Finding the Volt’s Traction Motor Redline

We all know that the Volt has two electric motors. These motors are blended to ensureefficient operation and so we don’t have any “overspeed” issues associated with having just one motor. Some may remember that the Tesla Roadster, during development, was originally supposed to have a 2-speed gearbox to help with motor overspeed issues. However, mechanical problems with this 2-speed box resulted in production delays and eventually the 2-speed box was eliminated and Tesla went with a custom, high rpm, hand wound motor eliminating need for the 2-speed gearbox.
 

 

What RPM limits do the Volt’s motors have?

I remember early forum discussions on this subject. Shortly after power split was announced, we speculated that the 70 mph transfer to power split was enacted because, at this speed, the main traction motor hit its redline at 6,200 rpm. (Also noted was the Prius 6,000 rpm limit.)

But how does the Volt really operate? When does it use one motor and when does it use two motors? Typically we would think that at lower speeds the Volt would be in one motor and at speeds above 70 mph it would be in two motor.

Much to my surprise and discovered using my newly acquired DashDAQ, under full throttle, the Volt stays in one motor operation ALL THE WAY TO 102 MPH! (Which is max governed speed.)

Discussion
 

 

Figure 1 shows the 4ET50’s planetary gear set configuration during one motor operation. In this mode, clutch 1 is closed locking the ring gear to ground and there is a single 3.24/1 gear reduction between the traction motor and the output (carrier) of the PG set.
 

 

Figure 2 shows the 4ET50’s configuration during two motor operation. Clutch 1 is disengaged and MGA blends to decrease MGB’s speed and allow efficient operation.

Volt Stays in 1 Motor Mode during Maximum Acceleration
 

 

The data taken with DashDAQ during max accel is presented in figure 3. The accelerator pedal was pushed to the floor. There are 2 very interesting things to note from this data:

1) The Volt stays in 1 motor mode all the way to 102 mph where MGB rpm hits 9,465 rpm …way in excess of the previously speculated 6,200 rpm limit.

2) Looking at the HV battery output, we see a fairly slow (1.2 second) ramp up to full power. Others (like DonC) have speculated about this “torque
softening” feature and, as we see here, it does exist. Taking some of this torque softening away could be a source of easily attained performance uprate for ELR or Volt SS models.

Volt shifts to 2 Motor after slowing to 60 MPH and holding Constant Speed
 

 

Following the accel, I slowed to the speed limit and held approximately 60 MPH on the flat and level road. Under these conditions, the Volt shifted to 2 motor operation. The shift is shown in figure 4.

The associated motor speeds for the 1 motor full accel and 2 motor steady state 60 mph condition are shown in figures 5 and 6.
 

 

Summary

Under full throttle the Volt stays in single motor and at steady state cruise it shifts to 2 motor operation. I can only speculate on the reason why the Volt operates this way:

During maximum acceleration we want maximum torque and this is provided by the 3/1 gear reduction of the sun/carrier speeds (versus the only 1.4/1 gear reduction of the ring/carrier speeds).

During steady state cruise we want “efficiency.” Maximum efficiency is attained by matching the motors at their most efficient operating point by “mixing” the two motors.

… What are your thoughts?

By: George Brewer

Source: GM-Volt.com