Page 143 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

be converted into linear motion or some other type of work). The degree of turning force delivered to the drive shaft is known as “motor torque.”
Motor torque is defined as the angular force the motor can deliver at a given distance from the
shaft. If a motor can lift 1 kg from a pulley with a radius of 1 m it would have a torque of 1 newton-
2
meter (1 N m). 1 N equals 1 kg-m/s , which is equal to 0.225 lb (1 in. equals 2.54 cm, 100 cm are in
a meter, and there are 2π radians in one revolution).
The formula for mechanical power in watts is equal to torque times the angular velocity in
radians per second. This formula is used to describe the power of a motor at any point in its work- ing range. A DC motor’s maximum power is at half its maximum torque and half its maximum rotational velocity (also known as the no-load velocity). This is simple to visualize from the discus- sion of the motor working range: Where the maximum angular velocity (highest revolutions per minute (rpm)) has the lowest torque, and where the torque is the highest, the angular velocity is zero (i.e., motor stall or start torque).
Note that as motor velocity (rpm) increases, the torque decreases; at some point the power stops rising and starts to fall, which is the point of maximum power.
When sizing a DC motor for an ROV, the motor should be running near its highest efficiency speed, rather than its highest power, in order to get the longest running time. In most DC motors, this will be at about 10% of its stall torque, which will be less than its torque at maximum power. So, if the maxi- mum power needed for the operation is determined, the motor can be properly sized. A measure of effi- ciency and operational life can then be obtained by oversizing the motor for the task at hand.
Brushless DC motors
For brushless DC motors, a sensor is used to determine the armature position. The input from the sensor triggers an external circuit that reverses the feed current polarity appropriately. Brushless motors have a number of advantages that include longer service life, less operating noise (from an electrical standpoint), and, in some cases, greater efficiency. Today’s brushless DC motors, with integrated control electronics, have been used to field efficient, highly reliable thrusters.
Gearing
DC motors can run from 8000 to 20,000 rpm and higher. Clearly, this is far too fast for ROV appli- cations if vehicle control is to be maintained. Thus, to match the efficient operational speed of the motor with the efficient speed of the thruster’s propeller, the motor will require gearing. Gearing allows two distinctive benefits—the power delivered to the propeller is both slower and more powerful. Further, with the proper selection of a gearbox with a proper reduction ratio, the maximum efficiency speed of the motor can match the maximum efficiency of the thruster’s propeller/kort nozzle combina- tion. However, the efficiency of the overall system may be lowered due to the energy necessary to drive the gearbox. Further, piloting technique (smooth operation of the thrusters versus over-controlling—i. e., constantly reversing thruster direction from full power in one direction to full power in the other) will determine both thruster efficiency as well as longevity of the gearing mechanism. Over-controlling a thruster will quickly destroy the gearing mechanism, resulting in both downtime and an angry client.
Drive shafts, seals, and couplings
The shafts, seals, and couplings for an ROV thruster are much like those for a motorboat. The shaft is designed to provide torque to the propeller while the seal maintains a watertight barrier that prevents water ingress into the motor mechanism.
6.1 Propulsion and thrust 131