Contactor/SSR pros & cons

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TnDrew

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Middle TN
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Systems R&D / Maintenance Manager
Pros/Cons: mechanical contactors vs solid state relays (24v control, 208 3 phase, 5hp or less)

and,

Pros/Cons: 100ft max distance: mounting the contactors/relays near the PLC vs controls near the motor (ie: control and power cable costs, signal issues etc)

Bonus Question: Does anyone make an "off the shelf" 3 phase solid state relay capable of controlling multiple motors from single device? (Boss says "If there ain't, there oughta be!").... I'm not convinced of the need for such other than saying it can be done. I would suspect it would be a fairly expensive device and only useful in limited situations.
 
Big Cons for SSRs:
1) Heat. They reject 1-1/2W per running load amp per pole. So a 3 pole SSR for a 5Hp 208V three phase motor rated 16.7A could reject 75W of heat into whatever enclosure it is put into. If that enclosure is not ventilated or seriously over sized at around 4x the necessary volume, with all sides able to dissipate the heat, it will cook itself.

2) Failure mode. SSRs fail shorted, so in a conducting state. That can lead to burning up a motor or not being able to shut it down unless you have additional protection, such as a line contactor or Shunt Trip breaker tied to a Shorted SCR detection feature of the SSR. If you are going to have the contactor anyway, what have you saved?

Edit: You can put as many motors as you want behind any kind of controller. You just have to have the correct Short Circuit and Overload protection for each individual motor and if the only way to kill power to them is the SSR, if one trips, you have to shut them all down. There are ways around that, but this gets into the weeds a bit.
 
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Jraef said:
Big Cons for SSRs:
1) Heat. They reject 1-1/2W per running load amp per pole. So a 3 pole SSR for a 5Hp 208V three phase motor rated 16.7A could reject 75W of heat into whatever enclosure it is put into. If that enclosure is not ventilated or seriously over sized at around 4x the necessary volume, with all sides able to dissipate the heat, it will cook itself.

2) Failure mode. SSRs fail shorted, so in a conducting state. That can lead to burning up a motor or not being able to shut it down unless you have additional protection, such as a line contactor or Shunt Trip breaker tied to a Shorted SCR detection feature of the SSR. If you are going to have the contactor anyway, what have you saved?

Edit: You can put as many motors as you want behind any kind of controller. You just have to have the correct Short Circuit and Overload protection for each individual motor and if the only way to kill power to them is the SSR, if one trips, you have to shut them all down. There are ways around that, but this gets into the weeds a bit.
Click to expand...
I agree on all of that, including the weeds. Thank you for those specifics!

These motors will be running off the Lenze VFD (the one you saw the CYA statement for in the other thread :ROFLMAO: ), so I'm not so much worried about the motors overloading the drive as I am making sure the controls don't do the same if/when they fail. (The motors have already been tested under load on a VFD and didn't seem stressed at all.)

For background, the boss wants a setup that everywhere I read says wouldn't/shouldn't be done. Budgetary reasons prevail, and my task is to make sure the budget has some common sense so it doesn't burn up later.

The single VFD is to provide power & speed control to up to 9 different motor driven augers... but only one auger should/will ever be powered up at a time.

The only way I know to do that is feed the VFD output to a group of contactors, one contactor for each motor, under the conditions that only one given contactor coil must be energized before the VFD output is active, and remain active until after the VFD output is turned off.

This will be controlled by a PLC/HMI, but also have a manual override where he can bypass the PLC with a jog button but still use the existing contactors and wiring vs a fully redundant hardwire using manual 3ph selector switches to bypass the contactors.
 
Jraef said:
That's a totally legit application, you just can't use an SSR on the output of a VFD.
Click to expand...
Thanks... That's one thing I had not run across in any of my research so far!

Would you mind sharing the details behind that for myself and others?
 
SSRs for AC loads use SCRs, which turn on by gating with a string of pulses until current starts to flow, then stay on until the sine wave crosses zero again. But in a VFD output, the AC is a “pseudo-sine wave” comprised of a string of DC pulses and gaps between them that the motor REACTS TO as if it was an AC sine wave. But to the SCRs in the SSR, it is not that true sine wave it is looking for, so the reaction is unpredictable.
 
I wonder if, and it's a really BIG If, there were a sufficient-sized load reactor between the VDF and the SSR it might maybe work.... but I wouldn't count on it. Also if the SSRs are using transistor switches (likely to be IGBTs) instead of SCRs, it also might maybe work. Again, I wouldn't count on it.

However you slice that, contactors are going to be cheaper and more reliable.
 
Jraef said:
SSRs for AC loads use SCRs, which turn on by gating with a string of pulses until current starts to flow, then stay on until the sine wave crosses zero again. But in a VFD output, the AC is a “pseudo-sine wave” comprised of a string of DC pulses and gaps between them that the motor REACTS TO as if it was an AC sine wave. But to the SCRs in the SSR, it is not that true sine wave it is looking for, so the reaction is unpredictable.
Click to expand...
Thank you, makes perfect sense!
 
Jraef said:
SSRs for AC loads use SCRs, which turn on by gating with a string of pulses until current starts to flow, then stay on until the sine wave crosses zero again. But in a VFD output, the AC is a “pseudo-sine wave” comprised of a string of DC pulses and gaps between them that the motor REACTS TO as if it was an AC sine wave. But to the SCRs in the SSR, it is not that true sine wave it is looking for, so the reaction is unpredictable.
Click to expand...
Could probably get around that by using a 1:1 transformer, or maybe a line reactor, but now you're adding cost, space, wasted energy in the windings.... totally obliterating any advantage you might have gained by using the SSRs in the first place.
 
zbang said:
I wonder if, and it's a really BIG If, there were a sufficient-sized load reactor between the VDF and the SSR it might maybe work.... but I wouldn't count on it. Also if the SSRs are using transistor switches (likely to be IGBTs) instead of SCRs, it also might maybe work. Again, I wouldn't count on it.

However you slice that, contactors are going to be cheaper and more reliable.
Click to expand...
SSRs with transistors are for DC loads. SSRs for AC use SCRs or Triacs.
 
zbang said:
Is there any reason you couldn't use a transistor for an AC load, especially where the AC is really switched DC? (I haven't thought about that too much.)
Click to expand...
Well sure, transistors are used in the output of a VFD, which is recreating a “pseudo” AC sine wave, but it is actually SWITCHING a DC voltage. Most transistors are not bi-directional in terms of blocking, so when the AC changes direction, the action that the transistor takes becomes non-linear, meaning unpredictable. This is why if you accidentally apply AC to the output terminals of a VFD, you instantly blow all the transistors. You CAN make transistors work, it’s just a LOT more expensive and complicated than using thyristors (SCRs and triacs) to accomplish the same goal.
 
Never worked with three pole solid state delays but got a small shock working on a single poke solid state relay that had no control voltage. Used my digital VOM meter to find out the type used had leakage current that produced 30 to 40 volts on a 240 volt circuit that was factory installed in new cooling tower immersion heaters. Amazing thing I could not pick up any current flow to the heaters while receiving the 30 to 40 volts.
 
garbo said:
Never worked with three pole solid state delays but got a small shock working on a single poke solid state relay that had no control voltage. Used my digital VOM meter to find out the type used had leakage current that produced 30 to 40 volts on a 240 volt circuit that was factory installed in new cooling tower immersion heaters. Amazing thing I could not pick up any current flow to the heaters while receiving the 30 to 40 volts.
Click to expand...
The leakage is mostly through the RC Snubber that goes across the SCR, so the only reason you read it with a DMM is bender there is no burden. Put a load on it, like a Wiggy, and it reads zero.
 
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