Ron,
I believe that what you are calling "... chopper drives we all use..." are more accurately called PWM BLDC controllers. If so, some of what you say is accurate, though with pulses in the 16kHz range, I wouldn't think that there is enough accel/deceleration in the stator to result in pulsed loading within a gearbox. That frequency would certainly be smoother (quieter) than the power from a 4 cylinder ICE runing at 3000rpm. There is noise from the BLDC motor itself and that noise is often tranmitted through the motormounts or gearbox to other parts of the boat which makes the concept of a small enclosure with sound damping fairly ineffective.
The drive system in my boat (and many others ehere) is a true A/C controller driving a PMAC motor, so what you describe is not really applicable. Here is a succinct (albeit lengthy) description of differences between the two types of motors (PMAC and BLDC) that are being used for many electric boat conversions.
Permanent magnet motors
In contrast to the ACIM, permanent magnet machines do not rely on induced AC currents to provide rotor magnetization. Fixed permanent magnets on the rotor take care of that task. As a result, permanent magnet motors use less overall current for a given torque, and provide superior performance down to zero speed, unlike the ACIM. This also means that permanent magnet motors have higher electrical efficiency than induction motors. Permanent magnet motors are synchronous, so there is no slip between the electrical and mechanical rotation speed.
One of the most commonly used permanent magnet motors is the brushless DC (BLDC) motor. Its name is a bit of a misnomer, as it is not truly a DC machine. The name was applied more than 30 years ago when it was compared to the brush-type DC motor for its similar speed-torque characteristic. At issue was mechanical commutation (brushes and commutator) versus electrical commutation. So the BLDC motor behaves like a DC (brush) motor as opposed to an AC (induction) motor, but the applied winding voltages are really AC for both.
Trapezoidal vs. sinusoidal commutation
Both trapezoidal six-step (BLDC) and sinusoidally commutated motors (permanent magnet AC or PMAC) most commonly have three stator windings arranged in a 3-wire WYE configuration. Both motor types are also driven from the same 3-phase Voltage-Source Inverter (VSI) topology. Phase-currents for both types of motors are controlled by pulse-width modulation of the applied voltage on each phase. Both BLDC and PMAC motors clearly have a lot in common. A casual observer might not even see any difference in the overall construction of these two types of motors.
The difference is in the "torque function" of each motor type, and how current is driven into the windings. If each motor type is spun, and the resulting back EMF examined on an oscilloscope, it would be seen that the BLDC motor has a flat-topped trapezoidal voltage, and the PMAC motor has a sinusoidal shape. In order to derive smooth torque from these motors, the applied current should be the same shape as the back EMF.
In the case of the BLDC motor, current is applied to only two of the three windings at a time. During one commutation interval, a steady current is injected into one motor lead, returning through a second lead – and the third lead is open. (It can be used for position estimate by measuring zero-cross). As the motor shaft rotates, the next set of windings is energized, sequencing through all six possible combinations (thus the name six-step) – at which point the motor has gone through one electrical revolution (fractional mechanical revolution depending on how many winding "poles" the motor has). An important implication of this commutation method is that only two of the six inverter switches are conducting at any time, and only two of the three windings carry current at any time. These two factors affect distribution of thermal loads in both the inverter and the motor.
In contrast, for a sinusoidally commutated PMAC motor, each of the three phases of the inverter is modulating (PWM) all the time, driving three out-of-phase sinusoidal currents into each winding. From an application standpoint, the biggest difference between trapezoidal and sinusoidal commutation is that sinusoidal commutation delivers absolutely smooth torque at any rotational angle or speed. Torque disturbances due to commutation, combined with magnetostriction in the stator generate acoustical commutation noise in BLDC motors. This effect is absent in sinusoidally commutated PMAC motors, making them virtually silent. Quiet operation is often a very important consideration for appliances.
Controlling synchronous motors
Based on the previous paragraph, it may seem that sinusoidal commutation would always be the preferred choice because of the smooth torque and lack of commutation noise. However, there is an additional control burden to consider as well. Simply put, trapezoidal control is simpler to implement than sinusoidal control. This added complexity must be considered in the decision of which motor and control type is best for an application.
For BLDC motors, a straightforward PWM current loop can control winding current, while commutation is handled by position sensors (Hall-effect or shaft encoder) or position estimate from winding voltage (from zero-crossing of unused winding). The current loop (which equals torque loop) is then controlled by a velocity control loop, and even an outer position control loop. Top speed of BLDC motor drive is reached when the motor back-EMF reaches the drive bus voltage. At that operating point, the drive cannot deliver enough current to increase motor speed, even at 100 percent duty-cycle.
Sinusoidal commutation offers some additional benefits. Simple open-loop variable-frequency sinusoidal drive control is used for ACIM, but not for permanent magnet synchronous motors. PMAC motors are driven with closed-loop sinusoidal current control. Maximum performance and efficiency is obtained with field-oriented control. Field orientation requires rotor position information, either through direct measurement (encoder) or estimation (sensorless). Motor currents are modeled in a two-axis format with orthogonal direct (D) and quadrature (Q) axis components. The D-axis vector controls the magnetization of the motor, independently of the Q-axis (torque-producing) vector. In normal operation of a PMAC motor, the D-axis command is zero, since the permanent magnets provide the necessary flux-levels. The Q-axis current is then the controlled parameter used to close the torque-loop.
For extended speed-range of a PMAC motor, the D-axis command can be set to reduce the motor flux (essentially opposing the flux generated by the permanent magnets). Reduced flux results in a reduced motor constant (both the speed constant Ke, and the torque constant Kt). So the net effect is to enable a higher motor top-speed from the same bus voltage. This method, known as Field Weakening, can practically extend the motor top speed to 3 times higher than without field weakening – at the direct expense of torque.
Fair winds,
Eric
Marina del Rey, CA
--- In electricboats@yahoogroups.com, Ronald Anderson <mcgyver28117@...> wrote:
>
> Hi Guys,
>
> This is my first post to the group... but the experience in regard to this subject is important.
>
> All Permanent Magnet motors have less inductance than series wound motor, ie no field windings to help run interference. The chopper drives we all use for traction drives really do pulse the electricity, only straight DC with no pulses like a relay across a battery is without pulses. These pulses generate torque pulses mechanical and these are real and very fast, same rate as the chopper drive, these will rattle all mechanical connections in a drive train, gears being noisier than belts.
>
> To lower noise we must acknowledge that source is the rapid rise in current when the chopper drive turns ON resulting in a mechanical torque signiture in sync with the applied electricity.
>
> Thus there are several ways to mitigate this real noise,
>
> a.) Increase the inductance of the motor by including an extra inductor in series with the motor, wire in between the motor and the controller. Do not install between battery and controller, this is a no-no. The inductor can be as simple as an aircore inductor, consisting of multiple turns of power wire all bundled into a circular winding, more the better for more uH. The inductance will lower the rate of current rise and increase the current available in the OFF phase of each chopper cycle.
>
> b.) Provide a torsional compliance damper coupling between the motor and the driveline/load to keep from banging the clearances between gears... A rubber damper dounut is a typical solution, but should not be too stiff, it must be torsionally compliant.
>
> c.) If the chopper drive has variable frequency adjustment, increase it to the mazimum available. Best to use this feature in conjuction with the two above.
>
> d.) If the above is not something you can implement, due to lack of materials then your left to simply enclose the motor in a box with a ventilation path the is surpentine to allow airflow yet block sound waves, the interior of the box is lined with sound absorbing materials.
>
> Sincerely,
> Ron
> Black Sheep Technology
> www.black-sheep.us
>
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