Fuel Economy, Hypermiling, EcoModding News and Forum - EcoModder.com - View Single Post - Hot-rodding the Toyota MGR

e*clipse · 11-07-2014, 02:52 AM

OK, this kind-of more on topic for the thread.

I started out saying we need to find a better way to cool it, because if we push it harder it will produce more heat. But it was all vague.

Well I've gathered a bunch of seemingly unrelated info from those ORNL test and the motor design papers I previously posted.

Also, I found a paper about how much current you can push through motor windings. This paper gives an excellent overview of how windings fail from both overcurrent and circulating currents at high frequencies.
http://www.smma.org/pdf/conferences/...%20Density.pdf

I based this hand-waving analysis on close equivelencies and similar designs. Unfortunately, there is no ORNL test of the MGR directly.

So, as a start, here is some technical info about the MGR.
1) each phase leg is made of 12 0.032" diameter wires.
2) rotor diameter: 14.79 cm
3) rotor laminate width: 4.57 cm
4) stator has 48 slots
5) motor has 8 poles

Interestingly, this is very similar to the 2010 Prius. The winding use the same gauge wire and have 12 in parallel. The motor is also uses a 48 slot 8 pole stator. The rotor is slightly smaller in diameter and length, so it's not a 1:1 comparison.

Other motors reviewed in the ORNL tests include the 2008 Lexus 600h and the Camry, which use different wiring and rotor dimensions, but still use the 48 slot 8 pole design. All of the new Toyota hybrid motors us a design similar (with different dimension) to the one in the paper detailing the 2005 SUV. Interestingly, that paper is actually talking about MG2 used in the Highlander hybrid.

So, I wanted to find some reasonable limits to the current this motor could be pushed to. The ORNL tests go WAY beyond just parroting the OE specs. Everything including thermal behavior (with a whole bunch of extra thermistors) and magnetic field strength are tested.

First, they did a series of tests to measure the BEMF. The tests were conducted to well over 12500 rpm. For the Prius, the test was run up to 14000 rpm! Considering that the MGR's rotor is the same design with a smaller diameter, it says to me that the MGR should be very capable of running the same speed - MUCH faster than Toyota's 10,500 rpm limit.

Also, BEMF - measured as Line - neutral RMS was completely linear. For the Prius it is 450V @ 12000 rpm. The Lexus LS 600h produced 300V BEMF @ 12,000 rpm.

Next, they did a number of locked rotor tests to determine torque for the amount of current. The MGR's stator windings are almost identical, but since the rotor is smaller, the torque numbers won't be usable. The current limit for the locked rotor torque was based on the stator winding's temperature. The Prius windings were fully tested at 150A, with brief tests (to not overheat the stator) all the way up to 300A. At 150A, the Prius produced 150 Nm of torque. To match the rated 207 Nm of torque, the full 300A would be required. Coolant temperature is an important part of these tests, and the coolant input temperature was kept to 25C for the locked rotor tests and 0C for the efficiency tests. Obviously we won't be able to keep the coolant temperature anywhere near 25C on a hot Norcal day. I'll be happy if I can keep the coolant at about 50C.

Since Toyota doesn't publish how much current is required to produce the rated torque, I used the MGR's rated maximum torque of 130Nm and compared ORNL's tests of the Prius, Camry, and Lexus. At 125Nm Torque, the Prius requires 125A, while the Lexus requires 175A. The Camry is in the middle. Without having anything better to use, I chose an average of 150A, hoping the MGR is somewhere in that range.

Another set of tests were done for the "continuous" operation. Note for a car, "continuous" operation is MUCH shorter than an industrial motor would require. The Prius was tested at 50kW and the 14 temperature sensors were monitored. An upper limit of 150C was used for the tests. At 5000 RPM using 50C coolant, the motor was run for about 4min 30sec before one of the stator's temperature sensors exceeded 150C. Meanwhile, the coolant exit temperature increased to 75C. So, 50kW = 67hp. At 5000 rpm, the torque is 70ft-lb, or 95 Nm. For the Prius locked rotor test, this requires about 100A.

The last relevant issue is - how much current can you shove through a copper wire? This is commonly referred to as "current density" and detailed in the paper at the top of this post. The wires used in the MGR and the Prius are 0.032" in diameter. The cross-sectional area is 0.000804 in^2. The total wire area per phase leg is 12*0.000804in^2 = 0.00965in^2.

According to this paper, a water cooled motor (industrial?) can run steady state with a winding current density of 12,000 A/in^2. Locked rotor tests can go up to 25,000 A/in^2. So, 12,000A/in^2 * 0.00965 in^2 = 115A. Pretty modest, and corresponds with ORNL's continuous operation test at 50kW. 25,000A/in^2 * 0.00965in^2 = 241 A. This is REALLY close to ORNL's 300A locked rotor test for the Prius. The paper showes the "darkened" windings that can result from this. Obviously, insulation rating and cooling play a large part in the winding limit.

It appears that for the MGR, current should be limited to 175A > 200A, just to give some headroom because we only have one thermocouple, and the coolant temperature could be hotter that that used by ORNL's tests. Based on this, a 25% or maybe 50% increase in current over that required to produce the 130Nm max torque spec. Increasing the maximum speed to 13000 rpm should be safe and allow those using smaller tires or trying for higher speeds. For example, it would allow a maximum speed of 145mph for a vehicle running 235/45R18 tires.

11-07-2014, 02:52 AM	#173 (permalink)
e*clipse Permanent Apprentice Join Date: Jul 2010 Location: norcal oosae Posts: 523 Thanks: 351 Thanked 314 Times in 215 Posts	OK, this kind-of more on topic for the thread. I started out saying we need to find a better way to cool it, because if we push it harder it will produce more heat. But it was all vague. Well I've gathered a bunch of seemingly unrelated info from those ORNL test and the motor design papers I previously posted. Also, I found a paper about how much current you can push through motor windings. This paper gives an excellent overview of how windings fail from both overcurrent and circulating currents at high frequencies. http://www.smma.org/pdf/conferences/...%20Density.pdf I based this hand-waving analysis on close equivelencies and similar designs. Unfortunately, there is no ORNL test of the MGR directly. So, as a start, here is some technical info about the MGR. 1) each phase leg is made of 12 0.032" diameter wires. 2) rotor diameter: 14.79 cm 3) rotor laminate width: 4.57 cm 4) stator has 48 slots 5) motor has 8 poles Interestingly, this is very similar to the 2010 Prius. The winding use the same gauge wire and have 12 in parallel. The motor is also uses a 48 slot 8 pole stator. The rotor is slightly smaller in diameter and length, so it's not a 1:1 comparison. Other motors reviewed in the ORNL tests include the 2008 Lexus 600h and the Camry, which use different wiring and rotor dimensions, but still use the 48 slot 8 pole design. All of the new Toyota hybrid motors us a design similar (with different dimension) to the one in the paper detailing the 2005 SUV. Interestingly, that paper is actually talking about MG2 used in the Highlander hybrid. So, I wanted to find some reasonable limits to the current this motor could be pushed to. The ORNL tests go WAY beyond just parroting the OE specs. Everything including thermal behavior (with a whole bunch of extra thermistors) and magnetic field strength are tested. First, they did a series of tests to measure the BEMF. The tests were conducted to well over 12500 rpm. For the Prius, the test was run up to 14000 rpm! Considering that the MGR's rotor is the same design with a smaller diameter, it says to me that the MGR should be very capable of running the same speed - MUCH faster than Toyota's 10,500 rpm limit. Also, BEMF - measured as Line - neutral RMS was completely linear. For the Prius it is 450V @ 12000 rpm. The Lexus LS 600h produced 300V BEMF @ 12,000 rpm. Next, they did a number of locked rotor tests to determine torque for the amount of current. The MGR's stator windings are almost identical, but since the rotor is smaller, the torque numbers won't be usable. The current limit for the locked rotor torque was based on the stator winding's temperature. The Prius windings were fully tested at 150A, with brief tests (to not overheat the stator) all the way up to 300A. At 150A, the Prius produced 150 Nm of torque. To match the rated 207 Nm of torque, the full 300A would be required. Coolant temperature is an important part of these tests, and the coolant input temperature was kept to 25C for the locked rotor tests and 0C for the efficiency tests. Obviously we won't be able to keep the coolant temperature anywhere near 25C on a hot Norcal day. I'll be happy if I can keep the coolant at about 50C. Since Toyota doesn't publish how much current is required to produce the rated torque, I used the MGR's rated maximum torque of 130Nm and compared ORNL's tests of the Prius, Camry, and Lexus. At 125Nm Torque, the Prius requires 125A, while the Lexus requires 175A. The Camry is in the middle. Without having anything better to use, I chose an average of 150A, hoping the MGR is somewhere in that range. Another set of tests were done for the "continuous" operation. Note for a car, "continuous" operation is MUCH shorter than an industrial motor would require. The Prius was tested at 50kW and the 14 temperature sensors were monitored. An upper limit of 150C was used for the tests. At 5000 RPM using 50C coolant, the motor was run for about 4min 30sec before one of the stator's temperature sensors exceeded 150C. Meanwhile, the coolant exit temperature increased to 75C. So, 50kW = 67hp. At 5000 rpm, the torque is 70ft-lb, or 95 Nm. For the Prius locked rotor test, this requires about 100A. The last relevant issue is - how much current can you shove through a copper wire? This is commonly referred to as "current density" and detailed in the paper at the top of this post. The wires used in the MGR and the Prius are 0.032" in diameter. The cross-sectional area is 0.000804 in^2. The total wire area per phase leg is 120.000804in^2 = 0.00965in^2. According to this paper, a water cooled motor (industrial?) can run steady state with a winding current density of 12,000 A/in^2. Locked rotor tests can go up to 25,000 A/in^2. So, 12,000A/in^2 0.00965 in^2 = 115A. Pretty modest, and corresponds with ORNL's continuous operation test at 50kW. 25,000A/in^2 * 0.00965in^2 = 241 A. This is REALLY close to ORNL's 300A locked rotor test for the Prius. The paper showes the "darkened" windings that can result from this. Obviously, insulation rating and cooling play a large part in the winding limit. It appears that for the MGR, current should be limited to 175A > 200A, just to give some headroom because we only have one thermocouple, and the coolant temperature could be hotter that that used by ORNL's tests. Based on this, a 25% or maybe 50% increase in current over that required to produce the 130Nm max torque spec. Increasing the maximum speed to 13000 rpm should be safe and allow those using smaller tires or trying for higher speeds. For example, it would allow a maximum speed of 145mph for a vehicle running 235/45R18 tires.