Strostkovy
Titanium
- Joined
- Oct 29, 2017
I have an RPC I built that has a 10hp motor and a 40hp motor started by a 5hp pony. The 10hp idler starts up without issue and I use it to power a 6.5HP brake just fine.
I only now got around to wiring the 40hp motor in and even though the pony brings it to very near synchronous speed it suddenly begins to stall out. When I first setup the smaller idler I wired it in a way the reconnected the start capacitors when the motor was shut off, which resulted in DC injection braking, and the big motor sounds exactly like that by louder.
I can't take a picture of the nameplate because it is obstructed but the important bits are as follows:
230/460 V
102-92.2/46.1 FLA
SF 1.15
PF 0.89
NEMA EFF 91.7%
The pony is a 5hp 230V 3450RPM compressor motor, connected by a 1:1 pulley arrangement.
The line impedence at the panel is 0.23 ohms, so my electrical service has a hard time delivering large surge currents.
My first attempt to start it was by disconnected the start capacitors but leaving the run capacitors connected. I started the pony, then the big idler, and it immediately slowed down. Power draw was around 230 Amps but it's hard to actually tell because the meters update slowly. The goal was to then bring on the small idler with no start capacitors and check rotation before reconnecting the capacitors, but that didn't happen.
Second attempt was like attempt one but with no capacitors.
Attempt three was to start the 10hp idler first, then blip on the big idler from standstill to find the direction of rotation. I swapped polarity so that the direction matched, then started the small idler, the pony, and then the large idler after it was up to speed. The voltage dropped low enough that some contactors dropped out and it stopped before I did.
It seems like the motor would start from the 10hp idler alone, with no pony, but that would take maybe 20 seconds at 250A which obviously isn't feasible.
My best guess is that this motor has very low resistance, especially relative to my high line impedance, which doesn't allow me to give it the inrush current it requires to establish the magnetic field in the rotor properly, and is likely just inducing an AC current into the rotor that often opposes the field windings.
If that is the case then starting the small idler first, then starting the pony and large idler at the same time should allow it to start, but I just don't have the power for that.
The thing I would try next would maybe be to put some beefy resistors from the pony motor contactor to feed a limited amount of power into the windings of the big idler to allow the magnetic field to form in the rotor as it was brought up to speed.
I checked and rechecked and then rechecked the wiring to the big idler and I'm positive I wired it to the correct voltage and in the correct way. It is delta only, no wye start.
The motor is too heavy to move somewhere with real three phase. It does turn over with the smaller RPC power alone, but the generated leg is too weak to bring it up to speed fast enough before burning up everything.
I only now got around to wiring the 40hp motor in and even though the pony brings it to very near synchronous speed it suddenly begins to stall out. When I first setup the smaller idler I wired it in a way the reconnected the start capacitors when the motor was shut off, which resulted in DC injection braking, and the big motor sounds exactly like that by louder.
I can't take a picture of the nameplate because it is obstructed but the important bits are as follows:
230/460 V
102-92.2/46.1 FLA
SF 1.15
PF 0.89
NEMA EFF 91.7%
The pony is a 5hp 230V 3450RPM compressor motor, connected by a 1:1 pulley arrangement.
The line impedence at the panel is 0.23 ohms, so my electrical service has a hard time delivering large surge currents.
My first attempt to start it was by disconnected the start capacitors but leaving the run capacitors connected. I started the pony, then the big idler, and it immediately slowed down. Power draw was around 230 Amps but it's hard to actually tell because the meters update slowly. The goal was to then bring on the small idler with no start capacitors and check rotation before reconnecting the capacitors, but that didn't happen.
Second attempt was like attempt one but with no capacitors.
Attempt three was to start the 10hp idler first, then blip on the big idler from standstill to find the direction of rotation. I swapped polarity so that the direction matched, then started the small idler, the pony, and then the large idler after it was up to speed. The voltage dropped low enough that some contactors dropped out and it stopped before I did.
It seems like the motor would start from the 10hp idler alone, with no pony, but that would take maybe 20 seconds at 250A which obviously isn't feasible.
My best guess is that this motor has very low resistance, especially relative to my high line impedance, which doesn't allow me to give it the inrush current it requires to establish the magnetic field in the rotor properly, and is likely just inducing an AC current into the rotor that often opposes the field windings.
If that is the case then starting the small idler first, then starting the pony and large idler at the same time should allow it to start, but I just don't have the power for that.
The thing I would try next would maybe be to put some beefy resistors from the pony motor contactor to feed a limited amount of power into the windings of the big idler to allow the magnetic field to form in the rotor as it was brought up to speed.
I checked and rechecked and then rechecked the wiring to the big idler and I'm positive I wired it to the correct voltage and in the correct way. It is delta only, no wye start.
The motor is too heavy to move somewhere with real three phase. It does turn over with the smaller RPC power alone, but the generated leg is too weak to bring it up to speed fast enough before burning up everything.