Re: SLO-SYN Motors

Charles Hopkins wrote:

> From: Charles Hopkins <chopkins@...>
>
> Where can I get supplies to work with my 20 amp motors?
>
> How much power saving do you think I can have by changing power supplies?
>
> I am confused on how to pick a power supply. The motors are rated at less than 2 volts, but are 12.7 amp and the larger ones are rated at 20 amps. I think the Ah-ha supplies are 48 volt. Can I send 48 volt to a motor rated at 2 volts?

The stepper driver usually uses a switching technique to convert the voltage.
Stepper drivers have some design maximum voltage that should not be
exceeded. But, you will get better motor performance by having something
near the maximum voltage available. the thing which kills high speed
operation of stepper motors is the winding inductance (thats why big
steppers use low voltage at high current, so they can have a few turns of heavy
wire, and low inductance). The higher power supply voltage allows the
current to build in those windings quickly. Once the current in the windings
reaches the current set into the driver, a switching transistor shuts off the
path from the power supply, and the current circulates in the winding.

> I have done a search for power supplies, but I don't understand just what I should be looking for.

If 48 Volts is your goal, then you want a NON-regulated power supply,
capable of delivering some current at roughly 48 volts, from the desired
line voltage. You might choose either 120 or 240 Volts, depending on
whether there is 240 available at that equipment location.

As for current - there is a trick. Switching power supplies convert
power, so if you are running a high current, low voltage motor from a
higher voltage supply, you will find the driver draws much less current
than the motor. (It sounds like something for nothing, but it is real.)

So, a stepper motor has 2 windings, each drawing up to 12.7 A at
2 V. That is 50.8 W, total, per motor. Now, to drive that from
48 V, you only need about 1 Amp. So, for a 3 axis system, if
you bought a 75 Amp power supply (for the 2 windings x 3 motors x
12.7 A) you would be wasting a lot of money. Due to switching
losses, and the energy dumped when a winding is turned off,
you actually need more than the theoretical minimum, but probably
2 Amps at 48 V per small motor, and 3 A for the big ones, should do it.
That sounds a lot more reasonable.

Jon