The Theory Behind the Behlman BL Motor Test Option

In our last blog post (please see Is Your Factory Power Source Corrupting Your Product Testing?), we discussed the importance of having a consistently reliable source of AC power when testing new electrical motors. In it, we mentioned that having a special motor-test option (with fold-back/soft-start capabilities such as in the BL 1350 and BL HP lines) can save you a lot of time and money when doing production testing on consumer products. In this post, we’ll explain how and why.

It is generally difficult to start motors because the peak current required to start a motor can be in the range of 3-6 times the current needed to run the motor at full load. After the peak current has just started the motor turning, there is a period of several seconds where the current falls to the run value. The problem is that if you size the power supply so that it has just sufficient capability to run the motor and load after start-up, you often find that it cannot start the motor without severe output voltage distortion or possibly even going into an over-current shut-down. If you size the power supply so that it can start the motor without significant output distortion, then you have purchased a power supply that is 3-5 times larger than what is needed for most of its operational time, which is very expensive. The ideal solution is to design a power supply that can deliver the extra, short-term current during startup but is basically sized for the long-term run current needed.

Electric motorIn any power amplifier there are two major factors that limit the output current. The first is the peak current handling capabilities of the output devices, in this case an output insulated-gate bipolar transistor (IGBT). The second is the requirement to dissipate the internal heating caused by losses in the amplifier. There are usually two types of current-limit features built into a power supply to protect it. One senses peak current through the output device and rapidly limits the peak output current to prevent damage to the IGBT. The other senses the RMS (root mean square) output current and, somewhat more slowly, limits the RMS output current to keep the unit from over-heating. In a typical Behlman BL unit, the RMS current limit is set at about 105-115% of the rated current.

A motor start sequence in a typical BL unit may go something like this: The motor is turned on. If the peak current is too high, the BL shuts off because it sees what it thinks is a short circuit. In this instance, there is no possibility that the BL can start the motor. If the BL doesn’t shut down because of the high peak current, then the motor wouldn’t start because the small initial peak current couldn’t overcome the bearing friction and get the motor turning, or the BL sees a short and just runs at zero output voltage and heats up the motor until the motor over-temperature sensor disconnects the power. The motor still wouldn’t start. The last possibility is that the motor starts turning but doesn’t get up to speed. The BL runs at full current and at some reduced voltage, but things just never get fully started.

The peak current capabilities of an AC power supply are usually limited by the size of the output device, in this case the output IGBT. Behlman doubles the rating of the output IGBT in BL units with the MT (motor test) option; and, since the circuitry that limits the peak current in the output is basically part of the IGBT, doubling the rating of the IGBT doubles the peak current available to provide the initial startup load. That large peak current rating is essential to getting a motor started.

The peak current gets the motor turning, but the power supply must be able to supply enough RMS current to get the motor up to speed. In MT option units, we also delay the action of the RMS limit a few tenths of a second to allow more time at a higher current before current limit restricts the output voltage. Several conditions allow this, first the larger IGBT allows more current. Second, internal heating takes some time to be a problem. Third, motors present a low power factor to the output, meaning that the internal heating of the amplifier is low compared to a resistive load of the same current.

For example, a 1-HP motor is rated at 746W but would need about 1000VA to run it due to motor losses and inefficiencies. A typical BL 1350 can provide 10Arms at 120V and approximately 25-30A peak. This may not be enough to start the motor. Yet, a BL 1350 MT can provide approximately 50-55A peak and a higher RMS current for a sufficiently period to have a much better chance of starting that motor.

We hope this brief explanation will encourage you to contact us to learn more about our BL Motor Test technology as an option in selecting a reliable power supply for your product testing needs.

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