Selecting the right driver

Hi,

What I would like to know is, when selecting a driver unit for use with stepper motors, should we be matching the driver amp rating to the same amp rating of the motor or do we really only need to select a driver that is half the amps of the motor ?

So if I had a motor that was rated at 4a, would that be 4a total regardless of the number of phase’s or divided by the number of phase’s?

So in this case it is a two phase motor rated at 4a, two phase’s at 4a = 8a so I would need an 8a driver or am I right in thinking only half of the motor (one phase) is in use at any one time?

What happen's when the current is moving from one phase to the next are they both in use at the cross over stage?

Thanks,
:feedback:

Quote:

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Originally Posted by Lee Roberts

Hi,

What I would like to know is, when selecting a driver unit for use with stepper motors, should we be matching the driver amp rating to the same amp rating of the motor or do we really only need to select a driver that is half the amps of the motor ?

So if I had a motor that was rated at 4a, would that be 4a total regardless of the number of phase’s or divided by the number of phase’s?

So in this case it is a two phase motor rated at 4a, two phase’s at 4a = 8a so I would need an 8a driver or am I right in thinking only half of the motor (one phase) is in use at any one time?

What happen's when the current is moving from one phase to the next are they both in use at the cross over stage?

Thanks,
:feedback:

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Lee,

Drivers are usually rated per phase...


Only one phase is on at a time in full step, wave drive. two phases are on in other modes, but the currents in each phase are reduced..

Drivers or motors ?

Quote:

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Originally Posted by Lee Roberts

Drivers or motors ?

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both...

If you look at the drivers from DIYCNC for example they are 2.5A drivers... which is per phase

"both..."

ok thanks mate !

A motor and driver is rated as per phase, so if a motor is 4A, then get a driver that is rated at 4A or over.
To get any power that is useful from the motor you also need volts, so dont just look at the current.
As an example if you see a 10A driver that is only rated at say 30V, this will be completely useless for larger motors, unless you are happy watching paint dry.
In other words you may get the torque, but you wont get the speed.

Gary's point is well made. The bigger the motor the higher the inductance of the windings, therefore the longer it takes for the magnetic field to build up. even small cheap steppers can have big inductances. A higher voltage 'forces' a higher current through the motor at the start of the step when the inductive impedance is highest. A simple rule of thumb is:

volts = sqrt(32 * H^2) where H is inductance in milliHenries.

so a 5mH motor would require ideally sqrt(32 * 5 * 5) = 30v, while a 10mH motor needs 56v (hence big motors on cheap drivers is a waste of time)

This is partly why the bipolar parallel arrangement give the best torque at high speeds. For typical motor windings that are bifilar wound (as most are) this halves the inductance as compared to bipolar series connection which quadruples it (all relative to unipolar).

So let’s take the PM542's (PDF), they have a Typical Supply Voltage of 36v with a MAX of 50v.

A late forum member told me I should supply them at the max (50v) is this the correct thing to do? Or would it be better to supply the driver with the Typical Rated Voltage?

I want to get the best speed I can from the motors, will supplying the driver at the MAX voltage mean the motor will also receive a higher voltage and give me better speeds?

If we take the Nema23 3Nm motor (PDF), the document said:

Bipolar Parallel 2.73v, 4.2a, 3Nm

So from that, i take it for the Motor to run at 3Nm Holding Torque it will require from the driver 2.73v and 4.2a ?

The other question i have is about the Motor, when a phase is "in use" is the Volts and Amps it is pulling from the driver divided by the amount of wires coming from the motor?

The max voltage is 50V, but over this voltage and the driver will trip into error, so i suggest to use a 40V PSU.

Forget the 3Nm holding torque, it is not the holdiong torque that you need, it is the torque while moving.
Also a stepper driver is a constant current device, so the motor does not pull current, the driver controls the current.

Quote:

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Originally Posted by Lee Roberts

So let’s take the PM542's (PDF), they have a Typical Supply Voltage of 36v with a MAX of 50v.

A late forum member told me I should supply them at the max (50v) is this the correct thing to do? Or would it be better to supply the driver with the Typical Rated Voltage?

I want to get the best speed I can from the motors, will supplying the driver at the MAX voltage mean the motor will also receive a higher voltage and give me better speeds?

If we take the Nema23 3Nm motor (PDF), the document said:

Bipolar Parallel 2.73v, 4.2a, 3Nm

So from that, i take it for the Motor to run at 3Nm Holding Torque it will require from the driver 2.73v and 4.2a ?

The other question i have is about the Motor, when a phase is "in use" is the Volts and Amps it is pulling from the driver divided by the amount of wires coming from the motor?

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Hi,

Well yea I know it would give error going over 50v, still need the questions answered please people.

Isn't the holdiong torque the same thing as "torque while moving" ?