Electronic Torque Limiting

[dknguyen]

Hi. So here is my scenario:

-I don't have a multi-speed transmission
-I need high speed, low torque on a motor for cruising on flat ground
-But I also need high torque at low speed on the same motor for climbing

So, what happens is that the motor I need is able to apply way more torque than is practically needed. So what happens is to "cruise" on flat ground, I apply a high voltage to the motor, and since it's flat ground there is low torques involved (and low currents). However, if I hit something in the road or skidding occurs, the motor will pull an excessive amount of current to drive the load (which it can do because a high voltage was being applied to achieve the speed in the first place). But this torque now exceeds the mechanical limits of the system. There are no problems with high currents and low-speed since much less power is being involved and it just indicates low-speed movement over treacherous terrain.

Having mechanical torque limiting is cumbersome (and impractical since I don't have the parts). I was wondering if it would be more, less, or equally effective if I limited the current of the motor at higher speeds, since at higher speeds the torque/current should be low anyways. I know it would work, but I'm worried about whether the response is fast enough to save the mechanical system, since mechanical torque limiters have a response time of zero by definition (right?)

I had another post asking a "similar" thing. Except it involved limiting the current when overvolting the motor in order to achieve extra speed but not burn the motor out if extra load was applied. In this case, the motor is not being overvolted, nor will it burn out if more load is applied at top speed- instead the motor will break the mechanics.

 

[Pommie]

If you put an analogue current limiter in the motor drive circuit (power feed to H-bridge) then this will act as a torque limiter. Analogue current limiters reduce the voltage in order to stop excessive current flowing. They unfortunately dissipate a lot of watts which may be a problem

Mike.

 

[dknguyen]

You mean stick an op-amp circuit in-line with the power source? That's the closest thing I've found in reference to analog current limiting. Yeah that won't work for me. Too much current going on. Also, the allowable current at low voltage/PWM duty cycle is the max motor current while the allowable current at full voltage is only a fraction of the max motor current since it's not a heat concern (where the current limit is just constant), but a torque one.

 

[kchriste]

Looks like a job for a micro controller like a PIC or something. First you need a method of monitoring motor current and voltage. Then program the PIC to calculate the max power (W = VI) that you want the motor to receive and control it with PWM. With a micro controller, you can then allow for a higher start current for a predetermined time frame so the vehicle has decent acceleration off the line.

 

[Hero999]

YOu could use fold-back current limiting or just cut off the power on an over current then turn it on (after a set time period) then off again if it's still too high; the circuit will oscillate until the motor isn't drawing an excessive current. I take it you're using a microcontroller so most of this can be done in software.

 

[dknguyen]

Ah, so there is a word for what it's called- foldback. That'll save a mouthful. Yeah, it's a uC. My question isn't as much how to do it, as whether it will stoip the torque before the drive train destroys itself. I guess it would depend on exactly how quickly the motor could react to appearance of this new load (bump or skid in the road).

 

[hjames]

Would it be possible to just (mis)use a stepper motor driver? It'll PWM the h-bridge to maintain a proper current, and the uC can just set the desired current.

As for the mechanical stuff - it depends on where the inertia is - if the motor's rotor has enough momentum, the gear train will happily flex trying to bring it to a stop. Still might be enough to strip gears if they aren't strong enough.

 

[dknguyen]

Well I'm building the driver from scratch anyways. So yeah, if I called it a stepper motor driver I guess it could be (it will have current sense either way).

I'm fine with stripped gears. Those are easy to replace. What I'm not okay with is cracked axles and exploding gearboxes.

 

[hjames]

What's the scale of the driver - If it's small enough, there are single chip stepper motor drivers that (might) save you the effort of doing a hardware current limit. But if a software solution is good enough, then there isn't any benefit to doing it that way.

 

[dknguyen]

It's on the order of 38A maximum. 25A is the continuous maximum.

 

[Pommie]

You mean stick an op-amp circuit in-line with the power source? That's the closest thing I've found in reference to analog current limiting. Yeah that won't work for me. Too much current going on. Also, the allowable current at low voltage/PWM duty cycle is the max motor current while the allowable current at full voltage is only a fraction of the max motor current since it's not a heat concern (where the current limit is just constant), but a torque one.

No, I mean something like this,
**broken link removed**

Obviously the values will need changing but the basic principle is the same.

As for requiring max current at low voltage, current = torque no matter what voltage is applied. Setting a current limit is the same as setting a torque limit.

Mike.

 

[dknguyen]

Oh yeah, motors have current constants and torque constants (which I guess are independent of each other). Doesn't the no-load current of a DC motor increase as the voltage is increased though? We've been doing so much on induction motors at school it's pushed all the DC motor stuff out of my head!

 

[hjames]

Oh yeah, motors have current constants and torque constants (which I guess are independent of each other). Doesn't the no-load current of a DC motor increase as the voltage is increased though? We've been doing so much on induction motors at school it's pushed all the DC motor stuff out of my head!

The higher voltage results in a higher speed which then results in higher frictional/viscous torque losses (plus eddy/electrical current losses), which then translates into higher currents.

Cough, a 40A driver changes the name of the game significantly - you'll end up having a milli-ohm level sense resistor. Switching had better be integrated into the h-bridge, either through software or some hardware override.

What's the voltage - is this a RC style motor, or some monster HP level motor?

 

[dknguyen]

It's an RC style motor, 18V max. I am using a hall current sensor from Allegro.
When you say switching better be integrated into the inverter (it's a 3 phase brushless motor)...isn't that a given? Are you referring to PWM or the current kill switch? BOth seem to be a given in my mind.

The largest issue for me (aside from the survival of the motor) is the survival of the DirectFETs. They can handle 25A at 70C ambient (great!), but of course, the motor can survive 60A at 38A. I need to find a way to get a heatsink onto those tiny tiny metal cases since it can handle 180A at a case temperature of 150C.

 

[Hero999]

I thought the current is near enough directly proportional to the torque.

My question isn't as much how to do it, as whether it will stoip the torque before the drive train destroys itself.

Providing you limit the current to level where the torque is low enough not to distroy the transmission I don't see how any harm can be done.

 

[dknguyen]

Yeah, but can does a given amount of current correspond to a given amount of torque regardless of the voltage being applied? HJames says it does.

 

[Hero999]

I both agree and disagree.

Let's ignore the hysteresis and eddy loses for the moment. This motor is perfect except for the copper losses and friction.

Stall
A huge current proportional to the applied voltage flows. The torque is directly proportional to the current which is directly proportional to the applied voltage up to the point where the core saturates.

Off load
Increasing the voltage increases the speed and therefore frictional losses which are effectively a load on our perfect motor; because this load has increased the effective torque has also increased causing a higher current draw. Note also that the increase in current doesn't obey ohm's law because the higher back EMF in the motor counteracts the increase.

In practice the eddy and hysteresis and eddy losses are pretty low and will just increase in a similar fashion to the frictional loss with increasing speed. For all intents and purposes the torque is proportional to the current draw.

 

[hjames]

It's an RC style motor, 18V max. I am using a hall current sensor from Allegro.
When you say switching better be integrated into the inverter (it's a 3 phase brushless motor)...isn't that a given? Are you referring to PWM or the current kill switch? BOth seem to be a given in my mind.

I was mostly trying to say that having another mosfet in series is a bad idea.

As for current/torque, I don't think anyone was saying that torque and current aren't proportional - just that as the motor spins at higher velocities, the torque required to maintain the steady-state velocity will increase, and will show up as an increase in no-load current.