Rockwell 3 Phase 3/4 HP Motor Questions

[Jim Peck]

I bought an old Delta drill press with a 3-phase Rockwell motor and ancient Arrow-Hart motor starter. I don't have 3-phase wiring in my shop, and the rubber around the wires in the starter was cracked, so the plan was to replace the starter with a VFD. To begin, I unhooked all the wires from the starter and dismounted the starter.

That was Dumb Move #1.

Now I have this lovely motor with 9 (count 'em, 9!) black wires coming out and no reference point for any of them.

The motor's data plate has a helpful chart indicating how the wires should be grouped for 220v vs 440v circuits, but I have no idea which is wire #1, which is #2, etc.

Question 1: Any idea how I can determine which wire is which?

Question 2: As you can see in the image below, on the data plate at the bottom of the red portion it says "connect line leads to 1..." then there is a big scratch obliterating the rest. Any idea what the rest of that sentence should say?

Thanks,

Jim

424846

[Malcolm McLeod]

...that sentence should say..
Fairly certain it should read “ 1 2 3 “

As for identifying the wires, a very careful check of the existing exposed wires with a magnifying glass could reveal a wisp of ink from numbers...? Or sometimes you can pull a bit more wire out of the motor housing and find wire numbers. Or might have gotten very hot and charred the insulation and ink with it.

Failing above, it’s either a meter looking at resistance of various pairs or motor repair shop.

[Jim Peck]

As for identifying the wires, a very careful check of the existing exposed wires with a magnifying glass could reveal a wisp of ink from numbers...? Or sometimes you can pull a bit more wire out of the motor housing and find wire numbers.

Thanks.

Extrapolating from this: Dumb Move #2 was not reading the wires before posting the question! I saw writing and figured it was the standard info you find on most wires--AWG and the like. Nope, looking closer it is indeed the wire numbers.

Appreciate your help!

Jim

[Malcolm McLeod]

Oh, but were they all so easy����! Glad you found them.

I’ve been casually looking for 3ph motor to do same (VFD) on my DP - so jealous.

[Jim Peck]

One other question while I'm here: As I said, the DP and the motor are ancient--probably early 1960s, which I guess means I'm ancient as well.

In any event, the motor has no ground wire. Should I attach a wire to the case of the motor and connect that to the ground terminal on the VFD (along with the ground wire from the wall to the VFD)? Or is the motor somehow magically grounded in some way?

[Steven Cooper2]

There may be a screw hole lug thing in the little wire box for ground. It is generally not a separate wire.

[Tom Bender]

Just a few cautions;

3/4 hp is not a lot, and it will be less at reduced speed HP = torque x rpm so half rpm = 3/8 hp

Assuming it has a fan, your motor will not cool well at reduced speed so limit run time

A vfd will not give you a lot of speed reduction, half speed is about as low as is practical

[Malcolm McLeod]

Just a few cautions;

3/4 hp is not a lot, and it will be less at reduced speed HP = torque x rpm so half rpm = 3/8 hp

Assuming it has a fan, your motor will not cool well at reduced speed so limit run time

A vfd will not give you a lot of speed reduction, half speed is about as low as is practical

In general, the above is all good advise, particularly on Hp.

But, I routinely run 7.5->400Hp motors at 25% of rated speed on VFDs with no supplemental cooling on the motor. Duty cycles are typically between 50-90%. In the case of a non-industrial DP, I’d think the duty cycle is 5% or less?? I can’t imagine overheating one based solely on low speed —- assuming the load is within the motor’s capacity (derated as Tom suggests).

If routinely running lower than 25% speed with a full load, I would highly recommend adding a supplemental cooling fan. ...And look for inverter-duty motor.