hbg6@citek.mcdphx.mot.com (05/09/91)
I am building a controller for the evaporative cooler on my house. The blower motor has two speeds, selected by providing power to either the HIGH terminal, or the LOW terminal. I really want to use solid state relays to avoid the cost and complexity of contactors and the associated drivers. The current spec. for the relays I have is in range but I have three concerns: 1 The SSR can easily handle the run current of the motor but is the starting current likely to kill it? 2 Is the normal failure mode for a SSR open or shorted? 3 What would happen if both the HIGH and LOW terminals on the motor are powered at the same time. Bottom line; if the SSR fails closed and power is applied to both terminals am I going to burn up a $200 blower motor? Thanks, John (everything over TTL is high voltage) Schuch ..................................................................... John Schuch - Motorola Inc., Computer Systems Division (602)438-3008 All opinions expressed are mine and not Motorolas, their loss. .....................................................................
youngqd@jacobs.CS.ORST.EDU (Dean Youngquist) (05/12/91)
In article <12500@qisoff.phx.mcd.mot.com> hbg6@citek.mcdphx.mot.com writes: >I am building a controller for the evaporative cooler on my house. The >blower motor has two speeds, selected by providing power to either >the HIGH terminal, or the LOW terminal. >text deleted >Bottom line; if the SSR fails closed and power is applied to >both terminals am I going to burn up a $200 blower motor? > The SSR could fail either open or shorted. I would suggest some old- fashioned slow-blow fuses in series with them power source before it splits into the high/low windings. If both windings were somehow powered simultainiously the fuse would blow. Make sure the fuse is not rated too high over normal maximam current. Dean Youngquist youngqd@jacobs.cs.orst.edu
wolfgang@wsrcc.com (Wolfgang S. Rupprecht) (05/14/91)
hbg6@citek.mcdphx.mot.com writes: > I am building a controller for the evaporative cooler on my house. The > blower motor has two speeds, selected by providing power to either > the HIGH terminal, or the LOW terminal. > I really want to use solid state relays to avoid the cost and > complexity of contactors and the associated drivers. The current > spec. for the relays I have is in range but I have three concerns: > 1 The SSR can easily handle the run current of the motor but is > the starting current likely to kill it? Yes. Of course. The more you beat on a device the faster it will wear out. While the SCR's (as any semiconductor) can tolerate a bit of extra surge current, it does stress them. Do this too often and you break them. > 2 Is the normal failure mode for a SSR open or shorted? Both. This is what fuses are made for. > 3 What would happen if both the HIGH and LOW terminals on the > motor are powered at the same time. Lots of current would be drawn. Smoke would be sure to follow. Use a simple interlock between the two SCR's, and USE A FUSE. > Bottom line; if the SSR fails closed and power is applied to > both terminals am I going to burn up a $200 blower motor? Not if you use a fuse. ;-) Oh one more thing. You should really think about what the voltage on the low speed SCR will look like when the high speed winding is energized. Hint: You will probably see quite a bit more than the input voltage across the low-speed SCR. Rate this SCR appropriately. You will also want to use a good snubber network and a zero crossing detector (for turn on). This will reduce RF bursts that will lead to radio interference and also an early destruction of the SCR's. -wolfgang -- Wolfgang Rupprecht wolfgang@wsrcc.com (or) uunet!wsrcc!wolfgang Snail Mail Address: Box 6524, Alexandria, VA 22306-0524
whit@milton.u.washington.edu (John Whitmore) (05/15/91)
In article <1991May14.135524.25014@wsrcc.com> wolfgang@wsrcc.com (Wolfgang S. Rupprecht) writes: >hbg6@citek.mcdphx.mot.com writes: >> I really want to use solid state relays to avoid the cost and >> complexity of contactors and the associated drivers. The current >> spec. for the relays I have is in range but I have three concerns: > >> 1 The SSR can easily handle the run current of the motor but is >> the starting current likely to kill it? >Yes. Of course. Nope, I don't agree. Unlike bipolar transistors, SCRs have no 'hot-spot' failure mode (they're like MOSFETs in that regard), so they are EXTREMELY tolerant of surges. Motorola gives their 12A SCRs a 300A surge rating; your motor is unlikely to kill 'em. Lightning strikes, however... >> Bottom line; if the SSR fails closed and power is applied to >> both terminals am I going to burn up a $200 blower motor? >Not if you use a fuse. ;-) Well said. John Whitmore
stevem@specialix.co.uk (Steven Murray) (05/15/91)
wolfgang@wsrcc.com (Wolfgang S. Rupprecht) writes: >hbg6@citek.mcdphx.mot.com writes: >> I am building a controller for the evaporative cooler on my house. The >> blower motor has two speeds, selected by providing power to either >> the HIGH terminal, or the LOW terminal. >> I really want to use solid state relays to avoid the cost and >> complexity of contactors and the associated drivers. >You will also want to use a good snubber network and a zero crossing >detector (for turn on). This will reduce RF bursts that will lead to >radio interference and also an early destruction of the SCR's. Expanding on this a bit, driving motors with SCR's or solid state relays, especially anything more than 'one motor, one relay' can be painful. You can tend to get problems with inductive spikes and back EMF - zero crossing switch-on is fine, but that doesn't control the switch off. Snubbers are great, VDR's aren't such a bad idea either. Aim to protect the SSR's with VDR's rather than stop interference from the load. -- Steven Murray uunet!slxsys!stevem stevem@specialix.co.uk I am speaking, but | If these are your opinions, then we are in agreement!! not for my employer.| Flames, spelling errors, complaints > /dev/null
winans@sirius.mcs.anl.gov (John Winans) (05/16/91)
In article <1991May15.132910.1396@specialix.co.uk> stevem@specialix.co.uk (Steven Murray) writes: [stuff deleted] >especially anything more than 'one motor, one relay' can be painful. You >can tend to get problems with inductive spikes and back EMF - zero crossing >switch-on is fine, but that doesn't control the switch off. Snubbers are >great, VDR's aren't such a bad idea either. Aim to protect the SSR's with >VDR's rather than stop interference from the load. Excuse my ignorance please... but... uhmmm..... what is a VDR? -- ! John Winans Advanced Computing Research Facility ! ! winans@mcs.anl.gov Argonne National Laboratory, Illinois ! ! ! !"The large print giveth, and the small print taketh away"-- Tom Waits !
jbertoia@medar.com (Jeffrey A. Bertoia) (05/18/91)
In article <1991May15.132910.1396@specialix.co.uk> stevem@specialix.co.uk (Steven Murray) writes: >wolfgang@wsrcc.com (Wolfgang S. Rupprecht) writes: > >>hbg6@citek.mcdphx.mot.com writes: >>> I am building a controller for the evaporative cooler on my house. The >>> blower motor has two speeds, selected by providing power to either >>> the HIGH terminal, or the LOW terminal. > >>> I really want to use solid state relays to avoid the cost and >>> complexity of contactors and the associated drivers. > Good move! >>You will also want to use a good snubber network and a zero crossing >>detector (for turn on). This will reduce RF bursts that will lead to >>radio interference and also an early destruction of the SCR's. >Expanding on this a bit, driving motors with SCR's or solid state relays, >especially anything more than 'one motor, one relay' can be painful. You >can tend to get problems with inductive spikes and back EMF - zero crossing >switch-on is fine, but that doesn't control the switch off. Snubbers are >great, VDR's aren't such a bad idea either. Aim to protect the SSR's with >VDR's rather than stop interference from the load. Expanding yet further and refuting some. In the following text I use SCR/SSR relatively interchangably. The zero crossing detector for turn on will definately reduce the RFI generated but will have little effect on the overall life of the SCR/SSR as long as it has been properly sized for the application. Note: In sizing you must consider not only the steady state but also the surge ratings. Medar's business is resistance welding controls which generally involves phase controlling a very large transformer. More often than not we swicth SCR's on at times other than the voltage zero-crossing. We rarely 'lose' SCRs due to current or voltage transients. A far more common failure mode is improper cooling. Now a snubber, will help extend the life of an SCR in it's off state (i.e. it will help protect the SCR for overvoltage transients in it's non-conducting mode). It will also assist in turn-on if the gate drive circuit is inadequate, however if I remember, the original post regarded SSRs which have an integral drive circuit which is properly sized. The problem with snubbers in these applications is that the off state over voltage protection is marginal due to the capacitors inability to store the energy. An additional consideration in using a snubber is that 'some' voltage will be present on the load side of the snubber at _all_ times do to the leakage through the snubber. This may or may not be tolerable. If overvoltage is the main concern a MOV (Metal Oxide Varistor) will do the job nicely. A MOV will 'break-over' at a specified voltage quencing the spike. Note: Size the MOV correctly otherwise things WILL get exciting... Lots of noise and smoke will follow. As I remember the original post... There was also a question about what if the SSR fails on. One of the frequent failure modes in SCRs is to fail on, but the is not necessarily a problem. The circuit can be properly fused to stop any potential problems. Most motors have sigificant thermal mass and can tolerate short over current conditions. For the very cautious a small protection circuit using (small) interlocking relays should be no problem to design thus preventing both SSRs from coming on simultaniously. This exercise is left to the reader. jeff -- Jeffrey A. Bertoia Medar, Inc. ...!uunet!medar!jbertoia 38700 Grand River Ave. jbertoia@medar.com Farmington Hills, MI 48335
jeffd@csd4.csd.uwm.edu (Jeffrey Alan Ding) (05/31/91)
In article <4814@cocoa46.UUCP> reichert@motcid.UUCP (Chuck KD9JQ) writes: >Tried E-Mail...but no luck > >John....What you want is a "Zero Crossing" Solid State Relay that operates >off +5VDC. These will switch the 240VAC or 120VAC line when the Voltage >when the line voltage is zero, therefore very little current is switched! > > Chuck Reichert KD9JQ Does anybody make a Solid State Relay that DOES NOT have the Zero Crossing circuit inside? If so I would be interested who and where I can get them. Please replay E-mail. Thanks. jeffd@csd4.csd.uwm.edu