Geared vs non geared Rpm limits - does the motor or controller limit it?

[Sprockett]

Hey guys

Do non geared and geared behave similarly in terms of RPM limits?

Here's the situation I'm facing:

I have a geared hub motor that was taken from a 36v electric bike.
I hooked the motor to another generic controller, but this time with 48v. Reason for this is to improve the top speed.

When I run it on no load, it appears to hit an artificial maximum, where the motor suddenly seem to pulse intermittently, limiting the rpm. This didn't seem to happen when it was driven using 36v.

So my questions:
1. Is this limit coming from the controller or motor? I've read that controllers also have their ERPM limits. And if the motor is geared, it is quite possible we are hitting these limits much earlier.

2. are motor rpms EVER limited at the motor level? Or can we just keep adding volts to increase rpm? If there is a forced limit at the motor level, are they the same for both geared and ungeared motors? In my experience I've never faced this issue (suddenly hitting an artificial limit where motor is pulsed) before when trying to overvolt ungeared motors.


Thanks in advance!

 

[docw009]

Approaching your second question from the design side, putting a speed limiter into a motor designed to work with arbitrary controllers would be quite expensive and overly complicate the design. This answers your first one. It's the controller. What you're seeing is either a speed limiter or a compatibility problem in the controller.

So what kind of controller and what kind of motor?

 

[amberwolf]

I have a geared hub motor that was taken from a 36v electric bike.
I hooked the motor to another generic controller, but this time with 48v. Reason for this is to improve the top speed.

When I run it on no load, it appears to hit an artificial maximum, where the motor suddenly seem to pulse intermittently, limiting the rpm. This didn't seem to happen when it was driven using 36v.

It's not the voltage, it's the new controller. Most likely it has a speed limiter built in, which may or may not be overridable.

It could also be the wrong phase/hall combo, which can cause all sorts of strange behavior.


Run the same controller/motor on 36v (if it doesn't have an LVC preventing this) and it'll probably do the same thing, as long as the voltage is sufficient to cause it to reach teh same RPM.

Use a bike computer with it's own wheel speed sensor (or other tachometer) to tell you what the RPM actually is in each combination of controller, motor, and voltage, and you can see where limits from voltage vs controller exist.

1. Is this limit coming from the controller or motor? I've read that controllers also have their ERPM limits. And if the motor is geared, it is quite possible we are hitting these limits much earlier.

I'ts possible it's an ERPM thing, but it's much more likely to be a simple speed limiter.

Either way, it's not the motor. The motor cannot know what RPM it is at, nor can it do anything at all to change the RPM it's at or limit it.

2. are motor rpms EVER limited at the motor level?

Not in the way you see, for any motor you're likely to run into. There could be some design that mechanically clutches a brake into place above a certain speed, by centrifugal force--but I don't think you're going to see that on a vehicle.

The only limit is mechanical strength of the various parts vs the forces against them caused by rotation.

So, for instance, an inrunner could have it's magnets fly off the rotor and jam against the stator, if those forces were high enough to overcome the glue (or other) fastening them to the rotor.

Or the commutator segments can come loose and "grenade" in a brushed motor.

Stuff liek that.

But either way, there's no RPM limiting per se, just that at some point the motor comes apart from "overspeed" mechanical failure.

 

[Sprockett]

Thank you all! Eventually realised it was a controller limit. I'm running geared and in sensorless mode the controller eRPM limit was reached at about 700 rpm. I attached a sensored version of the same motor to the same controller and managed to hit the kV rpm limit. Apparently sensored and sensorless has different eRPM limits at the Conrroller level.

Thanks all.