Motor Starting Torque (Peak Stall Torque) and Motor Types

Back talking about the Harley LiveWire, I commented that the bike, along with the Zero SR, didn’t seem to pull as hard as my PMDC-equipped bike right off the line.  Read: Wheelies.  I love me the wheelies.  So I got to thinking about the whole equation – motor types, starting torque, all that stuff.  One disclaimer though – my bike was running RC lipo, dumping huge amps, and was really light.  However, a mutually exclusive disclaimer, a buddy of mine has an AC20 bike also running RC lipo, is very light, and also has that AC-motor off the line, well, lag.

So it begs the question: does an AC motor have to spin up a bit before it develops full torque?  Which begs a bigger question: what are the typical characteristics of various types of motors?  I ask this because one of the quickest drag bikes on the planet is running series-DC.

The four basic types of motors I’m interested in are “permanent magnet DC” (PMDC), AC in general, but while we’re there, let’s look at the difference between “induction AC” and “synchronous AC”, and finally my drag-racing buddy’s “series DC” motors.

Once you dig into it, it appears that’s exactly what’s going on – an AC motor, because it needs to spin up to develop a field, doesn’t actually pull all of the torque it’s capable of at 0 RPM.  A PMDC motor does crank out high torque at zero, because the fields are already there by virtue of the magnets, and a series-DC motor, because of the way the fields are generated, can spool up starting torque like a bitch, if you want it to.  By the way…  any of these motors develop several times their “rated torque” at start-up, according to these sources, to break the stall torque.

Here are some references, with a little more detailed information.

PMDC motors

Here’s a great post telling you everything you need to know about PMDC motors, from MachineDesign.com.  Keeping the discussion to starting torque, they say this:

Because PM motors lack armature interaction, they can generate high momentary starting and acceleration torques, typically 10 to 12 times full rated torque. Thus, they suit applications requiring high starting torques or momentary bursts of power.

AC motors – Induction and Synchronous

AC motors are brushless, and can either use a magnetic field generated by coils, or by permanent magnets.  Here’s the most clear and simple explanation of that, from the Wikipedia:

There are two main types of AC motors, depending on the type of rotor used. The first type is the induction motor or asynchronous motor; this type relies on a small difference in speed between the rotating magnetic field and the rotor to induce rotor current. The second type is the synchronous motor, which does not rely on induction and as a result can rotate exactly at the supply frequency or a sub-multiple of the supply frequency. The magnetic field on the rotor is either generated by current delivered through slip rings or by a permanent magnet.

AC motors are a huge subject, but to keep it to starting torque, go here and read the primer on AC induction motors: Tesla Polyphase Induction Motors on the All About Circuits page.  Here’s the graph that got my attention:

Series-Wound DC motors

There’s a good description of a series-wound DC motor here, on the IMPhotonics page: Parts and Principles of operation of a Series DC Motor:

In series motors stator windings and field windings are connected in series with each other. As a result the field current and armature current are equal. Heavy currents flow directly from the supply to the field windings.

In a series motor electric power is supplied between one end of the series field windings and one end of the armature. When voltage is applied, current flows from power supply terminals through the series winding and armature winding. The large conductors present in the armature and field windings provide the only resistance to the flow of this current. Since these conductors are so large, their resistance is very low. This causes the motor to draw a large amount of current from the power supply. When the large current begins to flow through the field and armature windings, the coils reach saturation that results in the production of strongest magnetic field possible.

The strength of these magnetic fields provides the armature shafts with the greatest amount of torque possible. The large torque causes the armature to begin to spin with the maximum amount of power and the armature starts to rotate.

Here’s the simple explain from here, Engineer’s Edge:

The advantage of a Series Wound Motor is that it develops a large torque and can be operated at low speed. It is a motor that is well-suited for starting heavy loads; it is often used for industrial cranes and winches where very heavy loads must be moved slowly and lighter loads moved more rapidly.

With this graph:

Here’s some more from the Ohio Electric Motors page, with the title, “DC Series Motors – High Starting Torque but No Load Operation Ill Advised”.  OK, that about sums it up…

So, yeah, it seems that various motor designs have different characteristics when starting up from zero RPM.  What threw me are these motor torque curves I talked about back in this post, here.  From those, it looks like, for example, my ME1003 will pull just the same as an AC20 off the line.

For the complete discussion of these curves, go to the elmoto post, here.  From the followup I’ve done, I don’t think those take into account the real-world characteristics of breaking that “stall torque” point.  To be fair, an AC system is typically (in my experience) heavier as well – maybe up to 10% so.  All of those factors will play into the pull off the line, but breaking that first stall is, I’m guessing, huge.  Look at it this way – you’re comparing motors that range from being able to start a huge diesel truck from zero RPM, to motors that need capacitors and starting coils just to get themselves spinning up, so naturally it makes sense some motors are going to pull you off the line better than others.

Now, just to try this theory on for size, I looked at the Motenergy data sheets for two basically identical motors – the ME1003, a brushed, PMDC motor, and the ME0913, it’s brushless, PMAC cousin – note, both have permanent magnets.  We’re looking at the “Peak Stall Torque”, or, the torque the motor makes when you keep the spindle from turning.  On the ME0913, the Peak Stall Torque is 94Nm.  On the ME1003, it’s 108Nm.  That’s about a 10% difference, which is clearly significant.

There are several great analogies in gas motors, which I was using even before I saw this information.  An AC motor feels to me like a multi-cylinder, maybe even turbocharged 4 stroke.   It pulls OK, but when it really gets spinning is when you’re going to get the “hellbound train” feeling.  My PMDC felt more like my single-cylinder 600cc…  good low end, wheelies like you read about, but a limited top-end.

You might even go so far as to say an AC motor is like running a turbocharger – the motor needs to build exhaust pressure before it can deliver intake boost, otherwise known as “turbo lag”  – compared to a pulley-off-the-crankshaft supercharger, where as long as the motor is turning it’s delivering boost – more akin to the PMDC or series-DC power delivery.

So, as far as conclusions go, I think it’s an important thing to consider in the whole AC/DC/whatever motor decision – add “stall torque” to RPM, rated torque, HP, watts, weight…  and it’s something that contributes greatly to the basic feel of the bike, where the rubber hits the road.

More reading: Check out this page on MachineDesign.com for a complete discussion of AC and PMAC motors.