Author: webmaster

Stories from the rotabench lab: >>Let them (almost) burn!<<

The title picture is a photograph of the setup for testing the “freewheel” diodes on the rotabench 6P lab-inverter power stage. The purpose of this experiment was to find out, how much “reverse current” the power stage can handle in a fault condition. The experiment was motivated by a couple of destroyed power stages at the customer and in my lab.

One of the worst-case scenarios for an inverter power stage is, when suddenly the DC voltage drops while running a PMSM on a 4-quadrant test bench with a 2-quadrant PSU. When this happens, the brake engine delivers mechanical power to the DUT, the DUT becomes a generator, converts the mechanical energy into electrical energy and delivers current back to the inverter, which is converted in to heat (or fed back into the grid) by the load/PSU.

Another situation – with a similar outcome – would be if the PMSM is operating in the field weakening area and the DC-Bus voltage is lower than the theoretical Back-EMF voltage at a (theoretical) iD = 0 and for some reason the control fails, e.g. because the coupling slips. This could also lead to a situation where the DUT is forced into breaking operation and delivering current back to the inverter.

Such a sudden event – which happens typically within a timeframe < 1 second – is capable of wreaking havoc among the power electronic components, destroying the power stage within a blink of an eye and releasing the “magic dust”. The power stage takes the max. amount of damage possible in very little time.

Therefore such a situation needs to be addressed fast, fully automated and in the proper way. The most important countermeasure is to cut off the AC connections to the motor as fast as possible to stop the motor from feeding currents back into the inverter.

But: failure detection algorithms need time “to be sure”. No one want’s a device that shuts down into error mode at every little voltage spike, which simply could be a measurement error. Contactors need time for switching, you can’t stop a rotating machine within no time, and so on. So there is a short time span, where the power stage must be able to handle harsh overload conditions, without getting destroyed.

The power stage in the rotabench 6P lab-inverter has “freewheel” diodes arranged in a B6 configuration in parallel to the body-diodes of the MOSFETS. The main purpose of those (Schottky) diodes is to catch the (positive and negative) Back-EMF voltage spikes generated by the inductance of the DUT and the wiring (not to mistake with the Back-EMF of the motor!) during normal operation. In the scenarios described above those diodes become an involuntary B6 bridge rectifier, which rectify the (short circuit) current delivered the DUT and fed it into the DC Bus. There is not much you can do to prevent this, you just can design the PCB in a way that it is capable of handling this situation for the time necessary to separate the DUT from the inverter and shut everything down.

In order to check how much current the diodes can handle, I built a B6 rectifier bridge in the same configuration like on the rotabench 6P lab-inverter power stage but without the MOSFETs, short circuited the DC bus to allow the max. amount of current flow and used an EPS motor (Bosch, 4 pole-pairs, 12 Volt PMSM) driven by my test bench as generator. Then I measured the temperature on the diodes with a thermal camera, with the following results:

At a rectified DC current of slightly above 100 Ampere DC (200 Hz AC frequency), the temperature of the diodes increases from room temperature to ~80°C in one second and reaches ~120°C after two seconds, which is close to thermal destruction of the diodes. However, the encouraging result is: I have at least one second to detect an error condition in the motor control, switching off the AC contactor and shutting down the system. And in the microcontroller world, thinking in milliseconds, a second is a huge amount of time, even if the contactor is slow and needs 100 ms for a full separation of the electric circuit. So the conclusion is: 1 second is enough to shut down the test bench without major damage on the inverter.

Do I really need that much equipment? obviously yes.

With this setup, I can measure the group delay of the analog inputs for the voltage and current measurement of the rotabench 6P inverter.

The calibration process has two steps: in the first step, the outputs of a simulated PMSM are fed into the inverter. The device “thinks” it is driving a motor. The signals are generated by two Siglent Arbitrary Waveform generators, which are fed into a signal conditioning and amplifier PCB. The amplifier simulates the current output of the current sensors. A FPGA is used to simulate the encoder signals. With the known outputs of the signal generator the phase delay of the inverter measurement can be determined and compensated.

In the second step, I use a big choke to generate some currents around 100 A RMS. These currents are measured with a cRIO, a signal conditioning and amplifier device and Signaltec CT200 current sensors. With these data the D- and Q- currents are calculated, which can be compared with the inverter setpoint. As the first step does not use real currents and the current sensors of the inverter, some adjustmens for the group delay of the current sensors is necessary.

When anything is running smoothly, I can achieve a phase accuracy < 1° (electric) on a (simulated) PMSM with 4 pole pairs, running at 1000 Hz (electrical) frequency.

Hot on the bench: Signal Conditioning PCB for LEM CT-200 current sensors.

I just finished soldering the first samples of my signal conditioning PCB for LEM CT-200 current transducers. These devices converts the current output of up to 4 current CT-200 transducers into a voltage. For high precision I use Vishay Z-Foil burden resistors with an accuracy of 0.02% and a very low temperature coefficient. With some reduction in accuracy also CT-100 or CT-60 sensors could be used.

Format: 160 x 100 mm, 4 channels with Vishay Z-Foil 5 Ohm 0,02% burden resistors. Passive cooling. Power Supply: 2 x 18 Volt DC. The 5 Volt Rail for the Sensor Status output is generated by a DC-DC converter on the PCB.

Each Channel has a +-10 Volt (DC) Output and a 1 Volt RMS output on SMA connectors. The 1 Volt RMS outputs are intended to be used with my new-old Norma D6100, the 10 Volt Outputs go to a compact RIO.

Please welcome the newest member of the rotabench® family: the rotabench® 6P 100/60E power stage!

If you need a current amplifier, that converts the PWM signals from your DSP system into a high current 3-phase rotating field to power an electric motor, this new device may come handy.

It can generate currents up to 100 Ampere RMS per phase and can handle DC-Bus voltages up to 60 VDC nominal. This makes it the perfect tool for engineers who develop or test motors with a nom. Voltage of 12, 24, 36 or 48 Volt.

The typical application is in a lab or test bench environment, with the main purpose either to test the motors or to test the control algorithms for a particular motor. Typical motors are 3-phase automotive or eBike motors, or EC-motors for battery-powered tools.

It comes in a 3 HU 19’’ rack chassis with integrated cooling and D-Sub connectors, so you can easily assemble it in your test bench rig.

Go to the product page for more information

Here is a short video of the initial startup of one of our rotabench 6P 100/60 inverters with 100A RMS cont. and 60 Volt DC Bus voltage (max.). This device is going to Signaltec, where it will be used to test their current transducers under realistic conditions with pulsed currents. Signaltec already tests their current sensors in a wide frequency range from DC up to 500 kHz sinusoidal signals. The inverter gives them the possibility to test the sensors at different fundamental frequencies, variable switching frequency and distorted current waveshapes.

rotabench 6P inverters are designed for test bench and laboratory use with PMSMs, which need low voltage and high currents. Typical use cases are test benches for automotive electric motors or battery powered tools in the 12V to 48V range. The system is engineered by test bench developers for test engineers. Unlike other inverters, they provide a sophisticated user interface with an unparalleled usability and extensive visualization. Finally, an inverter is no longer a black box; you can see what it is doing, if you want to.

This is the first blog entry after our hompage re-design in July 2024