adjust transformer tap settings upstream of the drive (where possible) or use buck-boost transformers.
Improvements to steady-state voltage balance
The second line of defense against low rms voltage variations is to ensure adequate voltage balance between the phases at the drive terminals. DC drives can trip during unbalanced voltage sags because of low rms detection or armature current unbalances.
In addition, better voltage balance in an SCR-drive topology makes it easier for the SCR pairs to commutate. In theory, a drive that only runs at half load could ride-through a sag down to 50% of nominal voltage without problems, as long as the SCR pairs commutate properly. Unfortunately, the majority of voltage sags are single-line-to-ground, which usually results in a 2-phase sag at the drive terminals. The deeper the sag, the less likely the drive will be able to properly commutate the SCRs.
Using power conditioning devices
A number of power conditioning devices have been proposed for use with DC drives. Most are expensive because the initial drive installation involves sizing the power conditioning for the drive's entire load rating. The primary concept behind these solutions is to keep the voltage at the drive's input terminals at a level greater than 90% of the nameplate rating, with minimal voltage unbalance.
For fractional and small horsepower DC drives, the costs are easier to justify. Larger systems will require a comprehensive cost-payback analysis that compares the estimated annual losses with the cost of the power conditioning equipment under consideration. Some of the potential solutions available today include motor generator sets; dynamic voltage injection power conditioners; and uninterruptible power supplies with battery, flywheel, or ultracapacitor storage.
Control power and field winding solutions
Control power solutions include the logic and control circuits of DC drives as well as any external controls. Any external starters, contactors, emergency stop circuitry, and logic controller devices must also undergo conditioning. There are a number of low-cost power conditioners that can accomplish this task, including constant voltage transformers, UPS products, and coil hold-in devices.
It's important to recognize that conditioning the drive alone will not always work if the control power elements remain unconditioned. The recommended practice is for engineers to always apply power conditioning solutions to the drive controls, control power circuits, and field windings any time they apply power conditioning to the primary drive input.
Programmed ride-through solutions require dialogue between the process engineer and the drive manufacturer's application engineers. A few examples include:
- Changing the drive control algorithm to stop firing the rectifier during sags and interruptions.
- Turning off SCR firing controls to stop regenerating during sags.
- Delaying armature overcurrent and unbalance signals long enough to ride-through 10-cycle or 20-cycle voltage sags.
Many drive manufacturers have case studies showing the successful application of these techniques.
Programming opportunities involving changes to a drive's control algorithm or occasional power quality firmware upgrades also require interaction between drive manufacturer and process engineers. Once all parties clearly understand the torque and speed requirements of the process and the momentary nature of the disturbances, then a decision can be made as to whether or not the application is a good candidate for programmed ride-through.
Some manufacturers now offer products that will withstand short electrical disturbances or provide harmonic current minimization or noise isolation. These options are called “embedded solutions” and are typically offered as added-cost items.
Engineers should not expect embedded solutions to be built into the drive if the manufacturer was in a competitive bidding process, or if the customer did not request a specific performance capability in the original procurement specification. Most embedded solutions are similar to those described in the previous section on programming solutions, with the addition of built-in ride-through for the controls and field windings.
DC drive replacement with AC technology
Traditionally, DC motor drives have been used in applications requiring high starting torque, high torque at low speed, and precise speed and torque regulation. This is because of their low initial cost, excellent process-control performance, and relatively simple control characteristics. However, in recent years, new high-performance AC-drive technologies have become a practical alternative in many applications where DC drives were mainly used.
The new generation of “sensorless” AC drives, or vector control drives, provides users with all of the desirable DC motor-control characteristics, without the need for encoder feedback. AC drives use the measurements of two motor currents, plus motor-model parameters, to control the motor's speed and torque independently.
DC motor drives can be a source of complaint for end users who experience compatibility problems in their electrical environments. Still, the benefits of DC drives make them a good choice in a variety of applications.
To improve the power quality ride-through of DC drives, engineers must have a clear understanding of drive operation and the torque and speed requirements of facility processes. With that information secure, they can then decide on the best solution for their particular situation.
Doug Dorr is the business development manager for EPRI PEAC, located in Knoxville, Tenn. You can reach him at [email protected].
A DC-Drive Case Study
The printing presses at a major newspaper publishing facility were tripping offline because of voltage variations on the incoming power line. The facility has four printing presses, each of which is fed from 12 large spools of paper. A 12 hp DC motor and DC drive power each spool. If an undervoltage causes one of these drives to trip offline, paper is torn from the spool and the process is stopped. While the process is down, the paper must be untangled, removed, and rethreaded through a series of tensioning arms and the press itself. Each of these spooling machines, called reel tension pasters (RTPs), require approximately 15 min of preparation before restarting. In addition, fuses or SCRs in the affected DC drive may fail because of the regeneration that occurs when the inertia of the heavy spool keeps the motor spinning during voltage sags. Considering the tight printing schedules required to deliver the newspaper on time, downtime is a critical matter.
The manufacturer recommended a retrofit designed to initiate an orderly shutdown of the RTPs in the event of a voltage sag. The goal was not to prevent the drive from tripping, but to implement a smooth shutdown that would not tear the paper. If the printing presses and the RTPs could come to a controlled stop, then the process could be restarted easily with the push of a button.
The suggested retrofit was installed on one machine, and the serving utility asked EPRI PEAC engineers to test it with a portable sag generator during sag conditions. If one of the RTP drives tripped during a voltage sag, it could cause a sudden change in the speed of the belts. The heavy spool of paper cannot change speed instantly, so the belts end up shredding the paper where they make contact with the spool.
In addition, EPRI PEAC engineers needed to check the regeneration from the spinning DC motors, which had been known to cause SCRs and/or fuses in the DC drives to fail. To quickly detect voltage sags and break the electrical connection between the drives and motors, engineers installed an external 3-phase voltage monitor. When the monitor detects a voltage sag, it opens a set of contacts, which in turn opens the armature contactor allowing the DC motors to coast freely with the spools. A time-delay relay then keeps the armature contactor open until the motor stops spinning (currently set for approximately 20 sec). The main printing press was also retrofitted with the same type of voltage monitor so that it can issue controlled stop signals to its 12 drives.
During the controlled sag tests, the paper continued to tear and the testing team made adjustments to the settings of the voltage monitors and core brake. These adjustments, along with the installation of a new drive control card, led the system to perform a successful controlled stop, satisfying the manufacturer and printing facility operators.