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DC motor for EV, specs

2K views 5 replies 3 participants last post by  Tesseract 
#1 ·
Hello

For my master thesis I am repairing an old (1991) EV in an educational way. I am using a buck converter, commanded by an arduino to control the input voltage. So far, so good. This is already working quite fine. The problem that I am having is that i can't find the documentation of the motor that has been used in the EV. It's a DC series motor, 11kW from Leroy Somer model: T22, n° 716092/12, 85V, 150A, 2850tr/min. To calculate the ripple in the current etc. I would need the inductance L of the inductor .

Does anyone has experience with this kind of motors or could you give a guideline value, based on a similar motor?

Simon
 
#2 ·
You'd have to test the motor to get the inductance value and it is load dependent due to saturation.

Member Tesseract has posted inductance figures for the Warp9 motor he used for his control development. Maybe he'll chime in or you could search for it on this forum.
 
#3 ·
Ok but the biggest problem is that I only have the ends of the 4 wires. The motor is covered by the frame of the car, the wheels, the gearbox, .. and we don't have the necessary equiopment at school to get the motor out of it and attach it to a couple and tachometer :/ So is it possible to do this when the motor is still covered by all these things?
 
#4 ·
For an inductance measurement you would want a locked rotor so the armature generated voltage is zero. Low current testing should not damage the brushes or commutator. So work with under 10A from a variac and scope the AC voltage and current so you can see the phase shift. That would be good for starters. Calculate the inductance and plot against increasing currents. Once you have a good feeling for that, try a single DC pulse with a decay circuit and see how that correlates. With the locked rotor you can do short pulses of high current without damage. For the saturated inductance you'll need to go above maybe twice rated current. Keep the pulse short and run the motor at low voltage (low speed) unloaded between the locked rotor tests to keep the comm conditioned.
 
#6 ·
Yeah, I posted about this a few years ago... like 5 or 6... :D

Anyway, there are many ways to measure the inductance of a DC motor, but some methods are more useful/relevant than others. I'll list a few in no particular order and as major already noted, the rotor (armature) must be locked:

1. AC voltage divider method: connect a known resistance in series with motor then apply a known 50 or 60Hz AC voltage to the series RL circuit and measure the voltage across the resistor and across the motor. The voltage will split according to the resistance of the resistor and the inductive reactance of the motor (X_sub_L = 2*Pi*F*L). The problem with this method is that the typical EV motor doesn't exhibit much inductive reactance at 50/60Hz (e.g. - 100uH = 75 milliohms at 60Hz) and so the winding and interconnect resistances will contribute a significant error.

2. Straight current measurement method: similar to above, apply a known 50/60Hz AC voltage except just measure the current. For extra accuracy, follow by applying a known DC voltage to get the resistance and subtract that from the impedance measurement to get the pure inductive reactance component.

3. Phase angle: I personally would not bother with this one as it is difficult to make accurate measurements of phase displacement between a voltage and current waveform without very good current probes (e.g. - Pearson) and/or a vector network analyzer.

4. Current ripple fraction: use a motor controller (ie- buck converter without an output inductor or capacitor) to supply a chopped DC voltage of known amplitude and on time. Measure the change in current during the on time and plug that number, along with the applied voltage, into the inductor equation: L = (V * dt) / dI

Note that most of the inductance is in the field - around 80% seems typical - so you can just measure it. Also note that the inductance of the motor will decline significantly as the magnetic circuit approaches, then exceeds, saturation (the series motor is unique in this respect - it can deliver useful torque well past the point of magnetic saturation).
 
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