STMBL Driver DC Bus Options

I'm hoping to catch the eye of some of the experienced retrofitters here so I can learn some things. I'm in the process of retrofitting a Deckel FP4a with STMBL drives and LinuxCNC, it was originally 3 phase, but I want to run it on single phase 220, but not being an electrical engineer, I'm stumbling on the power supply for the drives.

I know I need to supply my motors with ~210VDC via a capacitor-smoothed DC bus, I'm just trying to find the best, safe option of doing so without breaking the bank, causing a fire, or killing myself and all my equipment, so I need to learn more.

From what I understand I have 3 options:

• An isolation transformer to drop the 220VAC to 150VAC, rectified to ~210VDC, but this needs an odd ~5KVA transformer that is likely very expensive and may be hard to source. I have the original, monstrous 3 phase transformer, but I have my doubts that it would work effectively in a single phase capacity.
• A skookum switch mode power supply that can handle the load of the 3 axis motors (2x 1.5KW, x1 1.75KW) and spindle (1.5KW). This is also likely to be expensive and hard to source.
• Direct rectification of the 220VAC to ~310VDC, very simple, but potentially quite dangerous, though I don't fully understand the risks and potential failure modes.

How are others handling this situation for their machines?


For a little background, the machine came to me free in a broken state, the original drives were questionable and the controller was non-functional, the machine is physically in great shape. I chose the STMBL drives because they can handle the resolvers in the stock Industrial Drives brushless axis motors and spindle motor, I couldn't find a decent, off-the-shelf drive that was cost competitive and they seemed like the ideal solution, provided I can power them.

Why do you need 210 VDC?

Normally you would use rectified 220V for the DC bus to run for 220V  motors

Are the motors 150 VAC?

That's an excellent question, the reason I think I need 210VDC is because my motors data plates show 210 as the rated voltage. Here's the info from the big Z axis motor:

Industrial Drives Brushless Motor
Model BRB3-2104-3027-A
Cont. 7.2A RMS Peak 26A
Rated RMS Volts Line to Line 210
Max speed 3900 RPM Freq. 195Hz
Rated Speed 3900 RPM Rated HP 3.1

You need ~300VDC for a 210 VRMS AC output

240VAC line rectified and filtered via DC capacitors to 340VDC to power a ~200VAC motor via inverter should be fine as long as the parameter(s) for setting motor rated voltage and v/f ramp (if applicable) are set accordingly. You just can't run such a motor across the line with a switch or contactor.

The few times I have seen this done using VFDs or servo drives, it was working well. I am not familiar enough with stmbl to tell you whether they have the same capability.

1. First confirm your 220VAC power. In my area our 220VAC usually measures around 240~244VAC, just make sure you're not using something like 380VAC.

2. The basic idea for your DC bus is AC through appropriately sized fuses or circuit breaker into the AC side of the bridge rectifier, the DC terminals of the rectifier feed the DC capacitor bank.

3. In practice you will also need a precharge circuit and a discharge resistor.

4. For the bridge rectifier I might use a GBPC3510 or comparable. It has to be bolted to an aluminum or copper heat sink, the required size of heat sink will depend on the ambient temperature and quantity of airflow. It is possible to calculate the heat sink requirement but there are so many variables, I simply use what I think and monitor the temperature during the first runs. If it gets hot the heat sink is too small. An IR temperature gun works great for this. For an application this size I might simply attach the bridge rectifier to the back or sub-panel of the steel electrical box housing the project.

5. I tend to size my capacitor banks by looking at comparably sized (Kw) VFDs or servo inverters and using about the same total uF. It can also be calculated using formulas. I keep calling it a capacitor bank. You might just need one single capacitor for this size project.

6. For the precharge circuits there are three popular ways. If your 220VAC power source has a high enough resistance you might do without the precharge circuit but I would not try that.

6.1. The cheap simple way is to insert an NTC thermistor in series with the AC input or DC output of the bridge rectifier feeding the capacitor bank. I have not attempted doing it this way but I see it a lot on mass produced inverter motor drives. The catch is appropriately sizing the NTC. I would tend to buy a couple different sizes so I can try each, measuring the start-up surge and running temperature. These WILL run HOT by design, probably over 100C.

6.2. I always install a single pole contactor or relay on one AC power lead for the bridge rectifier. An on-delay timer powers this contactor coil. A power resistor across the contactor points trickle charges the capacitor. After a few seconds the timer activates the contactor, allowing full power flow.

6.3. A third precharge method worth mentioning which I consider out of scope for a project of this size is the active rectifier.

7. A relatively high ohm resistor across the + and - of the capacitor bank will suffice for the discharge resistor, to ensure the capacitor is discharged in a few minutes after power is removed.

That's a very detailed answer, thank you! I'll read through that a few times in detail, much appreciated!

I went for the skookum SMPS on my Deckel FP2NC. 240VAC input, 160VDC 5KW output to match the existing Siemens servos. I found this item as cheap surplus, and bought two for insurance.

For the spindle I used a no-name import 220V-to-380V inverting VFD. Name-brand examples from ATO below:

www.ato.com/220v-to-380v-vfd

A 160V DC bus supply can only provide about 110 VAC to the motor
so you would be limited to about 1/2 the servo motors full speed.

PCW wrote:

A 160V DC bus supply can only provide about 110 VAC to the motor
so you would be limited to about 1/2 the servo motors full speed.

Can you clarify this? I had thought DC servo drives were typically an H-bridge, which would make for +/-160VDC output minus power semiconductor voltage drops. (Bipolar transistors in my Bosch drive, so not negligible.)

160V seemed reasonable for the 163V rating of my brushed DC servos. OP has 210V brushless servos, so their supply requirements are somewhat different.

reboots wrote:

160V seemed reasonable for the 163V rating of my brushed DC servos.
 

Brushed DC servos, yes, that is correct, but not for AC motors:

Look at an oscilloscope trace of 120VAC sine wave. The tops and bottoms of the trace touch +/- 170VDC, you can't create that off 120VDC with an h-bridge.

This will also cause a bridge rectifier+capacitor to produce ~170VDC from 120VAC.

I still remember my first experience with a rectifier, how amazed I was to get about 17VDC when I powered it with 12VAC from a transformer. The 12VAC is the actual power, sort of an average. "Average" is not technically the correct term here, but it is the idea. RMS is the term normally used.