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T200 High Voltage behavior


(Brian) #1

Hi,

Had a few quick questions for designing a battery system around a bank of T200 thrusters.

I saw in a previous response to another query that the T200 can technically stand up to 26Volts, and 22V is the official limit for the T200 (though your spec sheet says 6-20V).

The battery we’re looking at using is a 5 cell Lipo that will operate nominally at 18-22 Volts. And dropping that voltage down to 16V will introduce some inefficiencies and complexities we’d rather not deal with for various reasons

So the questions are:

First, is <22 volts still safe for use on the T200 with your BlueESC?

Second, what impact have you seen on the thruster operating at higher voltages? Specifically:

  1. Is the maximum thrust impacted (Raised/lowered)?
    2.Is the efficiency impacted (Thrust vs power raised/lowered)? And
    3.Does the maximum power draw (350W) remain static with a lower maximum amperage, or does the current draw remain static (25Amps) with a higher power consumption? Or a mix?

 

I understand if you haven’t thoroughly tested the thrusters beyond your nominal 12V and 16V operation. But any information or general indication on efficiency, maximum power, and maximum thrust as a result of higher voltage would be appreciated.

Obviously since I’m asking for incomplete and unpublished performance specs, I won’t hold anything you say as official or an endorsement on usage.

Congratulations on the company. Your T100 thrusters work great, and I’m really looking forward to getting my hands on the T200 series.

-Brian


(Rusty) #2

Hi Brian,

Thanks for the questions. I’d be happy to answer.

22 volts is the upper limit of what the BlueESC can handle. At that voltage, the T200 will draw quite a bit of current at full throttle and I would recommend limiting the throttle a bit so that it doesn’t draw too much.

The thrusters are affected in the following ways by increasing voltage:

  1. The maximum thrust is increased.

  2. The efficiency is negatively impacted. For the same amount of thrust, it will use a bit more power. At full throttle it will use dramatically more power. For example, with the T200 I think you could push 600+ watts at 22V. I wouldn’t recommend doing that for extended periods.

  3. The current draw actually goes up. This is caused by the fact that torque is only proportional to current. The speed, however, increases with voltage. The propeller requires a more or less fixed amount of torque at each speed. Because the motor tries to spin faster for a given input, it will draw more current to match the torque requirement of the propeller. With combined high voltage and high current you can end up with a very large power output.

We haven’t tested the thrusters for extended periods at these voltages and we don’t have any recorded data unfortunately. If there’s anything in particular that you want to know, we’d be happy to test it out!

Please let us know if you have any other questions.

Best

Rusty


(Joe) #3

Hi,

I would like to piggyback here to hear your thoughts on pushing the voltage range a little higher. Our vehicle runs off 6S packs so our max voltage would be 25.2V. Now this is obviously out of the range of the BlueESC, but given we are using and ESC that can handle these voltages, do you have major concerns about the thruster itself?

 

  1. From what you have said it seems that we should expect that the current draw at any specific speed should be roughly the same regardless of voltage due to the required torque. From this I would extrapolate that the thruster would use about 50% more power than what it would at 16V. Do you think that this might be the case?
  2. In terms of limiting the speed of the thruster, I assume that the more relevant limiting factor would be the max power and not the max current. Obviously you do not recommend running the motor outside of its rated operating conditions, but do you think that the motor can be reasonably run at higher than 350W? In terms of limiting the power, what would be the most likely thing to be damaged? I am thinking overheating leading to magnet demagnetization. On that note, do you have any max temperature that you would recommend?
  3. Do you have values for the inductance and resistance of the coils?
  4. Do you have estimates for the Kv and Kt of the T200 or are those too dependent on the application situation?
I understand that this is outside the max parameters of the thruster so obviously won't hold you responsible for any thrusters I might break, but your thoughts about this would be very helpful.

Thanks,

Joe

 


(Rusty) #4

Hi Joe,

The thruster will run, but it will draw a lot of current (over 30A). You’ll also get a bit more thrust, but overall it will quite a bit less efficient than if you were running at 12V or 24V.

  1. The current draw will be roughly the same at any RPM regardless of voltage since torque required to spin the prop is constant at each RPM and torque is only proportional to current. I agree that it will use roughly 50% more power for a given thrust at 24V.

  2. You can definitely run higher than our official rating of 350W. Higher currents can cause heating issues, which could lead to other issues. I’m not sure how well the heat will be dissipated in water at such high current, but I imagine it will still be pretty good. At higher thrust, the bearings will start to wear a little fast so you could expect reduced lifetime. That said, the T200 is really tough and normal expected lifetime is 600-1000 hours at least so a slight reduction may not be a big deal.

  3. Yes, for the T200 the phase resistance is 0.18 +/- 0.01 ohms and the inductance is 0.070 +/- 0.040 mH.

  4. The Kv (RPM/V) is about 490 Kv with no load. That of course changes with the load of the propeller. We have no empirical measurements for torque unfortunately.

I hope that helps!

Best,

Rusty


(Nicholas) #5
  1. From what you have said it seems that we should expect that the current draw at any specific speed should be roughly the same regardless of voltage due to the required torque. From this I would extrapolate that the thruster would use about 50% more power than what it would at 16V. Do you think that this might be the case?
At any speed, Faraday's Law says that motor back-EMF will be constant and equal to average (pwm) supplied voltage . At each speed, voltage only needs to be adjusted insofar as the winding resistance causes voltage drop. For example: 2000rpm at full load will require Vfl = Iphase * Rphase + Vno-load. You can test this by spinning a motor with a drill and watching the output on a scope, then hook it up to the ESC and you'll see the same average voltage at the same speed.

According to Lorentz Law (F=ILxB), current is directly proportional to force or torque. For example, if the motor were running in a vacuum, the only current drawn would be that needed to magnetize the motor core. An approximate current equation for one phase is something like T=k*(Iin - Imagnetization) or just T = k*I. Because of the winding inductance, even though the voltage signal is PWM, the current will be more or less constant while the phase is active. Testing this requires a swivel mounted motor held by a reaction arm, which requires some crafting, but you can do it if you’re funded.

So that’s some theory at least. In reality, the motor has an iron alloy core with certain characteristics, windings with certain characteristics, and there is an ESC in there that has its own characteristics and an absolute voltage limit. So, even though in theory the power draw should be the similar whether at 12 or 24 or 42V, the characteristics of the different parts could cause significant losses. You have to do testing to find out the ideal range of voltage and pwm frequency where each part runs best.

My recommendation is that props have a certain range where they work good, so there is no point going beyond that speed (speed=pwm*Vbatt). If you are planning on running higher voltage batteries, just keep in mind you need to limit your throttle setting so you don’t burn out the motor trying too hard to spin a lower RPM prop.