Motion Control Resources
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An ‘Old’ Technology Finds New Purpose - Saving Energy
by Mark Lewis, VP - Marketing & Sales - Dart Controls, Inc.
Dart Controls, Inc. Posted 01/28/2015
Pulse Width Modulation (PWM) used in motor speed control designs has been around since the 1960’s. First AC controls and later DC controls employed this method of changing speed of a motor. The technology has always produced energy savings especially when compared to other options such as running the motor at full supply voltage or dumping excess supply voltage across a resistor / rheostat. Today DC PWM drives are finding broad application in solar, portable equipment, agriculture, medical and food preparation equipment designs for several reasons including conservation of energy. Developers of DC PWM drives are adding features to their designs in recognition of the growing demand for these devices.
How Does PWM Work?
PWM is used to control the amount of voltage (in the case of a motor control) to change the motor speed. In the specific case of DC speed controls the supply voltage is DC - either from a battery or rectified and filtered AC (power supply). Solid state switching devices (MOSFET’s, IGBT’s) are used to switch this DC voltage on and off at a sufficiently high frequency such that the motor will not respond to the individual on and off cycles.
The resulting waveform looks something like this:
The motor will find the average voltage being delivered and run at that speed. The longer the voltage is turned on, the faster the motor will run. The time the voltage is turned off, supply voltage is not used and thus energy is saved. Effective PWM control designs use circuitry to drive switching devices on and off, reducing voltage drop losses across the control. Losses are further reduced using high-efficiency switching devices.
With this background we can explore how various industries are finding renewed interest in PWM controls for their motors. This article will focus on DC controls for brush and brushless motors.
Motors in solar applications are most often driving a pump of some description; irrigation and equipment lubrication are prime examples. The system consists of solar panels feeding a battery / set of batteries, which provide source power for the motor control and pump motor. The motor control may be used to run the pump at a continuously variable speed or preset speed (high / low) based upon demand conditions. Energy savings in this application would not seem to be needed (except on cloudy days); in fact the PWM control provides not only energy savings but important system life cycle savings:
- The battery in the system is used to maintain function when the sun is not strong enough to replenish the battery charge. The PWM control energy savings provides longer system operation in cloudy / night time operation
- A battery has a limited number of charge / discharge cycles in its life. The PWM control by saving energy prolongs the time between battery charge cycles and thus the overall battery life
- The PWM control has a positive effect on motor brush life by operating the motor/pump set at less than full speed continuously
DC PWM control designs used with a battery should include a feature to monitor the battery supply voltage. This feature is meant to protect the battery from a deep discharge cycle. When the battery supply voltage is determined to be too low, the control should shut off output to the motor/pump to prevent permanent damage to the battery (Under Voltage Lockout or UVL).
The recent surge in the domestic energy sector has caused increased demand for equipment to support the extraction and transportation of oil and gas. This remote located equipment requires lubrication and maintenance 24/7/365.
More work is being done today by equipment not located near a traditional line voltage (AC) power source. A mentioned, solar panels and batteries are more and more relied upon for energy to power motor driven systems. Examples include conveyors, augers, spreaders, sprayers, winches and other powered reels. Variable speed is used in these applications many times to reduce material waste and provide better application of materials in the case of spreaders and sprayers.
Whether connected to a solar panel or a truck/tractor, these systems have one or more batteries used in the source power train. In these applications the DC PWM control provides all the energy and system life cycle savings benefits described earlier.
Medical equipment applications bring a unique set of requirements when variable speed motor controls are needed. Most applications are moving fluid (pumps) or air (blowers/vacuums). Since this equipment is operating in a healthcare provider environment, quiet operation is a must. Low voltage systems are coveted for their inherent safe operation in environments where flammable gases may be present. Finally, operating a system at 24V (considered low voltage) has the advantage of simplified agency approval for the equipment.
The DC PWM control operates at switching frequencies above those generally audible to the human ear and thus provides the quiet operation necessary for these applications.
Food Preparation Applications
For manufacturers and users of automated food preparation equipment, using DC low voltage systems (as in medical applications) provides quiet operation and simplified agency approval. Compared to medical, food preparation (example: conveyor oven) equipment typically has a much higher duty cycle. Whereas a medical device in an operating room may be used 2-3 times in an 8 hour day, a conveyor oven is run nearly 16 hours/day, every day. For this reason (brush wear in the motor) food preparation equipment designers are moving toward brushless DC systems.
A Word on Brushless DC
Brushless DC (BLDC) motor speed controls use PWM technology as well. Many of the same general design features are present in BLDC as DC PWM controls:
· PWM switching
· High frequency switching
· Low power loss across the control
The primary difference a BLDC control employs a three-phase output much like an AC variable frequency drive. Commutation of the BLDC motor occurs in the control, compared to mechanical brushes / commutator bar in a brush motor. Electronic commutation of the BLDC motor by the control eliminates the maintenance of motor brushes, an issue in high duty cycle / remote located equipment.
PWM motor controls (for brush and brushless DC motors) achieve energy savings - and a lot more. Since fractional horsepower (FHP) DC motors are used in volume, the majority of the DC control / motor applications are not driven by energy savings regulations - yet. Depending on the application and the industry, factors other than energy savings may be the primary criteria for the motor and control technology chosen. Innovative DC control companies realize this and are including features in updated designs to meet the needs of the industries they serve.
Mark Lewis is Vice President - Marketing and Sales for Dart Controls, Inc., Zionsville, IN USA. He has a Bachelors degree in Electrical Engineering from Purdue University and Masters in Business Administration - Sales and Marketing from Indiana University. He has over 30 years experience in automation systems.
Dart celebrates 50 years in 2013 of designing and manufacturing DC motor controls for industrial and commercial use. Products include