Underwater Electric Motors

HOW TO SELECT THE PROPER UNDERWATER ELECTRIC MOTOR OR THRUSTER

THRUST and TORQUE

1.     THRUST: The ultimate desired feature of a thruster is THRUST.

2.     POWER LEVEL: Once the THRUST level is selected, the second issue is to find out how much POWER it will take to produce that thrust.

3.     MECHANICAL POWER is simply the rate of TORQUE and a certain Shaft Rotation (RPM). HP=TORQUE x SPEED

4.     ELECTRIC POWER: This means at least the same level of electric power will be required.

  1. MOTOR SIZE: The Electrical Power is set by the Thruster. The size of the Motor will now depend how this Electrical Power is generated.  Electrical POWER  = Voltage x Current. This means that if the VOLTAGE is LOW, then the CURRENT must be HIGH.
  2. SYSTEM VOLTAGE: Higher system voltages can make things easier. A higher Voltage will reduce the current levels in the wires. Lower current levels keep all wires, connectors and cables smaller.  Low Voltages are 120-240VDC and High Voltages are 300-600VDC.
  3. CURRENT CAPABILITY: It is possible to have low voltage, high power motors but these invariably require more space and weight. All Wiring must be larger and motors get bigger. Similarly, the electronics also get bigger to deal with transistors that can switch high currents at a high rate.
  4. WEIGHT & SIZE: Weight and Size always provide benefits of efficiencies for the same power level. Reducing size and weight is an advantage/benefit which means a designer will typically need to decide on a tradeoff.  If weight and size are paramount, it is best to plan on a high voltage power bus. However, with autonomous vehicles or submersibles running on Battery Banks, this is not always possible.
  5. VOLTAGE VARIATIONS: Motors can be designed to work precisely at very efficient levels if all parameters remain the same. However, in MANY cases, the bus voltage can swing over a wide range. This is particularly troublesome at low voltages (ie. 100-150V) which naturally occurs in Battery systems.

Power at MAX or MIN Voltage: The designer has the option to require the MAX POWER level at MAX Voltage, or at MIN Voltage. If this power level is required at MINIMUM Voltage, this requires a motor with much more current capacity, which affects size & weight. If this is only required at MAX Voltage, this will require lower currents, which can reduce Motor and Electronics Size & Weight, but it will mean that the net output power (and thrust) will drop slowly as the voltage level reduces.

It is possible to select a mid point on Voltage, such as to produce a steady power band over part of the voltage swing and allow a graceful degradation of thrust below that level. For example, a system with a voltage swing of 120-180VDC may select to operate at Max Power at 150VDC. This will guarantee uniform performance at all voltages above 150VDC, but will drop performance from 150-120VDC.

  1. ELECTRONIC DRIVERS: There are two basic drive technologies available: 1) Sensorless Drives and 2) Commutation Feedback Drives. All motors are THREE-PHASE and therefore will all require at least three (3) power lines and one (1) Ground.

SENSORLESS: These drivers use very sophisticated algorithms and prediction calculations to predict the motor rotor dynamics in order to control the current switching to keep it rotating. Part of the technology that makes this work is the driver’s ability to sense the motor Back EMF produced on the three power lines from the motor. When all setup, this works like magic. The cables to the motors are very simple and easy to deal with. However, this presents several tradeoffs.

    1. Since the system relies on Back EMF voltage sensing to regulate the switching, the process fails at motor speed = zero
    2. This works well for motors running at constant speed, preferably at high speed. (High Speed means High Back EMF signals and good speed control.
    3. Motor Speeds at less than 100 or 200RPM can become very rough
    4. Not recommended for use in any servo control loop.

COMMUTATED: Commutation is simply a term that provides position feedback of the motor rotor (magnets) so that the driver knows when to switch the currents. This commutation can be achieved in several fashions. The most common are: 1) HALL SENSORS and 2) SIN/COS Resolvers.

HALL SENSORS: Hall sensors are very simple and provide a discrete position signal that tells the driver when to switch currents. Its resolution of position indication is in the order of 20-60 steps per revolution. For speed control this is usually sufficient for a wide range of control. However, this system requires a PC Board in the motor with electronics on board. This can create extra risk when used in deep submergence motors with pressure balanced oil fill, a pressure effect that can affect the life of the electronic components (mostly the Hall sensor= solid state, and capacitors). This typically requires 6 feedback wires.

RESOLVERS: Resolvers are a clever rotational transformer that is able to generate a Sine and Cosine signal around a single rotation of the motor shaft. This level of feedback provides resolution of position indication in the order of 4000 to 10,000 steps per revolution. This high level of resolution offers the best control levels for a servo drive when precise motion is necessary from very low speeds all the way to full speed. The tradeoff is that the wiring is a little more complex.  This typically requires 7 feedback wires and requires one (1) more line than the HALL Feedback system. The advantage is that the device inside the motor is made entirely of steel and copper wire, a device that is not affected by pressure.