How to calculate AC current draw through a DC motor and its controller?

[mausaer]

I'm a novice who is rewiring an old machine that runs off of 2-phase 110VAC and has three motors. I want to balance the motor load between the two phases but am struggling because one motor is AC and two motors are DC.

Motor 1 is 220VAC 5A, motor 2 is 90VDC 5A, and motor 3 is 180VDC 7.6A.

How do I calculate the equivalent AC current for motor 2 and motor 3 so I can split the three motors as evenly as I can among the 2 phases?

 

[augie47]

Post approved as poster is student apprentice involved in machine wiring

 

[winnie]

By '2 phase' 110V, I presume you mean normal split phase 120/240V as found in a home. You should look up the difference between 'split phase' (common) and '2 phase' (rare) power.

Motor nominal utilization voltage is less than the corresponding supply voltage, so a 110V motor would be connected to a 120V supply, and 220V motor connected to a 240V supply.

The 220V motor is easy. It is connected L-L on the 240V supply, and evenly loads both legs.

The DC motors are harder to figure, because the controller is going to take input power and convert it to output power for the motor. You need to know the characteristics of the controller before you can really answer the question.

What is the AC supply voltage of the DC motor controllers?

-Jonathan

 

[Jraef]

Motor 1 is 220VAC 5A, motor 2 is 90VDC 5A, and motor 3 is 180VDC 7.6A.

Assuming Motor 1 is 220V single phase...

If you have a basic single phase "split phase" system that is 120/240V, then "balancing" is not the same here as it would be in other countries. For 240V, you have two poles that are either end of a SINGLE phase, so for the motors that are connected to that, there is no "balancing", the motors (or drives) are by definition connected to BOTH lines. In your case, that would be motors 1 and 3, because to run a 180VDC motor, the DC drive must be connected to 240V AC. That leaves only Motor 2 that is going to be connected to 120V (Line to Neutral), because a 90VDC controller is fed with 120V. So you can't "balance" a single device connection...

If you are still interested in determining power, just convert everything to kVA. There will be a small amount of added losses through the DC drives, but not enough to be concerned about.

 

[mausaer]

By '2 phase' 110V, I presume you mean normal split phase 120/240V as found in a home. You should look up the difference between 'split phase' (common) and '2 phase' (rare) power.

Motor nominal utilization voltage is less than the corresponding supply voltage, so a 110V motor would be connected to a 120V supply, and 220V motor connected to a 240V supply.

The 220V motor is easy. It is connected L-L on the 240V supply, and evenly loads both legs.

The DC motors are harder to figure, because the controller is going to take input power and convert it to output power for the motor. You need to know the characteristics of the controller before you can really answer the question.

What is the AC supply voltage of the DC motor controllers?

-Jonathan

The AC supply voltage for the 90VDC motor is 120 and for the 180VDC motor is 240.

 

[winnie]

Ok. @Jraef explained the best way to approximate this.

The 220 VAC 5A motor simply placed 5A on L1 and L2.

The 180 VDC 7.6A motor is 180 * 7.6 = 1368 VA / 220V = 6.2A placed on L1 and L2.

Finally the 90 VDC 5A motor is 450 VA / 110V = 4.1A placed on L1 and N (or L2 and N)

The above are only approximate values, for several reasons
1) I used 110/220V as 'utilization voltage' for the calculation, but the actual operating voltage is likely different.
2) I approximated the DC motor controllers are perfectly efficient.
3) I assumed that the motors were fully loaded; in reality the motor current consumption will change with the applied mechanical load.
4) I assumed that the power factor of the 220 VAC motor and the DC power supplies were the same.

But the above should get you reasonably close.

-Jonathan

 

[mausaer]

Thank you very much, Jonathan.