I wanted to post a solution to a pony start rpc problem that has gone on for weeks, hopefully it will save someone some time should they choose to go this route. There are many success stories using a pony start rpc and few reported problems which was one of the frustrating things about this project. I was at the point where i was going to throw in the towel and go the capacitor start, i even bought what i needed.
The pieces:
40hp rpc
1.5hp pony
50hp contactor
150amp breaker
50hp disconnect
200amp service
Problem:
1) Motor would blow the 150amp breaker with violent impact, sometimes, and this is the one of the main points of this post. Why would it start perfectly sometimes, but blow the breaker other times? There were times that it appeared to spin the rpc backward when the single phase was thrown.
2) in setting this all up, i blew the secondary fuse on the transformer.
3) i burned up one of the pony motors that i had trying to configure all of this.
Solution:
For a larger (40hp and up) pony start rpc certain conditions need to be met in order for a smooth startup without a violent breaker throw. First, bringing a pony start rpc online with grid power is similar to paralleling 2 generators. If you have two 5000w generators, when you parallel you effectively have a 10kw system. However, the manner in which you connect both generators is critical. Here is a list of requirements that need to be met to parallel, in 2 generators:
Frequency: the frequencies must be the same
Voltage: The generators should produce or be set to produce the same voltage
Phase number: The two systems should have the same number of phases, either three or single phase.
Phase rotation: For three phase systems, each of the three phases must be matched. This prevents excessive mechanical and electrical stresses as well as avoiding power surges.
Voltage Phase angle: The wave-forms should be matched such that they rise and fall together. There should be no angle difference and the potential difference between the phases should be zero.
This list of requirements is the same for a rpc/pony start
Frequency: the frequencies must be the same, we try and get rpc as close a match to the rpm rated for the motor as possible.
Voltage: the incoming voltage is the coming from the grid, so that is a match.
Phase number: we are trying to match the single phase.
Phase rotation: single
Voltage Phase angle: The wave-forms should be matched such that they rise and fall together. There should be no angle difference and the potential difference between the phases should be zero. I believe this the most important aspect of a large pony start rpc. When an attempt is made to try to synchronize an rpc and is out of phase with a grid , the magnetic fields of the rpc are in opposition (depending on the amount of "asynchronism") to those induced by the grid. So, the rpc motor has to speed up or slow down, sometimes by as much as 180 degrees, in a very short period of time. That's where the damage comes into the picture; as fast as it has to speed up or slow down to get into synchronism with the grid field it has to suddenly stop when synchronism is achieved. And, the damage can be caused by either or both of the sudden speed change and/or the sudden speed stop. I have noted that if i started when the there is a significant difference in the phase angle that the rpc will appear to turn in the opposite direction as the pony motor. This introduces a tremendous amount of current into the system which is why the breaker was getting thrown so violently. when the things were in-sync, there was very little in-rush current, and things would start smoothly.
Bring a rpc motor in phase with the grid power
This is the key to the entire setup. If you look at the drawing of the phase angle where they 2 sine waves are out of sync, you see the point where they cross the x axis, the current will be zero. If you connect a 240v light between the incoming single phase power l1 and the rpc l1, you will set the light turning on and off. the light is on when the sine waves are negative or positive, the light goes out when both sine waves cross zero. When they both cross zero axis, and you turn on the grid power to the rpc, you will force both waves to start at the same point, forcing them to be in sync. see video below:
Pony Start RPC - YouTube
Smaller pony start RPC.
There is little talk of problems with smaller RPC pony start motors. In fact, scouring the forum there was very little information on large RPC pony start motors. I believe larger RPC have tremendous inertia created by a significant mass of steel of bigger motors. Once going in one direction, it is very difficult to right the phase of the grid unless timing is correct. With smaller motors, i believe this can be easier to correct and the phase angle can be forced it to be in sync w/ the grid, even when the timing is off.
Things to watch for.
1) the second time i threw the switch to try and start the pony rpc motor, with out knowing about things being out of phase, i blew the secondary fuse on the main grid connected transformer. I had a 25kva transformer, when the power company came to take a look, i mentioned that i had some purchased some newer milling equipment and i believed that the transformer i had was a little undersized. They agreed and replaced it with a 50kva transformer.
2) i had a 100amp breaker on this setup. Even after i figured out the how to start the rpc in phase w/ the grid, it was still blowing breakers. Not nearly as violently when things were out of phase, but still blowing. I ended up putting a 150amp breaker and things worked fine.
3) I had an older pony motor setup. The violent shifting that occurred when the rpc was out of phase, eventually burned it up.
The pieces:
40hp rpc
1.5hp pony
50hp contactor
150amp breaker
50hp disconnect
200amp service
Problem:
1) Motor would blow the 150amp breaker with violent impact, sometimes, and this is the one of the main points of this post. Why would it start perfectly sometimes, but blow the breaker other times? There were times that it appeared to spin the rpc backward when the single phase was thrown.
2) in setting this all up, i blew the secondary fuse on the transformer.
3) i burned up one of the pony motors that i had trying to configure all of this.
Solution:
For a larger (40hp and up) pony start rpc certain conditions need to be met in order for a smooth startup without a violent breaker throw. First, bringing a pony start rpc online with grid power is similar to paralleling 2 generators. If you have two 5000w generators, when you parallel you effectively have a 10kw system. However, the manner in which you connect both generators is critical. Here is a list of requirements that need to be met to parallel, in 2 generators:
Frequency: the frequencies must be the same
Voltage: The generators should produce or be set to produce the same voltage
Phase number: The two systems should have the same number of phases, either three or single phase.
Phase rotation: For three phase systems, each of the three phases must be matched. This prevents excessive mechanical and electrical stresses as well as avoiding power surges.
Voltage Phase angle: The wave-forms should be matched such that they rise and fall together. There should be no angle difference and the potential difference between the phases should be zero.
This list of requirements is the same for a rpc/pony start
Frequency: the frequencies must be the same, we try and get rpc as close a match to the rpm rated for the motor as possible.
Voltage: the incoming voltage is the coming from the grid, so that is a match.
Phase number: we are trying to match the single phase.
Phase rotation: single
Voltage Phase angle: The wave-forms should be matched such that they rise and fall together. There should be no angle difference and the potential difference between the phases should be zero. I believe this the most important aspect of a large pony start rpc. When an attempt is made to try to synchronize an rpc and is out of phase with a grid , the magnetic fields of the rpc are in opposition (depending on the amount of "asynchronism") to those induced by the grid. So, the rpc motor has to speed up or slow down, sometimes by as much as 180 degrees, in a very short period of time. That's where the damage comes into the picture; as fast as it has to speed up or slow down to get into synchronism with the grid field it has to suddenly stop when synchronism is achieved. And, the damage can be caused by either or both of the sudden speed change and/or the sudden speed stop. I have noted that if i started when the there is a significant difference in the phase angle that the rpc will appear to turn in the opposite direction as the pony motor. This introduces a tremendous amount of current into the system which is why the breaker was getting thrown so violently. when the things were in-sync, there was very little in-rush current, and things would start smoothly.
Bring a rpc motor in phase with the grid power
This is the key to the entire setup. If you look at the drawing of the phase angle where they 2 sine waves are out of sync, you see the point where they cross the x axis, the current will be zero. If you connect a 240v light between the incoming single phase power l1 and the rpc l1, you will set the light turning on and off. the light is on when the sine waves are negative or positive, the light goes out when both sine waves cross zero. When they both cross zero axis, and you turn on the grid power to the rpc, you will force both waves to start at the same point, forcing them to be in sync. see video below:
Pony Start RPC - YouTube
Smaller pony start RPC.
There is little talk of problems with smaller RPC pony start motors. In fact, scouring the forum there was very little information on large RPC pony start motors. I believe larger RPC have tremendous inertia created by a significant mass of steel of bigger motors. Once going in one direction, it is very difficult to right the phase of the grid unless timing is correct. With smaller motors, i believe this can be easier to correct and the phase angle can be forced it to be in sync w/ the grid, even when the timing is off.
Things to watch for.
1) the second time i threw the switch to try and start the pony rpc motor, with out knowing about things being out of phase, i blew the secondary fuse on the main grid connected transformer. I had a 25kva transformer, when the power company came to take a look, i mentioned that i had some purchased some newer milling equipment and i believed that the transformer i had was a little undersized. They agreed and replaced it with a 50kva transformer.
2) i had a 100amp breaker on this setup. Even after i figured out the how to start the rpc in phase w/ the grid, it was still blowing breakers. Not nearly as violently when things were out of phase, but still blowing. I ended up putting a 150amp breaker and things worked fine.
3) I had an older pony motor setup. The violent shifting that occurred when the rpc was out of phase, eventually burned it up.