View Full Version : [Question(s)] Will this controller work?

05-11-2009, 09:37 AM
I am trying to build a ride-on train for my son.

I need to get a controller for the motor and I wanted to know if the SYREN 25 (http://www.trossenrobotics.com/store/p/5108-SyRen-25A-regenerative-motor-driver.aspx) would work for this motor.

Here are the motor specs: 36 Volt, 750 Watt, 2800 RPM, 27.4 Amp, permanent-magnet motor. It's original purpose was for a chinese scooter.

I am only running the motor at 24V.

What I cannot figure out is if a motor called a "permanent-magnet motor" is considered a BRUSHED motor.


05-11-2009, 09:46 AM
If it has two wires, it's brushed. If it has more, it prolly won't be :)
Since it's from a chinese scooter I'm thinking it is. Attach a car battery and see if it spins. If it does you're all set, though the controller you're linking to isn't going to be able to cope with the motor for very long, it might even kick into protection immediately preventing the motor to turn alltogether.

05-11-2009, 11:31 AM
Even if I am running at 24v, won't that reduce the amperage?

05-11-2009, 12:09 PM
I replied to this via PM, but once i hit the send button I started wondering..

I told you the amperage would increase since the power rating of the motor remains the same, yet the voltage is decreased, i.e. it would demand more current to overcome static and dynamic friction.

But as i said, ohm's law says otherwise, so I'm guessing there's something to do with inductance here.

I'm going to scratch my head a bit over this one and get back to you, but something tells me there's been posts about this on this very forum before

05-11-2009, 12:27 PM
If you lower the voltage to 24v the amps will go higher than 27.4Amps. Basiclly the motor compensates for the loss in power by pulling more amps. Even thou it sounds crazy your actually putting more strain on the motor running it at a lower voltage. You can run it at a lower voltage but the motor has a max amp rating once you go past this things start to wear out. And going higher than 36v is just a waste of power since the magnets in the motor will hit saturation. Basicllly its try to magnatize the iron beyond a reasonable amount.

You need a larger controller or a smaller motor. Since you have the motor already you need a beefier controller.


05-11-2009, 01:49 PM
Any suggestions on what controller to get? I need one that will work off a basic Potentiometer if possible.

05-11-2009, 02:35 PM
This IFI (http://www.ifirobotics.com/docs/36v-48v-victor-hv-users-manual.pdf) speedcontroller should do the thing... a tad over the top with 120A continuously, but it seems there's not a lot of 36v speed controllers out there.
If you're willing to undervolt (but live with the higher current draw) there's more options, yet they all come at a price.

05-11-2009, 02:57 PM
If you reduce the voltage supplied to the motor the current will also be reduced. The resistance of the motor is the constant factor. The 750 Watt rating is with the 36 volt supply, where it operates at peak efficiency. If compared to the 36 volts times the 27.4 Amp calculates out to 986 watt total of rated voltage and stall current, showing max values. There will be a linear reduction of of max current when reducing the voltage, so with 24 volts being 2/3 of 36, the stall current will also be 2/3 going down to 18.7 Amps. The Syren 25 is adequate to run the motor at the 24 volt level. Note the motor output would now be about 500 watts at peak efficiency.

05-11-2009, 03:22 PM
I'm actually not too sure about that. A motor is definetely not a resistive load, and as such its resistance is not a constant.
So too does the back-emf change as the load increases, and will also be different with another current and/or voltage.

It's been too long since I've had to do with the physics behind motors though, but there's something in the back of my head saying the current will increase with a lower voltage at the same mechanical load.

I wish I could explain why I think this way though... Where's Lynn when you need her? :p

05-11-2009, 03:36 PM
Right here... :P

The startup current will go up, otherwise I agree with Robologist. I have some 750wt @ 36v motors here, I run 'em on a dual 80A controller which is perfectly happy with 'em. That said - stall current IS a resistive load. Once it stops spinning, it's a giant power resistor.

For my money, the IFI Thor is on sale right now here: http://www.robotmarketplace.com/products/IFI-T883.html

Of course, that's only useful if you never plan to go to 36v.

The 4QD is a good pricepoint: http://www.robotmarketplace.com/products/4QD-VTX-40-36.html

Honestly, I've found the Dimension Engineering products to be over-rated. My 10A dual won't handle more than 7.5A reliably/continuously without shutting itself down repeatedly even with an enlarged cooler, fan, and heatsink. I was a fan until I started pushing them a bit harder... I wouldn't get behind them without reservation.

05-11-2009, 03:46 PM
I stand corrected.

I guess the current would indeed go up with lower voltage if the mechanical load is the same but this isn't the case as the amount of load it is capable of handling is limited by the lower voltage and thus current.

Quite dumb of me actually, if the batteries run low, the motor turns slower, simple as pie.. I'm going to bed now and reconsider the plan of going for that masters degree.

05-11-2009, 03:48 PM
Adrenalynn is exactly correct. When the rotor is not spinning the coils are essentially a resistor and you can calculate it using simple Ohms Law. For the rest of the calculations, this is a great page:

05-11-2009, 03:50 PM
Resistance is resistance is resistance. Copper doesn't change properties just because there is a magnetic field present. The magnetic field is also "permanent" and another constant.

I think the big problem comes when thinking that a particular torque output must be maintained. If a motor has to output 5 kg-cm with a 36 volt supply, it may draw 6 Amps to do that, as would be observable on it's torque-current graph. If the voltage supply is then reduced to 24 volts, to get the same 5 kg-cm torque output, the current does have to increase proportionately to 9 Amps, but would be placed at a further point on the new torque-current graph. Where the motor was able to output say 15 kg-cm with a 36 volt supply, it will only be able to output a max of 10 kg-cm with a 24 volt source. If you tried to pull an 11 kg-cm load at with a 24 volt supply, the motor wouldn't be able to move, where it could with the 36 volt supply.

Edit: ok, guess this wasn't needed, missed the other replies.

05-11-2009, 04:36 PM
Exactly. This motor is going in to a train that a person is going to sit on. To get the same out of the motor at a lower voltage you need to juice up the amps. You have to account for the load amps that get added to the no load rating. Its going to be higher than it would be at 36v. Lynn's recommendations would work for sure and as he gets bigger you could add the extra 12v and not worry about the controller since it will be able to handle it down the road.