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Unread 07-16-2018, 12:17 PM
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Spark motor controllers in series or parallel?

I have a drive with two CIM motors on each side attached to a dual cim input gearbox. I have 4 SPARK motor controllers.

Each side of the drive has the same signal with a Y-cable
Someone mentioned there might be issues putting them in parallel due to different internal resistances of the motors (manufacturer tolerance) and it will be putting constant strain on the motors because one is slightly going faster.

What do you guys normally do? Or is this considered negligible?

Thanks in advance
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Unread 07-16-2018, 12:21 PM
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Re: Spark motor controllers in series or parallel?

Quote:
Originally Posted by murdomeek View Post
I have a drive with two CIM motors on each side attached to a dual cim input gearbox. I have 4 SPARK motor controllers.

Each side of the drive has the same signal with a Y-cable
Someone mentioned there might be issues putting them in parallel due to different internal resistances of the motors (manufacturer tolerance) and it will be putting constant strain on the motors because one is slightly going faster.

What do you guys normally do? Or is this considered negligible?

Thanks in advance
It is standard practice to put the motors in parallel when using them in the configuration you described. (One Spark per motor, two motors per transmission.)
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Unread 07-16-2018, 01:22 PM
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Re: Spark motor controllers in series or parallel?

Assuming you're talking about the power wires: Wiring the motors in series would not comply with the rules (only one CIM per motor controller [R36], only one motor controller per breaker [R38]). Even if you aren't interested in the rules, this isn't a good idea. Wired in series, each motor would only get ~6V at full throttle instead of ~12V, reducing the top speed by 50%, the torque of each motor by 50%, and peak power by 75% compared to wired in parallel.

If you mean the splitting of the PWM wires to the SPARKs, parallel is again the way to go. Splitting the SPARK PWM voltage in half would PROBABLY work, but it would put you a lot closer to the tolerance values for those inputs, and therefore be less reliable.
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Last edited by GeeTwo : 07-16-2018 at 01:25 PM.
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Unread 07-16-2018, 01:29 PM
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Re: Spark motor controllers in series or parallel?

Quote:
Originally Posted by GeeTwo View Post
Assuming you're talking about the power wires: Wiring the motors in series would not comply with the rules (only one CIM per motor controller, only one motor controller per breaker). Even if you aren't interested in the rules, this isn't a good idea. Wired in series, each motor would only get ~6V at full throttle instead of ~12V, reducing the top speed by 50%, the torque of each motor by 50%, and peak power by 75% compared to wired in parallel.

If you mean the splitting of the PWM wires to the SPARKs, parallel is again the way to go. Splitting the SPARK PWM voltage in half would PROBABLY work, but it would put you a lot closer to the tolerance values for those inputs, and therefore be less reliable.
Quote:
Originally Posted by OP
I have a drive with two CIM motors on each side attached to a dual cim input gearbox. I have 4 SPARK motor controllers.
OP is fairly clear that he isn't asking about power wiring but instead putting the motors in parallel mechanically.

The answer to OPs question is simply, yes, the difference is negligible. Unless one motor has catastrophically failed then combining two identical or nearly identical (in the case of a mini cim+cim) is totally fine as long as your controlling them identically (like the Y PWM splitter you've suggested).
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Unread 07-16-2018, 01:42 PM
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Re: Spark motor controllers in series or parallel?

Quote:
Originally Posted by murdomeek View Post
Each side of the drive has the same signal with a Y-cable
Someone mentioned there might be issues putting them in parallel due to different internal resistances of the motors (manufacturer tolerance) and it will be putting constant strain on the motors because one is slightly going faster.
In short, this isn't really a thing when matching identical motors.

As long as a motor is not being forced to spin higher than its free speed, it is contributing meaningful torque. As the motors are identical, and since the motors are always experiencing some load (internal friction in the gearbox, at least), they will always be running at less than free speed.

The concern someone may have been talking about is when you match different motors together in a gearbox configuration where one motor is forced to run above its free speed - at which point that motor does not produce any torque at all. But this is fairly hard to do in FRC applications and essentially impossible to accidentally do just by mixing CIMs and/or miniCIMs together.

In short, you are totally fine using a Y cable - those motors should always be receiving the same command anyway.
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Unread 07-16-2018, 04:29 PM
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Re: Spark motor controllers in series or parallel?

Thats great.

Thanks for the clarification guys!
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