First step: receptacle replacement
Your existing receptacle is a 10-30R. This will need to be replaced with a 14-30R in order to plug either of your new appliances in. Furthermore, it appears you have aluminum wiring, so this will be a bit trickier than usual, but not impossible for a DIY job. Of course, if you are more comfortable calling in an electrician to deal with this, they'll be able to take care of it without too much trouble, and may be able to bring better stuff to bear on this job than you can.
For doing this yourself, you'll want:
- A NEMA 14-30R receptacle of the flushmount type
- Some short lengths of 10AWG copper THHN (black/red/white/bare if you want to insist on color codes, or you can use white as a universal donor with some tape flags for hots and wire stripping for the bare ground) for pigtailing the receptacle.
- 3 port Al7Cu or Al9Cu insulated screw lugs rated for 8AWG/10AWG wire
- Anti-oxidant compound if called for by the connectors used
- And a torque screwdriver, as the requirement for it got added in the 2017 NEC (you can get one at the orange borg or online for $60 these days)
Once you have the breaker off, you can undo and pull out the existing receptacle, and also undo the ground from the box lug at this point. Toss the old receptacle as you won't need any more of it. Fit the cut and stripped pigtails to the new receptacle, using your torque screwdriver to torque things down to specification. If called for in the connector instructions, apply anti-oxidant compound to the ends of the existing wires. Then, use the screw lugs to connect the pigtails to the existing wires, matching the pigtails up with the corresponding wires and using the torque screwdriver to torque the terminal screws down to specification. Stuff everything into the box (it may be tight) and screw the new receptacle in before turning the power back on and using a meter to check your work for the correct voltages.
Now: time for two 30A receptacles on one 30A circuit
There is absolutely nothing in the NEC that prohibits placing two 30A receptacles on the same 30A branch circuit. Neither 210.21(B)(3) nor 210.23(B) forbid this -- in fact, you could say that the Code explicitly provides for this exact case (otherwise there wouldn't be a line for 30A receptacles in Table 210.21(B)(3)).
Unfortunately, I don't know enough about how your house is built or where you want the 2nd receptacle on your wall to be able to instruct you further here. (This job will likely require you to tear out a bit of drywall, too.) The instructions above will leave you in a good position to do it or have an electrician do it, though.
Bonus! Your breaker box should be replaced at the earliest opportunity available!
You have a Zinsco panel -- while they don't quite have the sordid failure history of FPE's line of non-breakers, Zinscos should be replaced at the earliest opportunity available as their track record is poor, relative to something like a CH or a QO.
I do not see where you will have any issue with this arrangement - I am assuming your VFD has a means of control for motor speed ; whether that be a pot on the front, a PID loop control sensor or a PLC connected to it.
Not really something in Home Improvement as most homeowners don't have a VFD .
Since you are not going to use them at the same time - I think your setup will work with out issue.
You might want to look at the VFD specific requirements on grounding and neutrals.. depending on the VFD you might need to do some things different regarding your ground and neutral connections, also if using a PID loop control you might have some specific instructions..
I assume you know this info because you are using a VFD - but maybe not ..if you don't what make and model VFD ?? Yaskawa , Rockwell other ??
Best Answer
This is not legal or safe, but it's safe"er" at minimum cost.
Get a 120mm (4-11/16) junction box and lid, a cable clamp appropriate for that oven cord, 4 cable clamps appropriate for the heater power cables, and three rather large wire nuts, or alternately, 5/6-position lug splices that'll take up to #6. Also 1 foot of bare 10AWG wire, and a #10-32 ground screw (style points for green).
The 120mm box has a 10-32 tapped hole in it, put the ground screw in there and attach the pigtail.
Knock out holes for all the cable clamps that will make sense when it's mounted to the wall, and fit all the cable clamps, screws out so you can get to 'em.
Permanently mount the box on the wall within cord's reach of the 14-50 receptacle. Certainly do not leave it dangling loose. You'll pull cords out that way and give yourself a wallop.
Bring the dryer cord into its cable clamp. Tighten it down.
Bring each heater cord into a cable clamp. Tighten it down.
Use a splice to tie all the following wires: a) the stove cord ground, b) the pigtail from the junction box, and c) 4 ground wires from heater cords. There better be ground wires in those heater cords!
Use a splice to tie all the following: a) the heavy black wire in the stove cord, and b) one of the conductors from each heater cable (doesn't matter which).
Use a splice to tie all the following: a) the heavy white wire and b) the remaining conductor in each heater cable. Yes, you heard right. White.
Put a wire-nut cap on the red wire, and tape it on so it doesn't fall off. Wire-nuts don't like to stay on single wires.
Put the cover on the box.
Plug in the 14-50 plug. Light this baby up!
If you are getting the expected/advertised heat, then you are wrong about the heaters being 240V. They are 120V. By hooking up this way, we just saved the heaters.
If they're all heating but the heat is 1/4 of what you are expecting, then you passed the test. Unplug the 14-50 cord. Reopen the box, and redo the last 4 steps, but attaching the red wire to the heaters, and capping the white. Now you should get the works.
This isn't legal because these are not permissible uses for flexible cords, and we're abusing the tap rules, and these heaters are not listed to be portable or temporary heaters. The proper way would be a subpanel, 20A breakers feeding 6-20 receptacles, and proper cordage to heaters that are made to be portable.